Latest / Boring Science / How Many Black Holes Exist in the Milky Way

Transcript

How Many Black Holes Exist in the Milky Way · Boring Science

• 0:00
  Speaker 1: There are roughly 100 million black holes in the Milky Way,
• 0:04
  Speaker 1: and we have found fewer than 50 of them. Let
• 0:08
  Speaker 1: that sit for a moment. 100 million objects, each one
• 0:13
  Speaker 1: a place where gravity has crushed matter so completely that nothing,
• 0:18
  Speaker 1: not light, not radio waves, not any signal we can
• 0:22
  Speaker 1: send or receive, escapes. They are scattered across a galaxy, 100,000
• 0:29
  Speaker 1: light years wide, and we have confirmed fewer than 50.
• 0:33
  Speaker 1: That is not a rounding problem. That is not a
• 0:37
  Speaker 1: gap we are close to closing. That is a census
• 0:41
  Speaker 1: of almost total ignorance, and it is one of the
• 0:44
  Speaker 1: strangest situations in modern science. Here is what makes this
• 0:55
  Speaker 1: so strange. These are not rare objects. A star 20
• 1:00
  Speaker 1: times the mass of our sun burns through its fuel
• 1:04
  Speaker 1: in a few million years and then collapses, leaving a
• 1:09
  Speaker 1: black hole behind. That process has been running in this
• 1:13
  Speaker 1: galaxy for 13 billion years. The Milky Way contains somewhere
• 1:19
  Speaker 1: between 200 and 400 billion stars. The math is straightforward.
• 1:26
  Speaker 1: massive stars have lived and died here by the hundreds
• 1:29
  Speaker 1: of millions and the collapsed remnants of those stars did
• 1:33
  Speaker 1: not go anywhere they are still here they are everywhere
• 1:38
  Speaker 1: you are right now inside a galaxy absolutely packed with
• 1:43
  Speaker 1: them and yet we can count the confirmed ones on
• 1:48
  Speaker 1: two hands with room left over the number is somewhere
• 1:52
  Speaker 1: under 50 Depending on how strict you are with the
• 1:55
  Speaker 1: word confirmed the gap between 100 million and 50 is
• 2:00
  Speaker 1: not just large It is almost mocking It tells you
• 2:05
  Speaker 1: something fundamental about what a black hole is and why
• 2:10
  Speaker 1: finding one is so hard The problem is simple in
• 2:14
  Speaker 1: one sense and brutal in another black holes produce no
• 2:19
  Speaker 1: light A black hole sitting alone in empty space, with
• 2:24
  Speaker 1: nothing nearby to pull in and heat up, is completely dark.
• 2:29
  Speaker 1: No telescope picks it up. No detector registers it. It
• 2:34
  Speaker 1: has a gravitational field, and that is genuinely all it
• 2:38
  Speaker 1: offers the universe around it. If one drifted through the
• 2:42
  Speaker 1: space between stars a thousand light years from here, we
• 2:46
  Speaker 1: would have no way of knowing. It would pass through, invisible.
• 2:51
  Speaker 1: and keep going. What we have found, those few dozen
• 2:56
  Speaker 1: confirmed cases, are the loud ones. The ones that happen
• 3:01
  Speaker 1: to have a companion star close enough to steal gas from.
• 3:06
  Speaker 1: When a black hole pulls material off a nearby star,
• 3:10
  Speaker 1: that gas doesn't fall straight in. It spirals, flattens into
• 3:15
  Speaker 1: a disk, heats up through friction, until it reaches temperatures
• 3:20
  Speaker 1: of millions of degrees and blazes in X-rays. Space telescopes
• 3:25
  Speaker 1: can see those X-rays. That is how we found most
• 3:29
  Speaker 1: of the black holes on the confirmed list. We didn't
• 3:33
  Speaker 1: see the black hole. We saw what it was doing
• 3:36
  Speaker 1: to the star next to it. That is a detection
• 3:39
  Speaker 1: method with an obvious flaw. It only works when a
• 3:43
  Speaker 1: black hole has a companion. Most black holes don't. The
• 3:48
  Speaker 1: fraction of all black holes that happen to be in
• 3:51
  Speaker 1: close binary systems, actively pulling gas and glowing in X-rays,
• 3:56
  Speaker 1: is tiny. Maybe a few percent at most. The rest
• 4:01
  Speaker 1: are solitary, cold, and completely invisible to every instrument we
• 4:06
  Speaker 1: have ever built. They drift through the disc of the galaxy,
• 4:10
  Speaker 1: through the spiral arms, through the space between stars, and
• 4:15
  Speaker 1: we have no idea where any of them are. Now
• 4:18
  Speaker 1: think about what this means for the question in the title.
• 4:22
  Speaker 1: How many black holes exist in the Milky Way? That number,
• 4:27
  Speaker 1: roughly 100 million, doesn't come from counting. Nobody sat down
• 4:33
  Speaker 1: with a telescope and tallied them up. It comes from calculation,
• 4:39
  Speaker 1: from stellar population models and estimates of how many massive
• 4:43
  Speaker 1: stars the galaxy has produced over its lifetime. And what
• 4:47
  Speaker 1: fraction of those ended as black holes rather than neutron
• 4:51
  Speaker 1: stars or white dwarfs? It is a statistical argument applied
• 4:57
  Speaker 1: to a population that is almost entirely unobservable. It could
• 5:02
  Speaker 1: be off by a factor of 10 in either direction,
• 5:05
  Speaker 1: and we would have no direct way to know. So
• 5:09
  Speaker 1: what we are really asking, when we ask how many
• 5:12
  Speaker 1: black holes exist in the Milky Way, is something closer
• 5:16
  Speaker 1: to this. We know roughly how many there should be
• 5:21
  Speaker 1: based on what we understand about stars. We know roughly
• 5:25
  Speaker 1: where they should be based on where stars form and die.
• 5:30
  Speaker 1: We have confirmed a handful and we are now for
• 5:34
  Speaker 1: the first time beginning to develop methods that might actually
• 5:38
  Speaker 1: let us find isolated black holes, the quiet ones, the
• 5:43
  Speaker 1: dark ones, The ones with no companion and no X-ray
• 5:47
  Speaker 1: flare to give them away. Gravitational microlensing. Ultra-precise measurements of
• 5:54
  Speaker 1: stellar motion. Gravitational wave detections from mergers. Each method is
• 6:00
  Speaker 1: beginning to pull black holes out of the dark, one
• 6:03
  Speaker 1: at a time. This is going to be a long search.
• 6:07
  Speaker 1: The galaxy is enormous. The black holes are invisible. And
• 6:12
  Speaker 1: the tools we have are new. But the picture is
• 6:16
  Speaker 1: starting to come together, piece by piece, and what it
• 6:20
  Speaker 1: is showing us is a galaxy far more crowded with
• 6:24
  Speaker 1: these objects than most people realize. The rest are out
• 6:29
  Speaker 1: there somewhere, drifting through the galaxy in silence, and finding
• 6:34
  Speaker 1: them is one of the strangest problems in modern astronomy.
• 6:38
  Speaker 1: Before you can count something, you have to know what
• 6:41
  Speaker 1: you are counting. The word black hole gets used constantly,
• 6:46
  Speaker 1: but the actual physics behind it is stranger than most
• 6:50
  Speaker 1: descriptions let on. It is not a hole in the
• 6:54
  Speaker 1: way a pit is a hole. It is not empty space.
• 6:58
  Speaker 1: It is a region where matter has been compressed so
• 7:02
  Speaker 1: densely that gravity has become absolute. Absolute in the most
• 7:07
  Speaker 1: literal sense. Nothing that enters can leave. not gas, not light,
• 7:13
  Speaker 1: not a signal of any kind. Whatever crosses the threshold
• 7:18
  Speaker 1: is gone from the observable universe forever. To understand why
• 7:23
  Speaker 1: that happens you need escape velocity. Every massive object has one.
• 7:30
  Speaker 1: To escape Earth's gravity you need to be moving at
• 7:33
  Speaker 1: roughly 25,000 miles per hour. The more massive or compact
• 7:38
  Speaker 1: an object is the higher that velocity gets. For a
• 7:43
  Speaker 1: black hole, the escape velocity at a certain boundary has
• 7:47
  Speaker 1: reached the speed of light. Since nothing travels faster than light,
• 7:53
  Speaker 1: nothing can escape once it crosses that line. That boundary
• 7:58
  Speaker 1: has a name. Astronomers call it the event horizon. The
• 8:04
  Speaker 1: event horizon is not a surface you could land on
• 8:06
  Speaker 1: or touch. There is no wall. no barrier, no visible edge.
• 8:13
  Speaker 1: If you were falling toward a black hole, you would
• 8:16
  Speaker 1: cross the event horizon without feeling anything unusual in that instant.
• 8:21
  Speaker 1: Space looks normal from the inside of the crossing. What
• 8:25
  Speaker 1: changes is your future. Once you are past it, every
• 8:30
  Speaker 1: path available to you, every direction you could move, points inward.
• 8:36
  Speaker 1: There is no trajectory that leads back out. You are
• 8:40
  Speaker 1: not trapped the way a prisoner is trapped. You are
• 8:43
  Speaker 1: trapped the way time is trapped moving forward. There is
• 8:48
  Speaker 1: simply no other direction. Past the event horizon in the
• 8:53
  Speaker 1: models physicists work with, all the mass is concentrated at
• 8:58
  Speaker 1: a single point, the singularity, a place where density becomes
• 9:03
  Speaker 1: infinite and the equations of general relativity break down entirely.
• 9:10
  Speaker 1: What is actually happening at a singularity is one of
• 9:13
  Speaker 1: the genuinely open questions in physics. The math stops working,
• 9:19
  Speaker 1: which usually means the model is missing something important. But
• 9:23
  Speaker 1: whatever the singularity truly is, it sits at the center,
• 9:28
  Speaker 1: and everything that enters the event horizon is falling toward it.
• 9:34
  Speaker 1: Now consider the sun. It is enormous. Over a million
• 9:39
  Speaker 1: Earths could fit inside it. Its surface gravity is already
• 9:43
  Speaker 1: 28 times stronger than what you feel standing on Earth.
• 9:48
  Speaker 1: But the Sun is not a black hole because it
• 9:51
  Speaker 1: is spread out across a vast volume of space. The
• 9:56
  Speaker 1: surface is nearly a million miles from the center. Gravity
• 10:00
  Speaker 1: is strong, but it is not strong enough at the
• 10:03
  Speaker 1: surface to trap light. What would it take to make
• 10:08
  Speaker 1: the Sun a black hole? You would have to compress
• 10:12
  Speaker 1: the entire Sun, all of its mass, down into a
• 10:16
  Speaker 1: sphere roughly 3.7 miles across. that is smaller than most cities.
• 10:23
  Speaker 1: At that density, the escape velocity at the surface would
• 10:27
  Speaker 1: finally hit the speed of light, and the Sun would
• 10:30
  Speaker 1: vanish behind its own event horizon. The Solar System's planets
• 10:36
  Speaker 1: would still orbit normally. Gravity at a distance does not
• 10:41
  Speaker 1: change when you compress the object, but nothing could ever
• 10:45
  Speaker 1: receive light from it again. That 3.7 mile figure is
• 10:50
  Speaker 1: the Schwarzschild radius of the Sun. Every mass has one.
• 10:56
  Speaker 1: Earth's is about a third of an inch. A human
• 10:59
  Speaker 1: body's is so small, the number is essentially meaningless in
• 11:03
  Speaker 1: any physical sense. The point is that black holes are
• 11:08
  Speaker 1: not exotic alien objects with different physics. They are what
• 11:13
  Speaker 1: happens when ordinary matter gets pushed past the point where
• 11:17
  Speaker 1: any force can push back. Now, the Milky Way does
• 11:22
  Speaker 1: not contain just one kind of black hole. There are
• 11:26
  Speaker 1: three categories worth knowing. The distinction between these categories matters
• 11:32
  Speaker 1: because black holes of different masses behave differently and form
• 11:38
  Speaker 1: through different mechanisms. Understanding the differences changes how you interpret
• 11:44
  Speaker 1: observations and what those observations reveal about cosmic history. A
• 11:51
  Speaker 1: stellar mass black hole reflects the life and death of
• 11:55
  Speaker 1: a single star, while other types tell different stories about
• 11:59
  Speaker 1: their origins entirely. The first is stellar mass black holes.
• 12:05
  Speaker 1: These form when individual stars die in certain ways. They
• 12:10
  Speaker 1: typically range from about five solar masses on the low
• 12:14
  Speaker 1: end up to several dozen solar masses for the largest
• 12:18
  Speaker 1: known examples. they are by far the most numerous type.
• 12:24
  Speaker 1: When astronomers talk about 100 million black holes in the
• 12:28
  Speaker 1: Milky Way, they are talking almost entirely about these. The
• 12:33
  Speaker 1: second category is intermediate mass black holes. These sit in
• 12:39
  Speaker 1: a range from roughly 100 solar masses up to somewhere
• 12:43
  Speaker 1: around 100,000 solar masses. They are theoretically important because they
• 12:50
  Speaker 1: may represent a middle stage between stellar mass black holes
• 12:54
  Speaker 1: and the enormous ones found at galaxy centres. The frustrating
• 12:59
  Speaker 1: thing is that confirmed examples are rare. Astronomers believe they exist,
• 13:06
  Speaker 1: and the Milky Way is thought to contain somewhere between
• 13:09
  Speaker 1: a thousand and ten thousand of them, but pinning one
• 13:13
  Speaker 1: down with certainty has proven difficult. This is one of
• 13:18
  Speaker 1: the more persistent open problems in the field. The third
• 13:23
  Speaker 1: category is the supermassive black hole. Every large galaxy appears
• 13:29
  Speaker 1: to have one sitting at its centre, and the Milky
• 13:32
  Speaker 1: Way is no different. Ours is called Sagittarius A-star. It
• 13:38
  Speaker 1: contains the mass of roughly 4.3 million suns, packed into
• 13:44
  Speaker 1: a region smaller than our solar system. it is not
• 13:48
  Speaker 1: eating much right now. The galactic center is relatively quiet
• 13:53
  Speaker 1: compared to the active galactic nuclei seen in other galaxies,
• 13:58
  Speaker 1: but it is there, and it is real, and a
• 14:01
  Speaker 1: later part of this story covers it in detail. For now,
• 14:06
  Speaker 1: the thing to hold in your mind is the size
• 14:09
  Speaker 1: difference between these categories. Stellar mass black holes are the
• 14:15
  Speaker 1: remnants of single stars. Intermediate mass black holes are the
• 14:21
  Speaker 1: size of a small star cluster's worth of mass compressed down.
• 14:26
  Speaker 1: Sagittarius A star holds millions of solar masses in a
• 14:30
  Speaker 1: space you could describe with the scale of our own
• 14:33
  Speaker 1: solar neighborhood. These are not the same kind of object
• 14:37
  Speaker 1: scaled up. They likely form through different processes and have
• 14:42
  Speaker 1: different histories. Most of this story is about stellar mass
• 14:47
  Speaker 1: black holes the ones that form when individual stars die,
• 14:53
  Speaker 1: because those are the ones filling the galaxy by the millions,
• 14:57
  Speaker 1: and the ones we are still trying to track down.
• 15:01
  Speaker 1: Every stellar mass black hole in the Milky Way started
• 15:05
  Speaker 1: its life as an ordinary star, just a very large one,
• 15:09
  Speaker 1: not slightly large, not impressively large. We are talking about
• 15:15
  Speaker 1: objects born with 10, 20, 50, Sometimes over a hundred
• 15:20
  Speaker 1: times the mass of our sun, sitting at the centers
• 15:24
  Speaker 1: of young star-forming regions, burning so ferociously that they would
• 15:29
  Speaker 1: exhaust in a few million years what our sun will
• 15:33
  Speaker 1: take ten billion years to consume. The sun has been
• 15:37
  Speaker 1: quietly fusing hydrogen for nearly five billion years and is
• 15:42
  Speaker 1: not even halfway through its fuel supply. A star born
• 15:47
  Speaker 1: with 30 times that mass might burn through everything it
• 15:51
  Speaker 1: has in 3 million years and then face something that
• 15:55
  Speaker 1: no amount of energy can stop. Gravity. Pure, patient, total gravity.
• 16:04
  Speaker 1: The story of how that happens is one of the
• 16:07
  Speaker 1: most violent sequences of events in the known universe. It
• 16:12
  Speaker 1: starts with hydrogen. Every star begins its life fusing hydrogen
• 16:17
  Speaker 1: into helium in its core, releasing energy that pushes outward
• 16:22
  Speaker 1: and holds the star up against its own weight. This balance,
• 16:28
  Speaker 1: outward pressure from fusion against inward pull from gravity, is
• 16:33
  Speaker 1: what a star is. Remove the fusion and the whole
• 16:38
  Speaker 1: structure collapses. For most of a massive star's life, Hydrogen
• 16:44
  Speaker 1: does the job. But hydrogen does not last forever. When
• 16:49
  Speaker 1: the hydrogen in the core runs low, the core contracts
• 16:52
  Speaker 1: and heats up, and helium ignites. Helium fuses into carbon.
• 16:58
  Speaker 1: The star has bought itself more time, but not much.
• 17:02
  Speaker 1: Each new fuel source burns faster and leaves behind a
• 17:06
  Speaker 1: heavier ash. Carbon ignites into neon and oxygen. Oxygen burns
• 17:12
  Speaker 1: into silicon. Each phase is shorter than the last. Hydrogen
• 17:17
  Speaker 1: burning might take millions of years. Silicon burning, near the
• 17:21
  Speaker 1: very end, lasts about a day. Silicon burns into iron.
• 17:27
  Speaker 1: And that is where everything stops. Iron is the endgame
• 17:31
  Speaker 1: element in stellar physics. Every element lighter than iron releases
• 17:36
  Speaker 1: energy when you fuse it. That is the whole basis
• 17:40
  Speaker 1: of stellar power. but iron is different. Fusing iron does
• 17:46
  Speaker 1: not release energy. It consumes it. When a massive star
• 17:52
  Speaker 1: builds an iron core, it has reached a dead end.
• 17:56
  Speaker 1: There is no next fuel. There is no new ignition.
• 18:00
  Speaker 1: The core just sits there, growing heavier as the shells
• 18:04
  Speaker 1: above it keep dumping more iron downward. When that iron
• 18:08
  Speaker 1: core exceeds about one and a half times the mass
• 18:11
  Speaker 1: of the sun, something happens that takes less than a
• 18:14
  Speaker 1: quarter of a second the core collapses not slowly not
• 18:20
  Speaker 1: gradually in roughly 200 milliseconds a ball of iron the
• 18:26
  Speaker 1: size of earth compresses down to a sphere about 10
• 18:30
  Speaker 1: miles across the densities involved are almost beyond the reach
• 18:35
  Speaker 1: of normal language what happens next produces one of the
• 18:39
  Speaker 1: most energetic events in the universe the collapsing core overshoots,
• 18:45
  Speaker 1: reaches nuclear density, and bounces. That bounce sends a shockwave
• 18:52
  Speaker 1: outward through the star's outer layers, and when that shockwave
• 18:56
  Speaker 1: tears through the surface, the explosion releases more energy in
• 19:01
  Speaker 1: a few seconds than the Sun will release across its
• 19:05
  Speaker 1: entire 10 billion year life. This is a core collapse supernova.
• 19:12
  Speaker 1: From Earth it can outshine an entire galaxy. The Chinese
• 19:17
  Speaker 1: astronomers who recorded the supernova of 1054 watched a new
• 19:22
  Speaker 1: star appear in daylight and stay visible for nearly a month.
• 19:27
  Speaker 1: That explosion left behind the Crab Nebula, still expanding today,
• 19:32
  Speaker 1: nearly a thousand years later. Now here is where the
• 19:36
  Speaker 1: mass threshold matters. What survives at the centre after the
• 19:41
  Speaker 1: explosion depends almost entirely on how heavy the original star was.
• 19:47
  Speaker 1: For stars in roughly the 10 to 20 solar mass range,
• 19:52
  Speaker 1: the collapsed core stabilizes as a neutron star. A neutron
• 19:57
  Speaker 1: star is already one of the most extreme objects in
• 20:00
  Speaker 1: the universe. City-sized, roughly 10 miles across, containing more mass
• 20:07
  Speaker 1: than the Sun, so dense that a teaspoon of its
• 20:11
  Speaker 1: material would weigh about a billion tons. But it holds together.
• 20:17
  Speaker 1: The neutron pressure is enough to resist further collapse. For
• 20:21
  Speaker 1: stars above roughly 20 to 25 solar masses, the story
• 20:26
  Speaker 1: ends differently. The core is simply too heavy. Neutron pressure
• 20:32
  Speaker 1: cannot hold it. Nothing can hold it. The collapse continues
• 20:38
  Speaker 1: past the neutron star stage, past any density physics can support,
• 20:44
  Speaker 1: and a black hole forms. The exact threshold is not
• 20:48
  Speaker 1: perfectly pinned down. It sits somewhere between 20 and 25
• 20:53
  Speaker 1: solar masses, depending on the star's composition, rotation, and other factors.
• 21:00
  Speaker 1: But above that line, there is no rescue. The supernova
• 21:05
  Speaker 1: itself is not guaranteed either, and this is where things
• 21:09
  Speaker 1: get strange. Most of the time, the explosion works the
• 21:13
  Speaker 1: way the textbooks describe it. The shockwave forms, the outer
• 21:18
  Speaker 1: layers get blasted away, and the collapsed core sits exposed
• 21:23
  Speaker 1: at the center. But some very massive stars may not
• 21:27
  Speaker 1: follow that script at all. If the star is heavy enough,
• 21:32
  Speaker 1: or if the shock wave stalls before it reaches the surface,
• 21:36
  Speaker 1: the explosion might fail entirely. Instead of a supernova, the
• 21:41
  Speaker 1: whole star just falls inward. No blast, no flash, no
• 21:46
  Speaker 1: expanding nebula lighting up the neighborhood for centuries. The star disappears,
• 21:52
  Speaker 1: a black hole forms, and the only evidence is an absence,
• 21:57
  Speaker 1: a star that used to be there and suddenly is not.
• 22:03
  Speaker 1: Astronomers have a name for this. They call it a
• 22:06
  Speaker 1: failed supernova, or sometimes a direct collapse event, and they
• 22:11
  Speaker 1: have been watching for it. A team monitoring red supergiants
• 22:16
  Speaker 1: in nearby galaxies found what looked like exactly this kind
• 22:20
  Speaker 1: of event in 2009. A massive star in the galaxy
• 22:25
  Speaker 1: NGC 6946, already nicknamed the Fireworks Galaxy, because of how
• 22:32
  Speaker 1: many supernovae it produces, simply vanished between one observation and
• 22:37
  Speaker 1: the next. No explosion was recorded. When they looked at
• 22:42
  Speaker 1: the position again, the star was gone. A faint infrared
• 22:47
  Speaker 1: glow remained where it had been, consistent with dust heated
• 22:51
  Speaker 1: during the event, but there was no supernova. The star
• 22:56
  Speaker 1: had apparently fallen into itself. The object was designated N6946-BH1,
• 23:06
  Speaker 1: and while scientists are still working through alternative explanations, the
• 23:10
  Speaker 1: direct collapse interpretation remains the leading one. What this means
• 23:15
  Speaker 1: for counting black holes is significant. If a portion of
• 23:20
  Speaker 1: the most massive stars skip the explosion entirely and simply vanish,
• 23:25
  Speaker 1: those black holes form without any signals. No burst of light.
• 23:31
  Speaker 1: No expanding nebula to study later. No alert that anything
• 23:35
  Speaker 1: happened at all. You would never know a black hole
• 23:39
  Speaker 1: had formed unless you happened to be watching that specific
• 23:43
  Speaker 1: star continuously and noticed it was gone. And across a
• 23:48
  Speaker 1: galaxy containing hundreds of billions of stars, that is not
• 23:53
  Speaker 1: a practical monitoring strategy. The direct collapse channel may account
• 23:58
  Speaker 1: for a meaningful fraction of the most massive black holes
• 24:01
  Speaker 1: in the Milky Way, and those are the hardest ones
• 24:04
  Speaker 1: to find precisely because their births produce nothing memorable. The
• 24:10
  Speaker 1: picture that emerges from all of this is one of
• 24:12
  Speaker 1: enormous variety. Some black holes arrived with a brilliant announcement,
• 24:19
  Speaker 1: a supernova visible across millions of light years. a flash
• 24:23
  Speaker 1: that lit up the neighborhood for months. Others formed at
• 24:28
  Speaker 1: the end of a long and complicated process in which
• 24:32
  Speaker 1: the star ran through fuel after fuel, each stage shorter
• 24:37
  Speaker 1: and more desperate than the last, before quietly losing the
• 24:41
  Speaker 1: final battle. Some of the black holes in the galaxy
• 24:46
  Speaker 1: may have formed in total silence, their progenitor stars simply
• 24:51
  Speaker 1: vanishing from the sky one night with no explosion, no announcement, nothing.
• 24:58
  Speaker 1: Here is the fundamental problem. A black hole sitting by
• 25:02
  Speaker 1: itself in empty space produces no light, no heat, no
• 25:07
  Speaker 1: radio emission, nothing at all that any telescope on Earth
• 25:12
  Speaker 1: or in orbit can directly detect. That is not a
• 25:16
  Speaker 1: limitation of current technology. It is not something a bigger
• 25:20
  Speaker 1: mirror or a more sensitive detector will solve. It is physics.
• 25:26
  Speaker 1: A black hole is a region where gravity has pulled
• 25:29
  Speaker 1: so hard on the fabric of space that the escape
• 25:32
  Speaker 1: velocity exceeds the speed of light. Nothing gets out. Not x-rays,
• 25:39
  Speaker 1: not radio waves, not a single photon. The darkness is
• 25:45
  Speaker 1: total and it is permanent. There is a theoretical exception.
• 25:51
  Speaker 1: Stephen Hawking predicted in 1974 that black holes should slowly
• 25:56
  Speaker 1: leak energy through a quantum process at their event horizons,
• 26:01
  Speaker 1: a faint thermal glow now called Hawking radiation. For a
• 26:07
  Speaker 1: stellar mass black hole, the temperature of this radiation is
• 26:11
  Speaker 1: something like a hundred millionth of a degree above absolute zero.
• 26:17
  Speaker 1: The flux reaching any detector would be so vanishingly small
• 26:21
  Speaker 1: that no instrument built by humans could measure it. Not today,
• 26:27
  Speaker 1: not in any foreseeable future. Hawking radiation is real as
• 26:31
  Speaker 1: a mathematical result, but as a detection strategy, it is
• 26:36
  Speaker 1: essentially useless. It tells you nothing about where the black
• 26:40
  Speaker 1: holes are. So picture what an isolated stellar mass black
• 26:45
  Speaker 1: hole actually looks like from the outside. It is an
• 26:49
  Speaker 1: object somewhere between 5 and 30 times the mass of
• 26:53
  Speaker 1: our sun, compressed into a sphere roughly the width of
• 26:57
  Speaker 1: a small city. It drifts through the galaxy at whatever
• 27:01
  Speaker 1: velocity it inherited from its parent star, and whatever kick
• 27:06
  Speaker 1: it received during the supernova that created it. It warps
• 27:10
  Speaker 1: space around it. It has gravity, and it is completely,
• 27:15
  Speaker 1: perfectly dark. No glow, no halo, no shadow you could
• 27:20
  Speaker 1: photograph against the background sky. If one were drifting through
• 27:25
  Speaker 1: the space between stars a few hundred light years away,
• 27:29
  Speaker 1: and you aimed every space telescope humanity currently operates directly
• 27:34
  Speaker 1: at it, you would see nothing. The patch of sky
• 27:39
  Speaker 1: would look empty. the black hole would pass through your
• 27:43
  Speaker 1: field of view and you would never know. This is
• 27:47
  Speaker 1: not a small problem. Remember the estimate. Roughly 100 million
• 27:53
  Speaker 1: stellar mass black holes scattered through a galaxy roughly 100,000
• 27:59
  Speaker 1: light years across. Most of them formed from stars that
• 28:04
  Speaker 1: lived out their whole lives in isolation, collapsed, and went dark.
• 28:10
  Speaker 1: Those black holes are still out there, still moving, with
• 28:14
  Speaker 1: no companion to betray them, and no surrounding gas to
• 28:18
  Speaker 1: heat up. They are the vast majority of the population,
• 28:23
  Speaker 1: and we have essentially no way to find them by looking.
• 28:27
  Speaker 1: Now contrast that with what happens when a black hole
• 28:30
  Speaker 1: is not alone. When a black hole is orbiting close
• 28:34
  Speaker 1: enough to a normal star, gravity does something spectacular. The
• 28:40
  Speaker 1: black hole begins pulling material away from the star's outer layers.
• 28:45
  Speaker 1: That material does not fall straight in. It has angular momentum,
• 28:50
  Speaker 1: so it spirals inward, flattening into a disk. Layers of
• 28:55
  Speaker 1: gas inside that disk rub against each other as they
• 28:59
  Speaker 1: orbit at different speeds, and that friction heats the gas.
• 29:04
  Speaker 1: The temperatures in the inner regions of the disk reach
• 29:07
  Speaker 1: into the tens of millions of degrees. At those temperatures,
• 29:12
  Speaker 1: matter does not glow in visible light. It blazes in X-rays.
• 29:19
  Speaker 1: The disk becomes one of the most energetic environments in
• 29:22
  Speaker 1: the galaxy, pouring out radiation that space-based telescopes can detect
• 29:28
  Speaker 1: from thousands of light-years away. This is the situation astronomers
• 29:33
  Speaker 1: call an X-ray binary. One visible star, one hidden black hole,
• 29:39
  Speaker 1: and a river of stolen gas connecting them. The black
• 29:43
  Speaker 1: hole itself remains invisible, but the accretion disk surrounding it screams.
• 29:49
  Speaker 1: It pulses, flickers, and sometimes releases jets of plasma, moving
• 29:55
  Speaker 1: close to the speed of light. The signal is hard
• 29:59
  Speaker 1: to miss if you are looking in the right part
• 30:02
  Speaker 1: of the electromagnetic spectrum. This is how the overwhelming majority
• 30:08
  Speaker 1: of known stellar mass black holes were discovered. Not by
• 30:12
  Speaker 1: seeing the black hole, but by seeing the carnage it
• 30:16
  Speaker 1: leaves behind. Cygnus X-1 is the most famous example. In
• 30:22
  Speaker 1: the early 1960s, before any telescope had been placed in orbit,
• 30:28
  Speaker 1: Rocket-borne detectors were lofted briefly above Earth's atmosphere and scanned
• 30:33
  Speaker 1: the sky in X-rays. Earth's atmosphere absorbs X-rays completely, so
• 30:39
  Speaker 1: ground-based detection is impossible. Those early rocket flights caught something
• 30:45
  Speaker 1: bright in the constellation Cygnus, an intense source of X-ray
• 30:50
  Speaker 1: emission with no obvious optical counterpart. When the first dedicated
• 30:56
  Speaker 1: X-ray satellite Uhuru went up in 1970, it confirmed the
• 31:01
  Speaker 1: source and showed it was flickering on timescales of milliseconds.
• 31:07
  Speaker 1: That rapid variation told astronomers the emitting region was tiny, compact,
• 31:13
  Speaker 1: smaller than a planet. Something very small and very massive
• 31:18
  Speaker 1: was sitting inside all that radiation. The debate over whether
• 31:22
  Speaker 1: Cygnus X-1 contained a black hole or a neutron star
• 31:27
  Speaker 1: ran for years. What settled it was the companion star,
• 31:32
  Speaker 1: a blue supergiant called HDE2-26868, which turned out to be
• 31:40
  Speaker 1: orbiting an invisible object with a mass s of somewhere
• 31:44
  Speaker 1: around 15 solar masses. A neutron star cannot exceed roughly
• 31:49
  Speaker 1: two or three solar masses before collapsing further. Whatever HDE
• 31:55
  Speaker 1: 226868 was orbiting was too heavy to be anything other
• 32:00
  Speaker 1: than a black hole. Cygnus X-1 became the first object
• 32:06
  Speaker 1: most astronomers were willing to call a confirmed black hole,
• 32:11
  Speaker 1: and it got there entirely through the evidence of its
• 32:13
  Speaker 1: accretion disk. The problem is that Cygnus X-1 is an exception,
• 32:20
  Speaker 1: not a representative sample. X-ray binary systems require a very
• 32:26
  Speaker 1: specific set of circumstances. Two stars must have formed close
• 32:31
  Speaker 1: enough together to remain gravitationally bound. The more massive one
• 32:37
  Speaker 1: must have ended its life as a black hole without
• 32:40
  Speaker 1: completely disrupting the binary through the supernova kick. The surviving
• 32:46
  Speaker 1: companion must be transferring mass at a rate high enough
• 32:50
  Speaker 1: to keep the accretion disk lit up. Only a small
• 32:54
  Speaker 1: fraction of all black holes were born into systems that
• 32:58
  Speaker 1: met all those conditions. Estimates suggest that somewhere between a
• 33:04
  Speaker 1: few percent and maybe 10% of all black holes have
• 33:08
  Speaker 1: binary companions at all. Of those, only a fraction are
• 33:13
  Speaker 1: actively accreting right now. Many binary black hole systems are
• 33:18
  Speaker 1: in quiescence, meaning the mass transfer has temporarily stopped The
• 33:24
  Speaker 1: disk has cooled and the system goes quiet. During quiescence,
• 33:29
  Speaker 1: these systems look like ordinary stars. The black hole vanishes
• 33:34
  Speaker 1: back into invisibility. Astronomers have catalogued somewhere around 50 confirmed
• 33:41
  Speaker 1: or strongly suspected stellar mass black holes in the galaxy.
• 33:47
  Speaker 1: Against a backdrop of 100 million, That is like finding
• 33:51
  Speaker 1: 50 people in a country of 2 billion and declaring
• 33:56
  Speaker 1: you understand the population. The known black holes are the
• 34:01
  Speaker 1: loud ones, the ones with companions they are actively destroying,
• 34:06
  Speaker 1: and they make up a tiny slice of the total
• 34:09
  Speaker 1: population scattered across the galaxy. The first black hole humanity
• 34:15
  Speaker 1: ever confirmed was not found by looking for darkness, It
• 34:20
  Speaker 1: was found by looking at light, the wrong kind of light,
• 34:24
  Speaker 1: in the wrong part of the sky, coming from a
• 34:26
  Speaker 1: source nobody could explain. X-rays do not reach the ground.
• 34:33
  Speaker 1: Earth's atmosphere absorbs them completely before they get anywhere near
• 34:37
  Speaker 1: a telescope, which means that for most of human history,
• 34:41
  Speaker 1: an entire wavelength of the universe was simply invisible to us.
• 34:47
  Speaker 1: That changed in the early 1960s. Scientists started launching detectors
• 34:53
  Speaker 1: on sounding rockets, short arcing flights above the atmosphere that
• 34:58
  Speaker 1: lasted only a few minutes before falling back down. The
• 35:02
  Speaker 1: instruments were crude. The flight windows were short. But in 1962,
• 35:10
  Speaker 1: one of those flights detected something unexpected. a powerful source
• 35:14
  Speaker 1: of X-rays coming from somewhere outside our solar system. The
• 35:19
  Speaker 1: era of X-ray astronomy had started, not with a grand mission,
• 35:24
  Speaker 1: but with a few minutes of data from a rocket
• 35:27
  Speaker 1: that barely made it to space. By 1964, another rocket
• 35:33
  Speaker 1: flight had swept across a region of the constellation Cygnus
• 35:37
  Speaker 1: and found a source so bright in X-rays, it stood
• 35:41
  Speaker 1: out immediately. Astronomers called it Cygnus X-1, the first X-ray
• 35:48
  Speaker 1: source catalogued in that part of the sky. It was intense, variable,
• 35:55
  Speaker 1: and unlike anything they had a clean explanation for. When
• 35:59
  Speaker 1: NASA launched the Uhuru satellite in 1970, the first X-ray
• 36:04
  Speaker 1: telescope designed to operate in orbit for an extended period,
• 36:09
  Speaker 1: Cygnus X-1, became one of the most studied objects in
• 36:13
  Speaker 1: the sky almost immediately. Uhuru showed that the X-ray output
• 36:18
  Speaker 1: was flickering on timescales of hundredths of a second. That
• 36:23
  Speaker 1: was important. If a source of light is flickering that fast,
• 36:28
  Speaker 1: the region producing it cannot be larger than the distance
• 36:31
  Speaker 1: light travels in that time. Cygnus X-1 was producing enormous
• 36:37
  Speaker 1: energy from a region only a few hundred kilometers across
• 36:41
  Speaker 1: at most. Nothing ordinary could do that. The next clue
• 36:47
  Speaker 1: came from optical astronomers. Once the X-ray position was pinned
• 36:52
  Speaker 1: down precisely enough, they could point ground-based telescopes at the
• 36:56
  Speaker 1: same patch of sky and look for a visible counterpart.
• 37:01
  Speaker 1: What they found was a massive blue star called HDE
• 37:06
  Speaker 1: 226868 sitting right at the location of the X-ray source.
• 37:12
  Speaker 1: It was a supergiant, roughly 20 to 30 times the
• 37:16
  Speaker 1: mass of our Sun. And it was wobbling. The spectral
• 37:20
  Speaker 1: lines in its light were shifting back and forth on
• 37:23
  Speaker 1: a regular cycle, the classic fingerprint of a star being
• 37:28
  Speaker 1: gravitationally yanked by an unseen companion. The orbital period was 5.6 days.
• 37:36
  Speaker 1: From the motion of HDE 226868, astronomers could work backwards
• 37:44
  Speaker 1: and calculate what the invisible object had to weigh. The
• 37:48
  Speaker 1: answer kept coming out somewhere around 15 solar masses. The
• 37:54
  Speaker 1: implications of that calculation were immediately clear to anyone who
• 37:58
  Speaker 1: understood stellar physics. A neutron star The densest object that
• 38:04
  Speaker 1: physics allowed before crossing into black hole territory had a
• 38:08
  Speaker 1: strict mass limit. Theory predicted that neutron stars could not
• 38:13
  Speaker 1: exceed roughly two to three solar masses before the pressure
• 38:18
  Speaker 1: would overwhelm even the quantum forces holding them up. The
• 38:22
  Speaker 1: invisible object pulling HDE 226868 through its orbital dance was
• 38:30
  Speaker 1: five to seven times heavier than the maximum possible for
• 38:35
  Speaker 1: a neutron star. The math left no wiggle room for interpretation.
• 38:41
  Speaker 1: Either astronomers had discovered something that violated everything they understood
• 38:46
  Speaker 1: about the death of massive stars, or they had found
• 38:50
  Speaker 1: something far more extreme. There was only one candidate left
• 38:55
  Speaker 1: that could explain the observations. Not everyone accepted it right away.
• 39:02
  Speaker 1: Throughout the early 1970s, a real debate ran through the
• 39:06
  Speaker 1: astronomical community about whether Cygnus X-1 was actually a black hole,
• 39:12
  Speaker 1: or whether some exotic, poorly understood object could account for
• 39:17
  Speaker 1: the observations without invoking something as extreme as a singularity.
• 39:24
  Speaker 1: Black holes at that point were theoretical. General relativity predicted them.
• 39:31
  Speaker 1: Oppenheimer had worked through the math in 1939. But confirmation
• 39:36
  Speaker 1: was another matter entirely. Many physicists were deeply reluctant to
• 39:42
  Speaker 1: declare that a region of infinite density actually existed in nature.
• 39:47
  Speaker 1: based on a handful of x-ray measurements and an orbital wobble.
• 39:53
  Speaker 1: The skepticism was not irrational. It was caution applied to
• 39:58
  Speaker 1: an extraordinary claim. Kip Thorne was not one of the skeptics.
• 40:04
  Speaker 1: By 1974, Thorne was convinced the evidence pointed clearly at
• 40:09
  Speaker 1: a black hole. Stephen Hawking was not so sure, or
• 40:14
  Speaker 1: at least he was willing to publicly play both sides.
• 40:18
  Speaker 1: In what became one of the most famous wages in
• 40:20
  Speaker 1: the history of science, Hawking and Thorne made a formal bet.
• 40:26
  Speaker 1: Thorne put his money on Cygnus X, one being a
• 40:29
  Speaker 1: black hole. Hawking bet against it. His reasoning was almost playful.
• 40:36
  Speaker 1: He called it an insurance policy. He had spent years
• 40:40
  Speaker 1: building a theoretical framework around black holes. If Cygnus X-1
• 40:45
  Speaker 1: turned out to be something else entirely... at least he
• 40:49
  Speaker 1: would win a magazine subscription. The bet was written down
• 40:53
  Speaker 1: and signed. It sat there for 16 years. During those
• 40:59
  Speaker 1: 16 years, the evidence only deepened. Better satellites, sharper measurements,
• 41:06
  Speaker 1: more careful modeling of HDE2-26868's orbit all pointed the same direction.
• 41:15
  Speaker 1: the mass of the unseen object was not shrinking under
• 41:18
  Speaker 1: new analysis. If anything, tighter observations pushed it higher. The
• 41:25
  Speaker 1: accretion disk model, where gas stripped from the companion star
• 41:29
  Speaker 1: spirals inward and heats up to millions of degrees before
• 41:34
  Speaker 1: plunging past the event horizon, explained the X-ray variability with
• 41:39
  Speaker 1: a precision that competing models could not match. By the
• 41:43
  Speaker 1: late 1980s, The consensus among most researchers was no longer
• 41:48
  Speaker 1: seriously in question. Cygnus X-1 was a black hole. It
• 41:54
  Speaker 1: had been one all along. What the Cygnus X-1 story
• 41:58
  Speaker 1: established was not just the existence of one black hole.
• 42:02
  Speaker 1: It established a method. The approach, find an X-ray bright source,
• 42:08
  Speaker 1: identify its optical counterpart, measure the orbital dynamics. calculate the
• 42:14
  Speaker 1: mass of the invisible object and compare that mass against
• 42:19
  Speaker 1: the maximum possible for a neutron star became the template
• 42:23
  Speaker 1: for every stellar mass black hole discovery that followed. The
• 42:28
  Speaker 1: dozens of X-ray binary black holes confirmed in subsequent decades
• 42:34
  Speaker 1: were all found using variations of the same logic that
• 42:37
  Speaker 1: astronomers worked out while arguing about Cygnus X-1 through the 1970s.
• 42:45
  Speaker 1: Hawking eventually conceded the bet in 1990, signing over a
• 42:50
  Speaker 1: year's subscription to a certain magazine, and the object that
• 42:55
  Speaker 1: had started as an anomalous bright spot on a detector
• 42:59
  Speaker 1: became the F-oundation of an entirely new field. The number
• 43:03
  Speaker 1: 100 million does not come from counting black holes. It
• 43:08
  Speaker 1: comes from counting everything else and working backwards. After decades
• 43:13
  Speaker 1: of confirming a handful of black holes, the loud, bright,
• 43:18
  Speaker 1: X-ray blazing kind, astronomers faced an obvious problem. The confirmed
• 43:23
  Speaker 1: population was tiny. The galaxy is enormous. Something had to
• 43:29
  Speaker 1: bridge that gap, and the bridge turned out to be
• 43:32
  Speaker 1: mathematics rather than observation. Start with the galaxy itself. The
• 43:38
  Speaker 1: Milky Way contains somewhere between 200 and 400 billion stars.
• 43:44
  Speaker 1: That number is itself an estimate, derived from measuring stellar
• 43:49
  Speaker 1: densities in different regions and extrapolating outward. But it is
• 43:54
  Speaker 1: a reasonably well constrained estimate, and it gives astronomers a
• 43:59
  Speaker 1: foundation to work from. The question is, What fraction of
• 44:04
  Speaker 1: those stars are massive enough to end their lives as
• 44:07
  Speaker 1: black holes? Not every star can become one. A star
• 44:13
  Speaker 1: like our Sun will eventually swell into a red giant,
• 44:17
  Speaker 1: shed its outer layers, and leave behind a cold, dense
• 44:21
  Speaker 1: remnant called a white dwarf. That is the fate of
• 44:26
  Speaker 1: the majority. Black holes require something much more extreme. The
• 44:31
  Speaker 1: threshold is roughly 20 solar masses. Stars above that limit
• 44:37
  Speaker 1: burn hotter, faster, and more violently than anything in our
• 44:41
  Speaker 1: sun's league. They tear through their hydrogen in millions of
• 44:45
  Speaker 1: years rather than billions. When they finally run out of
• 44:50
  Speaker 1: fuel and collapse, the core is too massive for any
• 44:53
  Speaker 1: known force to halt. Neutron Star Formation requires a collapsing
• 44:59
  Speaker 1: core below roughly two to three solar masses. Above that,
• 45:04
  Speaker 1: the collapse keeps going and a black hole forms. So
• 45:09
  Speaker 1: the calculation becomes a question about how many stars in
• 45:13
  Speaker 1: the Milky Way have ever crossed that threshold. That is
• 45:17
  Speaker 1: where the initial mass function comes in. The initial mass
• 45:22
  Speaker 1: function is one of the most important relationships in all
• 45:25
  Speaker 1: of stellar astronomy. It describes how stars are distributed by
• 45:30
  Speaker 1: mass when they form. The pattern is consistent across nearly
• 45:36
  Speaker 1: every star-forming region astronomers have studied. Small stars vastly outnumber
• 45:42
  Speaker 1: large ones. For every star with 20 times the Sun's mass,
• 45:48
  Speaker 1: there are thousands of stars with roughly the Sun's mass,
• 45:52
  Speaker 1: and millions of stars smaller than that. The curve drops
• 45:56
  Speaker 1: steeply as mass increases. Massive stars are genuinely rare, but
• 46:03
  Speaker 1: the galaxy has been forming stars for 13 billion years
• 46:07
  Speaker 1: and rare things add up when you have that much
• 46:10
  Speaker 1: time and that many raw materials. Plug the initial mass
• 46:14
  Speaker 1: function into a model of the Milky Way's star formation
• 46:18
  Speaker 1: history and you can estimate how many high mass stars
• 46:21
  Speaker 1: have been born over the galaxy's lifetime. Star formation was
• 46:26
  Speaker 1: not constant. The early galaxy produced stars at a frantic rate.
• 46:32
  Speaker 1: Things have quieted down considerably since then. Astronomers model this
• 46:38
  Speaker 1: history using observations of stellar populations at different ages, chemical
• 46:44
  Speaker 1: abundances in old and young stars, and the current distribution
• 46:49
  Speaker 1: of stellar remnants. When you run those numbers, the fraction
• 46:54
  Speaker 1: of stars massive enough to become black holes is somewhere
• 46:58
  Speaker 1: around one in a thousand. Apply that fraction to the
• 47:03
  Speaker 1: total number of stars that have ever formed in the
• 47:06
  Speaker 1: Milky Way and you land near 100 million. This is
• 47:11
  Speaker 1: called stellar population synthesis. It is the same general approach
• 47:16
  Speaker 1: astronomers use to model distant galaxies, predict the chemical evolution
• 47:21
  Speaker 1: of stellar populations and estimate the abundance of other rare
• 47:26
  Speaker 1: stellar remnants. The technique is well tested. It produces results
• 47:32
  Speaker 1: that match observations in the cases where observations are possible.
• 47:37
  Speaker 1: But it rests on assumptions at every step, and those
• 47:41
  Speaker 1: assumptions carry real uncertainty. The initial mass function, for instance,
• 47:47
  Speaker 1: may not be perfectly universal. There is evidence it behaves
• 47:52
  Speaker 1: differently in extreme environments, very dense star clusters, or regions
• 47:58
  Speaker 1: with unusual chemical compositions. The threshold between neutron star and
• 48:04
  Speaker 1: black hole formation is not a hard line either. It
• 48:08
  Speaker 1: depends on the internal physics of the collapse, which is
• 48:12
  Speaker 1: genuinely complicated, and on the rotation and magnetic field of
• 48:17
  Speaker 1: the progenitor star. The star formation history of the Milky
• 48:21
  Speaker 1: Way is well constrained in broad strokes, but fuzzy in
• 48:25
  Speaker 1: the details. And there is a subtler problem. Not every
• 48:30
  Speaker 1: massive star that collapses necessarily produces a black hole with
• 48:35
  Speaker 1: the expected mass. Some may shed tremendous amounts of material
• 48:41
  Speaker 1: through stellar winds before they ever collapse, leaving a remnant
• 48:46
  Speaker 1: too lig-height to qualify. Metal-rich stars in particular tend to
• 48:50
  Speaker 1: lose more mass this way. All of those uncertainties stack.
• 48:55
  Speaker 1: The 100 million figure is the middle of a wide range.
• 49:00
  Speaker 1: The low end of serious estimates sits around 10 million.
• 49:04
  Speaker 1: The high end reaches toward a billion. That is a
• 49:08
  Speaker 1: factor of 100 between the floor and the ceiling. which
• 49:12
  Speaker 1: is a remarkable spread for a number that gets cited
• 49:15
  Speaker 1: with such confidence in astronomy headlines. The reason it still
• 49:20
  Speaker 1: gets cited is that the middle of the range is
• 49:23
  Speaker 1: genuinely informative. Even if the true number is 50 million
• 49:28
  Speaker 1: or 200 million, the conclusion is the same. There are
• 49:33
  Speaker 1: a staggering number of black holes out there. The vast
• 49:37
  Speaker 1: majority are invisible. and we have confirmed only a tiny
• 49:41
  Speaker 1: fraction of them. What makes this method powerful is exactly
• 49:46
  Speaker 1: what makes it limited. It is statistics applied to an
• 49:50
  Speaker 1: invisible population. You never see most of what you are counting.
• 49:56
  Speaker 1: You infer its existence from what you know about the
• 49:59
  Speaker 1: processes that created it and the objects that are similar
• 50:03
  Speaker 1: to it. A forensic accountant reconstructing finances from partial records
• 50:09
  Speaker 1: is doing something similar. The records are incomplete, the methodology
• 50:15
  Speaker 1: is sound, and the answer is probably in the right ballpark.
• 50:20
  Speaker 1: But you would not stake everything on the last two digits.
• 50:24
  Speaker 1: Astronomers are in the same position with black holes. The
• 50:29
  Speaker 1: confirmed population, fewer than 50 stellar mass black holes, with
• 50:34
  Speaker 1: reasonably well measured properties, is a sample so small it
• 50:39
  Speaker 1: barely represents the range of what should be out there.
• 50:43
  Speaker 1: Each one was found because it happened to have a
• 50:46
  Speaker 1: companion star close enough to feed from, or happened to
• 50:51
  Speaker 1: pass in front of a background star at the right moment.
• 50:55
  Speaker 1: The quiet ones, the isolated black holes drifting through the
• 51:00
  Speaker 1: galactic disk with nothing nearby to reveal them, are not
• 51:05
  Speaker 1: in the count. They are in the model. The honest
• 51:09
  Speaker 1: answer is that this estimate could be off by a
• 51:12
  Speaker 1: factor of 10 in either direction, and nobody will be
• 51:17
  Speaker 1: able to correct it until we find a way to
• 51:20
  Speaker 1: detect black holes that are not actively eating something. At
• 51:25
  Speaker 1: the exact centre of the Milky Way, hidden behind curtains
• 51:29
  Speaker 1: of gas and dust, Something sits that contains the mass
• 51:33
  Speaker 1: of four million suns compressed into a region smaller than
• 51:37
  Speaker 1: our solar system. You cannot see it with your eyes.
• 51:42
  Speaker 1: You cannot see it with an optical telescope aimed at
• 51:45
  Speaker 1: that patch of sky. This was because the galactic center
• 51:50
  Speaker 1: is wrapped in so much gas and dust that visible
• 51:53
  Speaker 1: light never makes it through. The region sits roughly 26,000
• 51:58
  Speaker 1: light years away. in the direction of the constellation Sagittarius.
• 52:05
  Speaker 1: It is one of the most chaotic environments in the
• 52:08
  Speaker 1: entire galaxy. Young massive stars orbit there in tight clusters.
• 52:15
  Speaker 1: Ancient stars trace slow ellipses through a region dense with
• 52:19
  Speaker 1: gas clouds and magnetic fields and high energy radiation. Everything
• 52:25
  Speaker 1: in that neighborhood is moving fast and being pulled by
• 52:28
  Speaker 1: something enormous that produces no light at all. The first
• 52:33
  Speaker 1: hint that something unusual lived at the center came from
• 52:37
  Speaker 1: radio astronomy. In 1974, two astronomers named Bruce Ballack and
• 52:44
  Speaker 1: Robert Brown were using a radio interferometer to map the
• 52:48
  Speaker 1: galactic center region when they spotted a compact. incredibly bright
• 52:54
  Speaker 1: radio source sitting almost exactly at the geometric center of
• 52:58
  Speaker 1: the Milky Way. It was tiny, far too small to
• 53:03
  Speaker 1: be a cloud of gas. It radiated with a consistency
• 53:07
  Speaker 1: that suggested something gravitationally extreme. They called it Sagittarius A star.
• 53:15
  Speaker 1: For years, astronomers debated what it actually was. The leading
• 53:20
  Speaker 1: hypothesis was a supermassive black hole. But a hypothesis is
• 53:25
  Speaker 1: not a measurement, and the evidence at that point was circumstantial.
• 53:30
  Speaker 1: What changed everything was stars. If you want to weigh
• 53:34
  Speaker 1: an invisible object, you watch what orbits around it. This
• 53:39
  Speaker 1: is the same logic that lets astronomers weigh planets and
• 53:43
  Speaker 1: measure the mass of our own sun. The problem at
• 53:46
  Speaker 1: the galactic centre was that the stars orbiting SGR A
• 53:51
  Speaker 1: were moving so fast in such tight orbits that tracking
• 53:56
  Speaker 1: them required resolving features at incredibly fine angular scales. Across 26,000
• 54:03
  Speaker 1: light years, through all that dust. Two independent research teams
• 54:08
  Speaker 1: took on that challenge and spent decades at it. Reinhard
• 54:13
  Speaker 1: Genzel at the Max Planck Institute in Germany and Andrea
• 54:17
  Speaker 1: Ghez at UCLA. Each built programs to track individual stars
• 54:23
  Speaker 1: at the galactic center using large telescopes equipped with a
• 54:27
  Speaker 1: technique called adaptive optics. That corrects in real time for
• 54:32
  Speaker 1: the blurring effect tease of Earth's atmosphere. Year after year
• 54:38
  Speaker 1: they watched. The stars moved. The orbits took shape. One
• 54:43
  Speaker 1: star in particular became the key to everything. Designated S2,
• 54:48
  Speaker 1: It follows an elliptical path that brings its swinging close
• 54:51
  Speaker 1: to SGR A every 16 years. That is a remarkably
• 54:56
  Speaker 1: short orbital period for a star. By comparison, it takes
• 55:01
  Speaker 1: our solar system roughly 225 million years to complete one
• 55:06
  Speaker 1: orbit around the galaxy. S2 finishes its entire loop around
• 55:12
  Speaker 1: the galactic center in the same time it takes a
• 55:15
  Speaker 1: child born today to finish high school. and graduate from college.
• 55:21
  Speaker 1: When researchers calculated the mass needed to produce that orbit
• 55:25
  Speaker 1: at that speed and that distance, the answer was clear
• 55:29
  Speaker 1: and unambiguous. The object at the center of S2' s orbit
• 55:35
  Speaker 1: contained approximately 4.3 million times the mass of our Sun,
• 55:40
  Speaker 1: and based on the precision of the positional measurements, that
• 55:44
  Speaker 1: mass was packed into a region far too small to
• 55:47
  Speaker 1: be any collection of ordinary stars. Nothing in known physics
• 55:53
  Speaker 1: other than a supermassive black hole could account for it.
• 55:58
  Speaker 1: Jensel and Ghez shared the 2020 Nobel Prize in Physics
• 56:02
  Speaker 1: for this work, alongside Roger Penrose, who had proven decades
• 56:08
  Speaker 1: earlier that black hole formation was a fundamental consequence of
• 56:12
  Speaker 1: general relativity. But a Nobel Prize for tracking stars is
• 56:17
  Speaker 1: not quite the same as a photograph. And for decades,
• 56:22
  Speaker 1: the actual visual confirmation of SGR-A remained out of reach.
• 56:28
  Speaker 1: Then came the Event Horizon Telescope. The EHT is not
• 56:33
  Speaker 1: a single telescope. It is a network of radio observatories
• 56:37
  Speaker 1: spread across multiple continents, including instruments in Chile, Hawaii, Spain, Greenland, Antarctica,
• 56:47
  Speaker 1: all synchronized to observe the same target at the same
• 56:50
  Speaker 1: moment by combining the data from all of those dishes.
• 56:56
  Speaker 1: Astronomers effectively created a telescope with a dish size equal
• 57:01
  Speaker 1: to the diameter of Earth itself. This is called very
• 57:05
  Speaker 1: long baseline interferometry. The resolution it achieves is extraordinary. Imaging
• 57:13
  Speaker 1: SGR-A from Earth using the EHT is roughly equivalent to
• 57:19
  Speaker 1: photographing a donut sitting on the surface of the Moon.
• 57:23
  Speaker 1: In May 2022, the collaboration released their result. The image
• 57:29
  Speaker 1: shows a bright, fuzzy ring of radio emission surrounding a
• 57:33
  Speaker 1: central dark region. That dark region is the shadow of
• 57:38
  Speaker 1: the black hole. the zone where the gravity is so
• 57:42
  Speaker 1: extreme that no light escapes. The bright ring is produced
• 57:47
  Speaker 1: by hot, glowing gas swirling just outside the event horizon,
• 57:52
  Speaker 1: heated to extreme temperatures as it spirals inward. The image
• 57:58
  Speaker 1: matched almost exactly what the equations of general relativity predicted
• 58:03
  Speaker 1: for a 4.3 million solar mass black hole at that distance.
• 58:09
  Speaker 1: It was the second black hole ever directly imaged, following
• 58:14
  Speaker 1: the 2019 image of the far more massive black hole
• 58:18
  Speaker 1: at the centre of the galaxy, Messier 87. SGRA star
• 58:25
  Speaker 1: does not look like an active monster. Compared to the
• 58:29
  Speaker 1: supermassive black holes in other galaxies that are actively consuming
• 58:33
  Speaker 1: enormous amounts of material, and blasting out jets of energy
• 58:37
  Speaker 1: across hundreds of thousands of light years, our galactic center
• 58:41
  Speaker 1: is quiet, calm even. SGRA star is mostly dormant. It
• 58:48
  Speaker 1: occasionally flares when a small amount of gas drifts close
• 58:51
  Speaker 1: enough to get pulled in. But these are minor events.
• 58:56
  Speaker 1: The energy output of SGRA HALM today is so low
• 59:01
  Speaker 1: that it barely registers compared to what is typical for
• 59:04
  Speaker 1: supermassive black holes elsewhere. Our galaxy has what some astronomers
• 59:10
  Speaker 1: describe as a sleeping giant at its centre. But sleeping
• 59:15
  Speaker 1: is not the same as always having been asleep. The
• 59:18
  Speaker 1: evidence suggests that SGRA star has not always been this quiet.
• 59:25
  Speaker 1: X-ray observations of the gas clouds surrounding the galactic centre
• 59:30
  Speaker 1: have revealed something puzzling. Some of those clouds are glowing
• 59:34
  Speaker 1: in X-rays in a way that only makes sense if
• 59:38
  Speaker 1: they were illuminated by an intense burst of radiation at
• 59:42
  Speaker 1: some point in the past. The clouds are too far
• 59:46
  Speaker 1: from SGR AR to be responding to current activity. They
• 59:51
  Speaker 1: are responding to something that happened long ago. The light
• 59:56
  Speaker 1: travel time tells you roughly when. The answer points to
• 1:00:01
  Speaker 1: an event around 200 years ago, meaning that sometime in
• 1:00:05
  Speaker 1: the early 1800s, Sagittarius A-star underwent a brief but violent
• 1:00:12
  Speaker 1: active period, tearing through a significant quantity of infalling material
• 1:00:19
  Speaker 1: and flooding the Soro-Unding region with radiation. No one alive
• 1:00:24
  Speaker 1: at the time had the instruments or the understanding to
• 1:00:28
  Speaker 1: know it was happening. Recent X-ray observations suggest that roughly
• 1:00:33
  Speaker 1: 200 years ago, Sagittarius A star briefly woke up and
• 1:00:39
  Speaker 1: tore through whatever material happened to drift within reach. The
• 1:00:44
  Speaker 1: light from that feeding event is still echoing through the
• 1:00:48
  Speaker 1: surrounding gas clouds today. 100 million is a large number,
• 1:00:54
  Speaker 1: but the Milky Way is an incomprehensibly large container. The
• 1:00:59
  Speaker 1: galaxy stretches roughly 100,000 light years across. Its disk is
• 1:01:05
  Speaker 1: a flattened spiral of gas, dust, and several hundred billion stars.
• 1:01:11
  Speaker 1: If you tried to drive across it at highway speed,
• 1:01:15
  Speaker 1: you would need something in the range of a trillion
• 1:01:17
  Speaker 1: years to finish the trip. Spreading 100 million black holes
• 1:01:22
  Speaker 1: across that volume sounds dense until you actually do the math.
• 1:01:28
  Speaker 1: On average, you get roughly one black hole for every
• 1:01:32
  Speaker 1: few cubic light years of space. The nearest one to
• 1:01:36
  Speaker 1: you at any given moment is probably hundreds of light
• 1:01:40
  Speaker 1: years away. These objects are not swarming around us. They
• 1:01:45
  Speaker 1: are scattered through an almost unimaginably large structure, and their
• 1:01:50
  Speaker 1: distribution across that structure is far from random. Black holes
• 1:01:56
  Speaker 1: tend to settle where the stars that made them were born.
• 1:02:00
  Speaker 1: This is not complicated physics. A star spends its life
• 1:02:05
  Speaker 1: in a particular place. It dies in roughly that same place.
• 1:02:10
  Speaker 1: The remnant, whether neutron star or black hole, inherits the address.
• 1:02:16
  Speaker 1: So the first thing to understand about where black holes
• 1:02:20
  Speaker 1: live is that they trace the history of star formation
• 1:02:23
  Speaker 1: in the Milky Way. Dense star-forming regions produced more massive stars.
• 1:02:30
  Speaker 1: More massive stars produced more black holes. The galactic disk
• 1:02:36
  Speaker 1: where the spiral arms carve through clouds of gas and
• 1:02:39
  Speaker 1: dust has always been the most active nursery for massive
• 1:02:43
  Speaker 1: star formation. That is where the majority of the galaxy's
• 1:02:48
  Speaker 1: black holes ended up. Most of those 100 million objects
• 1:02:53
  Speaker 1: are almost certainly embedded somewhere in that disk, spread across
• 1:02:57
  Speaker 1: the arms and the spaces between them, orbiting the galactic
• 1:03:02
  Speaker 1: center at speeds of hundreds of kilometers per second. But
• 1:03:06
  Speaker 1: the distribution is not flat. It thickens dramatically toward the center.
• 1:03:12
  Speaker 1: As you move inward toward the galactic core, the density
• 1:03:16
  Speaker 1: of stars rises steeply, The central bulge packs billions of
• 1:03:21
  Speaker 1: old and young stars into a relatively compact volume. More
• 1:03:26
  Speaker 1: stars means more generations of stellar death, more supernovae, more remnants.
• 1:03:33
  Speaker 1: A process called dynamical friction pulls compact objects inward over time.
• 1:03:40
  Speaker 1: When something dense and heavy moves through a background of
• 1:03:44
  Speaker 1: lighter objects. It experiences a kind of gravitational drag. It
• 1:03:50
  Speaker 1: loses energy. It sinks. Over billions of years, stellar-mass black
• 1:03:57
  Speaker 1: holes gradually migrate toward the center of the gravitational well.
• 1:04:02
  Speaker 1: The galactic center acts as a slow drain, and black
• 1:04:05
  Speaker 1: holes trickle toward it across cosmic time. Current estimates suggest
• 1:04:11
  Speaker 1: that roughly 25,000 stellar-mass black holes may have accumulated within
• 1:04:17
  Speaker 1: a single parsec of the galactic centre, which is about 3.26
• 1:04:22
  Speaker 1: light years. That is an extraordinary concentration. One parsec is
• 1:04:30
  Speaker 1: a region smaller than the distance between our Sun and
• 1:04:33
  Speaker 1: its nearest stellar neighbour, and it may contain tens of
• 1:04:37
  Speaker 1: thousands of black holes packed into that space. all orbiting
• 1:04:42
  Speaker 1: within the gravitational shadow of Sagittarius. A star. This creates
• 1:04:49
  Speaker 1: a region near the center unlike anything in the outer galaxy.
• 1:04:53
  Speaker 1: Dense clusters of black holes, neutron stars, and massive young
• 1:04:58
  Speaker 1: stars all sharing a relatively tight volume. The interactions between
• 1:05:04
  Speaker 1: them are complex. Black holes there can form binary pairs.
• 1:05:09
  Speaker 1: They can merge. They can be captured or ejected by
• 1:05:13
  Speaker 1: close gravitational encounters. Astronomers have only begun working out what
• 1:05:19
  Speaker 1: that environment actually looks like in detail, and the picture
• 1:05:23
  Speaker 1: keeps getting stranger with each new observation. Move outward from
• 1:05:29
  Speaker 1: the center and the density drops off. The disk of
• 1:05:33
  Speaker 1: the galaxy follows a profile that falls exponentially with distance
• 1:05:38
  Speaker 1: from the center. with a scale length of roughly 3 kiloparsecs.
• 1:05:44
  Speaker 1: That means as you move about 10,000 light years further
• 1:05:48
  Speaker 1: from the center, the density roughly drops by a fixed fraction.
• 1:05:54
  Speaker 1: The outer spiral arms are not empty, but they are
• 1:05:58
  Speaker 1: sparse compared to the inner galaxy. Star formation still happens
• 1:06:03
  Speaker 1: out there. Massive stars still form and die, but the
• 1:06:07
  Speaker 1: rate is lower and the history of star formation in
• 1:06:11
  Speaker 1: the outer disk is thinner than in the inner regions.
• 1:06:16
  Speaker 1: Fewer black holes per unit volume as you move outward
• 1:06:20
  Speaker 1: toward the galactic edge. Then there is the halo. This
• 1:06:25
  Speaker 1: is the part of the picture that surprises people. The
• 1:06:29
  Speaker 1: galactic halo is the vast, roughly spherical region that surrounds
• 1:06:34
  Speaker 1: the disk. It is mostly empty by disk standards. populated
• 1:06:39
  Speaker 1: by old globular clusters, ancient stars on wide orbits, and
• 1:06:44
  Speaker 1: the remnants of smaller galaxies the Milky Way has consumed
• 1:06:48
  Speaker 1: over billions of years. The stars in the halo are old.
• 1:06:54
  Speaker 1: They formed early in the galaxy's history, when the available
• 1:06:58
  Speaker 1: gas was mostly hydrogen and helium, with very little of
• 1:07:02
  Speaker 1: the heavier elements astronomers call metals. This matters because a
• 1:07:07
  Speaker 1: star's metal content affects how much mass it sheds during
• 1:07:12
  Speaker 1: its lifetime. Metal-rich stars like those in the disk drive
• 1:07:17
  Speaker 1: powerful stellar winds that blow off significant fractions of their
• 1:07:21
  Speaker 1: mass before they die. Metal-poor stars hold onto more of
• 1:07:26
  Speaker 1: their mass. A metal-poor star that starts at 30 solar
• 1:07:31
  Speaker 1: masses dies at something closer to 25 or 28 solar masses.
• 1:07:38
  Speaker 1: That heavier remnant is more likely to collapse into a
• 1:07:41
  Speaker 1: black hole rather than a neutron star, and if a
• 1:07:45
  Speaker 1: black hole does form, it tends to be heavier than
• 1:07:49
  Speaker 1: those produced by metal-rich stars. This matters for binary black
• 1:07:55
  Speaker 1: holes specifically. Models suggest that around half of all binary
• 1:08:01
  Speaker 1: black hole systems in the galaxy may reside in the
• 1:08:04
  Speaker 1: halo and in the satellite galaxies orbiting the Milky Way,
• 1:08:09
  Speaker 1: precisely because the low-metallicity environments there were better at making
• 1:08:14
  Speaker 1: the kinds of massive stars that left behind heavy remnants.
• 1:08:19
  Speaker 1: The disk produces more black holes in raw numbers, But
• 1:08:23
  Speaker 1: the halo produces a disproportionate share of the most massive ones.
• 1:08:29
  Speaker 1: So the galaxy does not have a single black hole population.
• 1:08:34
  Speaker 1: It has layers. A dense cluster near the center. A
• 1:08:39
  Speaker 1: broad population scattered through the disk. A diffuse but surprisingly
• 1:08:44
  Speaker 1: significant contribution from the ancient halo. The black holes are
• 1:08:49
  Speaker 1: not evenly spread. They cluster toward the center. thin out
• 1:08:54
  Speaker 1: toward the edges and pile up in specific regions where
• 1:08:58
  Speaker 1: the conditions for making massive stars were historically favorable. The
• 1:09:03
  Speaker 1: most elegant way to find a black hole that is
• 1:09:06
  Speaker 1: not eating anything is to wait for it to walk
• 1:09:10
  Speaker 1: in front of a star. That sounds almost too simple,
• 1:09:15
  Speaker 1: but it is built on something real, something that falls
• 1:09:19
  Speaker 1: directly out of Einstein's general relativity. Mass bends space. When
• 1:09:26
  Speaker 1: light from a distant star travels through curved space near
• 1:09:29
  Speaker 1: a massive object, the path of that light bends toward
• 1:09:34
  Speaker 1: the object. The effect is the same as a glass
• 1:09:38
  Speaker 1: lens focusing light, which is why astronomers call it gravitational lensing.
• 1:09:44
  Speaker 1: When the alignment is close to perfect, The lensing object
• 1:09:49
  Speaker 1: acts like a magnifying glass, and the background star suddenly
• 1:09:53
  Speaker 1: appears brighter than it really is. The brightening is temporary,
• 1:09:59
  Speaker 1: the lensing object keeps moving, the alignment fades, and the
• 1:10:03
  Speaker 1: background star dims back down. This entire event, from first
• 1:10:09
  Speaker 1: brightening to return to baseline, can last anywhere from a
• 1:10:14
  Speaker 1: few days to a few months, depending on how massive
• 1:10:18
  Speaker 1: the lensing OBJ-ECT is and how fast it is moving.
• 1:10:23
  Speaker 1: The shape of the brightening curve is not random. It
• 1:10:27
  Speaker 1: follows a very specific mathematical profile, and hidden inside that
• 1:10:31
  Speaker 1: profile is information. The peak brightness tells you how close
• 1:10:36
  Speaker 1: the alignment got. The width of the curve tells you
• 1:10:40
  Speaker 1: about the relative velocity between the lensing object and the
• 1:10:44
  Speaker 1: background star. The duration is tied to the mass of
• 1:10:49
  Speaker 1: the lensing object. If you can measure all of this
• 1:10:52
  Speaker 1: carefully enough, you can work backward and estimate what you
• 1:10:57
  Speaker 1: are actually looking at, even if you cannot see it directly.
• 1:11:02
  Speaker 1: This is called gravitational microlensing. and it is one of
• 1:11:07
  Speaker 1: the few tools astronomers have for detecting objects that produce
• 1:11:11
  Speaker 1: no light at all. The challenge is that these events
• 1:11:15
  Speaker 1: are rare. Most stars never produce a microlensing event on
• 1:11:20
  Speaker 1: any human timescale. To catch them in useful numbers, you
• 1:11:25
  Speaker 1: need to watch an enormous number of stars simultaneously and wait.
• 1:11:31
  Speaker 1: The logical place to look is toward the center of
• 1:11:34
  Speaker 1: the galaxy. The galactic bulge is packed with stars, and
• 1:11:39
  Speaker 1: anything drifting through the foreground between Earth and that dense
• 1:11:44
  Speaker 1: background population has a reasonable chance of drifting across the
• 1:11:48
  Speaker 1: line of sight to one of them. Two surveys have
• 1:11:53
  Speaker 1: spent decades doing exactly this. OGLE, the Optical Gravitational Lensing Experiment,
• 1:12:01
  Speaker 1: run by the University of Warsaw, has been monitoring tens
• 1:12:05
  Speaker 1: of millions of stars toward the galactic bulge since the
• 1:12:09
  Speaker 1: early 1990s. MOA, a Japan-New Zealand collaboration, runs a parallel
• 1:12:16
  Speaker 1: survey from a telescope in New Zealand. Together, these programs
• 1:12:22
  Speaker 1: have catalogued tens of thousands of microlensing events. Most of
• 1:12:27
  Speaker 1: them are unremarkable. An ordinary dim red dwarf passes in
• 1:12:32
  Speaker 1: front of a background star. The star brightens briefly, and
• 1:12:37
  Speaker 1: the event is logged and archived. Interesting to statisticians, not
• 1:12:42
  Speaker 1: headline news. But a small fraction of those events behave differently.
• 1:12:48
  Speaker 1: They are longer, wider. The mass implied by the curve
• 1:12:53
  Speaker 1: is too large to be any normal star. And some
• 1:12:56
  Speaker 1: of these events show something that regular stellar lensing does
• 1:13:00
  Speaker 1: not produce, an astrometric shift, meaning the apparent position of
• 1:13:05
  Speaker 1: the background star moves slightly as the lensing object passes.
• 1:13:12
  Speaker 1: The size of that shift depends on the mass of
• 1:13:15
  Speaker 1: the lensing object and its distance. When you combine the
• 1:13:20
  Speaker 1: photometric curve with the astrometric signals, you have a much
• 1:13:24
  Speaker 1: tighter grip on what you are dealing with. This is
• 1:13:28
  Speaker 1: where Hubble becomes essential. Ground-based telescopes can catch the brightening,
• 1:13:35
  Speaker 1: but measuring the tiny positional shift of a star requires
• 1:13:40
  Speaker 1: a level of precision that only space-based optics can deliver.
• 1:13:45
  Speaker 1: In 2022, a team using years of Hubble follow-up data
• 1:13:50
  Speaker 1: announced something remarkable. A microlensing event first flagged in 2011
• 1:13:56
  Speaker 1: by the MOA survey, designated MOA-2011-BLG-191, had finally yielded enough
• 1:14:06
  Speaker 1: data to characterize the lensing object conclusively. The team had
• 1:14:11
  Speaker 1: been tracking it for over six years with Hubble, measuring
• 1:14:15
  Speaker 1: the slow drift of the background star's position as the
• 1:14:18
  Speaker 1: lensing object pulled it out of alignment. When the numbers
• 1:14:22
  Speaker 1: came together, the mass of the lensing object came out
• 1:14:26
  Speaker 1: to roughly seven solar masses. It was dark. No companion star.
• 1:14:32
  Speaker 1: No X-ray emission. Nothing visible at any wavelength. The conclusion
• 1:14:38
  Speaker 1: was difficult to argue with. This was an isolated stellar
• 1:14:43
  Speaker 1: mass black hole drifting through the galaxy. It was the
• 1:14:47
  Speaker 1: first time one had ever been directly confirmed through microlensing
• 1:14:52
  Speaker 1: and the first time a lone black hole had been
• 1:14:55
  Speaker 1: identified without any binary companion or accretion disk to betray
• 1:15:00
  Speaker 1: its presence. The black hole has since been given the
• 1:15:04
  Speaker 1: designation OB110022. It is drifting somewhere about 5,000 light years
• 1:15:12
  Speaker 1: from Earth, entirely alone, entirely dark. giving nothing away except
• 1:15:19
  Speaker 1: a brief bend in starlight a decade ago that a
• 1:15:23
  Speaker 1: patient survey happened to catch. It is worth pausing on
• 1:15:27
  Speaker 1: what that patient's actually required. The initial brightening event lasted
• 1:15:33
  Speaker 1: a few months in 2011. That alone was suspicious, longer
• 1:15:38
  Speaker 1: than most stellar lensing events. But a suspicious microlensing curve
• 1:15:44
  Speaker 1: is not a confirmed black hole. To separate a black
• 1:15:48
  Speaker 1: hole from a heavy neutron star or an extremely faint
• 1:15:52
  Speaker 1: white dwarf, you need the astrometric signal, and that means
• 1:15:57
  Speaker 1: years of Hubble observations tracking a positional shift measured in
• 1:16:02
  Speaker 1: fractions of a milliac second. One milliac second is roughly
• 1:16:08
  Speaker 1: the angle subtended by a coin sitting on the surface
• 1:16:11
  Speaker 1: of the Moon, viewed from Earth. The precision required is extraordinary.
• 1:16:18
  Speaker 1: And this single confirmed event took over a decade from
• 1:16:22
  Speaker 1: detection to publication. This is the bottleneck. Ground surveys can
• 1:16:28
  Speaker 1: find hundreds of candidate events per year. The ones that
• 1:16:32
  Speaker 1: look massive enough to be black holes number in the
• 1:16:35
  Speaker 1: dozens annually. But following each one up with the depth
• 1:16:40
  Speaker 1: of Hubble observation needed to confirm the mass takes time,
• 1:16:44
  Speaker 1: telescope hours, and years of data reduction. The confirmed population
• 1:16:50
  Speaker 1: of isolated black holes found this way is still tiny.
• 1:16:55
  Speaker 1: Not because they are rare in the galaxy, because confirming
• 1:17:00
  Speaker 1: them one at a time with a single space telescope
• 1:17:03
  Speaker 1: is glacially slow. That is exactly what makes the Nancy
• 1:17:08
  Speaker 1: Grace Roman Space Telescope so significant to this field. Roman
• 1:17:15
  Speaker 1: is a NASA observatory with a wide field of view
• 1:17:18
  Speaker 1: roughly 100 times larger than Hubble's, and one of its
• 1:17:22
  Speaker 1: core survey programs is designed specifically to monitor the galactic
• 1:17:27
  Speaker 1: bulge in a way that no previous telescope has matched.
• 1:17:32
  Speaker 1: It will watch roughly 100 million stars simultaneously with the
• 1:17:37
  Speaker 1: cadence and resolution needed to catch microlensing events and begin
• 1:17:42
  Speaker 1: extracting astrometric signals far faster than any previous approach allowed.
• 1:17:49
  Speaker 1: The numbers astronomers project are striking. Where current surveys confirm
• 1:17:55
  Speaker 1: perhaps one or two isolated black holes per decade, Roman
• 1:17:59
  Speaker 1: is expected to identify dozens. The prediction is not that
• 1:18:04
  Speaker 1: it will find black holes by accident, but that it
• 1:18:07
  Speaker 1: will find them systematically, building a statistical sample large enough
• 1:18:12
  Speaker 1: to actually constrain the mass distribution of isolated black holes
• 1:18:17
  Speaker 1: across the galaxy. That sample matters enormously. Every confirmed isolated
• 1:18:24
  Speaker 1: black hole adds a data point to the question of
• 1:18:27
  Speaker 1: how black holes are distributed in mass, in space, in velocity.
• 1:18:33
  Speaker 1: It tests the stellar evolution models that produce the 100
• 1:18:37
  Speaker 1: million estimate. It checks whether direct collapse events really happen,
• 1:18:44
  Speaker 1: whether the kick velocities from supernova-borne black holes are large
• 1:18:48
  Speaker 1: or small, and whether the spatial distribution of lone black
• 1:18:53
  Speaker 1: holes matches what the simulations predict. Right now, all of
• 1:18:58
  Speaker 1: that is essentially unconstrained because the confirmed population is too
• 1:19:03
  Speaker 1: small to draw conclusions from. Roman changes that. The Roman
• 1:19:09
  Speaker 1: Space Telescope, once operational, is expected to catch thousands of
• 1:19:15
  Speaker 1: microlensing events per year, and among them, astronomers predict, will
• 1:19:21
  Speaker 1: be dozens of isolated black holes finally giving themselves away
• 1:19:26
  Speaker 1: through nothing more than the bending of light. In September
• 1:19:30
  Speaker 1: of 2015, two black holes roughly 1.3 billion light-years away
• 1:19:37
  Speaker 1: finished a billion-year spiral toward each other and merged in
• 1:19:41
  Speaker 1: a fraction of a second, and for the first time
• 1:19:44
  Speaker 1: in history, humanity felt the ground move. Not metaphorically, the
• 1:19:51
  Speaker 1: ground actually moved. Two detectors in the United States, one
• 1:19:56
  Speaker 1: in Louisiana and one in Washington state, each measuring the
• 1:20:00
  Speaker 1: distance between a pair of mirrors suspended four kilometers apart,
• 1:20:05
  Speaker 1: registered a change in that distance smaller than one thousandth
• 1:20:10
  Speaker 1: the width of a single proton. Think about that. A
• 1:20:15
  Speaker 1: proton is already so small that any comparison to everyday
• 1:20:19
  Speaker 1: objects becomes almost meaningless. And the signal that passed through
• 1:20:25
  Speaker 1: those detectors was a thousand times smaller still. Yet the
• 1:20:30
  Speaker 1: instruments caught it. They caught it because the people who
• 1:20:35
  Speaker 1: built LIGO, the Laser Interferometer Gravitational Wave Observatory, had spent
• 1:20:41
  Speaker 1: decades getting those mirrors stable enough, those lasers precise enough,
• 1:20:47
  Speaker 1: those sensors quiet enough, to hear something that should by
• 1:20:52
  Speaker 1: any R-easonable measure have been impossible to detect. The signal
• 1:20:58
  Speaker 1: lasted less than a second. It had been traveling for
• 1:21:01
  Speaker 1: over a billion years. What LIGO actually detected was a
• 1:21:06
  Speaker 1: ripple in space-time itself, not a wave moving through space
• 1:21:11
  Speaker 1: the way light does. A wave that is space, stretching
• 1:21:16
  Speaker 1: and squeezing the fabric of the universe as it passes.
• 1:21:20
  Speaker 1: Einstein predicted this in 1916, when he published his General
• 1:21:25
  Speaker 1: Theory of Relativity. He also thought the waves would never
• 1:21:30
  Speaker 1: be detected, that they would be too faint, that no
• 1:21:34
  Speaker 1: instrument humans could ever build would be sensitive enough. He
• 1:21:40
  Speaker 1: was wrong about the second part. Gravitational waves are produced
• 1:21:44
  Speaker 1: whenever massive objects accelerate. Two stars orbiting each other produce them.
• 1:21:51
  Speaker 1: A single spinning asymmetric object produces them. But by far
• 1:21:57
  Speaker 1: the most powerful sources are the final moments of two
• 1:22:01
  Speaker 1: compact objects. black holes or neutron stars, completing their long
• 1:22:07
  Speaker 1: in spiral and colliding. In those last fractions of a second,
• 1:22:13
  Speaker 1: the energy pouring out as gravitational waves exceeds the combined
• 1:22:18
  Speaker 1: light output of every star in the observable universe. And
• 1:22:23
  Speaker 1: it all passes through you without you feeling a thing,
• 1:22:27
  Speaker 1: or almost without you feeling a thing. The event that
• 1:22:31
  Speaker 1: LIGO caught in September of 2015 was later designated GW150914.
• 1:22:37
  Speaker 1: The two black holes involved had masses of roughly 29
• 1:22:46
  Speaker 1: and 36 solar masses. They merged into a single black
• 1:22:50
  Speaker 1: hole of about 62 solar masses, and the missing three
• 1:22:55
  Speaker 1: solar masses were radiated away as pure gravitational wave energy
• 1:23:00
  Speaker 1: in under a second. Three times the mass of the
• 1:23:04
  Speaker 1: sun converted entirely into ripples in space-time and flung across
• 1:23:10
  Speaker 1: the universe. The signal that reached Earth was a chirp,
• 1:23:15
  Speaker 1: rising in frequency and amplitude as the two objects spiraled
• 1:23:19
  Speaker 1: faster and faster before slamming together. When physicists played the
• 1:23:25
  Speaker 1: data as an audiophile, It sounded like a brief rising tone,
• 1:23:30
  Speaker 1: almost like a bird call. The countdown nobody expected. It
• 1:23:35
  Speaker 1: arrived at the Louisiana detector first, then seven milliseconds later
• 1:23:41
  Speaker 1: at THHE Washington detector. That tiny delay told the team
• 1:23:46
  Speaker 1: the approximate direction the wave had come from. The match
• 1:23:50
  Speaker 1: to Einstein's equations was near perfect. Here is what made
• 1:23:55
  Speaker 1: physicists stop and stare. Those black holes were heavy. X-ray
• 1:24:02
  Speaker 1: binary systems, the source of almost everything previously known about
• 1:24:06
  Speaker 1: stellar mass black holes, had suggested a typical mass range
• 1:24:11
  Speaker 1: of roughly 5 to 15 solar masses. The black holes
• 1:24:16
  Speaker 1: in GW150914 were 29 and 36 solar masses. that was
• 1:24:24
  Speaker 1: not supposed to happen under the standard picture. Stars that
• 1:24:29
  Speaker 1: massive were expected to shed enormous amounts of material through
• 1:24:34
  Speaker 1: stellar winds during their lifetimes, leaving behind smaller remnants. The
• 1:24:40
  Speaker 1: fact that two such heavy black holes existed and had
• 1:24:44
  Speaker 1: found each other raised immediate questions. Were these black holes
• 1:24:50
  Speaker 1: formed in a low metallicity environment? where stellar winds are
• 1:24:54
  Speaker 1: weaker and stars retain more mass before death. Were they
• 1:25:00
  Speaker 1: originally a stellar binary? Two massive stars born together? Or
• 1:25:05
  Speaker 1: had they formed separately and paired up through gravitational encounters
• 1:25:10
  Speaker 1: in a dense star cluster? The merger had answered one
• 1:25:15
  Speaker 1: question and opened a dozen more. More detections followed. By
• 1:25:21
  Speaker 1: the end of the first observing runs, the catalogue of
• 1:25:24
  Speaker 1: confirmed gravitational wave events had grown to nearly 90, almost
• 1:25:30
  Speaker 1: all of them black hole mergers. The mass distribution was strange.
• 1:25:36
  Speaker 1: Some mergers involved black holes in the expected range of
• 1:25:39
  Speaker 1: 5 to 20 solar masses. Others involved objects of 40, 50,
• 1:25:46
  Speaker 1: even 85 solar masses. One merger, detected in 2019, and
• 1:25:53
  Speaker 1: designated GW190521 involved a final black hole of roughly 142
• 1:26:02
  Speaker 1: solar masses, putting it in the intermediate mass range that
• 1:26:06
  Speaker 1: astronomers had been hunting for through other methods. The diversity
• 1:26:11
  Speaker 1: was not what anyone had cleanly predicted. The Universe appeared
• 1:26:16
  Speaker 1: to be making black holes across a much wider mass
• 1:26:19
  Speaker 1: range than the sample from X-ray binaries had indicated. That
• 1:26:25
  Speaker 1: earlier sample had been deeply biased. X-ray binaries show you
• 1:26:30
  Speaker 1: the black holes that are actively accreting in relatively tight orbits,
• 1:26:35
  Speaker 1: with companions bright enough to detect. They are not a
• 1:26:39
  Speaker 1: fair census. Gravitational waves are different. They do not care
• 1:26:45
  Speaker 1: whether a black hole has a companion. They fire off
• 1:26:49
  Speaker 1: at the moment of merger and travel in every direction
• 1:26:52
  Speaker 1: at once. LIGO does not work alone anymore. The Virgo
• 1:26:58
  Speaker 1: detector in Italy came online in 2017 and the Kagra
• 1:27:03
  Speaker 1: detector in Japan joined the network after that. Multiple detectors
• 1:27:08
  Speaker 1: working together do something a single instrument cannot. They triangulate.
• 1:27:15
  Speaker 1: When the same wave arrives at different detectors at slightly
• 1:27:19
  Speaker 1: different times, The time delays pin down the region of
• 1:27:23
  Speaker 1: the sky the wave came from, shrinking the error box
• 1:27:27
  Speaker 1: from a broad stripe across the sky to something closer
• 1:27:31
  Speaker 1: to a patch. That localization is what allows optical telescopes
• 1:27:37
  Speaker 1: and radio arrays to search the right region for any
• 1:27:41
  Speaker 1: electromagnetic counterpart to the merger. With neutron star collisions, this
• 1:27:47
  Speaker 1: follow-up has found the kilonova afterglow. the glowing debris cloud
• 1:27:53
  Speaker 1: of freshly forged heavy elements. With black hole mergers, there
• 1:27:58
  Speaker 1: is usually nothing to see, because nothing escapes. But the
• 1:28:03
  Speaker 1: localization still matters for understanding where in the universe these
• 1:28:08
  Speaker 1: mergers are happening, and what kind of galaxies they prefer.
• 1:28:13
  Speaker 1: What the full catalog of detections is building, event by event,
• 1:28:18
  Speaker 1: is a statistical portrait of black holes that no other
• 1:28:22
  Speaker 1: method can produce. X-ray binaries gave astronomers a sample of
• 1:28:27
  Speaker 1: dozens of black holes over decades of searching. Gravitational wave
• 1:28:33
  Speaker 1: detectors are adding confirmed mergers at a rate of several
• 1:28:37
  Speaker 1: per week during active observing runs. Each detection carries the
• 1:28:43
  Speaker 1: masses of both objects, the mass of the final merger product,
• 1:28:48
  Speaker 1: the spin parameters, the distance. Stacked together, these data are
• 1:28:54
  Speaker 1: sketching the shape of a population, and the shape keeps
• 1:28:58
  Speaker 1: coming back heavier broader and more varied than the models
• 1:29:02
  Speaker 1: built before September of 2015 had expected. The detectors are
• 1:29:07
  Speaker 1: getting more sensitive. The next generation of instruments, including proposed
• 1:29:14
  Speaker 1: underground observatories with arm lengths of tens of kilometers, will
• 1:29:18
  Speaker 1: push the detectable volume out far enough to catch mergers
• 1:29:22
  Speaker 1: happening almost anywhere in the observable universe. Every merger detected
• 1:29:29
  Speaker 1: since has added another data point to a growing picture
• 1:29:33
  Speaker 1: of a universe containing far more black holes and far
• 1:29:37
  Speaker 1: more massive ones than earlier generations of astronomers had assumed.
• 1:29:43
  Speaker 1: The closest known black hole to Earth is not a
• 1:29:46
  Speaker 1: dramatic X-ray blaze or a celebrated gravitational wave source. No
• 1:29:53
  Speaker 1: telescope caught it devouring a companion star. No detector felt
• 1:29:58
  Speaker 1: the ground shake when it formed. It was found because
• 1:30:02
  Speaker 1: a perfectly ordinary star in the constellation Ophiuchus was wobbling.
• 1:30:08
  Speaker 1: That wobble is the whole story. The star in question
• 1:30:12
  Speaker 1: is a G-type star, the same broad category as our
• 1:30:15
  Speaker 1: own Sun. Unremarkable in almost every way. Medium temperature. Medium brightness.
• 1:30:23
  Speaker 1: the kind of star that shows up by the thousands
• 1:30:26
  Speaker 1: in any decent survey of the galaxy. But when scientists
• 1:30:30
  Speaker 1: ran the data from the European Space Agency's Gaia satellite
• 1:30:34
  Speaker 1: through careful analysis in 2022, they noticed something wrong with
• 1:30:40
  Speaker 1: its path through space. It wasn't travelling in a straight
• 1:30:45
  Speaker 1: line the way a solitary star should. It was tracing
• 1:30:49
  Speaker 1: a small, repeating ellipse. Something invisible was pulling it in
• 1:30:55
  Speaker 1: a tight loop, roughly every 186 days, and that something
• 1:31:01
  Speaker 1: had a mass of around 10 times our sun. Gaia
• 1:31:05
  Speaker 1: was not designed to hunt black holes. Its mission is astrometry,
• 1:31:11
  Speaker 1: the precise mapping of stellar positions and motions across the sky.
• 1:31:16
  Speaker 1: Over the course of its operations, it has tracked the
• 1:31:20
  Speaker 1: movements of more than a billion stars. measuring their positions
• 1:31:24
  Speaker 1: to an accuracy that is genuinely difficult to put into
• 1:31:29
  Speaker 1: human terms. We're talking about detecting an angular shift equivalent
• 1:31:34
  Speaker 1: to watching a coin change position from across a continent.
• 1:31:39
  Speaker 1: That level of precision is what made the discovery possible.
• 1:31:44
  Speaker 1: You don't need a black hole to be eating something
• 1:31:47
  Speaker 1: to find it this way. You just need it to
• 1:31:51
  Speaker 1: be close enough to a star that the gravitational pull
• 1:31:54
  Speaker 1: is measurable. And with Gaia watching a billion stars at once,
• 1:32:00
  Speaker 1: the odds of catching that kind of signal improve enormously.
• 1:32:05
  Speaker 1: The black hole is now designated Gaia BH1. It sits
• 1:32:10
  Speaker 1: roughly 1,560 light years away, which in galactic terms is
• 1:32:15
  Speaker 1: essentially next door. The entire Milky Way is about 100,000
• 1:32:19
  Speaker 1: light years across. Gaia BH1 is close enough that, by comparison,
• 1:32:26
  Speaker 1: it's sitting on our front step. Yet nobody had found
• 1:32:30
  Speaker 1: it before. No X-ray signal. No unusual brightness. Nothing. Just
• 1:32:36
  Speaker 1: a star moving strangely, and a telescope precise enough to notice.
• 1:32:41
  Speaker 1: Follow-up observations confirmed what Gaia had hinted at. Ground-based spectrographs
• 1:32:48
  Speaker 1: measured the radial velocity of the companion star. the way
• 1:32:52
  Speaker 1: its light shifts toward red and then toward blue as
• 1:32:55
  Speaker 1: it orbits. Those measurements match the Gaia prediction almost exactly.
• 1:33:02
  Speaker 1: The invisible companion mass came in at roughly 9.6 solar masses,
• 1:33:08
  Speaker 1: far too heavy to be a neutron star, which tops
• 1:33:12
  Speaker 1: out around 2 to 3 solar masses. There was no
• 1:33:16
  Speaker 1: plausible explanation other than a black hole, Astronomers announced the
• 1:33:22
  Speaker 1: discovery in November 2022, and it immediately claimed the title
• 1:33:27
  Speaker 1: of nearest confirmed black hole to Earth. Then, a few
• 1:33:32
  Speaker 1: months later, the same team found another one. Gaia BH2
• 1:33:37
  Speaker 1: sits about 3,800 light-years away, also in a quiet corner
• 1:33:43
  Speaker 1: of the sky, also paired with an unassuming companion star.
• 1:33:48
  Speaker 1: This one is a red giant rather than a sun-like star.
• 1:33:52
  Speaker 1: The orbital period is longer, around 1,300 days. The black
• 1:33:58
  Speaker 1: hole mass came in at roughly 8.9 solar masses. Same method,
• 1:34:04
  Speaker 1: same discovery path, same verdict. A black hole hiding in
• 1:34:09
  Speaker 1: plain sight, detectable only because it was nudging its neighbor
• 1:34:13
  Speaker 1: through space. Both discoveries created immediate questions, and not small ones.
• 1:34:21
  Speaker 1: Standard models of stellar evolution predicted that these systems shouldn't exist.
• 1:34:27
  Speaker 1: When a massive star collapses into a black hole, the
• 1:34:31
  Speaker 1: core collapse and associated supernova should release an enormous asymmetric kick,
• 1:34:37
  Speaker 1: and that kick should tear the binary system apart. The
• 1:34:42
  Speaker 1: companion star should be flung free, leaving behind an isolated
• 1:34:46
  Speaker 1: black hole rather than a bound pair. Some simulations suggest
• 1:34:52
  Speaker 1: that even when a supernova doesn't fully destroy a binary,
• 1:34:56
  Speaker 1: it should leave the companion badly disrupted, chemically enriched by
• 1:35:01
  Speaker 1: the explosion, contaminated with elements from the dying star. The
• 1:35:06
  Speaker 1: companion stars in Gaia BH1 and Gaia BH2 show none
• 1:35:12
  Speaker 1: of that contamination. they look pristine. They look like nothing
• 1:35:18
  Speaker 1: violent happened anywhere near them. That is genuinely strange. It
• 1:35:24
  Speaker 1: means either the supernova was much weaker than standard models predict,
• 1:35:29
  Speaker 1: or the black hole formed through something closer to direct collapse,
• 1:35:34
  Speaker 1: the star simply shrinking inward without producing a major explosion.
• 1:35:40
  Speaker 1: If that second explanation is correct, it has implications for
• 1:35:45
  Speaker 1: how common these quiet systems might be. A dramatic supernova
• 1:35:50
  Speaker 1: throws light across the galaxy and announces the birth of
• 1:35:54
  Speaker 1: a compact remnant. A direct collapse does not. The black
• 1:36:00
  Speaker 1: hole simply appears. The companion star continues on its orbit, unaware,
• 1:36:07
  Speaker 1: and the whole system drifts through space putting out no
• 1:36:11
  Speaker 1: unusual signal at all. How many of those systems are
• 1:36:15
  Speaker 1: out there? That is the uncomfortable question sitting at the
• 1:36:20
  Speaker 1: center of these two discoveries. Gaia has only analyzed a
• 1:36:24
  Speaker 1: fraction of its full data set. The complete data release
• 1:36:29
  Speaker 1: will cover a far larger number of stars with far
• 1:36:32
  Speaker 1: more detailed motion measurements. If two systems turned up in
• 1:36:37
  Speaker 1: the early analysis, the expectation among researchers is that the
• 1:36:42
  Speaker 1: final catalog could reveal dozens or more. Maybe many more.
• 1:36:48
  Speaker 1: The companion stars in these systems look like ordinary stars
• 1:36:52
  Speaker 1: because they are ordinary stars. They are not being eaten.
• 1:36:57
  Speaker 1: They are not being irradiated. They are not showing any
• 1:37:01
  Speaker 1: sign of distress. The black hole sitting beside them is
• 1:37:05
  Speaker 1: just there, massive and invisible, completing its orbit like clockwork.
• 1:37:12
  Speaker 1: The fact that Gaia has found two of these systems
• 1:37:15
  Speaker 1: in a relatively short time suggests that black holes sitting
• 1:37:20
  Speaker 1: quietly beside ordinary stars may be far more common than
• 1:37:24
  Speaker 1: anyone previously guessed. Between the stellar mass black holes and
• 1:37:30
  Speaker 1: the supermassive monsters, there is a gap in the catalogue
• 1:37:34
  Speaker 1: that has bothered astronomers for decades. On one side, you
• 1:37:39
  Speaker 1: have stellar mass black holes the remnants of dead stars,
• 1:37:44
  Speaker 1: ranging from a few solar masses up to perhaps a hundred.
• 1:37:49
  Speaker 1: On the other side, you have supermassive black holes like
• 1:37:53
  Speaker 1: Sagittarius A star, clocking in at millions or billions of
• 1:37:57
  Speaker 1: solar masses. The middle ground is where intermediate mass black
• 1:38:02
  Speaker 1: holes should live. Hundreds of solar masses, thousands, maybe tens
• 1:38:09
  Speaker 1: of thousands. The math says they should be there. The
• 1:38:13
  Speaker 1: observations keep coming up nearly empty. This is not a
• 1:38:18
  Speaker 1: minor bookkeeping problem. It matters because intermediate mass black holes
• 1:38:23
  Speaker 1: are almost certainly the missing chapter in how supermassive black
• 1:38:27
  Speaker 1: holes got so large. Sagittarius A star contains the mass
• 1:38:33
  Speaker 1: of 4.3 million suns. It did not get there by
• 1:38:38
  Speaker 1: swallowing one star at a time. Something had to provide
• 1:38:42
  Speaker 1: the seeds, dense, massive objects that could sink to the
• 1:38:46
  Speaker 1: center of a young galaxy and grow. Intermediate mass black
• 1:38:51
  Speaker 1: holes are the leading candidate for what those seeds were.
• 1:38:56
  Speaker 1: If they existed in large numbers in the early universe,
• 1:39:00
  Speaker 1: the math of how supermassive black holes assembled starts to work.
• 1:39:05
  Speaker 1: If they did not, the origin story of every galaxy
• 1:39:09
  Speaker 1: centre becomes a lot harder to explain. The Milky Way
• 1:39:13
  Speaker 1: itself is estimated to contain somewhere between 1,000 and 10,000
• 1:39:17
  Speaker 1: of them. That range tells you something honest about the
• 1:39:22
  Speaker 1: state of the field. It is not a tight measurement.
• 1:39:27
  Speaker 1: It is a theoretical bracket built on stellar population models
• 1:39:32
  Speaker 1: and assumptions about dense star clusters, and it could be
• 1:39:36
  Speaker 1: wrong in either direction. What makes this especially frustrating is
• 1:39:42
  Speaker 1: that intermediate mass black holes should not be rare objects
• 1:39:47
  Speaker 1: hiding in exotic corners of the universe. They should be
• 1:39:51
  Speaker 1: forming right now, in the dense hearts of globular clusters,
• 1:39:56
  Speaker 1: where stars pack together tightly enough that collisions and mergers
• 1:40:00
  Speaker 1: happen on timescales short enough to pile up mass fast.
• 1:40:06
  Speaker 1: Globular clusters are one of the best places to look.
• 1:40:10
  Speaker 1: These are ancient, tightly bound collections of hundreds of thousands
• 1:40:14
  Speaker 1: of stars orbiting the Milky Way like satellites. Several of
• 1:40:20
  Speaker 1: them show something interesting when astronomers measure stellar velocities near
• 1:40:24
  Speaker 1: their centres. The stars are moving too fast. Not slightly fast,
• 1:40:31
  Speaker 1: noticeably fast, in a way that suggests something heavy is
• 1:40:35
  Speaker 1: sitting in the middle pulling on everything nearby. The globular
• 1:40:40
  Speaker 1: cluster M22 is one example. The velocity dispersion in its
• 1:40:46
  Speaker 1: core is consistent with a central object somewhere between a
• 1:40:50
  Speaker 1: few thousand and maybe a hundred thousand solar masses. But
• 1:40:55
  Speaker 1: velocity dispersion alone is not a confirmed detection. It is
• 1:40:59
  Speaker 1: a gravitational fingerprint without a face. The problem is that
• 1:41:05
  Speaker 1: ruling out alternatives is hard. A dense collection of stellar
• 1:41:10
  Speaker 1: mass black holes packed into the core of a cluster
• 1:41:14
  Speaker 1: can produce a similar velocity signal. Neutron stars can contribute.
• 1:41:20
  Speaker 1: You need to distinguish between one large invisible object and
• 1:41:24
  Speaker 1: a swarm of smaller ones, and right now the resolution
• 1:41:29
  Speaker 1: required to do that cleanly is at the edge of
• 1:41:32
  Speaker 1: what current instruments can manage. Some studies have argued for
• 1:41:37
  Speaker 1: a central massive object. Other studies using different data have
• 1:41:43
  Speaker 1: argued the same signal can be explained without one. The
• 1:41:47
  Speaker 1: debate has been running for years. Ultraluminous X-ray sources offer
• 1:41:52
  Speaker 1: another avenue. These are X-ray sources found in nearby galaxies
• 1:41:57
  Speaker 1: that shine far brighter than any single stellar mass black
• 1:42:01
  Speaker 1: hole should be able to manage, assuming it is pulling
• 1:42:05
  Speaker 1: in gas at a reasonable rate. Some of them are
• 1:42:09
  Speaker 1: bright enough that the most natural explanation is a black
• 1:42:13
  Speaker 1: hole in the intermediate mass range, somewhere between a few
• 1:42:18
  Speaker 1: hundred and a few thousand solar masses, actively accreting material.
• 1:42:24
  Speaker 1: A handful of these sources have been studied in detail.
• 1:42:29
  Speaker 1: The evidence is suggestive in several cases, but the interpretations
• 1:42:34
  Speaker 1: are contested and confirmed, airtight detections remain stubbornly rare. One
• 1:42:41
  Speaker 1: candidate stood out for years in the globular cluster 47 Tucani.
• 1:42:47
  Speaker 1: Radio and X-ray measurements pointed toward a central compact object.
• 1:42:52
  Speaker 1: Then deeper observations complicated the picture. Another candidate in the
• 1:42:58
  Speaker 1: galaxy NGC 1313 produced an X-ray signature that looked consistent
• 1:43:05
  Speaker 1: with a black hole of roughly 5,000 solar masses. If
• 1:43:10
  Speaker 1: these signals are real, the argument is over what is
• 1:43:13
  Speaker 1: producing them. Intermediate mass black holes are not confirmed simply
• 1:43:19
  Speaker 1: because the data fits. The data has to rule out
• 1:43:23
  Speaker 1: every other explanation too, and that bar keeps being difficult
• 1:43:27
  Speaker 1: to clear. What the field needs is a detection method
• 1:43:32
  Speaker 1: that does not depend on the black hole actively eating something.
• 1:43:37
  Speaker 1: Most of the best evidence so far comes from objects
• 1:43:41
  Speaker 1: that happen to be accreting gas or happen to sit
• 1:43:44
  Speaker 1: in clusters where stellar velocities can be measured. The vast
• 1:43:49
  Speaker 1: majority of intermediate mass black holes, if they exist in
• 1:43:54
  Speaker 1: the numbers the models predict, are sitting quietly in the
• 1:43:58
  Speaker 1: halos of galaxies or the remnants of dissolved clusters, producing
• 1:44:04
  Speaker 1: no X-rays, no obvious stellar wobble. nothing that current surveys
• 1:44:10
  Speaker 1: are designed to catch. Gravitational waves change that calculus. When
• 1:44:17
  Speaker 1: two intermediate mass black holes spiral together and merge, they
• 1:44:22
  Speaker 1: produce a gravitational wave signal in a frequency band that
• 1:44:26
  Speaker 1: sits below what ground-based detectors like LIGO can hear. LIGO
• 1:44:31
  Speaker 1: is tuned for stellar mass mergers. Supermassive mergers fall in
• 1:44:36
  Speaker 1: a completely different range. intermediate mass mergers fall in between,
• 1:44:42
  Speaker 1: in the millihertz band that only a space-based detector can access. LISA,
• 1:44:48
  Speaker 1: the space-based gravitational wave observatory planned for the 2030s, is
• 1:44:54
  Speaker 1: expected to detect mergers involving intermediate mass black holes at
• 1:44:59
  Speaker 1: distances so vast that it should finally settle the question
• 1:45:03
  Speaker 1: of how common they really are. The honest accounting of
• 1:45:08
  Speaker 1: what we do not know about black holes in the
• 1:45:10
  Speaker 1: Milky Way is longer than the accounting of what we do.
• 1:45:15
  Speaker 1: That is not a failure of the science. It is
• 1:45:18
  Speaker 1: just an honest description of where things stand when you
• 1:45:22
  Speaker 1: are trying to census a population of objects that produce
• 1:45:27
  Speaker 1: no light. Drift through a galaxy a hundred thousand light
• 1:45:32
  Speaker 1: years across. Spend most of their existence doing absolutely nothing detectable.
• 1:45:39
  Speaker 1: The 100 million figure gets repeated constantly. But it is
• 1:45:44
  Speaker 1: worth being clear about what that number actually is. It
• 1:45:48
  Speaker 1: is not a count. It is a calculation built on
• 1:45:52
  Speaker 1: assumptions about stellar masses, star formation rates. How often massive
• 1:45:58
  Speaker 1: stars successfully collapse into black holes rather than neutron stars.
• 1:46:05
  Speaker 1: change those assumptions slightly and the number shifts. Some models
• 1:46:10
  Speaker 1: come back with 10 million, others push toward a billion.
• 1:46:15
  Speaker 1: The true population could sit almost anywhere inside that range.
• 1:46:20
  Speaker 1: Right now, there is no observation that can settle the debate.
• 1:46:25
  Speaker 1: The mass distribution is another open problem. It turns out
• 1:46:30
  Speaker 1: to be a stranger one than astronomers expected. For decades,
• 1:46:35
  Speaker 1: the stellar mass black holes found in X-ray binaries clustered
• 1:46:39
  Speaker 1: in a fairly narrow range, mostly between 5 and 15
• 1:46:44
  Speaker 1: solar masses. Then gravitational wave detection started arriving, and the
• 1:46:50
  Speaker 1: picture got complicated fast. The first confirmed merger involved black
• 1:46:56
  Speaker 1: holes of roughly 36 and 29 solar masses, producing a
• 1:47:01
  Speaker 1: combined object of about 62 solar masses. That was surprising.
• 1:47:07
  Speaker 1: As more detections came in, the mass range kept expanding.
• 1:47:12
  Speaker 1: Black holes of 50, 70, even 85 solar masses appeared
• 1:47:17
  Speaker 1: in the data. These numbers do not fit neatly into
• 1:47:22
  Speaker 1: the standard models of how massive stars end their lives.
• 1:47:27
  Speaker 1: Either the formation pathways are more varied than the textbooks describe,
• 1:47:32
  Speaker 1: or there is a population of black holes that grew
• 1:47:35
  Speaker 1: through previous mergers, or both things are true at once.
• 1:47:40
  Speaker 1: Nobody has fully resolved this. Then there is the question
• 1:47:44
  Speaker 1: of quiet formation. Some massive stars may simply collapse without
• 1:47:49
  Speaker 1: producing any visible supernova at all. The outer layers fall inward.
• 1:47:55
  Speaker 1: There is no explosion. the star just disappears. Astronomers have
• 1:48:01
  Speaker 1: identified a handful of candidates, massive stars in nearby galaxies
• 1:48:06
  Speaker 1: that seem to vanish from survey images without any recorded outburst.
• 1:48:12
  Speaker 1: If direct collapse is common, it has two consequences. First,
• 1:48:17
  Speaker 1: it means a fraction of the galaxy's black holes were
• 1:48:20
  Speaker 1: born in total silence with no supernova kick. That affects
• 1:48:26
  Speaker 1: how they are distributed across the galaxy. Second, it means
• 1:48:31
  Speaker 1: the formation rate estimates are uncertain in ways that are
• 1:48:35
  Speaker 1: hard to correct without catching more stars in the act
• 1:48:39
  Speaker 1: of vanishing Surveys are watching, but the events are rare
• 1:48:44
  Speaker 1: enough that confirmed cases remain frustratingly few. The binary companion
• 1:48:50
  Speaker 1: problem runs deeper than it looks. Almost every confirmed stellar
• 1:48:56
  Speaker 1: mass black hole was found because it was actively pulling
• 1:49:00
  Speaker 1: gas from a companion star close enough to feed the
• 1:49:04
  Speaker 1: accretion disk. Gaia changed that slightly. by finding black holes
• 1:49:09
  Speaker 1: orbiting companions through gravitational wobble alone, without any X-ray signature
• 1:49:16
  Speaker 1: at all. But the systems Gaia can detect require companions
• 1:49:21
  Speaker 1: bright enough and wobbles large enough to register in the
• 1:49:25
  Speaker 1: astrometric data. Wide binaries where the companion is small, dim,
• 1:49:32
  Speaker 1: or very far from the black hole are still essentially invisible.
• 1:49:37
  Speaker 1: How many of those exist is genuinely unknown. The early
• 1:49:41
  Speaker 1: Gaia results suggested the number could be surprisingly large. But
• 1:49:47
  Speaker 1: translating those early results into a firm population estimate is
• 1:49:51
  Speaker 1: still ongoing work. Sagittarius A star sits at the centre
• 1:49:57
  Speaker 1: of this catalogue of uncertainties in its own category. It
• 1:50:01
  Speaker 1: is dormant now. But the X-ray echo evidence showing it
• 1:50:06
  Speaker 1: tore through surrounding material roughly 200 years ago is a
• 1:50:11
  Speaker 1: reminder that dormant and inactive are not the same thing.
• 1:50:16
  Speaker 1: Whether it will feed again in any timescale relevant to
• 1:50:19
  Speaker 1: human civilization, nobody can honestly say. The feeding depends on
• 1:50:26
  Speaker 1: whether any cloud of gas drifts close enough to fall
• 1:50:29
  Speaker 1: within reach. The galactic centre environment is chaotic enough that
• 1:50:35
  Speaker 1: predicting this is not yet possible. What astronomers do know
• 1:50:39
  Speaker 1: is that SGR A star is not uniquely quiet. It
• 1:50:44
  Speaker 1: is simply in a quiet phase. Quiet phases in supermassive
• 1:50:49
  Speaker 1: black holes can end. Intermediate mass black holes sit in
• 1:50:54
  Speaker 1: their own unresolved corner of the problem. Between stellar mass
• 1:50:59
  Speaker 1: objects and supermassive ones, There is a range where objects
• 1:51:03
  Speaker 1: should exist in meaningful numbers, forming in dense star clusters,
• 1:51:10
  Speaker 1: possibly seeding the growth of larger black holes over cosmic time.
• 1:51:16
  Speaker 1: The Milky Way is estimated to contain somewhere between a
• 1:51:19
  Speaker 1: thousand and ten thousand of them, but confirmed detections are
• 1:51:24
  Speaker 1: rare enough that those bounds are barely constrained by actual observation.
• 1:51:30
  Speaker 1: The signals that would indicate their presence, unusual velocity distributions
• 1:51:35
  Speaker 1: in globular clusters, unusually bright X-ray sources with characteristic spectral signatures,
• 1:51:43
  Speaker 1: are real but ambiguous. Lisa may eventually settle this by
• 1:51:49
  Speaker 1: detecting mergers involving these objects at distances where no other
• 1:51:53
  Speaker 1: method could reach. The deepest uncertainty is about the current
• 1:51:58
  Speaker 1: state of the population itself, Black holes do not just
• 1:52:02
  Speaker 1: sit still. They merge. A binary black hole system that
• 1:52:07
  Speaker 1: formed 8 billion years ago may have already merged once,
• 1:52:13
  Speaker 1: or twice, building up a remnant far more massive than
• 1:52:17
  Speaker 1: either original object. The 100 million estimate counts black holes
• 1:52:23
  Speaker 1: as if the galaxy's history is a simple ledger of
• 1:52:26
  Speaker 1: stars that lived and died. But the actual population has
• 1:52:32
  Speaker 1: been evolving through mergers, through accretion. Through the complex dynamics
• 1:52:38
  Speaker 1: of a galaxy that has been consuming smaller galaxies for
• 1:52:42
  Speaker 1: billions of years. The real number, whatever it is, belongs
• 1:52:48
  Speaker 1: to a system still in motion. The last decade changed
• 1:52:52
  Speaker 1: this field more than any decade before it. Gravitational wave
• 1:52:57
  Speaker 1: detection went from a theoretical ambition to a routine catalogue.
• 1:53:03
  Speaker 1: Gaia started turning up black holes through astrometry. Microlensing surveys
• 1:53:09
  Speaker 1: found the first isolated black holes confirmed through direct mass measurement.
• 1:53:16
  Speaker 1: The Event Horizon Telescope produced actual images of objects that
• 1:53:20
  Speaker 1: most physicists 50 years ago were still debating the existence of.
• 1:53:26
  Speaker 1: Ten years ago, the count of confirmed stellar mass black
• 1:53:30
  Speaker 1: holes in the galaxy stood at around 20. Today, it
• 1:53:35
  Speaker 1: is closer to 50, with more arriving every few months.
• 1:53:40
  Speaker 1: The rate is only accelerating as new instruments come online
• 1:53:45
  Speaker 1: and new detection methods mature.