Latest / Night Science Stories / What Is Pulling the Milky Way Through Space

Transcript

What Is Pulling the Milky Way Through Space · Night Science Stories

• 0:01
  Speaker 1: One small thing before we begin. If you regularly fall
• 0:05
  Speaker 1: asleep to these videos, there's a good chance your phone
• 0:08
  Speaker 1: spends the entire night awake beside you, screen glowing away
• 0:12
  Speaker 1: while you're doing the sensible thing and sleeping. There's now
• 0:15
  Speaker 1: a slightly better arrangement. The Sleepy Scientist is on Spotify.
• 0:20
  Speaker 1: Same episodes, same sleepy science, but you can play them
• 0:24
  Speaker 1: in the background, put your phone face down, and finally
• 0:27
  Speaker 1: allow the poor thing to get some rest as well.
• 0:30
  Speaker 1: If you find the show over there, a follow means
• 0:32
  Speaker 1: a lot. And if it's genuinely become part of your
• 0:35
  Speaker 1: nightly routine, one of those things that helps you drift off,
• 0:38
  Speaker 1: a five-star rating would be hugely appreciated. It really does
• 0:41
  Speaker 1: help more people find the show. The link is in
• 0:45
  Speaker 1: the description. Right then, get comfortable. Phone down. Worries temporarily postponed.
• 0:59
  Speaker 2: Hello, and welcome to the Sleepy Scientist. It's lovely to
• 1:02
  Speaker 2: have you along for tonight's journey. Before we go any further,
• 1:06
  Speaker 2: I'd love to know which part of the world you're
• 1:08
  Speaker 2: travelling with us from. Do leave your country or city
• 1:12
  Speaker 2: in the comments if you fancy it. I find it
• 1:15
  Speaker 2: rather wonderful to imagine people listening to the same story
• 1:18
  Speaker 2: from so many different places, some just getting ready for bed,
• 1:22
  Speaker 2: others already halfway to sleep, all of them quietly heading
• 1:25
  Speaker 2: in exactly the same direction. Wherever you are, I hope
• 1:29
  Speaker 2: you've found somewhere comfortable, and that the day's demands can
• 1:32
  Speaker 2: wait until morning. And if these evenings of science have
• 1:35
  Speaker 2: become a pleasant part of your routine, a like or
• 1:38
  Speaker 2: a subscribe is a lovely way to support the channel.
• 1:41
  Speaker 2: Consider it a small, gentle pull in our direction. To
• 1:45
  Speaker 2: everyone who already does, I'm genuinely grateful. You're what makes
• 1:48
  Speaker 2: these little journeys through the universe possible, because right now,
• 1:52
  Speaker 2: without moving an inch, you're on the move. Our entire
• 1:55
  Speaker 2: galaxy is racing through space at an astonishing speed, drawn
• 1:59
  Speaker 2: steadily towards something hundreds of millions of light-years away. And
• 2:04
  Speaker 2: for decades, astronomers have been trying to work out exactly
• 2:07
  Speaker 2: what it is, which is a slightly odd thing not
• 2:10
  Speaker 2: to know about your own destination. I'll try not to
• 2:13
  Speaker 2: make the mystery too difficult to sleep through, although I'll
• 2:16
  Speaker 2: admit discovering that the whole Milky Way is being tugged
• 2:19
  Speaker 2: towards an enormous gathering of matter we can barely see
• 2:22
  Speaker 2: does make an ordinary evening feel rather less ordinary. So,
• 2:27
  Speaker 2: close your eyes, settle back, enjoy the ride, and let's
• 2:31
  Speaker 2: find out. What is pulling the Milky Way through space,
• 2:35
  Speaker 2: and what exactly is the great attractor? Even when everything
• 2:39
  Speaker 2: around us seems perfectly still, we are travelling through space.
• 2:43
  Speaker 2: The ground beneath your feet, the room around you, and
• 2:46
  Speaker 2: every familiar place on Earth are moving together. carried by
• 2:50
  Speaker 2: a planet that is itself part of several much larger journeys.
• 2:54
  Speaker 2: Nothing about this requires us to feel a breeze or
• 2:57
  Speaker 2: hear an engine. We are already moving with our surroundings,
• 3:00
  Speaker 2: so most of these motions pass entirely unnoticed. The most
• 3:05
  Speaker 2: familiar journey is Earth's orbit around the Sun. Our planet
• 3:08
  Speaker 2: travels at an average speed of almost 30 kilometres per second.
• 3:12
  Speaker 2: In the time it takes to say a short sentence,
• 3:15
  Speaker 2: Earth has covered more than 100 kilometres along its path.
• 3:18
  Speaker 2: We do not feel that speed because our bodies, the
• 3:21
  Speaker 2: oceans and the atmosphere accompany the planet. What we experience
• 3:25
  Speaker 2: most directly is a change in motion, rather than a
• 3:28
  Speaker 2: steady speed shared with everything nearby. But the Sun is
• 3:32
  Speaker 2: not standing still while Earth circles it. Along with the planets,
• 3:37
  Speaker 2: it travels around the centre of the Milky Way at
• 3:40
  Speaker 2: roughly 230 kilometres per second. That is an enormous speed
• 3:45
  Speaker 2: by everyday standards. yet the orbit is so large that
• 3:48
  Speaker 2: one complete circuit takes roughly 230 million years. When the
• 3:54
  Speaker 2: Sun last occupied something like its present position along that journey,
• 3:58
  Speaker 2: dinosaurs were living on Earth. Almost everything familiar about our
• 4:02
  Speaker 2: planet has changed since then, while the Sun has continued
• 4:06
  Speaker 2: its long journey around the galaxy. The galaxy carrying us
• 4:10
  Speaker 2: is immense. The Milky Way is a spiral galaxy containing
• 4:14
  Speaker 2: hundreds of billions of stars, together with vast clouds of
• 4:18
  Speaker 2: gas and dust and a much larger distribution of invisible
• 4:22
  Speaker 2: dark matter. Its bright disk stretches across roughly 100,000 light-years,
• 4:28
  Speaker 2: although the galaxy's more diffuse outer regions extend much farther.
• 4:32
  Speaker 2: Our solar system lies in that disk, around 26,000 light-years
• 4:37
  Speaker 2: from the galactic centre. We are not close to its middle,
• 4:41
  Speaker 2: nor are we perched on the very edge. There is
• 4:43
  Speaker 2: an important detail here. Earth's speed around the Sun and
• 4:47
  Speaker 2: the Sun's speed around the galactic centre are measured relative
• 4:51
  Speaker 2: to different things. The first describes our motion compared with
• 4:55
  Speaker 2: the Sun. The second describes the Sun's motion compared with
• 4:58
  Speaker 2: the overall structure of the galaxy. Speeds cannot simply be
• 5:02
  Speaker 2: added together as though all these journeys follow one straight road.
• 5:06
  Speaker 2: Their directions change, and any answer to how far something
• 5:09
  Speaker 2: is moving must include what it is moving relative to.
• 5:13
  Speaker 2: This is the idea of a reference frame. one of
• 5:16
  Speaker 2: the most useful concepts in understanding motion. The same principle
• 5:20
  Speaker 2: applies when we consider the motion of the Milky Way itself.
• 5:23
  Speaker 2: Our galaxy is not alone. It belongs to the Local Group,
• 5:27
  Speaker 2: a collection of more than 50 known galaxies held together
• 5:30
  Speaker 2: by their mutual gravity. It's two largest members are the
• 5:34
  Speaker 2: Milky Way and the Andromeda Galaxy, with the smaller Triangulum
• 5:38
  Speaker 2: Galaxy and numerous Dwarf Galaxies also belonging to the neighbourhood.
• 5:42
  Speaker 2: These galaxies move in relation to one another, yet the
• 5:46
  Speaker 2: group as a whole also has a measurable movement relative
• 5:49
  Speaker 2: to the wider cosmos. Astronomers can describe that movement using
• 5:53
  Speaker 2: the cosmic microwave background. This is faint radiation reaching us
• 5:58
  Speaker 2: from an early period in the universe's history, spread across
• 6:01
  Speaker 2: the sky in almost every direction. It offers a particularly
• 6:05
  Speaker 2: useful reference against which large-scale motion can be measured. Relative
• 6:10
  Speaker 2: to the frame in which this radiation looks nearly the
• 6:12
  Speaker 2: same in every direction, The local group is travelling at
• 6:16
  Speaker 2: approximately 630km per second, that is more than 2 million
• 6:21
  Speaker 2: kilometres per hour. At that speed, a traveller could cross
• 6:25
  Speaker 2: the distance between Earth and the Moon in around 10 minutes,
• 6:28
  Speaker 2: if such a journey could be made in a straight line.
• 6:31
  Speaker 2: Yet nothing inside the Milky Way needs to be shaken
• 6:34
  Speaker 2: about by this shared motion. The stars, planets and interstellar
• 6:38
  Speaker 2: gas are all participating in it. Just as passengers on
• 6:41
  Speaker 2: a smoothly moving train share the train's speed without having
• 6:45
  Speaker 2: to race from one end of the carriage to the other,
• 6:48
  Speaker 2: does that mean the local group has a special speed
• 6:50
  Speaker 2: through space, measured against a universe that is absolutely at rest?
• 6:55
  Speaker 2: Not quite. The Cosmic Microwave Background gives astronomers a useful
• 7:00
  Speaker 2: cosmic reference frame, not a universal stationary platform. Motion still
• 7:06
  Speaker 2: depends on the frame chosen. What makes this measurement remarkable
• 7:11
  Speaker 2: is that it reveals a collective movement that extends beyond
• 7:14
  Speaker 2: the individual orbits of stars and the interactions between neighbouring galaxies.
• 7:19
  Speaker 2: And there is a reason that movement interests astronomers. Galaxies
• 7:23
  Speaker 2: respond to gravity. Mass is spread unevenly through the universe,
• 7:27
  Speaker 2: with some regions crowded by clusters of galaxies and others
• 7:31
  Speaker 2: containing relatively little matter. These imbalances can alter the movements
• 7:35
  Speaker 2: of galaxies over immense stretches of time. The Milky Way
• 7:39
  Speaker 2: is not being towed by a single hidden object. Its
• 7:42
  Speaker 2: movement reflects the combined gravitational influence of many concentrations of matter,
• 7:48
  Speaker 2: both near and far. Some of these concentrations contain thousands
• 7:53
  Speaker 2: of galaxies, while others form even larger arrangements stretching across
• 7:57
  Speaker 2: hundreds of millions of light years. It also helps to
• 8:00
  Speaker 2: separate speed from acceleration. Speed tells us how quickly a
• 8:05
  Speaker 2: distance is being covered, while velocity also includes direction. Acceleration
• 8:11
  Speaker 2: means a change in velocity, whether that change involves speeding up,
• 8:15
  Speaker 2: slowing down or turning. Gravity causes acceleration, but the velocity
• 8:20
  Speaker 2: we observe today preserves the consequences of gravitational influences acting
• 8:25
  Speaker 2: over much of cosmic history. The direction in which we
• 8:29
  Speaker 2: travel now is not necessarily the direction of the strongest
• 8:32
  Speaker 2: gravitational pull at this very moment. Our present movement carries
• 8:36
  Speaker 2: the accumulated influence of an enormous and changing cosmic environment.
• 8:41
  Speaker 2: So the question of what is pulling the Milky Way
• 8:43
  Speaker 2: through space has a more interesting answer than the image
• 8:47
  Speaker 2: of one gigantic object waiting somewhere ahead. Entire groups and
• 8:51
  Speaker 2: clusters of galaxies take part in related movements across distances
• 8:56
  Speaker 2: almost too large to imagine. Some of the matter responsible
• 8:59
  Speaker 2: is easy to see through telescopes. Some is difficult to
• 9:03
  Speaker 2: observe because our own galaxy stands in the way, and
• 9:06
  Speaker 2: much of it does not shine at all. From our
• 9:09
  Speaker 2: small position on Earth, the night sky can appear fixed
• 9:12
  Speaker 2: and peaceful. Yet the stars above us belong to a
• 9:16
  Speaker 2: galaxy in motion, surrounded by other galaxies moving through a
• 9:19
  Speaker 2: landscape shaped by gravity. Astronomers have learned to measure that
• 9:24
  Speaker 2: movement with extraordinary care, even when the matter responsible lies
• 9:28
  Speaker 2: far beyond anything the human eye could ever detect. What
• 9:32
  Speaker 2: appears to us as a perfectly ordinary patch of night
• 9:35
  Speaker 2: sky is part of a vast gravitational arrangement, one that
• 9:40
  Speaker 2: has been influencing the journey of our galactic home for
• 9:43
  Speaker 2: billions of years. Measuring the motion of an entire galaxy
• 9:46
  Speaker 2: presents an awkward problem. Every telescope we build travels with
• 9:51
  Speaker 2: the Earth, and the Earth travels with the Sun, which
• 9:54
  Speaker 2: itself belongs to the Milky Way. There is no convenient
• 9:58
  Speaker 2: observation deck outside our galaxy, where someone can watch us
• 10:01
  Speaker 2: pass by. Instead, astronomers need something visible across the sky
• 10:06
  Speaker 2: that provides a useful reference for our movement. Remarkably, one
• 10:11
  Speaker 2: of the best such references is light that has been
• 10:14
  Speaker 2: travelling towards us for almost the entire history of the universe.
• 10:18
  Speaker 2: This light is called the cosmic microwave background. It originated
• 10:24
  Speaker 2: when the universe was approximately 380,000 years old, long before
• 10:29
  Speaker 2: stars and galaxies had formed. Earlier than this, space was
• 10:33
  Speaker 2: filled with hot, dense material in which light repeatedly scattered
• 10:38
  Speaker 2: from free electrons. As the universe expanded and cooled, electrons
• 10:43
  Speaker 2: joined atomic nuclei to form mostly neutral hydrogen and helium.
• 10:48
  Speaker 2: Photons could then travel far more freely. Some of that
• 10:52
  Speaker 2: ancient radiation is still reaching Earth today, arriving from every direction.
• 10:58
  Speaker 2: The radiation did not begin its journey as microwaves. When
• 11:02
  Speaker 2: it was released, the universe was thousands of degrees hotter
• 11:05
  Speaker 2: than it is now, and the light had much shorter wavelengths.
• 11:09
  Speaker 2: Over billions of years, the expansion of space stretched those
• 11:13
  Speaker 2: wavelengths into the microwave part of the electromagnetic spectrum. The
• 11:18
  Speaker 2: resulting glow has an average temperature of approximately 2.7 Kelvin,
• 11:23
  Speaker 2: only a little above absolute zero. It is remarkably uniform,
• 11:28
  Speaker 2: a faint remnant of an early universe that looked very
• 11:31
  Speaker 2: similar in every direction. In 1965, Arne Penzias and Robert
• 11:37
  Speaker 2: Wilson reported detecting this radiation while working with a sensitive
• 11:41
  Speaker 2: radio antenna at Bell Telephone Laboratories in New Jersey. They
• 11:45
  Speaker 2: had been investigating an unwanted background signal that persisted wherever
• 11:49
  Speaker 2: they pointed their instrument. The signal was not explained by
• 11:53
  Speaker 2: ordinary equipment noise or familiar astronomical sources. At the same time,
• 11:59
  Speaker 2: researchers at Princeton University were preparing to search for radiation
• 12:03
  Speaker 2: left over from a hot early universe. The two groups
• 12:06
  Speaker 2: recognised that the unexpected signal was precisely the sort of
• 12:10
  Speaker 2: cosmic background the Princeton scientists had hoped to find. That
• 12:15
  Speaker 2: discovery gave powerful support to the Big Bang model, but
• 12:18
  Speaker 2: the background contained another useful clue. Although its temperature is
• 12:23
  Speaker 2: almost the same across the sky, one broad region appears
• 12:27
  Speaker 2: slightly warmer and the opposite region slightly cooler. The difference
• 12:31
  Speaker 2: is incredibly small, measured in thousandths of a Kelvin. Yet
• 12:37
  Speaker 2: the pattern is far too orderly to dismiss as random variation.
• 12:41
  Speaker 2: It is called the Cosmic Microwave Background Depole, and it
• 12:46
  Speaker 2: reveals something about the motion of the observer. Imagine travelling
• 12:50
  Speaker 2: through a gentle rain. Even if the rain falls evenly,
• 12:55
  Speaker 2: more drops seem to approach from the direction in which
• 12:57
  Speaker 2: you move. Light behaves differently from raindrops, but motion still
• 13:03
  Speaker 2: changes the radiation that an observer measures. In the direction
• 13:07
  Speaker 2: of travel, the microwave radiation is shifted towards slightly shorter
• 13:11
  Speaker 2: wavelengths and appears warmer. Behind the observer, it is shifted
• 13:15
  Speaker 2: towards longer wavelengths and appears cooler. This change is an
• 13:20
  Speaker 2: example of the Doppler effect, familiar in another form when
• 13:23
  Speaker 2: the pitch of a passing vehicle changes as it goes by.
• 13:27
  Speaker 2: By comparing the warmer and cooler regions, astronomers can calculate
• 13:31
  Speaker 2: both a speed and a direction. Crucially, the measurement made
• 13:35
  Speaker 2: near Earth first describes the movement of our own observing position.
• 13:40
  Speaker 2: The Sun and Solar System move at about 370 km
• 13:43
  Speaker 2: per second relative to the cosmic microwave background. Astronomers then
• 13:48
  Speaker 2: account for our motion within the Milky Way and the
• 13:51
  Speaker 2: motions of nearby galaxies to estimate how the local group
• 13:55
  Speaker 2: moves as a whole. That calculation gives a speed of
• 13:59
  Speaker 2: approximately 630 km per second. The accuracy of these measurements
• 14:04
  Speaker 2: improved enormously with spacecraft observations. In 1989, the National Aeronautics
• 14:11
  Speaker 2: and Space Administration launched the Cosmic Background Explorer satellite. John
• 14:16
  Speaker 2: Mather led work measuring the background spectrum, showing how closely
• 14:20
  Speaker 2: it matches the radiation expected from an almost perfect thermal source.
• 14:26
  Speaker 2: George Smoot led investigations of temperature variations across the sky.
• 14:31
  Speaker 2: the satellite helped establish that the dipole was a real,
• 14:34
  Speaker 2: measurable feature, while also revealing much smaller patterns linked to
• 14:38
  Speaker 2: the early distribution of matter. Those tiny temperature differences must
• 14:43
  Speaker 2: not be confused with an absolute cosmic compass. According to
• 14:48
  Speaker 2: Albert Einstein's theory of relativity, there is no universal platform
• 14:52
  Speaker 2: at rest against which every motion must be judged. The
• 14:56
  Speaker 2: cosmic microwave background instead supplies a particularly useful frame of reference,
• 15:01
  Speaker 2: one in which its large-scale temperature pattern is approximately the
• 15:05
  Speaker 2: same in every direction once the effect of our motion
• 15:08
  Speaker 2: is removed. It describes our movement relative to the average
• 15:12
  Speaker 2: cosmic environment, not motion against space itself. Nor does the
• 15:17
  Speaker 2: depot tell us what is producing that movement. A thermometer
• 15:21
  Speaker 2: pointed across the sky can reveal the speed of the
• 15:24
  Speaker 2: local group. but it cannot identify the particular galaxy clusters
• 15:28
  Speaker 2: or distant concentrations of matter that have accelerated us over
• 15:32
  Speaker 2: billions of years. That requires another kind of investigation, comparing
• 15:37
  Speaker 2: the positions and movements of galaxies across immense distances. Still,
• 15:42
  Speaker 2: the background radiation has given astronomers an extraordinary starting point.
• 15:48
  Speaker 2: A faint glow from the early universe allows us to
• 15:51
  Speaker 2: detect the journey of our entire galactic neighbourhood even while
• 15:55
  Speaker 2: everything immediately around us seems perfectly still. The universe is expanding,
• 16:01
  Speaker 2: but that does not mean every galaxy is moving away
• 16:04
  Speaker 2: from every other galaxy. In fact, some galaxies are approaching
• 16:08
  Speaker 2: one another, some are falling into enormous clusters, and others
• 16:12
  Speaker 2: are travelling together through space at hundreds of kilometres per second.
• 16:17
  Speaker 2: These movements might appear to contradict the expansion of the universe,
• 16:21
  Speaker 2: but they are perfectly compatible with it. To understand why,
• 16:26
  Speaker 2: we need to distinguish between the general expansion of the
• 16:29
  Speaker 2: cosmos and the additional movements produced by gravity. During the
• 16:33
  Speaker 2: early 20th century, astronomers began to discover that distant galaxies
• 16:38
  Speaker 2: were behaving in a rather remarkable way. In 1927, the
• 16:43
  Speaker 2: Belgian physicist and priest Georges Lemaître published a theoretical explanation
• 16:48
  Speaker 2: connecting an expanding universe with the observed movements of galaxies.
• 16:54
  Speaker 2: Two years later, the American astronomer Edwin Hubble presented observations
• 16:59
  Speaker 2: showing that more distant galaxies generally exhibited larger recession velocities. Together,
• 17:05
  Speaker 2: these developments helped establish one of the fundamental principles of
• 17:09
  Speaker 2: modern cosmology. Today, that relationship is known as the Hubble-Lemaître law.
• 17:15
  Speaker 2: It describes how the recession velocity associated with cosmic expansion
• 17:20
  Speaker 2: increases with distance. A galaxy twice as far away will
• 17:24
  Speaker 2: approximately have twice the expansion-related recession velocity, provided we are
• 17:29
  Speaker 2: considering distances where this simple relationship applies. The important point
• 17:34
  Speaker 2: is that the expansion is not caused by galaxies being
• 17:37
  Speaker 2: thrown outward from a central explosion. Instead, the distances between
• 17:42
  Speaker 2: sufficiently separated, gravitationally unbound objects increase as the universe evolves.
• 17:49
  Speaker 2: A useful comparison is a collection of marks on an
• 17:52
  Speaker 2: expanding rubber sheet. As the sheet stretches, the marks become
• 17:56
  Speaker 2: more widely separated, even though none needs to move independently
• 18:00
  Speaker 2: across the rubber. The comparison is imperfect, because the universe
• 18:05
  Speaker 2: is not a sheet stretching into some surrounding space. Nevertheless,
• 18:10
  Speaker 2: it captures the distinction between objects travelling through their surroundings
• 18:14
  Speaker 2: and the distances between them increasing through cosmic expansion. Real galaxies, however,
• 18:21
  Speaker 2: do not behave like perfectly stationary marks. They possess mass,
• 18:25
  Speaker 2: and mass produces gravitational effects. A galaxy can therefore move
• 18:30
  Speaker 2: relative to the average expansion, sometimes towards a neighbouring galaxy
• 18:35
  Speaker 2: and sometimes away from it. Astronomers call this additional movement
• 18:39
  Speaker 2: a peculiar velocity. Despite the slightly unusual name, there is
• 18:44
  Speaker 2: nothing particularly peculiar about it. It simply describes motion. that
• 18:49
  Speaker 2: differs from the smooth expansion expected at a particular location.
• 18:54
  Speaker 2: The Milky Way and the Andromeda Galaxy provide a familiar example.
• 18:59
  Speaker 2: Andromeda lies approximately 2.5 million light-years away, yet it is
• 19:04
  Speaker 2: approaching our galaxy rather than receding from it. The two
• 19:08
  Speaker 2: galaxies belong to the same gravitationally bound local group, where
• 19:12
  Speaker 2: their mutual attraction is sufficient to overcome the relatively modest
• 19:16
  Speaker 2: expansion effect across their separation. This does not mean cosmic
• 19:20
  Speaker 2: expansion has stopped. It means gravity dominates the movements of
• 19:25
  Speaker 2: these particular neighbours. The reason gravity can produce such movements
• 19:30
  Speaker 2: was first described mathematically by Isaac Newton during the 17th century.
• 19:36
  Speaker 2: His law of universal gravitation established that objects with mass
• 19:40
  Speaker 2: attract one another, with the strength of their mutual gravitational
• 19:44
  Speaker 2: force depending on their masses and the distance separating them.
• 19:48
  Speaker 2: increasing the mass stream the attraction, while increasing the separation
• 19:53
  Speaker 2: weakens it. Newton's description remains extraordinarily useful when calculating the
• 19:59
  Speaker 2: movements of galaxies and galaxy clusters under many ordinary astronomical conditions.
• 20:06
  Speaker 2: Albert Einstein provided a deeper understanding through his General Theory
• 20:10
  Speaker 2: of Relativity, published in 1915. In this description, gravity is
• 20:16
  Speaker 2: associated with the curvature of spacetime produced by mass and energy.
• 20:21
  Speaker 2: Objects move through that curved spacetime following paths determined by
• 20:25
  Speaker 2: its geometry. The mathematics is more sophisticated than Newton's original description,
• 20:31
  Speaker 2: but both approaches explain why concentrations of matter can influence
• 20:35
  Speaker 2: the movements of surrounding galaxies. For much of the relatively
• 20:39
  Speaker 2: slow motion involved in nearby galaxy flows, Newtonian calculations provide
• 20:45
  Speaker 2: very good approximations. Now consider what happens when the attracting
• 20:49
  Speaker 2: object is not one galaxy, but an entire galaxy cluster.
• 20:54
  Speaker 2: Some clusters contain thousands of galaxies, enormous quantities of hot gas,
• 20:59
  Speaker 2: and vast distributions of dark matter. their combined masses can
• 21:04
  Speaker 2: reach hundreds of trillions of times the mass of the Sun.
• 21:09
  Speaker 2: Such concentrations generate gravitational fields capable of influencing galaxies far
• 21:14
  Speaker 2: beyond the bright central regions of the clusters themselves. Larger
• 21:19
  Speaker 2: arrangements of clusters and filaments can produce effects extending across
• 21:24
  Speaker 2: hundreds of millions of light-years. These movements occur on several
• 21:28
  Speaker 2: different scales. Stars orbit within galaxies. Galaxies move within groups
• 21:34
  Speaker 2: and clusters, and entire groups can participate in broader patterns
• 21:38
  Speaker 2: of motion through the cosmic web. The Sun's movement around
• 21:42
  Speaker 2: the Milky Way is therefore a different phenomenon from the
• 21:44
  Speaker 2: movement of the local group, towards large concentrations of matter.
• 21:49
  Speaker 2: Both involve gravity, but their gravitational environments, characteristic distances and
• 21:55
  Speaker 2: timescales are very different. There is another essential ingredient. Matter
• 22:01
  Speaker 2: is not distributed evenly throughout the universe. Some regions contain
• 22:05
  Speaker 2: substantial concentrations of galaxies, while others are comparatively empty. If
• 22:11
  Speaker 2: equal gravitational attractions acted upon an object from every direction,
• 22:16
  Speaker 2: their effects could largely cancel. But the surrounding universe is
• 22:20
  Speaker 2: not arranged with such perfect symmetry. Stronger influences from particular
• 22:26
  Speaker 2: directions can leave a net gravitational acceleration. gradually changing the
• 22:31
  Speaker 2: motion of a galaxy or an entire group. Distance matters,
• 22:34
  Speaker 2: of course. The gravitational influence of an isolated, approximately spherical
• 22:40
  Speaker 2: mass decreases as the separation increases, but an extremely massive
• 22:45
  Speaker 2: concentration at a great distance can still produce a measurable effect.
• 22:50
  Speaker 2: This is why astronomers cannot explain the Milky Way's movement
• 22:54
  Speaker 2: simply by identifying the nearest large galaxy. They must consider
• 22:59
  Speaker 2: the combined influences of structures distributed across an enormous region,
• 23:05
  Speaker 2: including matter that cannot easily be observed. It is also
• 23:09
  Speaker 2: important to distinguish acceleration from velocity. Gravity changes velocity, but
• 23:15
  Speaker 2: a galaxy's present movement is not necessarily directed towards the
• 23:19
  Speaker 2: strongest gravitational influence acting upon it today. Its velocity has
• 23:25
  Speaker 2: developed through an extended history of acceleration. as the distribution
• 23:29
  Speaker 2: of matter evolved around it. A galaxy may continue travelling
• 23:34
  Speaker 2: in a particular direction, even while gravity gradually changes its
• 23:39
  Speaker 2: speed or bends its trajectory. The movement we measure is
• 23:44
  Speaker 2: therefore a record of gravitational influences accumulated over billions of years.
• 23:49
  Speaker 2: Astronomers can investigate these influences by comparing a galaxy's observed
• 23:54
  Speaker 2: recession velocity with the velocity expected from cosmic expansion at
• 23:59
  Speaker 2: its independently measured distance. A difference between the two can
• 24:04
  Speaker 2: reveal a peculiar velocity, although the calculation becomes more complicated
• 24:09
  Speaker 2: over very large cosmological distances. When many galaxies display related deviations,
• 24:16
  Speaker 2: the pattern can reveal a much broader gravitational disturbance. That
• 24:20
  Speaker 2: is how the movement of galaxies becomes a tool for
• 24:23
  Speaker 2: studying matter across the universe. An individual galaxy may tell
• 24:28
  Speaker 2: us comparatively little about the structures surrounding it, but the
• 24:32
  Speaker 2: collective movements of thousands of galaxies can reveal enormous concentrations
• 24:37
  Speaker 2: of mass, including regions that remain difficult to observe directly.
• 24:42
  Speaker 2: The expansion of the universe provides the general background against
• 24:46
  Speaker 2: which these movements are measured, while gravity adds the intricate
• 24:50
  Speaker 2: variations that make the cosmic landscape so interesting. Our own
• 24:55
  Speaker 2: galaxy participates in that enormous pattern, responding to gravitational influences
• 25:00
  Speaker 2: extending far beyond the stars we can see in the
• 25:03
  Speaker 2: night sky. A distant galaxy can tell us a remarkable
• 25:07
  Speaker 2: amount about itself through the light it sends across space.
• 25:12
  Speaker 2: Astronomers can estimate what kinds of stars it contains, how
• 25:16
  Speaker 2: quickly some of those stars are moving, and even something
• 25:19
  Speaker 2: about the chemical elements present within them. but discovering how
• 25:24
  Speaker 2: the entire galaxy is moving presents a rather different challenge.
• 25:29
  Speaker 2: Unlike a nearby car, a galaxy has no obvious speedometer,
• 25:33
  Speaker 2: and the light reaching our telescopes does not immediately reveal
• 25:37
  Speaker 2: whether it is moving faster or slower than the expansion
• 25:40
  Speaker 2: of the universe would predict. One of the most useful
• 25:44
  Speaker 2: measurements in astronomy is redshift. When astronomers spread the light
• 25:48
  Speaker 2: from a galaxy into a spectrum, They can identify distinctive
• 25:52
  Speaker 2: patterns associated with particular chemical elements. Hydrogen, for example, produces
• 25:58
  Speaker 2: recognisable spectral lines at known wavelengths. By comparing these lines
• 26:03
  Speaker 2: with measurements made in laboratories on Earth, researchers can determine
• 26:08
  Speaker 2: how much the incoming light has shifted towards longer or
• 26:11
  Speaker 2: shorter wavelengths. This shift provides valuable information about the changing
• 26:16
  Speaker 2: separation between the galaxy and the observer. For distant galaxies,
• 26:21
  Speaker 2: the dominant source of redshift is usually the expansion of
• 26:24
  Speaker 2: the universe. As light travels through expanding space, its wavelength increases.
• 26:32
  Speaker 2: The amount of stretching is related to how much the
• 26:34
  Speaker 2: universe has expanded during the light's journey. Motion through space
• 26:39
  Speaker 2: can also produce a Doppler shift, which adds to or
• 26:43
  Speaker 2: subtracts from the effect of cosmic expansion. Consequently, The redshift
• 26:48
  Speaker 2: observed from a galaxy can contain information about both processes.
• 26:53
  Speaker 2: Here is where the difficulty appears. Suppose astronomers observe a
• 26:57
  Speaker 2: galaxy with a redshift corresponding to a recession velocity of
• 27:01
  Speaker 2: several thousand kilometers per second. That measurement alone does not
• 27:05
  Speaker 2: establish how much of its recession is associated with cosmic
• 27:09
  Speaker 2: expansion and how much comes from the galaxy's own peculiar velocity.
• 27:15
  Speaker 2: To separate those contributions, researchers need another piece of information.
• 27:20
  Speaker 2: They must determine the galaxy's distance without simply calculating it
• 27:25
  Speaker 2: from the redshift they're trying to investigate. Fortunately, galaxies contain
• 27:30
  Speaker 2: physical properties that can be used to estimate their distances.
• 27:34
  Speaker 2: One particularly important discovery was made by the astronomers Richard
• 27:39
  Speaker 2: Brent Tully and Richard Fisher in 1977. They identified an
• 27:44
  Speaker 2: empirical relationship between the rotation speed of a spiral galaxy
• 27:49
  Speaker 2: and its intrinsic luminosity. This became known as the Tully-Fisher relation,
• 27:55
  Speaker 2: and it provided astronomers with a powerful method for estimating
• 27:58
  Speaker 2: distances far beyond the reach of many individual stellar measurements.
• 28:03
  Speaker 2: The principle is that spiral galaxies rotating more rapidly tend,
• 28:07
  Speaker 2: on average, to be intrinsically brighter. Their rotation speeds can
• 28:13
  Speaker 2: be measured using the movement of gas within their disks.
• 28:16
  Speaker 2: Neutral hydrogen is especially useful because it emits radio waves
• 28:21
  Speaker 2: with a characteristic wavelength of approximately 21 centimetres. As different
• 28:27
  Speaker 2: regions of a rotating galaxy move towards or away from
• 28:31
  Speaker 2: the observer, their radio emissions are shifted slightly in frequency.
• 28:36
  Speaker 2: The resulting broadening of the hydrogen emission line reveals information
• 28:40
  Speaker 2: about the galaxy's rotation, once its viewing angle has been
• 28:43
  Speaker 2: taken into account. With an estimate of the rotation speed,
• 28:48
  Speaker 2: astronomers can use the Tully-Fisher relation to infer the galaxy's
• 28:51
  Speaker 2: intrinsic brightness. They then compare that estimate with how bright
• 28:55
  Speaker 2: the galaxy actually appears from Earth. A distant galaxy appears
• 29:00
  Speaker 2: fainter than an otherwise identical nearby galaxy. because its light
• 29:04
  Speaker 2: spreads across an increasingly large area. This comparison gives an
• 29:10
  Speaker 2: independent estimate of distance. The method involves uncertainty, but it
• 29:16
  Speaker 2: makes something possible that redshift measurements alone cannot accomplish. Not
• 29:21
  Speaker 2: every galaxy is a spiral, however. Elliptical galaxies often contain
• 29:26
  Speaker 2: stars moving in many different directions rather than following the
• 29:30
  Speaker 2: orderly rotation of a thin disk. Sandra Moore. Faber and
• 29:35
  Speaker 2: Robert Jackson identified another useful relationship in 1976. connecting the
• 29:41
  Speaker 2: intrinsic luminosities of elliptical galaxies with the range of stellar
• 29:45
  Speaker 2: velocities inside them. This is called the Faber-Jackson relation. By
• 29:52
  Speaker 2: examining the broadening of absorption lines in a galaxy's spectrum,
• 29:55
  Speaker 2: astronomers can estimate how rapidly its stars move and use
• 29:59
  Speaker 2: that information to help determine its luminosity. A more sophisticated
• 30:04
  Speaker 2: approach is the fundamental plane relation. This combines three observable
• 30:10
  Speaker 2: properties of elliptical galaxies, their effective physical sizes, their average
• 30:15
  Speaker 2: surface brightnesses, and the velocity dispersions of their stars. Velocity
• 30:21
  Speaker 2: dispersion describes the spread of stellar velocities within a galaxy. Together,
• 30:27
  Speaker 2: these measurements define an empirical relationship that allows astronomers to
• 30:31
  Speaker 2: estimate distances with greater precision than the simplest luminosity relation alone.
• 30:38
  Speaker 2: The fundamental plane became especially valuable for investigating the movements
• 30:43
  Speaker 2: of large collections of elliptical galaxies. All these techniques depend
• 30:47
  Speaker 2: upon reliable calibrations. Astronomers establish the distance scale using objects
• 30:53
  Speaker 2: whose distances can be determined through other methods. Cepheid variable
• 30:58
  Speaker 2: stars are particularly important because their pulsation periods are related
• 31:02
  Speaker 2: to their intrinsic luminosities, a relationship discovered by Henrietta Swan Leavitt.
• 31:08
  Speaker 2: Certain supernovae, especially Type I supernovae, provide another valuable means
• 31:13
  Speaker 2: of estimating astronomical distances once their brightnesses have been properly calibrated. Together,
• 31:19
  Speaker 2: these methods help establish the cosmic distance ladder. Once a
• 31:23
  Speaker 2: galaxy's distance has been estimated independently, its observed redshift can
• 31:28
  Speaker 2: be compared with the recession expected from the Hubble-Lemaître law.
• 31:33
  Speaker 2: At relatively modest cosmological distances, the difference provides an estimate
• 31:38
  Speaker 2: of its radial peculiar velocity after accounting for the observer's
• 31:42
  Speaker 2: own motion. A galaxy receding more slowly than expected may
• 31:46
  Speaker 2: possess a peculiar velocity towards us, while one receding more
• 31:50
  Speaker 2: rapidly may have an additional component directed away from us.
• 31:54
  Speaker 2: There is an important limitation. These measurements reveal movement along
• 31:58
  Speaker 2: our line of sight, rather than the complete three-dimensional velocity
• 32:02
  Speaker 2: of a distant galaxy. A galaxy could also be travelling
• 32:06
  Speaker 2: sideways across the sky, but detecting that transverse motion directly
• 32:10
  Speaker 2: is extremely difficult at great distances. Astronomers must therefore reconstruct
• 32:16
  Speaker 2: much of the wider movement using statistical methods and information
• 32:20
  Speaker 2: from neighbouring galaxies. Individual distance estimates can also contain substantial errors.
• 32:26
  Speaker 2: A small percentage error in a galaxy's distance can translate
• 32:29
  Speaker 2: into hundreds of kilometres per second of uncertainty in its
• 32:33
  Speaker 2: inferred peculiar velocity. But when thousands of galaxies are measured,
• 32:39
  Speaker 2: broader patterns begin to emerge. Random uncertainties can be reduced statistically,
• 32:44
  Speaker 2: While coherent movements across large regions provide evidence of genuine
• 32:49
  Speaker 2: gravitational influences, this was the essential breakthrough behind the investigation
• 32:54
  Speaker 2: of the greater tractor. Astronomers discovered that many galaxies were
• 32:58
  Speaker 2: not following the smooth expansion pattern expected from their distances.
• 33:02
  Speaker 2: Their movements suggested the influence of enormous concentrations of matter,
• 33:07
  Speaker 2: including structures that were difficult to observe directly. By carefully
• 33:12
  Speaker 2: measuring how galaxies departed from the general cosmic expansion, researchers
• 33:17
  Speaker 2: could begin to reconstruct the invisible, gravitational landscape surrounding our
• 33:22
  Speaker 2: own galactic home. The Milky Way is surrounded by a
• 33:25
  Speaker 2: remarkably varied collection of galaxies, stretching from our nearest companions
• 33:30
  Speaker 2: to enormous clusters tens of millions of light-years away. These
• 33:36
  Speaker 2: galaxies are not scattered randomly through an otherwise featureless universe.
• 33:40
  Speaker 2: They gather into groups, form great concentrations of matter, and
• 33:45
  Speaker 2: trace enormous arrangements that extend far beyond the boundaries of
• 33:48
  Speaker 2: individual galaxies. Our own galactic neighbourhood is part of this
• 33:53
  Speaker 2: complicated structure, and its uneven distribution of matter plays an
• 33:57
  Speaker 2: important role in determining how the Milky Way moves through space.
• 34:01
  Speaker 2: The closest of these arrangements is the Local Group, a
• 34:05
  Speaker 2: gravitationally associated collection containing the Milky Way. the Andromeda Galaxy,
• 34:10
  Speaker 2: the Triangulum Galaxy and dozens of smaller galaxies. Its two
• 34:15
  Speaker 2: largest members, the Milky Way and Andromeda, dominate much of
• 34:18
  Speaker 2: the group's gravitational behaviour. Around them are numerous dwarf galaxies,
• 34:23
  Speaker 2: some travelling as satellites of the larger systems and others
• 34:27
  Speaker 2: occupying more distant parts of the group. Although the local
• 34:31
  Speaker 2: group extends across millions of light years, its members remain
• 34:34
  Speaker 2: connected by their mutual gravitational attraction. Andromeda provides a particularly
• 34:40
  Speaker 2: clear demonstration of this relationship. Despite being approximately 2.5 million
• 34:46
  Speaker 2: light-years away, it is approaching the Milky Way rather than
• 34:49
  Speaker 2: receding from it with the expansion of the universe. Its
• 34:53
  Speaker 2: motion reflects the gravitational interaction between two enormous galaxies and
• 34:58
  Speaker 2: their surrounding dark matter. This approach is quite different from
• 35:02
  Speaker 2: the movement of the Local Group as a whole through
• 35:04
  Speaker 2: the wider cosmos. One concerns the changing separation between neighbouring galaxies.
• 35:10
  Speaker 2: The other describes the collective movement of our entire galactic
• 35:14
  Speaker 2: neighbourhood relative to more distant structures. Beyond the Local Group
• 35:19
  Speaker 2: lies a somewhat larger arrangement known as the Local Sheet.
• 35:22
  Speaker 2: This is a relatively flattened distribution of nearby galaxies, including
• 35:27
  Speaker 2: our own group and several neighbouring systems, extending across tens
• 35:31
  Speaker 2: of millions of light-years. The galaxies within it occupy something
• 35:36
  Speaker 2: resembling a broad, thin layer, rather than a roughly spherical collection.
• 35:41
  Speaker 2: Although they have their own individual movements, many share a
• 35:44
  Speaker 2: substantial component of motion through the surrounding universe. The local
• 35:49
  Speaker 2: sheet is therefore useful for understanding how our immediate neighbourhood
• 35:52
  Speaker 2: behaves within a much larger gravitational environment. Such flattened arrangements
• 35:58
  Speaker 2: are common features of the cosmic web. On enormous scales,
• 36:02
  Speaker 2: Galaxies collect along filaments and sheets, with dense clusters appearing
• 36:07
  Speaker 2: where substantial concentrations intersect. Between them lie regions containing comparatively
• 36:14
  Speaker 2: little matter. The result is a universe that resembles a
• 36:17
  Speaker 2: vast network of connected structures, although the pattern is three-dimensional
• 36:22
  Speaker 2: and considerably more complicated than any ordinary web. This arrangement
• 36:26
  Speaker 2: emerged gradually as gravity amplified small differences in the density
• 36:31
  Speaker 2: of matter during the universe's history. One of the most
• 36:34
  Speaker 2: important nearby concentrations is the Virgo Cluster, situated approximately 50
• 36:40
  Speaker 2: to 60 million light-years from Earth. It contains well over
• 36:43
  Speaker 2: a thousand identified galaxies, ranging from enormous elliptical systems to
• 36:48
  Speaker 2: comparatively small dwarf galaxies. Its central regions contain substantial quantities
• 36:55
  Speaker 2: of extremely hot gas, while an extensive distribution of dark
• 36:59
  Speaker 2: matter contributes most of the cluster's gravitational mass. The Virgo
• 37:03
  Speaker 2: cluster is one of the dominant concentrations of matter in
• 37:06
  Speaker 2: our relatively nearby cosmic environment. The gravitational influence of Virgo
• 37:11
  Speaker 2: extends beyond the galaxies visibly gathered within the cluster. Nearby
• 37:16
  Speaker 2: systems can acquire peculiar velocities directed partly towards its concentration
• 37:21
  Speaker 2: of mass, producing departures from the smooth recession expected from
• 37:26
  Speaker 2: cosmic expansion. The local group itself has a component of
• 37:31
  Speaker 2: motion associated with attraction towards Virgo. However, the influence of
• 37:37
  Speaker 2: this single cluster cannot fully explain the movement of our
• 37:40
  Speaker 2: galactic neighbourhood relative to the cosmic microwave background. Other structures,
• 37:46
  Speaker 2: extending considerably farther away, must also be considered. Historically, astronomers
• 37:52
  Speaker 2: described the Milky Way as belonging to the Virgo supercluster,
• 37:55
  Speaker 2: a much larger concentration of galaxy groups and clusters associated
• 37:59
  Speaker 2: with the Virgo region. The distinction between the Virgo cluster
• 38:03
  Speaker 2: and Virgo supercluster is important. The cluster is a substantial
• 38:07
  Speaker 2: gravitationally bound system, whereas the traditional supercluster encompasses a much
• 38:12
  Speaker 2: wider and more loosely connected arrangement. More recent attempts to
• 38:17
  Speaker 2: classify cosmic structures using the movement of galaxies have introduced
• 38:21
  Speaker 2: different boundaries, demonstrating that our cosmic address can depend on
• 38:25
  Speaker 2: how these immense arrangements are defined. Not every region surrounding
• 38:30
  Speaker 2: the Milky Way is crowded with galaxies. Close to the
• 38:33
  Speaker 2: local sheet lies the local void, an enormous volume of
• 38:36
  Speaker 2: space containing far fewer galaxies than the cosmic average. It
• 38:41
  Speaker 2: is not entirely empty. nor is it separated from the
• 38:44
  Speaker 2: surrounding universe by a physical wall. Instead, it is an
• 38:48
  Speaker 2: underdense region, occupying part of the space between the more
• 38:51
  Speaker 2: substantial structures of the cosmic web. Its existence is important
• 38:56
  Speaker 2: because gravity responds to the distribution of matter, including places
• 39:00
  Speaker 2: where that matter is relatively scarce. A galaxy beside a
• 39:04
  Speaker 2: large void may acquire a peculiar velocity directed away from
• 39:08
  Speaker 2: the underdense region. This does not mean the void possesses
• 39:11
  Speaker 2: an unusual repulsive force. Rather, the surrounding concentrations of matter
• 39:16
  Speaker 2: exert a stronger gravitational influence, while the under-dense region contributes
• 39:21
  Speaker 2: less attraction than an average region would. The resulting imbalance
• 39:25
  Speaker 2: can produce movement away from the void relative to the
• 39:28
  Speaker 2: general cosmic expansion. It is a subtle effect that across
• 39:32
  Speaker 2: millions of light years and billions of years, its consequences
• 39:36
  Speaker 2: become measurable. Astronomical reconstructions indicate that the local sheet has
• 39:41
  Speaker 2: a significant component of motion away from the local void.
• 39:45
  Speaker 2: At the same time, the Virgo cluster contributes attraction in
• 39:49
  Speaker 2: another direction, while more distant concentrations influence the movement of
• 39:53
  Speaker 2: the entire region. These effects are combined as velocities with
• 39:57
  Speaker 2: both magnitude and direction. They cannot be understood simply by
• 40:02
  Speaker 2: adding their speeds as ordinary numbers, because gravitational influences arriving
• 40:06
  Speaker 2: from different directions can partly reinforce or cancel one another,
• 40:11
  Speaker 2: The motion of the Milky Way therefore reflects several overlapping
• 40:14
  Speaker 2: gravitational environments. Within the Local Group, the Milky Way interacts
• 40:19
  Speaker 2: strongly with Andromeda and its smaller companions. Across the Local Sheet,
• 40:24
  Speaker 2: neighbouring galaxies participate in broader collective movements. Farther away, clusters
• 40:30
  Speaker 2: such as Virgo and the large concentrations beyond it contribute
• 40:34
  Speaker 2: additional gravitational influences. The importance of each structure depends upon
• 40:38
  Speaker 2: its mass, distance and position relative to our galaxy. This
• 40:42
  Speaker 2: is why the question of what moves the Milky Way
• 40:44
  Speaker 2: cannot be answered by looking for one particularly large neighbouring galaxy.
• 40:50
  Speaker 2: Gravity acts across the entire surrounding landscape and our present
• 40:53
  Speaker 2: velocity has accumulated through an extended history of changing gravitational influences.
• 41:00
  Speaker 2: The Virgo cluster and local void help explain important parts
• 41:04
  Speaker 2: of our local motion. but the much greater velocity measured
• 41:07
  Speaker 2: relative to the cosmic microwave background points towards influences extending
• 41:12
  Speaker 2: far beyond them. Our immediate galactic neighbourhood is only one
• 41:16
  Speaker 2: small region within a gravitational landscape of extraordinary size. During
• 41:21
  Speaker 2: the 1980s, astronomers encountered a rather unexpected problem. Galaxies across
• 41:27
  Speaker 2: a remarkably large region of the nearby universe did not
• 41:31
  Speaker 2: appear to be following the smooth pattern of expansion that
• 41:35
  Speaker 2: researchers had anticipated. Their movements suggested that something was influencing
• 41:40
  Speaker 2: them on a scale far greater than the gravitational attraction
• 41:43
  Speaker 2: of a single galaxy cluster. The evidence was not a
• 41:47
  Speaker 2: strange object appearing in a telescope, it was a pattern
• 41:50
  Speaker 2: hidden in measurements of distance and motion. Much of the
• 41:54
  Speaker 2: investigation was carried out by seven astronomers, who became informally
• 41:58
  Speaker 2: known as the Seven Samurai. They were David Burstein, Roger Davies,
• 42:04
  Speaker 2: Alan Dressler, Sandra Morfaber, Donald Lindenbell, Roberto Terlovich and Gary Wegner.
• 42:11
  Speaker 2: The nickname was memorable, although their actual work involved considerably
• 42:15
  Speaker 2: more patient measuring than sword fighting. Their collaboration brought together
• 42:20
  Speaker 2: observations of elliptical galaxies from different observatories with the aim
• 42:24
  Speaker 2: of establishing how those galaxies were moving relative to the
• 42:27
  Speaker 2: expansion of the universe. The researchers faced a familiar difficulty.
• 42:33
  Speaker 2: A galaxy's redshift could reveal its apparent recession, but redshift
• 42:37
  Speaker 2: alone could not tell them whether the galaxy was moving
• 42:40
  Speaker 2: unusually quickly or slowly. They needed independent distances. For elliptical galaxies,
• 42:46
  Speaker 2: they used relationships involving the sizes of galaxies, their surface
• 42:50
  Speaker 2: brightnesses and the spread of speeds among their stars. By
• 42:54
  Speaker 2: comparing these measurements with their observed redshifts, they could estimate
• 42:58
  Speaker 2: each galaxy's peculiar velocity. the component of motion that differed
• 43:03
  Speaker 2: from the general cosmic expansion. This required care. Elliptical galaxies
• 43:08
  Speaker 2: do not all have the same intrinsic size or brightness,
• 43:12
  Speaker 2: and a small error in the estimated distance could substantially
• 43:16
  Speaker 2: alter the inferred velocity. Rather than rely on a handful
• 43:20
  Speaker 2: of objects, the team assembled observations of approximately 400 elliptical
• 43:25
  Speaker 2: galaxies distributed across much of the sky. Taken together, Their
• 43:29
  Speaker 2: measurements allowed them to search for movements shared by an
• 43:32
  Speaker 2: entire population, instead of fluctuations that might belong to individual galaxies.
• 43:38
  Speaker 2: What emerged was striking. Numerous galaxies appeared to possess substantial
• 43:44
  Speaker 2: peculiar velocities with broadly similar directions. They were not merely
• 43:49
  Speaker 2: orbiting their neighbours or falling towards the centre of their
• 43:52
  Speaker 2: own clusters. Their motion suggested a collective movement extending across
• 43:57
  Speaker 2: an enormous volume of space. Astronomers call this phenomenon bulk flow.
• 44:03
  Speaker 2: It describes the shared component of velocity of many galaxies
• 44:07
  Speaker 2: or galaxy systems measured after accounting for the average expansion
• 44:11
  Speaker 2: of the universe. A bulk flow does not mean every
• 44:15
  Speaker 2: galaxy travels in exactly the same direction at exactly the
• 44:19
  Speaker 2: same speed. Within a moving region, individual galaxies continue to orbit,
• 44:25
  Speaker 2: interact and respond to nearby masses. But beneath this variety
• 44:30
  Speaker 2: of local motions there can be a larger pattern. Imagine
• 44:33
  Speaker 2: several boats drifting with a broad current while each also
• 44:37
  Speaker 2: moves about on the water. The boats need not follow
• 44:40
  Speaker 2: identical paths for the shared current to be apparent. The
• 44:44
  Speaker 2: measured flow was especially interesting because the nearby Virgo cluster
• 44:47
  Speaker 2: did not seem sufficient to explain it. Virgo certainly influences
• 44:52
  Speaker 2: the movement of our own galactic neighbourhood. These observations suggested
• 44:57
  Speaker 2: that a substantial gravitational contribution must arise from farther away.
• 45:02
  Speaker 2: In their published analysis in 1988, the seven samurai described
• 45:06
  Speaker 2: a model in which the galaxy's movements were consistent with
• 45:10
  Speaker 2: attraction towards a massive region beyond the better-known nearby structures.
• 45:15
  Speaker 2: The inferred direction lay towards the southern sky, broadly in
• 45:19
  Speaker 2: the region of the constellations Centaurus and Norma. The investigators
• 45:24
  Speaker 2: estimated that the gravitational influence involved an immense excess of matter,
• 45:29
  Speaker 2: comparable in scale to a major supercluster. Their model attributed
• 45:34
  Speaker 2: a streaming contribution of roughly 570 km per second at
• 45:38
  Speaker 2: the Sun's position, although this estimate depended on assumptions about
• 45:43
  Speaker 2: the unseen mass distribution. The name Greater Tractor was attached
• 45:47
  Speaker 2: to this proposed concentration. It was an arresting name. but
• 45:51
  Speaker 2: it did not refer to the discovery of a single
• 45:54
  Speaker 2: gigantic black hole or some previously unknown kind of gravity.
• 45:59
  Speaker 2: It described the gravitational effect of matter distributed across a
• 46:02
  Speaker 2: large region. There was an added complication. From Earth, much
• 46:07
  Speaker 2: of that direction lies close to the crowded plane of
• 46:10
  Speaker 2: our own Milky Way. Foreground stars and interstellar dust make
• 46:14
  Speaker 2: it difficult to see the distant galaxies behind them in
• 46:17
  Speaker 2: visible light. Astronomers could detect unusual movements across the surrounding region,
• 46:23
  Speaker 2: while still lacking a clear view of all the matter
• 46:26
  Speaker 2: that might be producing those movements. Our galaxy was, rather inconveniently,
• 46:31
  Speaker 2: obscuring part of the evidence about its own journey. The
• 46:35
  Speaker 2: original estimates were necessarily uncertain. Distance measurements had sizeable errors.
• 46:41
  Speaker 2: The survey sampled only some of the available galaxies, and
• 46:44
  Speaker 2: a model with one dominant attractor was a simplification of
• 46:48
  Speaker 2: a complicated three-dimensional mass distribution. Later studies revised the inferred
• 46:54
  Speaker 2: location and strength of the attraction. Other distant concentrations could
• 47:00
  Speaker 2: also contribute to the same observed movement, so a shared
• 47:03
  Speaker 2: flow did not prove that every galaxy was heading towards
• 47:06
  Speaker 2: a single eventual destination. Nevertheless, the discovery changed the way
• 47:12
  Speaker 2: astronomers thought about our surrounding universe. the motions of hundreds
• 47:17
  Speaker 2: of galaxies had revealed a gravitational pattern far larger than
• 47:21
  Speaker 2: the familiar neighbourhood of the Milky Way. What first appeared
• 47:25
  Speaker 2: as a puzzling deviation from cosmic expansion became evidence that
• 47:30
  Speaker 2: enormous concentrations of matter were shaping the movement of whole
• 47:34
  Speaker 2: galactic regions. The Great Attractor was not initially something astronomers
• 47:40
  Speaker 2: could simply photograph. It was something they learned to recognise
• 47:43
  Speaker 2: through the way gravity had influenced the galaxies around it.
• 47:47
  Speaker 2: The name Great Attractor gives the impression that somewhere beyond
• 47:50
  Speaker 2: the Milky Way, there must be an extraordinary object, something
• 47:54
  Speaker 2: so enormous that it is drawing entire galaxies towards itself.
• 47:59
  Speaker 2: It sounds rather like a cosmic giant, hidden in the
• 48:02
  Speaker 2: darkness and exerting its influence across unimaginable distances. the reality
• 48:08
  Speaker 2: is more interesting and considerably less like a single object.
• 48:12
  Speaker 2: The Great Attractor is not one enormous body. It is
• 48:16
  Speaker 2: a region of the universe containing substantial concentrations of matter,
• 48:21
  Speaker 2: whose combined gravitational influence helps explain the movements of galaxies
• 48:26
  Speaker 2: across an enormous volume of space. When astronomers first identified
• 48:31
  Speaker 2: evidence for this attraction, they were not looking directly at
• 48:35
  Speaker 2: its source. They were examining the peculiar velocities of galaxies
• 48:40
  Speaker 2: and discovering that many shared a component of motion in
• 48:43
  Speaker 2: roughly the same direction. Early models suggested a major concentration
• 48:48
  Speaker 2: of matter approximately 200 million light-years away, although the estimated
• 48:53
  Speaker 2: position depended on the measurements and assumptions used. The distance
• 48:57
  Speaker 2: itself is worth considering. Light travelling at almost 300,000 kilometres
• 49:01
  Speaker 2: per second requires approximately 200 million years to cross it.
• 49:07
  Speaker 2: The gravitational structures being investigated were therefore unimaginably remote by
• 49:12
  Speaker 2: ordinary standards, yet close enough on cosmological scales to influence
• 49:18
  Speaker 2: the movements of our surrounding galaxies. neighbourhood. What makes the
• 49:22
  Speaker 2: Greater Tractor difficult to describe is that its mass is
• 49:25
  Speaker 2: not gathered into one conveniently identifiable object. Instead, it is
• 49:30
  Speaker 2: distributed among galaxy clusters, smaller groups, extended filaments and the
• 49:35
  Speaker 2: enormous quantities of matter associated with them. Some of these
• 49:40
  Speaker 2: structures contain hundreds or thousands of galaxies. Others are long
• 49:44
  Speaker 2: arrangements of galaxies stretching between denser concentrations. Their combined gravitational
• 49:50
  Speaker 2: influence creates an extensive region in which the movements of
• 49:54
  Speaker 2: surrounding matter are affected by an uneven distribution of mass.
• 49:58
  Speaker 2: Astronomers often describe such regions using the concept of a
• 50:02
  Speaker 2: gravitational potential well. The term refers to the way gravitational
• 50:07
  Speaker 2: potential varies across space. with deeper regions associated with stronger
• 50:11
  Speaker 2: concentrations of attracting matter. Objects can accelerate towards these regions
• 50:16
  Speaker 2: as gravity changes their motion. A useful comparison is a
• 50:21
  Speaker 2: landscape containing hills and valleys, where the shape of the
• 50:24
  Speaker 2: terrain influences which direction an object might travel. The comparison
• 50:29
  Speaker 2: is not literal, of course. There is no enormous physical
• 50:32
  Speaker 2: hollow in space into which galaxies are falling. The gravitational
• 50:36
  Speaker 2: potential describes the influence of the mass distribution, rather than
• 50:41
  Speaker 2: an actual depression carved into the universe. Importantly, a gravitational
• 50:46
  Speaker 2: potential well does not require all the contributing matter to
• 50:49
  Speaker 2: occupy one central location. Several substantial concentrations can produce a
• 50:54
  Speaker 2: combined gravitational landscape, whose influence extends across distances much greater
• 51:00
  Speaker 2: than the sizes of the individual structures. Each galaxy cluster
• 51:05
  Speaker 2: contributes to the overall gravitational field, while material distributed between
• 51:10
  Speaker 2: clusters also plays a part. The resulting pattern can be complicated,
• 51:15
  Speaker 2: with galaxies experiencing the combined effects of several different concentrations
• 51:20
  Speaker 2: at once. This also explains why the greater tractor is
• 51:24
  Speaker 2: not a gigantic black hole. A black hole is an
• 51:27
  Speaker 2: object containing mass within an event horizon. a boundary beyond
• 51:32
  Speaker 2: which light cannot escape to distant observers. The greater tractor
• 51:36
  Speaker 2: has no such defining boundary, nor does it require an
• 51:40
  Speaker 2: unfamiliar form of gravity. The movements associated with it can
• 51:45
  Speaker 2: be investigated using the established principles of gravitational physics, together
• 51:49
  Speaker 2: with observations of the matter distributed throughout the region. One
• 51:54
  Speaker 2: of the important concentrations associated with the greater tractor lies
• 51:59
  Speaker 2: in the Hydrocentaurus region, This broad arrangement contains several substantial
• 52:04
  Speaker 2: galaxy clusters and groups, including the Hydra Cluster and the
• 52:08
  Speaker 2: Centaurus Cluster. The structures are separated by enormous distances, but
• 52:14
  Speaker 2: their collective gravitational effects contribute to the wider pattern of
• 52:18
  Speaker 2: galactic motion. Historically, observations of this region helped astronomers recognise
• 52:24
  Speaker 2: that the apparent attraction involved something much larger than an
• 52:27
  Speaker 2: isolated cluster. Another particularly important concentration is the Norma Cluster,
• 52:34
  Speaker 2: situated approximately 220 million light years from Earth. It is
• 52:40
  Speaker 2: a massive galaxy cluster containing numerous galaxies, extremely hot gas
• 52:45
  Speaker 2: and a substantial distribution of dark matter. The cluster lies
• 52:49
  Speaker 2: in a direction where the foreground Milky Way makes observations
• 52:53
  Speaker 2: especially difficult. Nevertheless, Astronomers have identified it as one of
• 52:58
  Speaker 2: the major concentrations associated with the greater tractor region. Its
• 53:03
  Speaker 2: enormous mass makes it an important contributor to the surrounding
• 53:07
  Speaker 2: gravitational environment. The Norma cluster should not, however, be mistaken
• 53:13
  Speaker 2: for the entire greater tractor. It represents one substantial concentration
• 53:18
  Speaker 2: within a much larger arrangement. This distinction matters because the
• 53:22
  Speaker 2: location where galaxy motions appear to converge is need not
• 53:26
  Speaker 2: coincide precisely with the brightest galaxy or most impressive cluster
• 53:32
  Speaker 2: visible through a telescope. Gravity responds to the complete distribution
• 53:37
  Speaker 2: of matter, including material spread across intergalactic space and matter
• 53:42
  Speaker 2: that produces little or no detectable light. This is one
• 53:47
  Speaker 2: reason astronomers find it difficult to assign the greater tractor
• 53:50
  Speaker 2: a single precise mass. Different investigations define the relevant region differently.
• 53:56
  Speaker 2: and including additional clusters or filaments, naturally changes the total.
• 54:01
  Speaker 2: Earlier models sometimes represented the attraction through one idealised concentration,
• 54:06
  Speaker 2: while more detailed reconstructions reveal a much less symmetrical distribution.
• 54:12
  Speaker 2: There are also gravitational influences from structures beyond the original
• 54:16
  Speaker 2: greater tractor region, making it difficult to separate their contributions perfectly.
• 54:22
  Speaker 2: The observed motion of our local group therefore cannot be
• 54:25
  Speaker 2: treated as though a single object were pulling it along
• 54:28
  Speaker 2: a straight line. Its present velocity reflects the gravitational influence
• 54:33
  Speaker 2: of multiple structures acting over billions of years. Some contributions
• 54:38
  Speaker 2: arise from relatively nearby matter concentrations, while others come from
• 54:43
  Speaker 2: much farther away. These effects combine according to their strengths
• 54:47
  Speaker 2: and directions. and the resulting movement is not necessarily directed
• 54:51
  Speaker 2: towards one permanent destination. The greater tractor remains an essential
• 54:57
  Speaker 2: part of this explanation. It identifies a major region of
• 55:01
  Speaker 2: excess matter and gravitational influence within the cosmic environment surrounding
• 55:06
  Speaker 2: the Milky Way, but it is not the complete explanation
• 55:09
  Speaker 2: for the approximately 630 km per second motion of the
• 55:15
  Speaker 2: local group relative to the cosmic microwave background. Perhaps the
• 55:19
  Speaker 2: most remarkable aspect of the Great Attractor is that astronomers
• 55:22
  Speaker 2: first recognised its influence through movement rather than appearance. They
• 55:27
  Speaker 2: could measure the behaviour of galaxies and infer that substantial
• 55:31
  Speaker 2: concentrations of matter must be affecting them even before those
• 55:35
  Speaker 2: concentrations were adequately mapped. The Great Attractor is therefore not
• 55:40
  Speaker 2: a single hidden giant waiting somewhere in space. It is
• 55:44
  Speaker 2: part of an immense gravitational landscape, in which the distribution
• 55:48
  Speaker 2: of matter across hundreds of millions of light years helps
• 55:53
  Speaker 2: determine the motion of galaxies, including our own. The Milky
• 55:58
  Speaker 2: Way creates an unusual difficulty for anyone trying to understand
• 56:01
  Speaker 2: the universe beyond it. We live inside a vast spiral galaxy,
• 56:06
  Speaker 2: surrounded by its stars, gas and dust. From our position,
• 56:13
  Speaker 2: Looking towards some distant galaxies means looking through much of
• 56:16
  Speaker 2: our own galactic disk first, and rather inconveniently, the direction
• 56:21
  Speaker 2: of the greater tractor lies behind one of these crowded regions.
• 56:26
  Speaker 2: Our galaxy partly conceals the enormous structures that help determine
• 56:30
  Speaker 2: its own motion. On a clear night, the Milky Way
• 56:33
  Speaker 2: appears as a pale band across the sky. That band
• 56:37
  Speaker 2: is the combined light of countless distant stars, seen along
• 56:41
  Speaker 2: the flattened disc of the galaxy. From within it, our
• 56:45
  Speaker 2: view is much like looking across a forest from among
• 56:48
  Speaker 2: the trees. Some directions offer relatively open views between the trunks.
• 56:53
  Speaker 2: Others are so crowded that whatever lies beyond becomes difficult
• 56:57
  Speaker 2: to distinguish. Astronomers call the most heavily obscured band the
• 57:01
  Speaker 2: Zone of Avoidance. The name arose because early catalogues of
• 57:05
  Speaker 2: galaxies contained remarkably few objects near the plane of the
• 57:09
  Speaker 2: Milky Way. It might once have seemed as though distant
• 57:12
  Speaker 2: galaxies simply avoided this part of the sky. But the
• 57:16
  Speaker 2: apparent gap is not an immense empty trench running through
• 57:19
  Speaker 2: the universe. It is largely a problem with the view
• 57:22
  Speaker 2: from Earth. Galaxies are present beyond the foreground material, even
• 57:27
  Speaker 2: when our telescopes struggle to recognise them. Two obstacles are
• 57:31
  Speaker 2: particularly important. The first is the sheer number of foreground stars.
• 57:37
  Speaker 2: Against a dense background of points of light, The faint,
• 57:41
  Speaker 2: extended glow of a distant galaxy can be remarkably hard
• 57:44
  Speaker 2: to identify. The second is interstellar dust. Despite its name,
• 57:50
  Speaker 2: this dust is not ordinary household dust floating between the stars.
• 57:55
  Speaker 2: It consists of tiny, solid particles containing materials such as
• 57:59
  Speaker 2: silicates and carbon-rich compounds mixed with interstellar gas. These particles
• 58:05
  Speaker 2: absorb and scatter visible light, a process astronomers call Extinction
• 58:11
  Speaker 2: Shorter visible wavelengths are generally affected more strongly, so stars
• 58:16
  Speaker 2: seen through moderate amounts of dust often appear dimmer and
• 58:19
  Speaker 2: redder than they otherwise would. Behind sufficiently thick clouds, an
• 58:24
  Speaker 2: entire distant galaxy may disappear from an ordinary optical image.
• 58:29
  Speaker 2: The amount of obscuration varies greatly across the sky, which
• 58:34
  Speaker 2: makes it difficult to correct every observation with one simple calculation.
• 58:38
  Speaker 2: The greater tractor is particularly troublesome because much of the
• 58:42
  Speaker 2: relevant sky lies towards the southern constellations of Norma and Centaurus,
• 58:48
  Speaker 2: close to the galactic plane. Massive clusters and groups may
• 58:52
  Speaker 2: be present behind a dazzling and dusty foreground. A region
• 58:56
  Speaker 2: capable of exerting an important gravitational influence need not look
• 59:00
  Speaker 2: impressive at all from our vantage point. An absence of
• 59:04
  Speaker 2: visible galaxies can tell us more about what blocks their
• 59:07
  Speaker 2: light than about how much matter actually lies beyond. Fortunately,
• 59:12
  Speaker 2: visible light is only a small part of the electromagnetic spectrum.
• 59:17
  Speaker 2: Infrared radiation has longer wavelengths, and some infrared wavelengths pass
• 59:22
  Speaker 2: through interstellar dust more readily than visible light. Infrared surveys
• 59:27
  Speaker 2: can therefore reveal galaxies otherwise hidden in optical observations. The
• 59:32
  Speaker 2: 2-micron all-sky survey, for example, provided astronomers with a much
• 59:37
  Speaker 2: broader infrared view of the sky and helped uncover the
• 59:40
  Speaker 2: distribution of galaxies near the obscured galactic plane. Infrared astronomy
• 59:46
  Speaker 2: is not a perfect solution. Dense dust clouds can still
• 59:50
  Speaker 2: hide objects, while the crowded foreground of stars and emissions
• 59:54
  Speaker 2: the Milky Way itself complicate the search. Astronomers also need
• 59:59
  Speaker 2: to know whether a detected source is a distant galaxy,
• 1:00:02
  Speaker 2: or something within our own. Different observing techniques are useful
• 1:00:07
  Speaker 2: precisely because each encounters a different set of limitations. Radio
• 1:00:11
  Speaker 2: astronomy provides another way to look through the foreground. Many
• 1:00:15
  Speaker 2: galaxies contain neutral hydrogen, whose atoms can emit radio waves
• 1:00:20
  Speaker 2: at a wavelength of approximately 21 centimetres, This emission arises
• 1:00:25
  Speaker 2: from a tiny change in the relative orientation of the
• 1:00:28
  Speaker 2: magnetic properties of the atom's proton and electron. Radio waves
• 1:00:33
  Speaker 2: at this wavelength pass through much of the galactic dust
• 1:00:36
  Speaker 2: that absorbs visible light. Their observed frequencies can also reveal
• 1:00:41
  Speaker 2: the redshifts of distant hydrogen-rich galaxies. The Parkes Radio Telescope
• 1:00:47
  Speaker 2: in Australia played an important part in this work. Its
• 1:00:50
  Speaker 2: large dish, approximately 64 metres across, was used in the
• 1:00:54
  Speaker 2: park's Zone of Avoidance survey to search for hydrogen emissions
• 1:00:59
  Speaker 2: behind the southern galactic plane. One blind survey detected 883 galaxies,
• 1:01:05
  Speaker 2: many previously unknown. Each detection helped fill a gap in
• 1:01:10
  Speaker 2: the map, showing that the apparent emptiness behind the Milky
• 1:01:13
  Speaker 2: Way concealed genuine galactic structures. There is a limitation here
• 1:01:18
  Speaker 2: as well. Galaxies containing little neutral hydrogen may be difficult
• 1:01:22
  Speaker 2: to detect with this method, including some large elliptical galaxies
• 1:01:27
  Speaker 2: found in rich clusters. X-ray astronomy supplies a complementary clue.
• 1:01:32
  Speaker 2: Massive galaxy clusters often contain gas heated to tens of
• 1:01:36
  Speaker 2: millions of degrees, producing X-ray emission. Orbiting X-ray observatories can
• 1:01:41
  Speaker 2: detect this radiation and help locate clusters even where their
• 1:01:45
  Speaker 2: individual galaxies are hard to distinguish in visible light. Absorption
• 1:01:50
  Speaker 2: by foreground gas can still be troublesome, so these observations
• 1:01:54
  Speaker 2: need careful interpretation. Observatories associated with the European Southern Observatory
• 1:02:01
  Speaker 2: have contributed optical and infrared measurements, while the Hubble Space
• 1:02:05
  Speaker 2: Telescope has helped examine selected crowded or obscured fields in
• 1:02:09
  Speaker 2: greater detail. Combined with radio and X-ray surveys, These observations
• 1:02:15
  Speaker 2: gradually turn a poorly understood band across the sky into
• 1:02:18
  Speaker 2: a more useful map of the galaxies beyond it. No
• 1:02:22
  Speaker 2: individual telescope sees through every obstruction, and no wavelength reveals
• 1:02:27
  Speaker 2: every kind of galaxy equally well. The result matters for
• 1:02:31
  Speaker 2: more than completing a catalogue. Every newly identified galaxy has
• 1:02:36
  Speaker 2: a position, and measurements of its distance and motion can
• 1:02:40
  Speaker 2: help reveal the distribution of matter around the great attractor.
• 1:02:44
  Speaker 2: Astronomers can compare that distribution with the gravitational flows inferred
• 1:02:48
  Speaker 2: from neighbouring galaxies. What appeared to be an unexplained attraction
• 1:02:53
  Speaker 2: becomes easier to investigate once the concealed structures are brought
• 1:02:56
  Speaker 2: into view. The Milky Way still hides part of the answer,
• 1:03:01
  Speaker 2: but by observing the sky in several different ways, researchers
• 1:03:04
  Speaker 2: have learned to see far more of the universe behind
• 1:03:07
  Speaker 2: our own galaxy. Hidden behind the crowded stars and dust
• 1:03:12
  Speaker 2: of the southern Milky Way lies one of the most
• 1:03:14
  Speaker 2: substantial concentrations of galaxies in our relatively nearby universe. It
• 1:03:19
  Speaker 2: is called the Norma Cluster, and although it is extremely
• 1:03:23
  Speaker 2: difficult to observe through ordinary visible light telescopes, it has
• 1:03:27
  Speaker 2: become one of the most important structures in our understanding
• 1:03:30
  Speaker 2: of the Great Attractor. For many years, astronomers could measure
• 1:03:34
  Speaker 2: the unusual movements of galaxies in this direction without possessing
• 1:03:38
  Speaker 2: a complete picture of the matter responsible. The Norma Cluster
• 1:03:42
  Speaker 2: helped turn that gravitational puzzle into something considerably more tangible.
• 1:03:48
  Speaker 2: Astronomers also know the cluster by its catalogue designation, ABEL 3627.
• 1:03:55
  Speaker 2: It lies approximately 220 million light-years from Earth, in the
• 1:04:00
  Speaker 2: direction of the constellation Norma. This places it several times
• 1:04:04
  Speaker 2: farther away than the Virgo Cluster. yet still within the
• 1:04:08
  Speaker 2: relatively nearby universe when compared with galaxies whose light has
• 1:04:12
  Speaker 2: travelled for billions of years. The distance is enormous by
• 1:04:16
  Speaker 2: any human measure. The light reaching us from Norma began
• 1:04:20
  Speaker 2: its journey long before the earliest dinosaurs appeared on Earth.
• 1:04:23
  Speaker 2: The cluster itself contains hundreds of galaxies held together by
• 1:04:27
  Speaker 2: their collective gravity. Some are enormous elliptical galaxies containing vast
• 1:04:32
  Speaker 2: populations of ageing stars while others are spiral galaxies whose
• 1:04:37
  Speaker 2: disks contain stars, gas and dust. These galaxies are not
• 1:04:42
  Speaker 2: simply gathered in the same direction by coincidence. They belong
• 1:04:45
  Speaker 2: to a gravitationally associated system, moving through an environment dominated
• 1:04:50
  Speaker 2: by an immense concentration of matter. Studies of the Norma
• 1:04:54
  Speaker 2: Cluster indicate a total mass approaching a quadrillion times the
• 1:04:58
  Speaker 2: mass of the Sun, making it one of the most
• 1:05:00
  Speaker 2: massive known clusters in the greater Tractor region. Such a
• 1:05:05
  Speaker 2: quantity is difficult to picture. Even if every star in
• 1:05:08
  Speaker 2: the Milky Way contributed its entire mass, a single galaxy
• 1:05:13
  Speaker 2: would represent only a fraction of the material involved, and
• 1:05:16
  Speaker 2: much of the cluster's mass is not contained in its
• 1:05:19
  Speaker 2: stars at all. Between the galaxies lies an extensive atmosphere
• 1:05:24
  Speaker 2: of extremely hot gas, known as the intracluster medium. This
• 1:05:28
  Speaker 2: gas can reach temperatures of tens of millions of degrees.
• 1:05:32
  Speaker 2: At such temperatures, atoms are largely stripped of their electrons
• 1:05:36
  Speaker 2: forming an electrically charged plasma. The particles move rapidly and
• 1:05:41
  Speaker 2: the gas produces X-ray radiation that can be detected by
• 1:05:45
  Speaker 2: specialised telescopes operating above Earth's atmosphere. This X-ray emission provides
• 1:05:51
  Speaker 2: important information about the cluster's gravitational environment. The hot gas
• 1:05:56
  Speaker 2: occupies a deep gravitational potential well created by the combined
• 1:06:01
  Speaker 2: mass of the galaxies, the gas itself, and a much
• 1:06:04
  Speaker 2: larger distribution of dark matter. By measuring the gas temperature
• 1:06:09
  Speaker 2: and how its density changes across the cluster, astronomers can
• 1:06:13
  Speaker 2: estimate the gravitational mass required to support it, provided the
• 1:06:17
  Speaker 2: assumptions about its physical condition are appropriate. The movements of
• 1:06:22
  Speaker 2: the galaxies provide another method. Individual galaxies travel through the
• 1:06:26
  Speaker 2: cluster with substantial velocities, often approaching 1,000 km per second
• 1:06:31
  Speaker 2: relative to the cluster's average motion. Their distribution of speeds
• 1:06:36
  Speaker 2: reveals how strongly gravity binds the system together. When these
• 1:06:40
  Speaker 2: measurements are combined with observations of the X-ray emitting gas,
• 1:06:44
  Speaker 2: they help establish the Norma Cluster as an exceptionally massive
• 1:06:48
  Speaker 2: concentration rather than merely a striking gathering of bright galaxies.
• 1:06:53
  Speaker 2: Its discovery was especially significant because the inferred greater tractor
• 1:06:58
  Speaker 2: had originally been recognised through gravitational motion rather than through
• 1:07:02
  Speaker 2: a clearly identified physical structure. Astronomers had evidence that substantial
• 1:07:07
  Speaker 2: quantities of matter must be present, but much of the
• 1:07:11
  Speaker 2: relevant sky was obscured. Detailed surveys of the Norma region
• 1:07:16
  Speaker 2: revealed a rich cluster positioned close to the inferred centre
• 1:07:19
  Speaker 2: of the attraction. The gravitational influence was no longer associated
• 1:07:24
  Speaker 2: solely with an uncertain location hidden behind the Milky Way. Nevertheless,
• 1:07:29
  Speaker 2: the Norma Cluster does not account for the entire greater tractor.
• 1:07:33
  Speaker 2: Several other massive systems occupy the wider region, including the
• 1:07:37
  Speaker 2: Hydra Cluster and the Centaurus Cluster. These are important members
• 1:07:41
  Speaker 2: of the broad Hydra-Centaurus concentration, an arrangement of galaxy groups
• 1:07:46
  Speaker 2: and clusters whose gravitational influence extends across an enormous volume
• 1:07:51
  Speaker 2: of space. The Centaurus Cluster is particularly interesting. because it
• 1:07:56
  Speaker 2: contains multiple populations of galaxies with different average velocities. Such
• 1:08:01
  Speaker 2: complexity suggests that even a major cluster can possess internal structure,
• 1:08:07
  Speaker 2: reflecting the gradual assembly of matter through gravitational interactions. The
• 1:08:11
  Speaker 2: Hydra cluster forms another substantial concentration within the same general
• 1:08:16
  Speaker 2: cosmic environment, together with smaller groups and extended distributions of galaxies,
• 1:08:22
  Speaker 2: These systems contribute to the uneven gravitational landscape surrounding the
• 1:08:26
  Speaker 2: greater tractor. Farther across this region lie structures associated with
• 1:08:31
  Speaker 2: Parvo and Indus. Galaxy surveys have revealed connections between some
• 1:08:36
  Speaker 2: of these concentrations, including an extensive arrangement known as the
• 1:08:40
  Speaker 2: Norma Wall. This structure contains galaxies distributed along a broad,
• 1:08:45
  Speaker 2: elongated region, with the Norma Cluster occupying an important position
• 1:08:49
  Speaker 2: within it, The Parvo II cluster forms part of this
• 1:08:53
  Speaker 2: connected arrangement, illustrating how substantial concentrations can be linked by
• 1:08:58
  Speaker 2: filaments and walls extending across many millions of light years.
• 1:09:03
  Speaker 2: These connections are important because gravity responds to the complete
• 1:09:07
  Speaker 2: distribution of matter. A collection of clusters spread along a
• 1:09:11
  Speaker 2: filament does not produce precisely the same gravitational field as
• 1:09:16
  Speaker 2: an identical amount of mass concentrated at one point. The
• 1:09:20
  Speaker 2: direction and strength of the resulting acceleration depend on where
• 1:09:24
  Speaker 2: the matter lies. Several neighbouring concentrations can therefore create a
• 1:09:28
  Speaker 2: complicated gravitational potential, shaping the motions of galaxies throughout the
• 1:09:33
  Speaker 2: surrounding region. There is also a distinction between a gravitationally
• 1:09:37
  Speaker 2: bound cluster and the larger structures connecting different clusters. Within
• 1:09:43
  Speaker 2: the Norma Cluster, gravity is sufficiently strong to hold many
• 1:09:46
  Speaker 2: galaxies together despite their considerable internal velocities. Across much greater distances, however,
• 1:09:53
  Speaker 2: neighbouring clusters may remain separated by the continuing expansion of
• 1:09:57
  Speaker 2: the universe. Being part of the same wall or supercluster
• 1:10:01
  Speaker 2: does not necessarily mean those structures will eventually merge. The
• 1:10:05
  Speaker 2: enormous masses associated with Norma Hydra Centaurus and the surrounding
• 1:10:10
  Speaker 2: filaments demonstrate that the greater tractor is not an inexplicable
• 1:10:15
  Speaker 2: gravitational anomaly. Much of its influence can be associated with
• 1:10:20
  Speaker 2: substantial concentrations of ordinary and dark matter distributed across a
• 1:10:25
  Speaker 2: vast region. Yet identifying these structures does not mean that
• 1:10:29
  Speaker 2: every component of the local group's motion has been fully explained.
• 1:10:34
  Speaker 2: More distant concentrations also contribute to the gravitational environment, and
• 1:10:39
  Speaker 2: separating their individual effects remains difficult. What once appeared to
• 1:10:44
  Speaker 2: be an unseen source of attraction has gradually become a
• 1:10:47
  Speaker 2: recognisable landscape of galaxy clusters, hot gas, dark matter and
• 1:10:53
  Speaker 2: connecting filaments. The Norma Cluster is one of its most
• 1:10:57
  Speaker 2: impressive features, but the larger gravitational influence emerges from the
• 1:11:01
  Speaker 2: arrangement as a whole. The Milky Way's movement is therefore
• 1:11:05
  Speaker 2: connected not to one hidden giant, but to an immense
• 1:11:08
  Speaker 2: collection of structures whose combined gravity reaches across distances far
• 1:11:13
  Speaker 2: greater than the galaxies themselves. The galaxies gathered around the
• 1:11:17
  Speaker 2: Great Attractor contain an almost unimaginable number of stars. Some
• 1:11:22
  Speaker 2: shine brightly enough to be detected across hundreds of millions
• 1:11:25
  Speaker 2: of light years, while others belong to faint galaxies hidden
• 1:11:29
  Speaker 2: among the foreground stars of the Milky Way. Yet even
• 1:11:32
  Speaker 2: if astronomers could record every one of those stars, they
• 1:11:36
  Speaker 2: would still be missing most of the matter responsible for
• 1:11:38
  Speaker 2: the gravitational forces in this region. Much of the mass
• 1:11:42
  Speaker 2: shaping the movements of galaxies does not produce any detectable light.
• 1:11:47
  Speaker 2: This unseen component is known as dark matter. The name
• 1:11:50
  Speaker 2: is a description of its behaviour rather than an explanation
• 1:11:54
  Speaker 2: of what it is made from. Dark matter appears to
• 1:11:57
  Speaker 2: exert gravity but does not emit, absorb or scatter light
• 1:12:02
  Speaker 2: to any appreciable degree. Astronomers therefore cannot photograph a cloud
• 1:12:07
  Speaker 2: of dark matter in the way they photograph a cloud
• 1:12:10
  Speaker 2: of hydrogen gas. They investigate it by observing what gravity
• 1:12:14
  Speaker 2: does to the material they can see. Evidence for missing
• 1:12:18
  Speaker 2: mass emerged long before the Great Attractor was recognised. During
• 1:12:23
  Speaker 2: the 1930s, the Swiss astronomer Fritz Zwicky investigated the Coma Cluster,
• 1:12:28
  Speaker 2: a rich collection of galaxies hundreds of millions of light-years away.
• 1:12:33
  Speaker 2: By examining the motions of its members, he estimated how
• 1:12:36
  Speaker 2: much mass would be needed to hold the cluster together.
• 1:12:39
  Speaker 2: His estimate was much greater than the mass suggested by
• 1:12:42
  Speaker 2: the visible galaxies. Zwicky's calculations had substantial uncertainties, but they
• 1:12:48
  Speaker 2: raised a question that grew increasingly important. Could the universe
• 1:12:52
  Speaker 2: contain far more matter than its starlight revealed? Decades later,
• 1:12:58
  Speaker 2: the American astronomer Vera Rubin and her colleague Kent Ford
• 1:13:01
  Speaker 2: helped strengthen the evidence by studying the rotation of spiral galaxies.
• 1:13:06
  Speaker 2: Stars and gas orbiting in the outer regions of many
• 1:13:09
  Speaker 2: galaxies were travelling faster than astronomers would expect if most
• 1:13:12
  Speaker 2: of the gravitational mass were concentrated in their bright central regions.
• 1:13:17
  Speaker 2: Instead of falling off steeply with distance, many measured rotation
• 1:13:21
  Speaker 2: curves remained approximately flat. The most widely accepted explanation is
• 1:13:26
  Speaker 2: that galaxies are surrounded by extensive halos of dark matter,
• 1:13:30
  Speaker 2: reaching well beyond the visible stellar disks. Dark matter is
• 1:13:34
  Speaker 2: not believed to be a rare peculiarity of a few galaxies.
• 1:13:38
  Speaker 2: In the standard cosmological model, it accounts for approximately 85%
• 1:13:42
  Speaker 2: of all matter in the universe. Ordinary matter, made from
• 1:13:45
  Speaker 2: particles such as protons, neutrons and electrons, supplies the remaining 15%.
• 1:13:51
  Speaker 2: This percentage refers to matter, not everything in the universe.
• 1:13:57
  Speaker 2: Dark energy is a separate component, associated with the observed
• 1:14:01
  Speaker 2: acceleration of cosmic expansion. Despite their similar names, dark matter
• 1:14:07
  Speaker 2: and dark energy play very different roles. Dark matter helps
• 1:14:11
  Speaker 2: gravity assemble cosmic structures. In the early universe, small variations
• 1:14:17
  Speaker 2: in matter density grew under their own gravitational influence. Regions
• 1:14:21
  Speaker 2: containing slightly more matter attracted additional material, gradually developing into
• 1:14:27
  Speaker 2: the halos, galaxy groups, clusters and connecting filaments that form
• 1:14:32
  Speaker 2: the cosmic web. Ordinary gas accumulated within many of those
• 1:14:36
  Speaker 2: gravitational structures, where it could cool and form stars. The
• 1:14:40
  Speaker 2: visible arrangement of galaxies therefore gives us a partial guide
• 1:14:43
  Speaker 2: to a much larger underlying distribution of mass. A great
• 1:14:47
  Speaker 2: deal of this unseen mass is concentrated in galaxy clusters.
• 1:14:51
  Speaker 2: Within structures such as the Norma cluster, dark matter contributes
• 1:14:55
  Speaker 2: substantially to the gravitational potential that holds the system together.
• 1:15:00
  Speaker 2: Hot gas fills the spaces between its galaxies, and its
• 1:15:03
  Speaker 2: temperature offers one clue to the cluster's total mass. The
• 1:15:07
  Speaker 2: individual galaxies provide another through their orbital motions. Both kinds
• 1:15:12
  Speaker 2: of observations indicate that the stars alone cannot account for
• 1:15:15
  Speaker 2: the strength of the gravitational field. There is a third
• 1:15:19
  Speaker 2: approach that is particularly elegant. Albert Einstein's general theory of
• 1:15:23
  Speaker 2: relativity predicts that mass changes the geometry of spacetime, causing
• 1:15:28
  Speaker 2: light to follow curved paths. When light from a distant
• 1:15:32
  Speaker 2: galaxy passes through the gravitational field of a closer galaxy cluster,
• 1:15:36
  Speaker 2: its image may appear stretched, distorted or multiplied. This phenomenon
• 1:15:41
  Speaker 2: is called gravitational lensing. Even distortions too subtle to notice
• 1:15:46
  Speaker 2: in one galaxy can be measured statistically across thousands of
• 1:15:50
  Speaker 2: background galaxies, producing maps of the intervening mass. Gravitational lensing
• 1:15:55
  Speaker 2: is valuable because it responds to mass whether or not
• 1:15:58
  Speaker 2: that mass shines. Astronomers can compare lensing measurements with observations
• 1:16:03
  Speaker 2: of stars, hot gas and galaxy movements. Each method has
• 1:16:07
  Speaker 2: its own uncertainties, but together they allow researchers to estimate
• 1:16:11
  Speaker 2: where matter is concentrated and how much of it cannot
• 1:16:14
  Speaker 2: be accounted for by luminous material. For the great attractor,
• 1:16:18
  Speaker 2: this distinction is essential. Some galaxies in its direction are
• 1:16:22
  Speaker 2: difficult to observe because the Milky Way's dust hides their light.
• 1:16:26
  Speaker 2: Dark matter presents a different challenge. It does not become
• 1:16:30
  Speaker 2: visible simply because astronomers use a telescope capable of looking
• 1:16:33
  Speaker 2: through the dust. Nevertheless, its gravity contributes to the motions
• 1:16:38
  Speaker 2: of galaxy groups and clusters, and consequently to the large-scale
• 1:16:42
  Speaker 2: patterns astronomers are trying to explain. There is still much
• 1:16:46
  Speaker 2: that scientists do not know. The particles, or other physical constituents,
• 1:16:51
  Speaker 2: responsible for dark matter have not been identified conclusively. Its
• 1:16:55
  Speaker 2: inferred gravitational effects are strongly supported by several independent observations,
• 1:17:00
  Speaker 2: but its microscopic nature remains an open question. Explaining the
• 1:17:05
  Speaker 2: Great Attractor does not require inventing a special substance unique
• 1:17:09
  Speaker 2: to that region. The same gravitational principles and the same
• 1:17:12
  Speaker 2: widespread dark matter component apply throughout the cosmic web. This
• 1:17:17
  Speaker 2: gives galaxy motions an unexpected value. When astronomers measure peculiar velocities,
• 1:17:23
  Speaker 2: They are not merely recording how quickly distant objects are travelling.
• 1:17:27
  Speaker 2: They are gathering evidence about the mass distribution that has
• 1:17:31
  Speaker 2: influenced those movements over cosmic time. The visible galaxies serve
• 1:17:35
  Speaker 2: as markers of a gravitational landscape that is only partly illuminated.
• 1:17:40
  Speaker 2: Much of what guides the Milky Way through the universe
• 1:17:43
  Speaker 2: may never appear as a bright object in the night sky,
• 1:17:47
  Speaker 2: yet its influence can be traced in the movements of
• 1:17:49
  Speaker 2: galaxies across enormous distances. The Milky Way belongs to a
• 1:17:54
  Speaker 2: structure so enormous that its full extent remained unrecognised until
• 1:17:58
  Speaker 2: relatively recently. It contains vast numbers of galaxies, including our own,
• 1:18:04
  Speaker 2: together with the Virgo Cluster, the Greater Tractor Region, and
• 1:18:08
  Speaker 2: many other concentrations of matter. This immense arrangement is called Laniakea,
• 1:18:13
  Speaker 2: and its discovery provided astronomers with a remarkable new perspective
• 1:18:17
  Speaker 2: on the gravitational environment surrounding our galactic home. Rather than
• 1:18:22
  Speaker 2: examining individual clusters in isolation, researchers could begin to see
• 1:18:27
  Speaker 2: how entire regions of the universe were connected through their movements.
• 1:18:31
  Speaker 2: In 2014, an international team of astronomers published a study
• 1:18:36
  Speaker 2: identifying the boundaries of this enormous structure. The researchers were
• 1:18:40
  Speaker 2: Richard Brent Tully, Hélène Courtois, Yehuda Hoffman, and Daniel Pomerade.
• 1:18:45
  Speaker 2: Their work combined measurements of galaxy distances and peculiar velocities
• 1:18:49
  Speaker 2: to reconstruct the movement of matter across a substantial portion
• 1:18:53
  Speaker 2: of the nearby universe. What emerged was not simply another
• 1:18:57
  Speaker 2: catalogue of galaxies, but a map showing how those galaxies
• 1:19:01
  Speaker 2: participated in a much larger gravitational pattern. The scale of
• 1:19:05
  Speaker 2: Lanarkia is extraordinary. In the original reconstruction, The supercluster extended
• 1:19:11
  Speaker 2: approximately 500 million light-years across. It encompassed roughly 100,000 substantial galaxies,
• 1:19:18
  Speaker 2: with an estimated total mass equivalent to around 100 quadrillion suns.
• 1:19:23
  Speaker 2: Even the Milky Way, containing hundreds of billions of stars,
• 1:19:27
  Speaker 2: represents only a small contribution to that enormous total. Yet
• 1:19:32
  Speaker 2: the most interesting feature of Laniakea is not simply its size.
• 1:19:36
  Speaker 2: It is the way astronomers determined which galaxies belonged to it. Traditionally,
• 1:19:41
  Speaker 2: superclusters were identified by looking for large concentrations of galaxies.
• 1:19:46
  Speaker 2: Astronomers mapped their positions and recognised regions where clusters and
• 1:19:50
  Speaker 2: groups appeared unusually crowded. This approach revealed important structures, but
• 1:19:56
  Speaker 2: it created a problem with boundaries. The cosmic web consists
• 1:20:00
  Speaker 2: of interconnected filaments, walls and clusters. without obvious dividing lines
• 1:20:05
  Speaker 2: between many neighbouring concentrations. There is no physical fence marking
• 1:20:09
  Speaker 2: where one supercluster ends and another begins. The Laniakea researchers
• 1:20:14
  Speaker 2: approached the problem differently. Instead of relying entirely on the
• 1:20:18
  Speaker 2: positions of galaxies, they examined how those galaxies were moving.
• 1:20:23
  Speaker 2: Using measurements of peculiar velocity, they reconstructed the flow of
• 1:20:27
  Speaker 2: matter relative to the average expansion of the universe. They
• 1:20:31
  Speaker 2: also separated the movements associated with the surrounding region from
• 1:20:35
  Speaker 2: broader influences originating farther away. The resulting patterns revealed where
• 1:20:41
  Speaker 2: gravitational flows converged and where neighbouring flows began to diverge.
• 1:20:46
  Speaker 2: This led to the idea of a basin of attraction.
• 1:20:49
  Speaker 2: In a gravitational flow reconstruction, a basin of attraction is
• 1:20:54
  Speaker 2: a region where the calculated peculiar velocity streams converge towards
• 1:20:58
  Speaker 2: a common area of gravitational influence. It is not necessarily
• 1:21:02
  Speaker 2: a place where every galaxy will eventually arrive. Rather, it
• 1:21:06
  Speaker 2: describes the organisation of the reconstructed motion at the time
• 1:21:10
  Speaker 2: being examined, once the general cosmic expansion and relevant larger-scale
• 1:21:14
  Speaker 2: flows have been accounted for. A useful comparison is a
• 1:21:17
  Speaker 2: landscape containing hills, valleys and rivers. Rain falling on different
• 1:21:22
  Speaker 2: slopes may follow very different routes, yet streams belonging to
• 1:21:26
  Speaker 2: the same drainage basin eventually converge towards a shared lower region.
• 1:21:32
  Speaker 2: Along the boundary between two drainage basins, water travels in
• 1:21:36
  Speaker 2: different directions depending on which side it falls. Astronomers can
• 1:21:40
  Speaker 2: identify something similar in reconstructed galaxy flows, although the actual
• 1:21:45
  Speaker 2: movements occur through three-dimensional space rather than across a landscape.
• 1:21:50
  Speaker 2: These dividing regions are especially useful. Where reconstructed flows separate
• 1:21:55
  Speaker 2: towards different gravitational basins, researchers can define a boundary between
• 1:22:00
  Speaker 2: the corresponding superclusters. The original Laniakea map enclosed a vast
• 1:22:05
  Speaker 2: region of converging motion, with the Milky Way situated well
• 1:22:09
  Speaker 2: away from its central attraction. Our galaxy was not at
• 1:22:13
  Speaker 2: the centre of this arrangement. It belonged to an outer
• 1:22:16
  Speaker 2: part of a structure extending hundreds of millions of light-years
• 1:22:19
  Speaker 2: beyond our immediate surroundings. The name Laniakea comes from Hawaiian
• 1:22:24
  Speaker 2: and is commonly translated as immense heaven. It was suggested
• 1:22:28
  Speaker 2: by Noah Napoleon, a scholar of Hawaiian language, In recognition
• 1:22:32
  Speaker 2: of Polynesian navigators who used their knowledge of the stars
• 1:22:35
  Speaker 2: to travel across the Pacific Ocean, the name is particularly
• 1:22:39
  Speaker 2: appropriate for a structure that gives our galaxy a wider
• 1:22:42
  Speaker 2: cosmic address. Where ancient navigators identified their position through the heavens,
• 1:22:47
  Speaker 2: modern astronomers have used the movements of galaxies to establish
• 1:22:51
  Speaker 2: our place within an enormous gravitational landscape. Near the main
• 1:22:56
  Speaker 2: convergence region of the original Laniakea reconstruction lies the greater tractor,
• 1:23:01
  Speaker 2: associated with the Norma Cluster and the broader Hydra-Centaurus concentration.
• 1:23:07
  Speaker 2: This relationship helped explain why astronomers had detected such extensive
• 1:23:11
  Speaker 2: peculiar motions in that direction. The Great Attractor was not
• 1:23:15
  Speaker 2: an isolated gravitational curiosity. It occupied an important position within
• 1:23:21
  Speaker 2: a much larger system of interconnected flows, involving galaxies separated
• 1:23:26
  Speaker 2: by extraordinary distances. There is, however, An important distinction between
• 1:23:30
  Speaker 2: belonging to the same flow-defined supercluster and being gravitationally bound together.
• 1:23:37
  Speaker 2: The galaxies within a rich cluster can remain associated because
• 1:23:40
  Speaker 2: their mutual gravity is strong enough to overcome the expansion
• 1:23:43
  Speaker 2: of the universe. Laniakea is vastly larger and not every
• 1:23:48
  Speaker 2: structure within its proposed boundaries is destined to collapse towards
• 1:23:52
  Speaker 2: one common centre. Cosmic expansion continues to influence the separations
• 1:23:57
  Speaker 2: between distant, unbound regions. Nor are the boundaries of Laniakea
• 1:24:02
  Speaker 2: permanent physical surfaces. They depend upon reconstructed velocities, observational uncertainties,
• 1:24:09
  Speaker 2: and the methods used to separate local motion from larger-scale influences.
• 1:24:14
  Speaker 2: The original map was a major scientific achievement, but its
• 1:24:17
  Speaker 2: boundaries represent a particular interpretation of the measured gravitational flow,
• 1:24:23
  Speaker 2: More extensive observations can refine that interpretation, especially where distant
• 1:24:28
  Speaker 2: or obscured galaxies remain poorly measured. Nevertheless, Laniakea transformed the
• 1:24:34
  Speaker 2: way astronomers could describe our galactic surroundings. The Milky Way
• 1:24:38
  Speaker 2: is part of the local group, which belongs to a
• 1:24:40
  Speaker 2: wider arrangement of nearby galaxies, which in turn occupies a
• 1:24:44
  Speaker 2: region of the enormous cosmic web. The gravitational influences associated
• 1:24:49
  Speaker 2: with the Great Attractor become more understandable when viewed within
• 1:24:53
  Speaker 2: this larger setting. What appears from Earth to be a
• 1:24:57
  Speaker 2: collection of distant, unrelated galaxies is actually part of a
• 1:25:01
  Speaker 2: complicated network of matter and motion. The galaxies do not
• 1:25:05
  Speaker 2: need to communicate or follow any predetermined route. They respond
• 1:25:09
  Speaker 2: to gravity, and their movements reveal something about the distribution
• 1:25:13
  Speaker 2: of mass surrounding them. Laniakir provides one particularly beautiful example.
• 1:25:18
  Speaker 2: of how those movements can be used to map structures
• 1:25:21
  Speaker 2: far larger than anything visible in an ordinary image of
• 1:25:24
  Speaker 2: the night sky. Across the universe, galaxies are travelling through
• 1:25:29
  Speaker 2: an enormous gravitational landscape. Some move towards neighbouring clusters, others
• 1:25:34
  Speaker 2: drift away from comparatively empty regions, and many participate in
• 1:25:39
  Speaker 2: broad patterns of movement extending across hundreds of millions of light-years.
• 1:25:45
  Speaker 2: These motions are far too slow to watch directly through
• 1:25:48
  Speaker 2: a telescope over a human lifetime. Nevertheless, astronomers have developed
• 1:25:53
  Speaker 2: ways to reconstruct them, producing extraordinary three-dimensional maps that reveal
• 1:25:58
  Speaker 2: how matter moves through the cosmic web. The process begins
• 1:26:03
  Speaker 2: with observations of individual galaxies. Astronomers measure their positions in
• 1:26:07
  Speaker 2: the sky, estimate their distances and examine the redshifts of
• 1:26:11
  Speaker 2: their light. A galaxy's redshift provides information about its recession,
• 1:26:17
  Speaker 2: while an independently measured distance allows researchers to calculate how
• 1:26:21
  Speaker 2: much recession would be expected from the expansion of the universe.
• 1:26:26
  Speaker 2: The difference provides an estimate of its radial peculiar velocity.
• 1:26:30
  Speaker 2: Repeating these measurements across thousands of galaxies produces a collection
• 1:26:34
  Speaker 2: of clues about the gravitational forces shaping the surrounding cosmos.
• 1:26:39
  Speaker 2: One of the most important efforts to assemble these measurements
• 1:26:41
  Speaker 2: is the Cosmic Flows Research Programme. Its purpose is to
• 1:26:45
  Speaker 2: build increasingly comprehensive catalogues of galaxy distances and velocities, allowing
• 1:26:50
  Speaker 2: astronomers to investigate the distribution of matter throughout our cosmic neighbourhood.
• 1:26:55
  Speaker 2: Each successive catalogue has brought together additional observations, more reliable
• 1:26:59
  Speaker 2: distance estimates and a wider view of the universe. These
• 1:27:03
  Speaker 2: measurements are particularly valuable because the movements of galaxies contain
• 1:27:07
  Speaker 2: information about matter that may be difficult to observe directly.
• 1:27:11
  Speaker 2: In 2023, Richard Brent Tully, Esan Korkji, Hélène Courtois and
• 1:27:15
  Speaker 2: their collaborators published Cosmic Flows 4, an extensive compilation containing
• 1:27:20
  Speaker 2: distance estimates for approximately 56,000 galaxies, organised into roughly 38,000
• 1:27:23
  Speaker 2: groups or individual systems. The catalogue brought together measurements obtained
• 1:27:32
  Speaker 2: through several astronomical techniques, including the Tully-Fisher relation, the fundamental
• 1:27:37
  Speaker 2: plane relation, and observations of particular kinds of supernovae. Combining
• 1:27:43
  Speaker 2: these methods allowed researchers to examine peculiar velocities across a
• 1:27:47
  Speaker 2: much larger region than earlier investigations. But constructing a catalogue
• 1:27:52
  Speaker 2: is not quite the same as creating a complete map
• 1:27:55
  Speaker 2: of galactic motion. Most observations reveal only the component of
• 1:28:00
  Speaker 2: a galaxy's velocity, directed towards or away from Earth. They
• 1:28:04
  Speaker 2: do not directly measure its movement sideways across the sky.
• 1:28:08
  Speaker 2: To reconstruct a three-dimensional velocity field, astronomers must combine the
• 1:28:13
  Speaker 2: observations with mathematical models describing how gravity influences matter on
• 1:28:18
  Speaker 2: large scales. This process is known as velocity field reconstruction.
• 1:28:24
  Speaker 2: Researchers use the measured peculiar velocities of galaxies to estimate
• 1:28:28
  Speaker 2: the broader pattern of movement throughout the surrounding volume of space.
• 1:28:32
  Speaker 2: The underlying principle is that neighboring regions are not gravitationally independent.
• 1:28:37
  Speaker 2: Their motions are connected through the distribution of matter, allowing
• 1:28:41
  Speaker 2: measurements at many different locations to constrain the likely movement
• 1:28:44
  Speaker 2: in regions where direct observations are unavailable. The standard cosmological
• 1:28:50
  Speaker 2: model provides an important foundation for these calculations. It describes
• 1:28:54
  Speaker 2: a universe containing ordinary matter, cold, dark matter and dark energy.
• 1:28:59
  Speaker 2: with structures developing as gravity amplifies variations in density. In
• 1:29:04
  Speaker 2: regions where those variations remain relatively modest, the peculiar velocity
• 1:29:09
  Speaker 2: field is mathematically related to the underlying matter density field.
• 1:29:14
  Speaker 2: Astronomers can therefore use observed motions to estimate where substantial
• 1:29:18
  Speaker 2: concentrations of matter and comparatively empty regions are likely to exist.
• 1:29:23
  Speaker 2: The resulting computer visualisations are remarkably beautiful. Streams of movement
• 1:29:28
  Speaker 2: curve through three-dimensional space, sometimes converging towards enormous concentrations, and
• 1:29:34
  Speaker 2: sometimes separating around regions containing relatively little matter. These reconstructed
• 1:29:40
  Speaker 2: paths resemble rivers flowing through an invisible landscape. Instead of
• 1:29:45
  Speaker 2: water responding to slopes and valleys, the galaxies respond to
• 1:29:49
  Speaker 2: gravitational influences produced by the distribution of matter across the universe.
• 1:29:54
  Speaker 2: There is an important distinction, however. The flowing lines are
• 1:29:58
  Speaker 2: not physical trails left behind by galaxies. They are representations
• 1:30:03
  Speaker 2: of a reconstructed velocity field at a particular cosmic epoch.
• 1:30:08
  Speaker 2: A galaxy following one of these directions today is not
• 1:30:11
  Speaker 2: guaranteed to continue along precisely the same path forever. The
• 1:30:15
  Speaker 2: gravitational landscape changes as matter moves, and the expansion of
• 1:30:19
  Speaker 2: the universe continues to influence the distances between sufficiently separated structures.
• 1:30:25
  Speaker 2: Where the reconstructed streams converge, astronomers may identify regions associated
• 1:30:30
  Speaker 2: with substantial gravitational attraction. Where they spread apart, the pattern
• 1:30:35
  Speaker 2: can indicate an underdense region, such as a cosmic void.
• 1:30:40
  Speaker 2: These behaviours are called flow convergence and divergence. Their importance
• 1:30:45
  Speaker 2: is that they allow researchers to investigate the gravitational environment
• 1:30:49
  Speaker 2: even when the matter responsible is not clearly visible through telescopes.
• 1:30:52
  Speaker 2: The measurements are not without difficulties. Estimating a galaxy's distance
• 1:30:57
  Speaker 2: is considerably less precise than measuring its redshift. A modest
• 1:31:01
  Speaker 2: percentage error in distance can produce an uncertainty of hundreds
• 1:31:05
  Speaker 2: of kilometres per second in the inferred peculiar velocity. For
• 1:31:09
  Speaker 2: an individual galaxy, that uncertainty may be comparable to the
• 1:31:13
  Speaker 2: very movement astronomers are attempting to measure. Researchers reduce these
• 1:31:17
  Speaker 2: problems by combining independent distance indicators. Grouping galaxies that belong
• 1:31:22
  Speaker 2: to the same physical systems and comparing the results of
• 1:31:25
  Speaker 2: different reconstruction methods. Grouping is particularly useful because individual galaxies
• 1:31:31
  Speaker 2: inside a cluster can have substantial orbital motions that do
• 1:31:35
  Speaker 2: not represent the movement of the cluster as a whole.
• 1:31:38
  Speaker 2: Statistical techniques help separate these local variations from the broader
• 1:31:42
  Speaker 2: flows associated with large-scale structure. Some regions remain harder to
• 1:31:47
  Speaker 2: reconstruct than others. The crowded plane of the Milky Way
• 1:31:51
  Speaker 2: conceals distant galaxies, while measurements become increasingly uncertain at great distances.
• 1:31:57
  Speaker 2: Where observations are sparse, the resulting maps depend more heavily
• 1:32:01
  Speaker 2: on assumptions about the statistical behaviour of cosmic matter. Astronomers
• 1:32:05
  Speaker 2: therefore examine alternative reconstructions and their uncertainties, rather than treating
• 1:32:11
  Speaker 2: every inferred feature as an established physical boundary. The Cosmic
• 1:32:15
  Speaker 2: Flow's measurements have transformed the investigation of the Greater Tractor
• 1:32:19
  Speaker 2: and Laniakea. Instead of identifying one apparent destination for a
• 1:32:24
  Speaker 2: collection of moving galaxies, researchers can examine an interconnected network
• 1:32:29
  Speaker 2: of gravitational influences extending across enormous distances. The resulting maps
• 1:32:35
  Speaker 2: reveal how our galactic neighbourhood participates in movements associated with
• 1:32:40
  Speaker 2: nearby clusters. distant concentrations and vast underdense regions. What makes
• 1:32:46
  Speaker 2: this achievement particularly remarkable is that much of the underlying
• 1:32:49
  Speaker 2: gravitational landscape cannot be observed directly. Its structure must be
• 1:32:54
  Speaker 2: inferred from the motion of the galaxies distributed throughout it.
• 1:32:58
  Speaker 2: By measuring their distances and velocities, astronomers can gradually reconstruct
• 1:33:03
  Speaker 2: the immense pattern of matter surrounding the Milky Way. revealing
• 1:33:08
  Speaker 2: a universe whose gravitational connections extend far beyond anything visible
• 1:33:12
  Speaker 2: in the night sky. Gravity is usually described as a
• 1:33:16
  Speaker 2: force that brings things together, stars gather into galaxies, galaxies
• 1:33:21
  Speaker 2: assemble into clusters, and enormous concentrations of matter influence the
• 1:33:26
  Speaker 2: movements of objects across millions of light years. Yet there
• 1:33:30
  Speaker 2: is another side to this picture. The motion of the
• 1:33:33
  Speaker 2: Milky Way is influenced not only by where matter is concentrated,
• 1:33:37
  Speaker 2: but also by where matter is missing. Some of the
• 1:33:40
  Speaker 2: most important features shaping our journey through the universe are
• 1:33:44
  Speaker 2: enormous regions containing remarkably little of anything. These regions are
• 1:33:49
  Speaker 2: called cosmic voids. They occupy the spaces between the great filaments,
• 1:33:54
  Speaker 2: walls and clusters of the cosmic web. A void can
• 1:33:57
  Speaker 2: extend across tens or even hundreds of millions of light years,
• 1:34:01
  Speaker 2: containing far fewer galaxies than a typical region of comparable size.
• 1:34:05
  Speaker 2: They are not completely empty. Scattered galaxies, diffuse gas and
• 1:34:10
  Speaker 2: dark matter can still be present. Nevertheless, their average density
• 1:34:14
  Speaker 2: is substantially lower than that of the surrounding universe. The
• 1:34:18
  Speaker 2: existence of these immense under-dense regions is a natural consequence
• 1:34:22
  Speaker 2: of cosmic structure formation. During the early history of the universe,
• 1:34:27
  Speaker 2: matter was distributed almost uniformly with small variations in density.
• 1:34:32
  Speaker 2: Gravity gradually amplified those differences. Slightly denser regions attracted additional material,
• 1:34:39
  Speaker 2: while regions containing less matter lost ground to their increasingly
• 1:34:43
  Speaker 2: massive surroundings. Over billions of years, matter accumulated into the
• 1:34:48
  Speaker 2: filaments and clusters we observe today, leaving vast, under-dense regions
• 1:34:53
  Speaker 2: between them. At first, it might seem strange to suggest
• 1:34:56
  Speaker 2: that a nearly empty region could influence the movement of
• 1:34:59
  Speaker 2: a galaxy. After all, gravity is attractive. A void does
• 1:35:04
  Speaker 2: not contain some exotic substance that pushes galaxies away, nor
• 1:35:09
  Speaker 2: does it possess a special form of anti-gravity. The explanation
• 1:35:13
  Speaker 2: comes from comparing the actual gravitational environment with what would
• 1:35:17
  Speaker 2: exist if matter were distributed uniformly throughout space. Imagine a
• 1:35:22
  Speaker 2: galaxy surrounded by equal amounts of matter in every direction.
• 1:35:26
  Speaker 2: The gravitational attractions from different sides would largely balance. Now
• 1:35:31
  Speaker 2: suppose one side contains a substantial concentration of matter, while
• 1:35:36
  Speaker 2: the opposite side contains an enormous underdense region. The balance
• 1:35:40
  Speaker 2: is disturbed. More gravitational attraction comes from the crowded side,
• 1:35:45
  Speaker 2: and less comes from the comparatively empty side. The galaxy
• 1:35:49
  Speaker 2: acquires a peculiar acceleration towards the denser region, and away
• 1:35:53
  Speaker 2: from the void, relative to the motion expected in a
• 1:35:57
  Speaker 2: uniform expanding universe. This is why astronomers sometimes describe cosmic
• 1:36:02
  Speaker 2: voids as gravitational repellers. The name is convenient, but the
• 1:36:06
  Speaker 2: underlying physics remains ordinary gravitational attraction. An under-dense region acts
• 1:36:12
  Speaker 2: as a repeller in the reconstructed peculiar velocity field because
• 1:36:15
  Speaker 2: it contains less attracting matter than the cosmic average. Nothing
• 1:36:19
  Speaker 2: inside the void needs to exert an outward force. One
• 1:36:23
  Speaker 2: particularly important example lies relatively close to home. The local
• 1:36:28
  Speaker 2: void is an immense underdense region adjacent to the local sheet.
• 1:36:32
  Speaker 2: The flattened arrangement of nearby galaxies containing our local group.
• 1:36:37
  Speaker 2: Astronomers have found that the local sheet possesses a substantial
• 1:36:40
  Speaker 2: component of motion directed away from this relatively empty region.
• 1:36:45
  Speaker 2: Research led by Richard Brent Tully identified a movement of
• 1:36:49
  Speaker 2: approximately 260 km per second away from the local void,
• 1:36:54
  Speaker 2: in a particular decomposition of nearby galactic motions. That is
• 1:36:58
  Speaker 2: nearly a million kilometres per hour. The figure does not
• 1:37:01
  Speaker 2: mean the local void is directly pushing the Milky Way
• 1:37:04
  Speaker 2: at that speed. It represents an inferred component of the
• 1:37:08
  Speaker 2: local sheet's peculiar motion associated with the surrounding density imbalance.
• 1:37:14
  Speaker 2: This movement becomes understandable when the local void is considered
• 1:37:17
  Speaker 2: alongside the other structures surrounding us. Matter is concentrated unevenly
• 1:37:22
  Speaker 2: across the nearby universe. The Virgo cluster provides attraction in
• 1:37:26
  Speaker 2: one direction, while the under-density of the local void contributes
• 1:37:30
  Speaker 2: to motion away from another. The resulting velocity depends on
• 1:37:34
  Speaker 2: the combination of these influences, along with contributions from more
• 1:37:38
  Speaker 2: distant structures. The importance of cosmic voids became particularly apparent
• 1:37:43
  Speaker 2: in research published in 2017 by Yehuda Hoffman, Daniel Pomered,
• 1:37:48
  Speaker 2: Richard Brent Tully and Hélène Courtois. The researchers reconstructed the
• 1:37:54
  Speaker 2: large-scale velocity field surrounding the local group and identified evidence
• 1:37:58
  Speaker 2: for an extensive underdense region that appeared to play a
• 1:38:02
  Speaker 2: significant role in our galactic neighbourhood's movement. They called this
• 1:38:06
  Speaker 2: inferred feature the dipole repeller. The name refers to its
• 1:38:10
  Speaker 2: relationship with the large-scale dipole pattern of cosmic motion. The
• 1:38:15
  Speaker 2: local group travels through the universe at approximately 630 km
• 1:38:20
  Speaker 2: per second relative to the cosmic microwave background. Earlier explanations
• 1:38:25
  Speaker 2: had concentrated primarily on the attraction of massive galaxy concentrations.
• 1:38:30
  Speaker 2: The new reconstruction suggested that an enormous underdense region, lying
• 1:38:35
  Speaker 2: broadly opposite the direction of the Local Group's movement, could
• 1:38:39
  Speaker 2: be equally important. The Dipole Repeller was inferred from the
• 1:38:43
  Speaker 2: reconstructed motions of galaxies rather than identified as a sharply
• 1:38:48
  Speaker 2: defined empty region through direct observation. Its direction lies broadly
• 1:38:53
  Speaker 2: opposite the substantial gravitational influence associated with the distant Shapley concentration.
• 1:39:00
  Speaker 2: In the researcher's model, attraction associated with Shapley and the
• 1:39:03
  Speaker 2: relative repelling effect of the under-dense region contributed roughly comparable
• 1:39:08
  Speaker 2: amounts to the observed large-scale flow. This does not mean
• 1:39:12
  Speaker 2: that the dipole repeller and the local void are the
• 1:39:14
  Speaker 2: same structure. The local void belongs to our relatively nearby
• 1:39:18
  Speaker 2: cosmic environment, while the dipole or repeller describes a much
• 1:39:22
  Speaker 2: more distant and extensive underdensity inferred from large-scale velocity reconstructions.
• 1:39:29
  Speaker 2: Both illustrate how regions containing less matter than average can
• 1:39:32
  Speaker 2: influence the movement of galaxies, but they operate within different
• 1:39:36
  Speaker 2: parts of the surrounding gravitational landscape. Mapping an underdense region
• 1:39:41
  Speaker 2: is especially challenging. A rich galaxy cluster contains numerous visible
• 1:39:47
  Speaker 2: objects that reveal its location. A void, by definition, provides
• 1:39:52
  Speaker 2: relatively few galaxies to measure. Astronomers must therefore rely heavily
• 1:39:56
  Speaker 2: on the movements of surrounding structures and on statistical models
• 1:40:00
  Speaker 2: connecting peculiar velocities with the underlying distribution of matter. The
• 1:40:05
  Speaker 2: inferred location and boundaries of a repeller consequently carry uncertainties.
• 1:40:11
  Speaker 2: What emerges is a picture of the universe in which
• 1:40:14
  Speaker 2: dense and under-dense regions are equally necessary for understanding the
• 1:40:18
  Speaker 2: overall gravitational pattern. The Milky Way's motion is not simply
• 1:40:23
  Speaker 2: a journey towards the largest concentration of galaxies somewhere ahead.
• 1:40:27
  Speaker 2: It reflects an imbalance extending across an enormous region, with
• 1:40:31
  Speaker 2: matter pulling more strongly from some directions than others. The
• 1:40:35
  Speaker 2: most surprising feature is that absence itself can leave a
• 1:40:38
  Speaker 2: measurable signature. A region containing relatively little matter may appear
• 1:40:44
  Speaker 2: unremarkable in a telescope, yet its place within the cosmic
• 1:40:47
  Speaker 2: web helps determine how neighbouring galaxies move. Our own galaxy
• 1:40:52
  Speaker 2: is travelling through a gravitational landscape shaped both by immense
• 1:40:56
  Speaker 2: concentrations of matter and by the enormous spaces between them.
• 1:41:01
  Speaker 2: The Great Attractor is an enormous gravitational feature associated with
• 1:41:06
  Speaker 2: galaxy clusters spread across hundreds of millions of light-years Yet
• 1:41:10
  Speaker 2: the concentrations of matter surrounding it are not the largest
• 1:41:14
  Speaker 2: structures influencing the movement of our galactic neighbourhood. Farther into
• 1:41:19
  Speaker 2: the universe, in broadly the same direction, lies something even
• 1:41:23
  Speaker 2: more substantial. It is a vast gathering of galaxy clusters
• 1:41:27
  Speaker 2: known as the Shapley Supercluster, or more commonly, the Shapley Concentration.
• 1:41:33
  Speaker 2: Its existence reveals just how far the gravitational influences shaping
• 1:41:37
  Speaker 2: the Milky Way's movement may extend. The Shapley concentration lies
• 1:41:42
  Speaker 2: approximately 650 million light-years from Earth, more than three times
• 1:41:48
  Speaker 2: the distance commonly associated with the central Great Attractor region.
• 1:41:52
  Speaker 2: At such a distance, even the enormous Milky Way would
• 1:41:55
  Speaker 2: appear as a relatively modest patch of light through a
• 1:41:58
  Speaker 2: sufficiently powerful telescope. Yet the Shapley region contains not just
• 1:42:03
  Speaker 2: one galaxy, or one rich cluster, but an extensive collection
• 1:42:08
  Speaker 2: of clusters and groups occupying a substantial volume of space.
• 1:42:13
  Speaker 2: It is among the richest concentrations of galaxy clusters known
• 1:42:17
  Speaker 2: in the relatively nearby universe. The first indications of this
• 1:42:21
  Speaker 2: extraordinary structure appeared during the 1930s. The American astronomer Harlow
• 1:42:26
  Speaker 2: Shapley was studying the distribution of galaxies when he noticed
• 1:42:30
  Speaker 2: an unusual concentration in the direction of the constellation Centaurus.
• 1:42:35
  Speaker 2: In 1930, he described what appeared to be a distant
• 1:42:38
  Speaker 2: cloud of galaxies, although the available observations could not establish
• 1:42:42
  Speaker 2: its complete three-dimensional structure. At the time, astronomers were only
• 1:42:47
  Speaker 2: beginning to understand the immense scale of the universe beyond
• 1:42:50
  Speaker 2: the Milky Way, and reliable distances to such remote systems
• 1:42:54
  Speaker 2: were difficult to obtain. Several decades passed before the significance
• 1:42:59
  Speaker 2: of this concentration became clearer. During the late 1980s, Roberto
• 1:43:04
  Speaker 2: Scaramella and his collaborators examined catalogues of galaxy clusters and
• 1:43:10
  Speaker 2: identified a remarkable excess of rich clusters in the same
• 1:43:14
  Speaker 2: broad region. Their research, published in 1989, raised the possibility
• 1:43:21
  Speaker 2: that this enormous concentration of matter could contribute to the
• 1:43:25
  Speaker 2: large-scale movements of galaxies. The timing was particularly interesting because
• 1:43:30
  Speaker 2: astronomers were already investigating the gravitational influence associated with the
• 1:43:35
  Speaker 2: Great Attractor. The Shapley Concentration contains numerous massive galaxy clusters,
• 1:43:42
  Speaker 2: each holding hundreds or thousands of galaxies within a shared
• 1:43:45
  Speaker 2: gravitational environment. Between these clusters lie smaller groups, diffuse matter
• 1:43:51
  Speaker 2: and connecting structures. The total mass is thought to reach
• 1:43:55
  Speaker 2: tens of thousands of trillions of times the mass of
• 1:43:58
  Speaker 2: the Sun, although estimates depend on the region and methods considered.
• 1:44:03
  Speaker 2: Much of this mass belongs to dark matter, which contributes
• 1:44:06
  Speaker 2: to the gravitational field without producing the light observed from
• 1:44:09
  Speaker 2: stars and hot gas. The important question is whether a
• 1:44:13
  Speaker 2: concentration so far away can meaningfully influence the movement of
• 1:44:17
  Speaker 2: the local group. Gravity becomes weaker with distance, and the
• 1:44:21
  Speaker 2: gravitational acceleration produced by an isolated mass decreases according to
• 1:44:26
  Speaker 2: the inverse square of the separation. But distance is only
• 1:44:29
  Speaker 2: part of the calculation. The amount of matter involved also matters,
• 1:44:34
  Speaker 2: and the Shapley concentration is exceptionally massive. Its distributed gravitational
• 1:44:40
  Speaker 2: influence can therefore contribute to peculiar motions across an enormous
• 1:44:44
  Speaker 2: region of the universe. There is a useful distinction between
• 1:44:48
  Speaker 2: the greater tractor and Shapley. The Great Attractor is associated
• 1:44:53
  Speaker 2: with the nearer Norma and Hydra Centaurus concentrations, while Shapley
• 1:44:57
  Speaker 2: lies considerably farther away. They are not two names for
• 1:45:01
  Speaker 2: the same structure, nor does the presence of both in
• 1:45:05
  Speaker 2: a broadly similar direction mean that one must be physically
• 1:45:08
  Speaker 2: contained within the other. Their locations are determined through measurements
• 1:45:12
  Speaker 2: of galaxy distances and redshifts, which reveal how the concentrations
• 1:45:17
  Speaker 2: are arranged in three-dimensional space. Their gravitational effects, however, can overlap.
• 1:45:24
  Speaker 2: A galaxy situated between two substantial concentrations of matter responds
• 1:45:29
  Speaker 2: to the attraction of both. Depending on the relative directions
• 1:45:33
  Speaker 2: and strengths of those influences, its acceleration may reflect a
• 1:45:37
  Speaker 2: combination of nearby and more distant structures. The resulting peculiar
• 1:45:42
  Speaker 2: velocity is further shaped by the gravitational forces that acted
• 1:45:45
  Speaker 2: upon the galaxy during its earlier history. Consequently, a large-scale
• 1:45:51
  Speaker 2: flow does not necessarily point towards one isolated source. This
• 1:45:55
  Speaker 2: complicates the attempt to explain the movement of the Milky Way.
• 1:45:59
  Speaker 2: The local group travels at approximately 630 kilometres per second
• 1:46:05
  Speaker 2: relative to the cosmic microwave background, but that velocity cannot
• 1:46:09
  Speaker 2: simply be assigned to the greater tractor or the Shapley
• 1:46:12
  Speaker 2: concentration alone. The Virgo cluster surrounding galaxy groups Distant mass
• 1:46:19
  Speaker 2: concentrations and enormous underdense regions all contribute to the gravitational environment.
• 1:46:25
  Speaker 2: Different reconstruction methods can assign different relative contributions to these influences.
• 1:46:31
  Speaker 2: The Shapley concentration is especially interesting because its gravitational reach
• 1:46:35
  Speaker 2: may be reflected in movements extending beyond the original greater
• 1:46:39
  Speaker 2: tractor region. Some studies have identified large-scale streaming motions consistent
• 1:46:45
  Speaker 2: with significant attraction from distant concentrations, while others have found
• 1:46:50
  Speaker 2: smaller contributions from Shapley to the local group's particular velocity.
• 1:46:55
  Speaker 2: These differences reflect the difficulty of reconstructing an immense gravitational
• 1:47:00
  Speaker 2: landscape from incomplete observations. The direction of a galaxy's motion
• 1:47:05
  Speaker 2: is measurable. but identifying exactly how much of that motion
• 1:47:09
  Speaker 2: was produced by each distant structure is considerably harder. There
• 1:47:14
  Speaker 2: is also the possibility that the apparent boundaries between gravitational
• 1:47:18
  Speaker 2: regions depend on the scale being examined. A group of
• 1:47:22
  Speaker 2: galaxies might participate in a relatively local flow towards the
• 1:47:26
  Speaker 2: greater tractor, while that entire region also shares a broader
• 1:47:30
  Speaker 2: movement associated with more distant concentrations. These patterns can coexist
• 1:47:35
  Speaker 2: without contradictions. The cosmic web is not divided into perfectly
• 1:47:40
  Speaker 2: independent gravitational territories, and an influence that appears dominant over
• 1:47:44
  Speaker 2: one distance may become less important when a much larger
• 1:47:47
  Speaker 2: volume is considered. The discovery of the Shapley concentration therefore
• 1:47:53
  Speaker 2: changed the interpretation of the greater tractor. What initially seemed
• 1:47:57
  Speaker 2: to be a largely localised gravitational mystery became part of
• 1:48:02
  Speaker 2: a wider question about how matter is distributed across hundreds
• 1:48:05
  Speaker 2: of millions of light-years. The movement of the Milky Way
• 1:48:09
  Speaker 2: is connected to structures far beyond the familiar galaxies of
• 1:48:13
  Speaker 2: our immediate neighbourhood, and their combined influence has developed over
• 1:48:17
  Speaker 2: billions of years. Seen from Earth, the Shapley Concentration is
• 1:48:22
  Speaker 2: an extraordinarily distant arrangement of galaxies, too faint and remote
• 1:48:27
  Speaker 2: to be recognised without powerful instruments, yet its immense mass
• 1:48:31
  Speaker 2: makes it relevant to understanding the gravitational landscape surrounding us.
• 1:48:36
  Speaker 2: The greater tractor remains an important concentration, but it is
• 1:48:40
  Speaker 2: not necessarily the final or largest feature within the pattern
• 1:48:43
  Speaker 2: of cosmic motion. Across these enormous distances, galaxies respond to
• 1:48:48
  Speaker 2: a distribution of matter whose gravitational influence extends far beyond
• 1:48:53
  Speaker 2: any single cluster, linking our own galactic neighbourhood to some
• 1:48:57
  Speaker 2: of the greatest structures in the nearby universe. For many years,
• 1:49:02
  Speaker 2: astronomers believed they had identified an enormous gravitational region surrounding
• 1:49:07
  Speaker 2: our galactic neighbourhood, It was called Lniakea, a supercluster extending
• 1:49:12
  Speaker 2: roughly 500 million light-years across, with the greater tractor occupying
• 1:49:17
  Speaker 2: an important position within its reconstructed pattern of galactic motion.
• 1:49:23
  Speaker 2: Yet as measurements improved, a fascinating possibility emerged. What if
• 1:49:28
  Speaker 2: this immense structure was not an independent gravitational region after all?
• 1:49:33
  Speaker 2: What if the movements of its galaxies belonged to something
• 1:49:36
  Speaker 2: considerably larger? The question arose because the boundaries of a
• 1:49:41
  Speaker 2: supercluster are not always as definite as those of a
• 1:49:44
  Speaker 2: planet or a star. A planet has a surface, and
• 1:49:48
  Speaker 2: a star has a recognisable concentration of luminous material. But
• 1:49:52
  Speaker 2: the cosmic web consists of interconnected galaxies, clusters, filaments, and
• 1:49:58
  Speaker 2: enormous underdense regions. These structures blend into one another across
• 1:50:04
  Speaker 2: vast distances. Deciding where one arrangement ends and another begins
• 1:50:09
  Speaker 2: depends partly on how astronomers choose to examine it. The
• 1:50:13
  Speaker 2: original identification of Laniakea relied upon a particularly interesting approach.
• 1:50:19
  Speaker 2: Instead of grouping galaxies together simply because they occupied neighbouring
• 1:50:23
  Speaker 2: regions of space, researchers reconstructed their peculiar velocities. These are
• 1:50:29
  Speaker 2: the movements of galaxies relative to the average expansion of
• 1:50:32
  Speaker 2: the universe. where the calculated streams of motion converge towards
• 1:50:37
  Speaker 2: a shared region of gravitational influence, astronomers identified what they
• 1:50:42
  Speaker 2: called a basin of attraction. In 2024, a new investigation
• 1:50:47
  Speaker 2: revisited these gravitational basins using a substantially expanded collection of observations.
• 1:50:54
  Speaker 2: The research was published in the journal Nature Astronomy by
• 1:50:57
  Speaker 2: Aurelien Vallade, Noam Libeskind, Daniel Pomeréde, Richard Brent Tully, Yehuda Hoffman,
• 1:51:04
  Speaker 2: Simon Pfeiffer, and Esen Korchi. Their study examined the large-scale
• 1:51:10
  Speaker 2: structure of the nearby universe using the Cosmic Flows 4 catalogue,
• 1:51:14
  Speaker 2: which contained distance and velocity information for tens of thousands
• 1:51:18
  Speaker 2: of galaxies and galaxy groups. The researchers wanted to determine
• 1:51:23
  Speaker 2: how confidently astronomers could identify separate gravitational basins when uncertainties
• 1:51:29
  Speaker 2: in the observations were properly considered. galaxy distances are difficult
• 1:51:33
  Speaker 2: to measure precisely, and even relatively modest errors can alter
• 1:51:38
  Speaker 2: the peculiar velocities inferred from them. These uncertainties become particularly
• 1:51:43
  Speaker 2: important when trying to define the boundaries of structures extending
• 1:51:48
  Speaker 2: across hundreds of millions of light years. To investigate the problem,
• 1:51:53
  Speaker 2: the team reconstructed the distribution of matter and the associated
• 1:51:57
  Speaker 2: velocity field using the standard cosmological model. This model describes
• 1:52:02
  Speaker 2: a universe containing ordinary matter, cold, dark matter and dark energy,
• 1:52:08
  Speaker 2: with gravitational attraction governing the growth of cosmic structures. The
• 1:52:13
  Speaker 2: researchers employed a statistical technique called Hamiltonian Monte Carlo, which
• 1:52:19
  Speaker 2: allowed them to explore many possible reconstructions consistent with the
• 1:52:23
  Speaker 2: available measurements. Rather than producing one map and treating its
• 1:52:27
  Speaker 2: boundaries as unquestionably correct, they generated a collection of plausible
• 1:52:32
  Speaker 2: gravitational landscapes. Each reconstruction represented a possible arrangement of matter
• 1:52:38
  Speaker 2: and motion compatible with the observations and the assumptions of
• 1:52:42
  Speaker 2: the model. By comparing these alternatives, the scientists could estimate
• 1:52:47
  Speaker 2: how frequently particular regions belong to the same basin of attraction.
• 1:52:52
  Speaker 2: This approach introduced the concept of probabilistic basins of attraction.
• 1:52:57
  Speaker 2: The underlying idea is straightforward– If nearly every plausible reconstruction
• 1:53:02
  Speaker 2: places a galaxy within the same basin, its membership is
• 1:53:06
  Speaker 2: relatively secure. If different reconstructions assign that galaxy to different basins,
• 1:53:12
  Speaker 2: the classification is more uncertain. The boundaries, therefore, become regions
• 1:53:17
  Speaker 2: associated with different probabilities, rather than perfectly sharp surfaces dividing
• 1:53:22
  Speaker 2: one enormous structure from another. The results suggested that Laniakea
• 1:53:27
  Speaker 2: might not be as independent as the original reconstruction implied.
• 1:53:32
  Speaker 2: The researchers found a slight probabilistic preference for the region
• 1:53:36
  Speaker 2: previously identified as Laniakea to belong to a much larger
• 1:53:40
  Speaker 2: gravitational basin associated with the Shapley concentration. This was an
• 1:53:47
  Speaker 2: important distinction, but not a definitive overturning of the earlier discovery.
• 1:53:52
  Speaker 2: A slight statistical preference means the available evidence favoured one
• 1:53:56
  Speaker 2: interpretation over another, without establishing that interpretation beyond reasonable scientific uncertainty.
• 1:54:03
  Speaker 2: Laniakea remained a recognisable feature of the reconstructed cosmic landscape.
• 1:54:09
  Speaker 2: What had become less certain was whether it represented a
• 1:54:12
  Speaker 2: separate major basin or formed part of a more extensive arrangement.
• 1:54:18
  Speaker 2: The difference is easier to understand by considering gravitational flows
• 1:54:22
  Speaker 2: at different scales. A group of galaxies may move towards
• 1:54:26
  Speaker 2: a nearby concentration of matter, while that entire region participates
• 1:54:31
  Speaker 2: in a broader pattern associated with a much more distant concentration.
• 1:54:36
  Speaker 2: The local flow remains real, even when it belongs to
• 1:54:40
  Speaker 2: a larger gravitational environment. Similarly, the greater tractor can remain
• 1:54:45
  Speaker 2: significant for the movements of surrounding galaxies without being the
• 1:54:49
  Speaker 2: dominant influence across every distance considered. The Shapley concentration is
• 1:54:54
  Speaker 2: particularly relevant because it contains an extraordinary accumulation of galaxy
• 1:55:00
  Speaker 2: clusters situated considerably farther away than the original greater tractor region.
• 1:55:06
  Speaker 2: Its enormous mass makes it an important feature of the
• 1:55:09
  Speaker 2: surrounding gravitational landscape. If Laniakea forms part of the wider
• 1:55:14
  Speaker 2: Shapley Basin, the movements of galaxies throughout our neighbourhood may
• 1:55:19
  Speaker 2: be connected to a gravitational structure extending substantially beyond the
• 1:55:24
  Speaker 2: boundaries originally assigned to our supercluster. The 2024 study also
• 1:55:30
  Speaker 2: identified another remarkable feature associated with the Sloan Great Wall.
• 1:55:36
  Speaker 2: This immense arrangement of galaxies and galaxy groups forms part
• 1:55:40
  Speaker 2: of the larger cosmic web extending across an extraordinary distance.
• 1:55:46
  Speaker 2: The reconstructed basin associated with it was the largest identified
• 1:55:50
  Speaker 2: within the volume examined by the researchers, occupying more than
• 1:55:54
  Speaker 2: twice the sampled volume of the Shapley Basin. However, a
• 1:55:58
  Speaker 2: reconstructed gravitational basin must not be confused with a single
• 1:56:03
  Speaker 2: gravitationally bound object. The Sloan Great Wall is a real
• 1:56:08
  Speaker 2: observed concentration of galaxies while the basin associated with it
• 1:56:12
  Speaker 2: is defined through inferred patterns of motion and gravitational potential.
• 1:56:18
  Speaker 2: Its enormous size does not mean that every galaxy within
• 1:56:22
  Speaker 2: the region will eventually collapse into one gigantic cluster. Cosmic
• 1:56:26
  Speaker 2: expansion continues to influence widely separated structures, and their future
• 1:56:32
  Speaker 2: movements are more complicated than a simple journey towards one destination.
• 1:56:36
  Speaker 2: There are also limits to what the available observations can establish.
• 1:56:41
  Speaker 2: Some galaxies remain hidden behind the Milky Way, while measurements
• 1:56:44
  Speaker 2: become less complete and more uncertain at greater distances. Different
• 1:56:49
  Speaker 2: statistical assumptions can influence the reconstructed velocity field, particularly near
• 1:56:55
  Speaker 2: the boundaries of poorly observed regions. The resulting maps are
• 1:56:59
  Speaker 2: therefore scientific estimates constrained by real measurements, rather than direct
• 1:57:04
  Speaker 2: photographs of every gravitational feature. The greater tractor and the
• 1:57:09
  Speaker 2: Shapley concentration should consequently not be regarded as rival explanations
• 1:57:14
  Speaker 2: for the Milky Way's motion. They are different concentrations within
• 1:57:18
  Speaker 2: a much larger distribution of matter, and their gravitational influences
• 1:57:22
  Speaker 2: can contribute simultaneously. The Virgo cluster, surrounding cosmic voids and
• 1:57:29
  Speaker 2: numerous smaller structures, also participate in shaping the movement of
• 1:57:33
  Speaker 2: our local group. What once appeared to be a fairly
• 1:57:36
  Speaker 2: straightforward question about one enormous attractor, has gradually revealed a
• 1:57:41
  Speaker 2: far more intricate cosmic arrangement. The Milky Way does not
• 1:57:46
  Speaker 2: travel through a landscape divided into a few perfectly separate
• 1:57:49
  Speaker 2: gravitational territories. It belongs to an interconnected network whose movements
• 1:57:55
  Speaker 2: reflect the combined influence of matter across extraordinary distances. The
• 1:58:01
  Speaker 2: remarkable achievement is that astronomers can investigate this landscape at all.
• 1:58:06
  Speaker 2: From measurements of distant galaxies, they reconstruct structures that cannot
• 1:58:10
  Speaker 2: be viewed directly as complete objects. And with every improvement
• 1:58:15
  Speaker 2: in those measurements, the apparent boundaries of our cosmic neighbourhood
• 1:58:18
  Speaker 2: become a little better understood. The Great Attractor remains an
• 1:58:22
  Speaker 2: important part of that picture, but the gravitational environments surrounding
• 1:58:27
  Speaker 2: our galaxy may be larger and more interconnected than its
• 1:58:30
  Speaker 2: original discovery suggested. After examining the immense gravitational structures surrounding
• 1:58:36
  Speaker 2: the Milky Way, one question remains particularly interesting. Where is
• 1:58:42
  Speaker 2: our galaxy actually going? We know that it is moving
• 1:58:46
  Speaker 2: through the universe, and astronomers can measure that movement with
• 1:58:49
  Speaker 2: remarkable precision. But knowing how fast something is travelling is
• 1:58:54
  Speaker 2: not quite the same as knowing where it will eventually arrive.
• 1:58:58
  Speaker 2: For the Milky Way, the distinction is especially important because
• 1:59:03
  Speaker 2: its movement is influenced by a gravitational landscape that continues
• 1:59:07
  Speaker 2: to change over time. The local group, which contains the
• 1:59:11
  Speaker 2: Milky Way and Andromeda, travels at approximately 630 km per
• 1:59:17
  Speaker 2: second relative to the cosmic microwave background. This measurement provides
• 1:59:22
  Speaker 2: a reliable description of its present motion within a useful
• 1:59:25
  Speaker 2: cosmological reference frame. It does not, however, identify a permanent destination.
• 1:59:31
  Speaker 2: The universe contains no absolute stationary platform, and a galaxy's
• 1:59:36
  Speaker 2: velocity must always be described relative to something else. Nor
• 1:59:41
  Speaker 2: does its present direction necessarily reveal where it will be
• 1:59:44
  Speaker 2: travelling billions of years from now. Consider a spacecraft moving
• 1:59:48
  Speaker 2: through the solar system. Knowing its current speed and direction
• 1:59:53
  Speaker 2: would allow us to estimate its position a short time later.
• 1:59:57
  Speaker 2: But predicting its journey over many years would require accounting
• 2:00:00
  Speaker 2: for the gravitational influences of the Sun, planets and other objects.
• 2:00:06
  Speaker 2: The same principle applies to galaxies. Although the distances and
• 2:00:11
  Speaker 2: timescales are enormously greater, a galaxy's future trajectory depends upon
• 2:00:16
  Speaker 2: its present velocity and the gravitational forces that continue to
• 2:00:20
  Speaker 2: alter that velocity. The Milky Way experiences influences from several
• 2:00:25
  Speaker 2: different directions. The Virgo cluster contributes to the gravitational environment
• 2:00:30
  Speaker 2: of our nearby region. Farther away, the concentrations associated with
• 2:00:35
  Speaker 2: the greater tractor affect the movements of galaxies across a
• 2:00:39
  Speaker 2: much larger volume. Beyond them lies the Shapley concentration, whose
• 2:00:44
  Speaker 2: extraordinary mass may contribute to still broader patterns of motion.
• 2:00:49
  Speaker 2: Enormous cosmic voids also influence the resulting velocity field through
• 2:00:54
  Speaker 2: the relative absence of matter. These effects do not combine
• 2:00:58
  Speaker 2: into a simple journey towards one particular object. Gravity acts
• 2:01:03
  Speaker 2: in three dimensions, and its influence depends on the distribution
• 2:01:08
  Speaker 2: of matter throughout space. Moreover, our galaxy's present velocity reflects
• 2:01:13
  Speaker 2: accelerations accumulated over billions of years. Even if the strongest
• 2:01:18
  Speaker 2: gravitational influence acting today were directed towards one concentration, The
• 2:01:23
  Speaker 2: Milky Way would not necessarily be moving directly towards it.
• 2:01:27
  Speaker 2: Its existing velocity would also matter. The gravitational landscape itself
• 2:01:33
  Speaker 2: is not permanent. Galaxies continue to move within clusters, smaller
• 2:01:38
  Speaker 2: systems merge, and matter flows along the filaments of the
• 2:01:41
  Speaker 2: cosmic web. These developments gradually change the gravitational fields surrounding
• 2:01:47
  Speaker 2: different regions. The movements of galaxy clusters are therefore part
• 2:01:51
  Speaker 2: of an evolving arrangement. rather than a collection of fixed
• 2:01:55
  Speaker 2: destinations towards which everything travels. There is also the expansion
• 2:01:59
  Speaker 2: of the universe to consider. On sufficiently large scales, the
• 2:02:04
  Speaker 2: average distances between gravitationally unbound galaxies increase over time. Gravity
• 2:02:10
  Speaker 2: can oppose this expansion locally, but it does not necessarily
• 2:02:14
  Speaker 2: overcome it across the enormous separations between superclusters. Two concentrations
• 2:02:19
  Speaker 2: may influence one another gravitationally, without eventually coming together. Their
• 2:02:24
  Speaker 2: future relationship depends on the amount of mass involved, their
• 2:02:28
  Speaker 2: relative motion, and the continuing evolution of cosmic expansion. This
• 2:02:34
  Speaker 2: distinction becomes particularly important when considering Lanarkia. Although the original
• 2:02:40
  Speaker 2: reconstruction identified a vast region of connected galactic flows, that
• 2:02:44
  Speaker 2: does not mean every galaxy within it belongs to one
• 2:02:48
  Speaker 2: permanently gravitationally bound system. The local group is held together
• 2:02:53
  Speaker 2: by gravity, but the much larger supercluster-scale environment contains structures
• 2:02:59
  Speaker 2: separated by distances across which expansion remains significant. There is
• 2:03:04
  Speaker 2: no requirement for everything inside Laniakea to collapse into one
• 2:03:09
  Speaker 2: gigantic collection of galaxies. Dark energy adds another consideration. In
• 2:03:15
  Speaker 2: the standard cosmological model, dark energy is associated with the
• 2:03:19
  Speaker 2: accelerating expansion of the universe. As expansion continues, sufficiently distant
• 2:03:25
  Speaker 2: gravitationally unbound structures can become increasingly separated. This does not
• 2:03:30
  Speaker 2: mean dark energy is pulling stars out of galaxies or
• 2:03:34
  Speaker 2: forcing the Milky Way apart. Gravitationally bound systems can remain intact,
• 2:03:41
  Speaker 2: while the distances between much larger independent structures continue to increase. Consequently,
• 2:03:47
  Speaker 2: The distant Shapley concentration should not automatically be regarded as
• 2:03:52
  Speaker 2: our eventual destination. Its gravity can contribute to the local
• 2:03:56
  Speaker 2: group's observed peculiar velocity without guaranteeing that our galaxy will
• 2:04:01
  Speaker 2: ever reach it. The same applies to the greater tractor.
• 2:04:06
  Speaker 2: A present contribution to galactic motion is not a promise
• 2:04:10
  Speaker 2: of a future collision, particularly when the intervening distances extend
• 2:04:14
  Speaker 2: across hundreds of millions of light-years. Astronomers investigate possible future
• 2:04:19
  Speaker 2: trajectories using computer simulations that model gravity, cosmic expansion and
• 2:04:26
  Speaker 2: the evolving distribution of matter. These calculations can reveal plausible outcomes,
• 2:04:32
  Speaker 2: but their reliability depends on the completeness of the observations
• 2:04:36
  Speaker 2: and the assumptions used. Predicting the detailed movements of enormous
• 2:04:41
  Speaker 2: cosmic structures billions of years into the future is considerably
• 2:04:45
  Speaker 2: more difficult than measuring their present velocities. None of this
• 2:04:49
  Speaker 2: makes the Great Attractor a threat to the Milky Way.
• 2:04:52
  Speaker 2: It is not an enormous object racing towards us, nor
• 2:04:56
  Speaker 2: is our galaxy being dragged helplessly towards an approaching catastrophe.
• 2:05:01
  Speaker 2: It is part of the gravitational environment in which our
• 2:05:03
  Speaker 2: cosmic neighbourhood has evolved. Its influence operates over enormous distances
• 2:05:09
  Speaker 2: and timescales, producing changes that are completely imperceptible during an
• 2:05:14
  Speaker 2: ordinary human lifetime. Indeed, the universe has no single gravitational
• 2:05:19
  Speaker 2: centre towards which every galaxy must eventually travel. Different regions
• 2:05:24
  Speaker 2: contain different concentrations of matter, and galaxies respond to the
• 2:05:28
  Speaker 2: particular environment surrounding them. On sufficiently large scales, the universe
• 2:05:33
  Speaker 2: has no preferred central location. The movements we observe arise
• 2:05:38
  Speaker 2: from regional differences in the distribution of matter rather than
• 2:05:42
  Speaker 2: attraction towards one universal destination. So the Milky Way's journey
• 2:05:47
  Speaker 2: is both measurable and uncertain. We can describe its present
• 2:05:52
  Speaker 2: motion relative to the cosmic microwave background, identify many of
• 2:05:56
  Speaker 2: the structures contributing to that motion, and reconstruct the gravitational
• 2:06:00
  Speaker 2: landscape surrounding our galaxy. But there is no single address
• 2:06:06
  Speaker 2: in the distant universe that we can confidently identify as
• 2:06:09
  Speaker 2: its final destination. Meanwhile, the Earth continues around the Sun.
• 2:06:14
  Speaker 2: The Sun continues its long orbit within the Milky Way,
• 2:06:18
  Speaker 2: and our entire galactic neighbourhood participates in the movement of
• 2:06:22
  Speaker 2: the cosmic web. Beneath our feet, everything seems perfectly still.
• 2:06:28
  Speaker 2: Yet we're travelling through an enormous universe. guided by gravitational
• 2:06:32
  Speaker 2: influences extending far beyond the stars visible in our sky.
• 2:06:38
  Speaker 2: The motion is real, the forces are understandable, and the
• 2:06:41
  Speaker 2: journey has been unfolding for billions of years without any
• 2:06:45
  Speaker 2: need for a final destination. Somewhere beneath the familiar movements
• 2:06:50
  Speaker 2: of our everyday lives, an extraordinary journey is taking place.
• 2:06:55
  Speaker 2: The Earth travels around the Sun, the Sun follows its
• 2:06:58
  Speaker 2: long orbit through the Milky Way, and our entire galactic
• 2:07:01
  Speaker 2: neighbourhood moves relative to the wider universe. We cannot feel
• 2:07:06
  Speaker 2: that last movement. The stars do not appear to rush
• 2:07:09
  Speaker 2: across the sky, and nothing on Earth provides a sign
• 2:07:12
  Speaker 2: of the immense speed involved. Yet measurements of the cosmic
• 2:07:16
  Speaker 2: microwave background show that the local group is travelling at
• 2:07:20
  Speaker 2: approximately 630 kilometres per second, relative to that ancient radiation.
• 2:07:27
  Speaker 2: The cosmic microwave background offers a reference frame, not a
• 2:07:30
  Speaker 2: universal state of absolute rest. Its faint radiation reaches us
• 2:07:35
  Speaker 2: from an early era of the universe, and the slight
• 2:07:38
  Speaker 2: difference in its temperature across the sky reveals our movement
• 2:07:42
  Speaker 2: relative to the large-scale cosmic average. The warmer direction and
• 2:07:47
  Speaker 2: the cooler direction form a pattern called the cosmic microwave
• 2:07:51
  Speaker 2: background dipole. From this delicate signal, astronomers can measure a
• 2:07:55
  Speaker 2: velocity greater than 2 million kilometres per hour. The remaining
• 2:08:00
  Speaker 2: question is, what gravitational history has given our neighbouring galaxies
• 2:08:05
  Speaker 2: this collective motion? The answer is not one enormous object
• 2:08:09
  Speaker 2: dragging the Milky Way through otherwise empty space. Gravity acts
• 2:08:13
  Speaker 2: wherever matter exists. and the universe contains matter in an
• 2:08:17
  Speaker 2: extraordinarily uneven arrangement. Galaxies gather in groups. Groups form parts
• 2:08:24
  Speaker 2: of clusters, and clusters connect through filaments and walls stretching
• 2:08:28
  Speaker 2: across vast distances. Between them are regions containing far less
• 2:08:33
  Speaker 2: matter than average. The combined influence of all these structures
• 2:08:38
  Speaker 2: changes the velocities of galaxies over billions of years. Our
• 2:08:42
  Speaker 2: present movement is the result of that long history. rather
• 2:08:46
  Speaker 2: than a direct pointer towards a single destination. Astronomers first
• 2:08:51
  Speaker 2: recognised an especially important part of this landscape by observing
• 2:08:55
  Speaker 2: how galaxies moved. During the 1980s, measurements of galaxy distances
• 2:09:01
  Speaker 2: and redshifts revealed that many galaxies shared an unexpected component
• 2:09:06
  Speaker 2: of motion. The smooth expansion of the universe could not
• 2:09:09
  Speaker 2: explain the full pattern. The investigators inferred a major excess
• 2:09:13
  Speaker 2: of gravitating matter in the direction of the southern constellations
• 2:09:17
  Speaker 2: Centaurus and Norma. That inferred region became known as the
• 2:09:21
  Speaker 2: Great Attractor. It was discovered in a sense through its
• 2:09:25
  Speaker 2: effects before its underlying structures could be mapped in sufficient detail.
• 2:09:30
  Speaker 2: The Great Attractor is not a single galaxy, a gigantic
• 2:09:34
  Speaker 2: black hole or a compact object with a surface. It
• 2:09:38
  Speaker 2: describes a broad concentration of mass associated with a pattern
• 2:09:42
  Speaker 2: of gravitational flows. The Norma Cluster, roughly 220 million light-years away,
• 2:09:49
  Speaker 2: is one of its most substantial identified features. Other galaxy
• 2:09:54
  Speaker 2: concentrations in the Hydrocentaurus region contribute to the wider gravitational landscape.
• 2:10:00
  Speaker 2: These systems contain galaxies, extremely hot gas and extensive distributions
• 2:10:06
  Speaker 2: of dark matter. Their combined influence helps explain why surrounding
• 2:10:11
  Speaker 2: galaxies have not simply followed the average cosmic expansion. For
• 2:10:15
  Speaker 2: a long time, understanding this region was made harder by
• 2:10:19
  Speaker 2: our position inside the Milky Way. Dust and countless foreground
• 2:10:23
  Speaker 2: stars obscure part of the southern sky, creating the observational
• 2:10:27
  Speaker 2: band known as the Zone of Avoidance. The name does
• 2:10:31
  Speaker 2: not describe an empty region that distant galaxies somehow refuse
• 2:10:34
  Speaker 2: to enter. It describes a part of the sky where
• 2:10:37
  Speaker 2: those galaxies are difficult to see. The remarkable complication is
• 2:10:41
  Speaker 2: that our own galaxy conceals some of the structures whose
• 2:10:44
  Speaker 2: gravity has helped shape its movement. Different kinds of telescopes
• 2:10:49
  Speaker 2: have gradually opened that obscured view. Infrared observations penetrate some
• 2:10:53
  Speaker 2: of the dust that blocks visible light. Radio telescopes detect
• 2:10:58
  Speaker 2: emission from neutral hydrogen in galaxies otherwise hidden behind the
• 2:11:03
  Speaker 2: galactic plane. X-ray telescopes reveal hot gas within massive clusters.
• 2:11:08
  Speaker 2: Each method has limitations, but together they uncover a richer
• 2:11:12
  Speaker 2: distribution of matter than ordinary optical images can provide. Astronomers
• 2:11:18
  Speaker 2: can then compare the location of those structures with the
• 2:11:21
  Speaker 2: patterns of peculiar velocity revealed by the movement of galaxies
• 2:11:25
  Speaker 2: around them. Even a complete map of shining galaxies would
• 2:11:29
  Speaker 2: not show all the matter involved. dark matter accounts for
• 2:11:33
  Speaker 2: most of the matter in the standard cosmological model, and
• 2:11:36
  Speaker 2: its gravity helps shape galaxy halos, clusters and the larger
• 2:11:40
  Speaker 2: cosmic web. It does not reveal itself through ordinary starlight. Instead,
• 2:11:46
  Speaker 2: astronomers infer its presence through galaxy motions, the behaviour of
• 2:11:51
  Speaker 2: hot cluster gas and gravitational lensing in which mass bends
• 2:11:55
  Speaker 2: the paths of light. The movements of galaxies therefore offer
• 2:11:59
  Speaker 2: information about a landscape much more extensive than the luminous
• 2:12:03
  Speaker 2: structures alone. In 2014, astronomers gave part of that landscape
• 2:12:08
  Speaker 2: a name, Laniakea. By reconstructing peculiar velocity flows, they identified
• 2:12:15
  Speaker 2: an immense region extending approximately 500 million light-years across, containing
• 2:12:21
  Speaker 2: the Milky Way and the Greater Tractor region. Its proposed
• 2:12:24
  Speaker 2: boundaries followed patterns of gravitational motion rather than visible walls
• 2:12:29
  Speaker 2: in space. The picture was a useful reminder that our
• 2:12:33
  Speaker 2: galactic neighbourhood forms part of something far larger. Although such
• 2:12:37
  Speaker 2: a flow-defined supercluster is not necessarily one gravitationally bound object
• 2:12:42
  Speaker 2: destined to collapse together, observations and reconstructions have continued to
• 2:12:47
  Speaker 2: refine that picture. Beyond the nearer greater tractor lies the
• 2:12:51
  Speaker 2: Shapley Concentration, an exceptionally rich gathering of galaxy clusters hundreds
• 2:12:57
  Speaker 2: of millions of light-years away. Studies using more extensive galaxy
• 2:13:02
  Speaker 2: distance catalogues have suggested that the original Laniakea region may
• 2:13:07
  Speaker 2: be part of a still larger basin of attraction associated
• 2:13:11
  Speaker 2: with Shapley. That interpretation remains uncertain. What matters is that
• 2:13:16
  Speaker 2: gravity does not respect the boundaries of our diagrams. Influences
• 2:13:21
  Speaker 2: from nearby clusters and more distant concentrations can operate together
• 2:13:25
  Speaker 2: across overlapping regions. The comparatively empty regions matter too. The
• 2:13:31
  Speaker 2: local void lies beside our galactic neighbourhood, while a more
• 2:13:35
  Speaker 2: distant underdense region has been inferred and named the dipole repeller.
• 2:13:40
  Speaker 2: Neither represents a source of unusual antigravity. A void contains
• 2:13:44
  Speaker 2: less attracting matter than the cosmic average. so neighbouring regions
• 2:13:49
  Speaker 2: can acquire peculiar velocities directed away from it, as denser
• 2:13:53
  Speaker 2: surroundings exert the greater pull. Motion away from an underdensity
• 2:13:58
  Speaker 2: and towards an overdensity are complementary parts of the same
• 2:14:02
  Speaker 2: gravitational explanation. The universe's apparent emptiness can be as informative
• 2:14:07
  Speaker 2: as its brightest clusters. Throughout all of this, cosmic expansion continues.
• 2:14:14
  Speaker 2: At large distances, Unbound galaxies generally become more widely separated
• 2:14:19
  Speaker 2: as the universe evolves. Peculiar velocities describe their additional movements
• 2:14:24
  Speaker 2: relative to that expansion. Gravity can bring nearby galaxies together,
• 2:14:29
  Speaker 2: as it does within the local group, without reversing the
• 2:14:32
  Speaker 2: expansion everywhere else. Nor does our present velocity mean the
• 2:14:36
  Speaker 2: Milky Way must eventually collide with the greater tractor or
• 2:14:39
  Speaker 2: reach the Shapley concentration. The future depends on evolving gravitational fields.
• 2:14:45
  Speaker 2: the velocities of surrounding structures and the continuing expansion of space.
• 2:14:50
  Speaker 2: So the Milky Way has no known final cosmic address.
• 2:14:54
  Speaker 2: It carries hundreds of billions of stars, including the Sun
• 2:14:57
  Speaker 2: and its planets, through an environment shaped by matter extending
• 2:15:01
  Speaker 2: far beyond its visible disk. The Great Attractor is an
• 2:15:05
  Speaker 2: important part of that environment, but not a concealed giant
• 2:15:09
  Speaker 2: controlling our destiny. Every galaxy follows a history written partly
• 2:15:13
  Speaker 2: by the gravity of its neighbours, and partly by structures
• 2:15:17
  Speaker 2: much farther away. From one small planet, astronomers have learned
• 2:15:21
  Speaker 2: to read that history in the faint light and measured
• 2:15:24
  Speaker 2: motions of distant galaxies. And beneath a night sky that
• 2:15:29
  Speaker 2: appears perfectly still, our own great galaxy continues its journey
• 2:15:33
  Speaker 2: through the connected landscape of the universe.