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Sagittarius A Might Not Be a Black Hole · Boring Science

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  Speaker 1: Hello and welcome to Boring Science to Sleep, a quiet
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  Speaker 1: place where science becomes slow, soft, and easy to drift through. Tonight,
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  Speaker 1: you're going to gently explore Sagittarius A-star, the strange and
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  Speaker 1: deeply mysterious object resting at the very center of the
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  Speaker 1: Milky Way. Not to learn everything about it, but simply
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  Speaker 1: to notice it slowly, calmly, one quiet idea at a time.
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  Speaker 1: And before you settle in, If you truly enjoy these
• 0:30
  Speaker 1: calm journeys through interesting facts about science, you're welcome to
• 0:35
  Speaker 1: like this video and subscribe, only if it feels right
• 0:38
  Speaker 1: for you. And when you're ready, let me know in
• 0:41
  Speaker 1: the comments where you're listening from and what time it
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  Speaker 1: is right now. It's always a quiet joy to see
• 0:47
  Speaker 1: how many places and moments gather here together. Now, dim
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  Speaker 1: the lights. Maybe let a fan hum softly in the background.
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  Speaker 1: Allow your body to become a little heavier against whatever
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  Speaker 1: is supporting you, and remember that there is nothing to do.
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  Speaker 1: You are safe. There is nowhere you need to go.
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  Speaker 1: You can simply listen and drift as we slowly make
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  Speaker 1: our way toward the quiet centre of your galaxy. Imagine
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  Speaker 1: that you are floating very gently in the darkness, far
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  Speaker 1: above the familiar world beneath you, far beyond the clouds,
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  Speaker 1: far beyond the blue atmosphere. until the earth itself becomes
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  Speaker 1: a small and distant sphere of light. You continue drifting outward, slowly,
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  Speaker 1: without effort, leaving behind the familiar sounds and movements of
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  Speaker 1: your everyday life. The farther you travel, the more of
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  Speaker 1: the night sky begins to surround you. Stars appear in
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  Speaker 1: every direction, some bright, some faint, some so distant that
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  Speaker 1: their light has been travelling for thousands, or millions, or
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  Speaker 1: even billions of years before reaching you. And among all
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  Speaker 1: of those stars is your galaxy, the Milky Way, a
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  Speaker 1: vast gathering of stars, gas, dust, and something much harder
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  Speaker 1: to see. At its centre, about 27,000 light-years from Earth,
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  Speaker 1: there is a region that has fascinated astronomers for generations. There,
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  Speaker 1: hidden behind thick clouds of interstellar dust, sits something extremely
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  Speaker 1: massive and and extraordinarily compact. You cannot simply look toward
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  Speaker 1: it and see the object itself. Instead, you have to
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  Speaker 1: listen to what the surrounding universe is quietly telling you.
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  Speaker 1: The story begins almost a century ago. In the 1930s,
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  Speaker 1: the astronomer Carl Jansky detected an unusual radio signal coming
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  Speaker 1: from the direction of the constellation Sagittarius. It was not
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  Speaker 1: a bright star shining visibly in the night. It was
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  Speaker 1: something quieter, something revealed through radio waves rather than ordinary light.
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  Speaker 1: At the time, nobody knew exactly what was producing the signal.
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  Speaker 1: Black holes were still mostly theoretical ideas, strange possibilities emerging
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  Speaker 1: from the mathematics of gravity. So astronomers had other possibilities
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  Speaker 1: in mind. Perhaps the source was related to clouds of material.
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  Speaker 1: Perhaps it was a collection of stars. Perhaps it was
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  Speaker 1: the remains of an ancient stellar explosion. There was no
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  Speaker 1: clear answer. There was simply a faint signal arriving from
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  Speaker 1: a particular direction in the sky. And sometimes, science begins
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  Speaker 1: exactly this way. Not with an answer, but with something
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  Speaker 1: that refuses to fit neatly into the world you already understand.
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  Speaker 1: Decades passed. Observations became more precise. In 1974... Bruce Ballack
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  Speaker 1: and Robert Brown identified the source as a single, remarkably
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  Speaker 1: compact object, and Brown later gave it the name Sagittarius A-star.
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  Speaker 1: By then, the idea of black holes had become much
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  Speaker 1: more familiar, but the most revealing evidence was still waiting
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  Speaker 1: in the darkness around the object. As telescopes improved during
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  Speaker 1: the 1990s, astronomers began to follow individual stars moving around
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  Speaker 1: what appeared to be an empty region of space. These
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  Speaker 1: stars became known as the S-stars. They were close enough
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  Speaker 1: to the galactic centre for astronomers to follow their movements
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  Speaker 1: with extraordinary precision. And what they were doing was remarkable.
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  Speaker 1: They were moving very, very quickly. Some of these stars
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  Speaker 1: can travel at speeds of around 24,000 kilometres per second,
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  Speaker 1: which is roughly 8% of the speed of light. You
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  Speaker 1: can pause there for a moment and simply imagine the motion.
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  Speaker 1: A star... enormous by human standards, moving through the darkness
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  Speaker 1: at a speed that is difficult to hold in your mind,
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  Speaker 1: tracing a path around something you cannot see. And yet
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  Speaker 1: the path is not random. The star follows an orbit.
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  Speaker 1: Again and again, the invisible gravity at the centre guides
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  Speaker 1: its movement. Among these stars is one called S2. S2
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  Speaker 1: completes a full journey around Sagittarius. a star in less
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  Speaker 1: than 16 years. For you, 16 years might feel like
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  Speaker 1: a long stretch of life. But on the scale of
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  Speaker 1: a galaxy, it is almost nothing. Our own Sun, moving
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  Speaker 1: around the centre of the Milky Way, takes more than
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  Speaker 1: 200 million years to complete a single orbit. So while
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  Speaker 1: the Sun makes one enormously slow journey around the galaxy,
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  Speaker 1: S2 circles the galactic centre again and again. moving through
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  Speaker 1: its much smaller path in the deep darkness. And by
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  Speaker 1: carefully watching that motion, astronomers can begin to weigh something
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  Speaker 1: they cannot see. This is one of the quiet beauties
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  Speaker 1: of astronomy. You do not always need to see an
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  Speaker 1: object directly to discover how much it weighs. Sometimes you
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  Speaker 1: can watch the things around it. You can watch how
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  Speaker 1: they move. You can watch how quickly they travel, how
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  Speaker 1: tightly they curve, how long they take to return. and
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  Speaker 1: from those movements you can reconstruct the invisible source of
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  Speaker 1: the gravity. The result is extraordinary. At the heart of
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  Speaker 1: the Milky Way is an object with a mass of
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  Speaker 1: about 4.3 million times the mass of our Sun. 4.3
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  Speaker 1: million Suns, all concentrated into an incredibly compact region. Try
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  Speaker 1: to imagine that without needing to calculate anything. You do
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  Speaker 1: not need numbers filling your mind. Just imagine millions of
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  Speaker 1: stars like our Sun, each one enormous, each one carrying
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  Speaker 1: its own warmth and light, somehow gathered into a region
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  Speaker 1: so compact that its characteristic scale is comparable to the
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  Speaker 1: orbit of Venus. And around this hidden centre, the stars
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  Speaker 1: continue to move. They do not stop. They do not
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  Speaker 1: fall inward. They simply follow their paths, quietly tracing the
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  Speaker 1: shape of the gravity around them. From far away, The
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  Speaker 1: galactic centre may seem like just another point in the sky,
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  Speaker 1: but when you look more closely, it becomes something much stranger,
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  Speaker 1: a place where an enormous amount of mass is gathered
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  Speaker 1: into a very small space. A place where stars race
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  Speaker 1: through darkness. A place where the universe seems to reveal
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  Speaker 1: something invisible by showing you only its effects. For decades,
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  Speaker 1: this has been the strongest evidence that Sagittarius A star
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  Speaker 1: is a supermassive black hole. and that interpretation remains the
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  Speaker 1: leading one. But there is something subtle hiding inside that conclusion.
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  Speaker 1: The stars tell you that something massive is there. They
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  Speaker 1: tell you that it is extraordinarily compact. They tell you
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  Speaker 1: how its gravity affects the space around it. But they
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  Speaker 1: do not simply hand you a label and say, this
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  Speaker 1: is exactly what the object must be. So for now,
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  Speaker 1: you can let that thought rest quietly. 4.3 million suns
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  Speaker 1: gathered near the centre of your galaxy. S2 moving through
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  Speaker 1: the darkness. Thousands of years of light travelling toward you.
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  Speaker 1: And somewhere beyond all of that distance, something invisible continues
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  Speaker 1: to hold its place. You do not have to solve
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  Speaker 1: it tonight. There is nothing to figure out. You can
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  Speaker 1: simply remain here for a moment, floating quietly beneath the stars,
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  Speaker 1: knowing that even in the deepest darkness... Motion can reveal
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  Speaker 1: what the eye cannot see, and as the Milky Way
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  Speaker 1: turns slowly around you, the question becomes a little softer
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  Speaker 1: and a little more interesting. What, exactly, is sitting there
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  Speaker 1: at the centre? As you remain there, drifting quietly near
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  Speaker 1: the centre of the Milky Way, the stars continue along
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  Speaker 1: their paths around that hidden mass. Nothing dramatic happens. There
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  Speaker 1: is no sudden flash. no great movement, no sound coming
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  Speaker 1: through the darkness. There is only gravity, quietly shaping everything nearby.
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  Speaker 1: And for many years, this simple fact has offered astronomers
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  Speaker 1: a remarkably powerful clue. If something contains about 4.3 million
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  Speaker 1: times the mass of our Sun, and if that enormous
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  Speaker 1: amount of mass is confined to such a small region,
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  Speaker 1: then a supermassive black hole becomes the most natural explanation.
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  Speaker 1: It is an idea that fits remarkably well. A black
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  Speaker 1: hole is not simply an empty hole in space. It
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  Speaker 1: is a region where matter has become so concentrated that
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  Speaker 1: gravity becomes extraordinarily strong. Around a sufficiently massive black hole,
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  Speaker 1: there is a boundary known as the event horizon. You
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  Speaker 1: can imagine this boundary as a quiet point of no return.
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  Speaker 1: If light passes beyond it, the light cannot come back
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  Speaker 1: out to tell you what lies inside and because light
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  Speaker 1: cannot escape from within that boundary you cannot look directly
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  Speaker 1: at the black hole in the same way you might
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  Speaker 1: look at a star. You cannot simply photograph its surface.
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  Speaker 1: There is no ordinary surface waiting there glowing gently in
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  Speaker 1: the darkness. Instead, you watch what happens around it. You
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  Speaker 1: watch the stars. You watch clouds of gas. You watch
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  Speaker 1: the way light bends. You watch matter moving through the
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  Speaker 1: intense gravitational field. And slowly, from these surrounding movements, you
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  Speaker 1: build a picture of the invisible object. This is why
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  Speaker 1: Sagittarius A star is such a fascinating case. You do
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  Speaker 1: not see a giant black sphere sitting at the centre
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  Speaker 1: of the Milky Way. You see stars moving around an
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  Speaker 1: apparently empty region. You see their paths responding to something
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  Speaker 1: enormously massive. You see gas and dust responding to gravity.
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  Speaker 1: And when all of these observations are combined, the black
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  Speaker 1: hole interpretation becomes extraordinarily compelling. The central object has to
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  Speaker 1: be compact. It cannot simply be a loose collection of
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  Speaker 1: millions of ordinary stars. If all of that mass were
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  Speaker 1: spread over a much larger region... The nearby stars would
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  Speaker 1: move differently. The paths of S2 and the other S
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  Speaker 1: stars would not behave in quite the same way. So
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  Speaker 1: the observations gradually close the space available for other explanations.
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  Speaker 1: Something very massive must be there, and it must be
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  Speaker 1: very compact. That is the part we know with remarkable confidence.
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  Speaker 1: But there is a subtle difference between knowing what an
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  Speaker 1: object does and knowing exactly what the object is. You
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  Speaker 1: can know the gravity. You can measure the motion. You
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  Speaker 1: can estimate the mass. You can measure the size of
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  Speaker 1: the region in which that mass must be contained. And still,
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  Speaker 1: the deepest question can remain slightly beyond direct observation. What
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  Speaker 1: is actually there? This is why the phrase supermassive compact
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  Speaker 1: object is so interesting. It describes the observational fact without
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  Speaker 1: forcing every detail of the interpretation into a single word. Sagittarius,
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  Speaker 1: a star, is certainly a supermassive compact object. The question
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  Speaker 1: is whether that compact object must have an event horizon.
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  Speaker 1: That distinction is very small in language, but very large
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  Speaker 1: in physics. A true black hole has an event horizon.
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  Speaker 1: An alternative compact object could be incredibly dense without having
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  Speaker 1: that boundary. From far away, the difference might be surprisingly
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  Speaker 1: difficult to notice. Imagine that you are standing far out
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  Speaker 1: in space, watching an object surrounded by stars. You cannot
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  Speaker 1: see its surface. You cannot reach toward it, you can
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  Speaker 1: only observe the gravity it produces. If two different objects
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  Speaker 1: created almost the same gravitational field in the region where
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  Speaker 1: the stars are moving, then the stars might follow almost
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  Speaker 1: the same paths. They would not know what name you
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  Speaker 1: had given the object. They would simply respond to gravity.
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  Speaker 1: This is where alternative ideas begin to enter the story.
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  Speaker 1: Physicists have imagined other kinds of compact objects, including unusual
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  Speaker 1: configurations made from hypothetical particles or other exotic forms of matter.
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  Speaker 1: Some would have no event horizon, some would have no singularity.
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  Speaker 1: And yet, if they were sufficiently compact, they might imitate
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  Speaker 1: some of the behaviour expected from a black hole. This
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  Speaker 1: does not mean that Sagittarius A star is secretly something else.
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  Speaker 1: It does not mean that astronomers have discovered a mistake.
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  Speaker 1: And it certainly does not mean that the black hole
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  Speaker 1: interpretation has failed. The black hole model remains the leading explanation.
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  Speaker 1: The important point is simply that science sometimes leaves a
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  Speaker 1: small door open. That door exists because observations have limits.
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  Speaker 1: The closer you look, the more information you can gather.
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  Speaker 1: but there are always regions that remain hidden. For Sagittarius
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  Speaker 1: A star, the deepest region is hidden behind dust, distance,
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  Speaker 1: extreme gravity, and the fundamental difficulty of observing something from
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  Speaker 1: which light itself may not escape. There is another question
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  Speaker 1: here too. If Sagittarius A star really is a supermassive
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  Speaker 1: black hole, how did it become so large? A normal
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  Speaker 1: stellar black hole can form when a massive star reaches
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  Speaker 1: the end of its life and collapses. But Sagittarius A
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  Speaker 1: star contains the mass of millions of suns. It could
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  Speaker 1: not have simply appeared from the collapse of one ordinary star.
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  Speaker 1: It must have grown, perhaps through the gradual accumulation of matter,
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  Speaker 1: perhaps through the merging of smaller black holes, perhaps through
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  Speaker 1: some combination of processes that took place over billions of years.
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  Speaker 1: and the early universe presents an even quieter version of
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  Speaker 1: the same mystery. Astronomers have found very massive black holes
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  Speaker 1: at surprisingly early times in cosmic history. They appear to
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  Speaker 1: have grown enormously while the universe was still young. That
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  Speaker 1: does not make black holes impossible. But it leaves astronomers
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  Speaker 1: wondering how quickly the first massive black holes could have
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  Speaker 1: formed and grown. And so, as you float near the
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  Speaker 1: centre of the Milky Way, two questions begin to sit
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  Speaker 1: quietly beside one another. The first is familiar. Is Sagittarius
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  Speaker 1: A star a supermassive black hole? The evidence strongly suggests
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  Speaker 1: that it is. The second question is more delicate. Could
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  Speaker 1: something else, something extraordinarily dense and compact, create almost the
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  Speaker 1: same effects without possessing an event horizon? For a long time,
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  Speaker 1: this remained mostly a theoretical possibility. But then another mystery
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  Speaker 1: begins to enter the picture. Dark matter. You have already
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  Speaker 1: seen the stars moving around an invisible mass at the centre.
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  Speaker 1: Now imagine widening your view. Far beyond those stars, far
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  Speaker 1: beyond the bright disk of the Milky Way, there appears
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  Speaker 1: to be another invisible presence surrounding the entire galaxy. It
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  Speaker 1: does not announce itself with light, it does not glow,
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  Speaker 1: it does not form a visible surface. And yet, through gravity,
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  Speaker 1: it seems to be there. Perhaps, somewhere within this invisible substance,
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  Speaker 1: there is a clue to the object at the centre.
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  Speaker 1: Perhaps the thing hidden in the darkness is not completely
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  Speaker 1: separate from the darkness around it. And as you slowly
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  Speaker 1: drift outward from Sagittarius A star, leaving the fast-moving stars behind,
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  Speaker 1: the Milky Way begins to reveal another mystery, one that
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  Speaker 1: has been quietly surrounding the galaxy all along. For a
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  Speaker 1: little while, let Sagittarius, a star, remain behind you. You
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  Speaker 1: do not need to leave it completely. You can simply
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  Speaker 1: allow your attention to widen, slowly, as though the view
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  Speaker 1: in front of you is becoming larger and quieter. The
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  Speaker 1: stars around the galactic centre become smaller. The dense central
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  Speaker 1: region becomes a distant point. And gradually, the shape of
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  Speaker 1: the Milky Way begins to appear around you. A great
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  Speaker 1: spiral of stars, gas and dust. turning slowly through the darkness.
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  Speaker 1: From where you are now, the galaxy seems almost complete.
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  Speaker 1: You can see its glowing disk. You can see the
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  Speaker 1: countless stars gathered along its spiral arms. You can see
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  Speaker 1: regions where clouds of gas are forming new stars, and
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  Speaker 1: other regions where older stars have been travelling for billions
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  Speaker 1: of years. But there is something else here, something you
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  Speaker 1: cannot see. It does not shine. It does not reflect starlight.
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  Speaker 1: It does not form a bright disk around the galaxy.
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  Speaker 1: And yet, according to the motion of stars and galaxies,
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  Speaker 1: something invisible appears to be surrounding everything you can see.
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  Speaker 1: This is dark matter. The name sounds almost mysterious by itself,
• 18:24
  Speaker 1: but the basic idea is surprisingly quiet. Dark matter is
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  Speaker 1: not called dark because it is simply very far away,
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  Speaker 1: or because it is hidden behind ordinary dust. It is
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  Speaker 1: called dark because it does not appear to interact with
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  Speaker 1: light in the familiar ways. You cannot look up and
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  Speaker 1: see a dark matter star. You cannot point a telescope
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  Speaker 1: toward the sky and watch dark matter glowing. Instead, you
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  Speaker 1: notice its gravity. You notice what happens to things that
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  Speaker 1: move through it. You notice that galaxies rotate as though
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  Speaker 1: more mass is present than the visible stars and gas
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  Speaker 1: can account for. And you notice the same kind of
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  Speaker 1: quiet imbalance on much larger scales, where groups and clusters
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  Speaker 1: of galaxies move under the influence of more gravity than
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  Speaker 1: the luminous matter seems able to provide. The story goes
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  Speaker 1: back to the 1930s, when the astronomer Fritz Zwicky studied
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  Speaker 1: the motions of galaxies within the Coma Cluster. The galaxies
• 19:24
  Speaker 1: were moving too quickly for the visible matter alone to
• 19:28
  Speaker 1: keep the cluster together in the way that was expected.
• 19:32
  Speaker 1: Something unseen seemed to be contributing to additional gravity. At first,
• 19:38
  Speaker 1: the idea was only a possibility, but over the decades
• 19:42
  Speaker 1: more observations began to point in the same general direction.
• 19:46
  Speaker 1: Again and again, the visible universe seemed to be surrounded
• 19:50
  Speaker 1: by something that could not be seen directly. And when
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  Speaker 1: you look at the Milky Way itself, the picture becomes
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  Speaker 1: even more interesting. The stars you can see are only
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  Speaker 1: part of the galaxy. Far beyond the bright spiral disk,
• 20:04
  Speaker 1: there appears to be a vast region of invisible matter.
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  Speaker 1: Astronomers call this the dark matter halo. You can imagine
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  Speaker 1: it as a very faint, enormous sphere surrounding the galaxy,
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  Speaker 1: extending far beyond the region where most of the visible
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  Speaker 1: stars live. The stars are like the glowing surface you
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  Speaker 1: notice first. The dark matter is the much larger, invisible
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  Speaker 1: structure around them, and this halo may contain most of
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  Speaker 1: the matter in the galaxy. On the scale of the universe,
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  Speaker 1: Dark matter is thought to account for roughly 85% of
• 20:39
  Speaker 1: all matter. That is an extraordinary number. And yet, despite
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  Speaker 1: its apparent abundance, you still do not know exactly what
• 20:48
  Speaker 1: dark matter is. You know what it seems to do.
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  Speaker 1: You know that gravity responds to it. You know that
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  Speaker 1: it helps shape galaxies and the larger cosmic structures around them.
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  Speaker 1: But its physical identity remains unknown. Scientists have proposed many possibilities.
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  Speaker 1: Among the better-known ideas are particles called axions, and another family,
• 21:13
  Speaker 1: known as weakly interacting massive particles, often shortened to WIMPs.
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  Speaker 1: There are other possibilities too. The important thing is not
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  Speaker 1: the names. The important thing is that the universe seems
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  Speaker 1: to contain something real, something with gravitational influence, while its
• 21:31
  Speaker 1: microscopic nature remains hidden. And now, you can let your
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  Speaker 1: attention become smaller again. Much smaller. Imagine that the enormous
• 21:41
  Speaker 1: dark matter halo around the Milky Way is made from
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  Speaker 1: countless tiny particles. They drift through space. They gather under gravity.
• 21:50
  Speaker 1: They spread outward. They move through the galaxy without producing
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  Speaker 1: the familiar glow of stars. What if these particles belonged
• 21:58
  Speaker 1: to a special family? What if they were fermions? Fermions
• 22:02
  Speaker 1: are a broad class of particles. Some familiar particles in
• 22:06
  Speaker 1: ordinary matter, including electrons and the tiny building blocks inside atoms,
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  Speaker 1: belong to this family. And fermions obey a quiet rule
• 22:16
  Speaker 1: of quantum physics. Identical fermions cannot all occupy exactly the
• 22:22
  Speaker 1: same quantum state. You do not need to calculate anything
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  Speaker 1: to appreciate what this means. Just imagine a very large
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  Speaker 1: invisible crowd. If gravity tries to squeeze that crowd closer
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  Speaker 1: and closer together, the particles cannot all simply settle into
• 22:39
  Speaker 1: one identical arrangement. There is a kind of quantum resistance
• 22:43
  Speaker 1: to being compressed without limit. In ordinary matter, this principle
• 22:48
  Speaker 1: is part of the reason atoms have structure. It helps
• 22:51
  Speaker 1: keep electrons from simply collapsing into the same state. And now,
• 22:56
  Speaker 1: very gently, you can extend the idea into the darkness
• 23:00
  Speaker 1: around the Milky Way. If dark matter were made from fermions,
• 23:05
  Speaker 1: perhaps this same quantum rule could influence the way dark
• 23:08
  Speaker 1: matter gathers. Perhaps gravity would pull the particles inward, and perhaps,
• 23:14
  Speaker 1: at the same time, the quantum nature of those particles
• 23:17
  Speaker 1: would resist unlimited compression. Most of the time, the effect
• 23:22
  Speaker 1: could be almost invisible. Spread across the enormous halo of
• 23:26
  Speaker 1: a galaxy, it would simply contribute to to the gravitational
• 23:31
  Speaker 1: structure around the stars. But near the centre, where gravity
• 23:35
  Speaker 1: becomes much stronger and matter is allowed to gather more closely,
• 23:39
  Speaker 1: something different might happen. The invisible particles could become much denser.
• 23:45
  Speaker 1: A broad halo could gradually lead inward toward a much
• 23:49
  Speaker 1: more concentrated region. And if enough of these particles were
• 23:52
  Speaker 1: gathered together, the central region could become extraordinarily compact. You
• 23:58
  Speaker 1: can think of the galaxy, then, not as one bright
• 24:01
  Speaker 1: structure surrounded by something completely separate, but as a layered arrangement,
• 24:07
  Speaker 1: a diffuse, invisible halo far from the centre, a denser
• 24:11
  Speaker 1: region closer in, and perhaps, at the very heart, an
• 24:15
  Speaker 1: incredibly compact core. This is where the quiet mystery surrounding
• 24:20
  Speaker 1: Sagittarius A-star begins to connect with the much larger mystery
• 24:25
  Speaker 1: of dark matter. The object at the centre of the
• 24:28
  Speaker 1: Milky Way may be one question. The invisible halo surrounding
• 24:33
  Speaker 1: the galaxy may be another. But if dark matter is
• 24:36
  Speaker 1: made from fermions, perhaps the two questions do not have
• 24:40
  Speaker 1: to remain separate. Perhaps the same invisible substance could exist
• 24:44
  Speaker 1: in two very different forms, a wide, gentle distribution across
• 24:49
  Speaker 1: the galaxy, and a dense, concentrated structure at its centre.
• 24:54
  Speaker 1: You can let that possibility rest for a moment. There
• 24:57
  Speaker 1: is no need to decide whether it is true. The
• 25:00
  Speaker 1: black hole interpretation remains the leading explanation for Sagittarius A star.
• 25:06
  Speaker 1: Dark matter has not been proven to be made of fermions,
• 25:10
  Speaker 1: and the proposed central core remains a hypothesis. But it
• 25:14
  Speaker 1: is an intriguing hypothesis because it connects two mysteries that
• 25:18
  Speaker 1: have spent so long living beside one another. One mystery
• 25:21
  Speaker 1: sits at the heart of the Milky Way. The other
• 25:24
  Speaker 1: surrounds the entire galaxy. And perhaps, somewhere in the darkness
• 25:29
  Speaker 1: between the two, there is a quiet connection waiting to
• 25:33
  Speaker 1: be understood. As you continue drifting through the darkness around
• 25:38
  Speaker 1: the Milky Way, the enormous invisible halo remains almost impossible
• 25:43
  Speaker 1: to see. There is no bright edge, no glowing surface,
• 25:48
  Speaker 1: no boundary that your eyes could follow. And yet, if
• 25:51
  Speaker 1: you could somehow sense gravity itself, You might feel that
• 25:55
  Speaker 1: the galaxy is surrounded by something vast, something quietly spread
• 26:00
  Speaker 1: through an enormous region of space. Far away from the centre,
• 26:04
  Speaker 1: this invisible matter can remain diffuse. The particles are separated
• 26:09
  Speaker 1: by great distances, moving slowly through the galaxy, following the
• 26:14
  Speaker 1: gentle pull of gravity. But as you move inward, toward
• 26:18
  Speaker 1: the galactic centre, the story can become different. Gravity becomes stronger.
• 26:25
  Speaker 1: Matter can gather more closely, and if the invisible particles
• 26:29
  Speaker 1: are fermions, there is another quiet influence that begins to matter.
• 26:33
  Speaker 1: The particles cannot all simply occupy the same quantum state.
• 26:38
  Speaker 1: That rule, which seemed almost abstract when you first encountered it,
• 26:42
  Speaker 1: can have a very physical consequence. It means that as
• 26:46
  Speaker 1: gravity tries to squeeze fermionic matter into a smaller and
• 26:50
  Speaker 1: smaller region, the particles begin to resist being packed together
• 26:55
  Speaker 1: without limit. You can imagine the difference very softly. Gravity
• 27:00
  Speaker 1: is always asking the matter to move inward. The quantum
• 27:04
  Speaker 1: nature of the fermions is quietly answering, not all the way,
• 27:08
  Speaker 1: not infinitely close, not into a single identical state. There
• 27:13
  Speaker 1: is a limit to how tightly the particles can be arranged.
• 27:17
  Speaker 1: This effect is sometimes described using the language of quantum pressure,
• 27:22
  Speaker 1: but you do not need to picture a hard surface
• 27:25
  Speaker 1: or an ordinary pressure pushing outward. It is more subtle
• 27:29
  Speaker 1: than that. It comes from the quantum rules governing the
• 27:32
  Speaker 1: particles themselves, and under suitable conditions that quiet resistance could
• 27:37
  Speaker 1: become important on an enormous scale. Instead of allowing the
• 27:41
  Speaker 1: dark matter to collapse without limit, the fermions could settle
• 27:45
  Speaker 1: into an extremely dense configuration, a compact core. a region
• 27:50
  Speaker 1: where an enormous amount of invisible matter is gathered together,
• 27:54
  Speaker 1: yet where the particles have not been compressed into an
• 27:57
  Speaker 1: infinitely small point. This possibility changes the picture of Sagittarius
• 28:02
  Speaker 1: A star. In the familiar black hole picture, gravity has
• 28:06
  Speaker 1: compressed matter so strongly that an event horizon forms. Beyond
• 28:11
  Speaker 1: that boundary, light cannot return. Deep within the classical description,
• 28:17
  Speaker 1: the black hole contains a singularity, a place where the
• 28:21
  Speaker 1: usual description of space and gravity reaches an extreme limit.
• 28:25
  Speaker 1: But the hypothetical fermionic core would be different. It could
• 28:30
  Speaker 1: be extraordinarily dense without having an event horizon. It could
• 28:34
  Speaker 1: be compact without becoming a black hole. And it could
• 28:37
  Speaker 1: contain a tremendous amount of mass while remaining, in principle,
• 28:42
  Speaker 1: an extended object rather than a single mathematical point. From
• 28:47
  Speaker 1: far away, however... the difference might be difficult to notice.
• 28:51
  Speaker 1: This is the important part. The stars orbiting Sagittarius. A
• 28:56
  Speaker 1: star do not care what name you give to the
• 28:58
  Speaker 1: object in the centre. They respond to gravity. If a
• 29:02
  Speaker 1: dense fermionic core produces nearly the same gravitational influence in
• 29:07
  Speaker 1: the region where those stars travel, then their orbits can
• 29:10
  Speaker 1: look remarkably similar. The same enormous central mass can guide them.
• 29:15
  Speaker 1: The same deep gravitational well can shape their paths. The
• 29:19
  Speaker 1: same invisible centre can remain surrounded by rapidly moving stars.
• 29:24
  Speaker 1: And so, from a great distance, two very different objects
• 29:29
  Speaker 1: could begin to resemble one another. One would be a
• 29:32
  Speaker 1: black hole. The other would be an extraordinarily dense core
• 29:36
  Speaker 1: of hypothetical dark matter. One would possess an event horizon.
• 29:41
  Speaker 1: The other would not. One would contain a singularity in
• 29:45
  Speaker 1: the classical picture. The other would remain a finite extended structure.
• 29:50
  Speaker 1: Yet the stars around them might trace almost the same paths.
• 29:54
  Speaker 1: This is where the idea becomes particularly interesting. The fermionic
• 29:58
  Speaker 1: core model is does not necessarily ask you to replace
• 30:02
  Speaker 1: one strange object with another unrelated strange object. Instead, it
• 30:08
  Speaker 1: suggests that the central core could be connected to the
• 30:11
  Speaker 1: much larger dark matter halo around the Milky Way. Imagine
• 30:15
  Speaker 1: beginning far from the galactic centre. There, the dark matter
• 30:20
  Speaker 1: is spread thinly across a huge region. You move inward.
• 30:25
  Speaker 1: The density gradually increases. you continue toward the centre. The
• 30:30
  Speaker 1: invisible matter becomes more concentrated, and eventually, at the heart
• 30:35
  Speaker 1: of the galaxy, the distribution becomes extremely dense. In this picture,
• 30:41
  Speaker 1: there is no sharp conceptual separation between the halo and
• 30:44
  Speaker 1: the core. They are different regions of the same hypothetical
• 30:48
  Speaker 1: fermionic structure. The outer region is diffuse, the inner region
• 30:53
  Speaker 1: is dense, and at the very centre, sits the compact
• 30:57
  Speaker 1: object that we currently call Sagittarius A-star. This is one
• 31:02
  Speaker 1: of the most appealing features of the idea. The universe
• 31:06
  Speaker 1: would not need one mysterious substance to explain the dark
• 31:09
  Speaker 1: halo and another completely separate explanation for the central object.
• 31:15
  Speaker 1: The same hypothetical particles could, in principle, create both. The
• 31:20
  Speaker 1: galaxy would have a kind of invisible architecture. a broad
• 31:24
  Speaker 1: outer presence surrounding the stars, a denser inner region, and
• 31:29
  Speaker 1: a deeply concentrated core at the centre. This family of
• 31:33
  Speaker 1: ideas has been explored in what is known as the
• 31:36
  Speaker 1: Ruffini-Arguelles-Rueda framework. You do not need to remember that name.
• 31:42
  Speaker 1: What matters is the picture. A fermionic dark matter halo
• 31:47
  Speaker 1: surrounding a much denser central core. and that core could
• 31:51
  Speaker 1: become massive enough to resemble a supermassive black hole when
• 31:54
  Speaker 1: viewed from far away. There is something almost peaceful about
• 31:59
  Speaker 1: this possibility. The same invisible particles that are spread across
• 32:03
  Speaker 1: the galaxy could be quietly gathering at its centre, not suddenly,
• 32:08
  Speaker 1: not violently, but through the slow influence of gravity over
• 32:12
  Speaker 1: immense stretches of cosmic time. The galaxy itself becomes a
• 32:17
  Speaker 1: place of gradual concentration. Outward, the dark matter is diffuse. Inward,
• 32:23
  Speaker 1: it becomes denser. Closer still, it becomes extremely compact. And there,
• 32:29
  Speaker 1: beneath the darkness at the heart of the Milky Way,
• 32:32
  Speaker 1: the hypothetical core waits. If this picture were correct, it
• 32:37
  Speaker 1: would mean that Sagittarius A star is not an isolated mystery.
• 32:42
  Speaker 1: It would be the densest part of something much larger.
• 32:46
  Speaker 1: The central object and the surrounding halo would be connected,
• 32:50
  Speaker 1: The darkness at the centre would belong to the same
• 32:53
  Speaker 1: invisible structure as the darkness surrounding the galaxy. But there
• 32:58
  Speaker 1: is an important question still waiting. A beautiful model is
• 33:02
  Speaker 1: not enough. A model has to make predictions. It has
• 33:06
  Speaker 1: to survive observation. And the most natural place to begin
• 33:10
  Speaker 1: testing this idea is with the very stars that first
• 33:14
  Speaker 1: revealed the enormous mass at the centre. S2 is still there.
• 33:18
  Speaker 1: Its orbit is still being measured. The strange G-objects are
• 33:22
  Speaker 1: still moving through the same gravitational field. And each small
• 33:27
  Speaker 1: movement offers another opportunity to ask whether the invisible centre
• 33:31
  Speaker 1: is truly a black hole, or whether something quieter, something
• 33:35
  Speaker 1: made from fermionic dark matter, could be guiding the stars instead.
• 33:41
  Speaker 1: For now, you can simply let that possibility drift beside you.
• 33:45
  Speaker 1: A vast invisible halo, a dense invisible core, and between them,
• 33:51
  Speaker 1: perhaps no dividing line at all, only a gradual change
• 33:55
  Speaker 1: in density, unfolding across the enormous darkness of the Milky Way.
• 34:03
  Speaker 1: You can return now to the stars. After drifting through
• 34:06
  Speaker 1: the enormous invisible halo, after moving inward toward the dense
• 34:11
  Speaker 1: hypothetical core, You can let your attention settle once again
• 34:15
  Speaker 1: on the small points of light moving through the darkness
• 34:18
  Speaker 1: around Sagittarius A star. There is something comforting about returning
• 34:23
  Speaker 1: to the same stars. They have not changed their paths
• 34:27
  Speaker 1: simply because you have been thinking about them. S2 is
• 34:30
  Speaker 1: still moving. The other stars are still moving, and the
• 34:34
  Speaker 1: gravity at the centre is still quietly guiding them. This
• 34:37
  Speaker 1: is useful, because if you want to know what the
• 34:40
  Speaker 1: hidden object really is, You can ask the stars. Not
• 34:44
  Speaker 1: by asking them directly, of course. They cannot tell you
• 34:47
  Speaker 1: whether they are orbiting a black hole or something else.
• 34:51
  Speaker 1: But their movements can carry the answer. Every orbit contains information.
• 34:56
  Speaker 1: The speed of a star. The shape of its path.
• 34:59
  Speaker 1: The distance it reaches from the centre. The time it
• 35:02
  Speaker 1: takes to complete its journey. Even the tiny changes that
• 35:06
  Speaker 1: appear from one orbit to another can reveal something about
• 35:10
  Speaker 1: the invisible mass beneath them. and this is where the
• 35:13
  Speaker 1: idea of a fermionic dark matter core becomes more than
• 35:17
  Speaker 1: a beautiful possibility. It has to reproduce what the stars
• 35:21
  Speaker 1: actually do. The first important star is S2. You have
• 35:26
  Speaker 1: already watched it travel around Sagittarius, a star in less
• 35:31
  Speaker 1: than 16 years. Its orbit brings it relatively close to
• 35:34
  Speaker 1: the galactic centre, making it one of the most useful
• 35:38
  Speaker 1: natural tracers of the gravity in that region. If Sagittarius
• 35:43
  Speaker 1: A star is a black hole, the path of S2
• 35:46
  Speaker 1: can be calculated. If Sagittarius A star is instead a
• 35:50
  Speaker 1: dense fermionic core, the path can also be calculated. And then,
• 35:56
  Speaker 1: very quietly, you can place the two predictions beside each other.
• 36:01
  Speaker 1: What happens? Would the star suddenly move along a completely
• 36:04
  Speaker 1: different path? Would the two models separate clearly? At least
• 36:09
  Speaker 1: in the observations tested so far, The answer is no.
• 36:13
  Speaker 1: The predicted motions can be remarkably similar. The difference is
• 36:17
  Speaker 1: small enough that current measurements do not easily tell the
• 36:21
  Speaker 1: two possibilities apart. And that is an important kind of
• 36:25
  Speaker 1: scientific result. It is not a discovery that says, this
• 36:29
  Speaker 1: is definitely a dark matter core. It is something more careful.
• 36:34
  Speaker 1: It says that the dark matter core model can imitate
• 36:37
  Speaker 1: the gravitational behaviour expected from a black hole surprisingly well
• 36:41
  Speaker 1: in the region that has been tested. Imagine drawing two
• 36:45
  Speaker 1: paths across a dark sheet. One path represents the black
• 36:49
  Speaker 1: hole prediction. The other represents the fermionic core prediction. You
• 36:54
  Speaker 1: might expect the lines to separate dramatically. Instead, they can
• 36:58
  Speaker 1: lie almost on top of each other, so close that,
• 37:02
  Speaker 1: at the precision of the observations being considered, distinguishing them
• 37:06
  Speaker 1: becomes extremely difficult. The difference in the tested orbital behaviour
• 37:11
  Speaker 1: is less than 1%. You do not need to hold
• 37:14
  Speaker 1: that number in your mind. It is enough to imagine
• 37:17
  Speaker 1: two almost identical journeys through space. Two curves traced around
• 37:22
  Speaker 1: the same invisible centre. Two possibilities producing almost the same motion.
• 37:28
  Speaker 1: And this is where another group of unusual objects becomes important.
• 37:33
  Speaker 1: They are known as the G-objects. They were discovered in
• 37:36
  Speaker 1: the early years of the 21st century, and they are
• 37:40
  Speaker 1: among the stranger residents of the galactic centre. At first glance,
• 37:44
  Speaker 1: they can look somewhat like ordinary stars, but when astronomers
• 37:49
  Speaker 1: observe them more carefully, their appearance can be much less certain.
• 37:54
  Speaker 1: Some seem surrounded by gas and dust. Some behave as
• 37:58
  Speaker 1: though they contain a compact stellar object inside a larger
• 38:02
  Speaker 1: cloud of material. and their shapes can change as they
• 38:05
  Speaker 1: travel around Sagittarius A star. They are not simple points
• 38:10
  Speaker 1: of light following perfectly neat paths. They can stretch. They
• 38:15
  Speaker 1: can become more extended. They can appear almost cloud-like, and
• 38:20
  Speaker 1: that makes them especially interesting, because now, instead of testing
• 38:24
  Speaker 1: a model against only one star, astronomers can ask, whether
• 38:28
  Speaker 1: it can describe the motions of several different objects moving
• 38:31
  Speaker 1: through the same intense gravitational environment. The 2026 study considered
• 38:37
  Speaker 1: S2 together with five known G objects. The researchers compared
• 38:42
  Speaker 1: their observed behaviour with predictions from the familiar black hole picture,
• 38:47
  Speaker 1: and with predictions from a model containing a dense fermionic
• 38:51
  Speaker 1: dark matter core. Again, the difference remained remarkably small. The
• 38:56
  Speaker 1: alternative model was able to reproduce the tested orbital behaviour,
• 39:00
  Speaker 1: with differences of less than 1% from the black hole model.
• 39:05
  Speaker 1: That does not mean that the stars are secretly orbiting
• 39:07
  Speaker 1: dark matter. It means something quieter. The available observations have
• 39:13
  Speaker 1: not yet created a large enough difference between the predictions
• 39:16
  Speaker 1: to make the answer obvious. And perhaps that is one
• 39:19
  Speaker 1: of the most interesting things about the centre of the
• 39:22
  Speaker 1: Milky Way. you are looking at an environment where nature
• 39:25
  Speaker 1: has created conditions so extreme that two very different theories
• 39:30
  Speaker 1: objects can begin to look almost identical from a distance.
• 39:35
  Speaker 1: One has an event horizon. One does not. One is
• 39:38
  Speaker 1: a black hole. The other is a hypothetical concentration of
• 39:42
  Speaker 1: invisible fermions. And yet, to S2, the gravitational pull may
• 39:47
  Speaker 1: be almost indistinguishable, To a G object drifting through the region,
• 39:52
  Speaker 1: the difference may also be very difficult to notice. The
• 39:56
  Speaker 1: stars simply move, they follow the gravity, they trace their
• 40:00
  Speaker 1: quiet paths through space. This reminds you of something important
• 40:04
  Speaker 1: about astronomy. Sometimes the universe does not give you a
• 40:07
  Speaker 1: direct view of the thing you want to understand. Instead,
• 40:11
  Speaker 1: it gives you motion. It gives you light. It gives
• 40:14
  Speaker 1: you timing. It gives you small changes repeated over many years.
• 40:19
  Speaker 1: and from those small changes you slowly build an understanding
• 40:23
  Speaker 1: of something hidden. The G-objects make this even more interesting,
• 40:27
  Speaker 1: because they are mysterious in their own right. Their changing
• 40:31
  Speaker 1: shapes remind you that the galactic centre is not an empty,
• 40:35
  Speaker 1: perfectly ordered place. It is filled with stars, gas, dust, radiation,
• 40:42
  Speaker 1: and gravity, all moving together in a region that is
• 40:46
  Speaker 1: difficult to observe. Yet, Beneath all that complexity, the central
• 40:52
  Speaker 1: mass remains remarkably consistent. Whatever it is, it has enough
• 40:56
  Speaker 1: gravity to guide these objects. It has enough mass to
• 41:00
  Speaker 1: shape their paths. And it remains concentrated in a tiny region.
• 41:05
  Speaker 1: The black hole model explains this naturally. The fermionic core
• 41:09
  Speaker 1: model attempts to explain it in another way. For now,
• 41:13
  Speaker 1: the stars have not clearly chosen between them. And perhaps
• 41:16
  Speaker 1: you can let that uncertainty feel peaceful rather than frustrating.
• 41:21
  Speaker 1: You do not need an answer immediately. Science does not
• 41:24
  Speaker 1: always move by replacing one idea with another overnight. Sometimes
• 41:28
  Speaker 1: it moves by making the measurements more precise. By watching
• 41:32
  Speaker 1: one orbit again. By watching another object. By waiting for
• 41:37
  Speaker 1: a small difference to become large enough to see. S2
• 41:41
  Speaker 1: continues its journey. The G-objects continue drifting through the galactic centre,
• 41:46
  Speaker 1: and somewhere beneath their paths, the invisible object remains quiet.
• 41:52
  Speaker 1: Perhaps it is a black hole. Perhaps, in a much
• 41:55
  Speaker 1: stranger possibility, it is a dense core of dark matter.
• 42:00
  Speaker 1: From where you are floating now, the two possibilities still overlap.
• 42:05
  Speaker 1: Their paths almost touch. Their predictions almost agree. And the
• 42:10
  Speaker 1: Milky Way keeps turning gently around them, carrying this small
• 42:15
  Speaker 1: unanswered question through the darkness. You can let the stars
• 42:21
  Speaker 1: near the galactic centre become smaller now. S2 is still
• 42:25
  Speaker 1: moving through its quiet orbit. The G objects are still
• 42:29
  Speaker 1: drifting through the crowded region around Sagittarius A star. But
• 42:33
  Speaker 1: your view does not need to remain there. Slowly, gently,
• 42:38
  Speaker 1: you can widen it again. The central darkness becomes a
• 42:41
  Speaker 1: small point. The surrounding stars spread outward. The spiral arms
• 42:46
  Speaker 1: begin to appear. And before long, you are looking at
• 42:50
  Speaker 1: the Milky Way as a whole. From this distance, the
• 42:53
  Speaker 1: galaxy seems beautifully complete. A bright disk of stars and
• 42:57
  Speaker 1: glowing clouds, turning slowly through space. But most of what
• 43:02
  Speaker 1: surrounds it is still invisible. There is the familiar stellar disk,
• 43:06
  Speaker 1: where much of the light comes from. There are enormous
• 43:09
  Speaker 1: clouds of gas and dust. There are older stars moving
• 43:13
  Speaker 1: through the outer regions. And around all of this, there
• 43:17
  Speaker 1: is the much larger dark matter halo. You cannot see
• 43:20
  Speaker 1: the halo directly. There is no glowing border around it.
• 43:24
  Speaker 1: There is no surface that would tell you where it ends.
• 43:27
  Speaker 1: Yet its gravity influences the movement of the visible galaxy.
• 43:32
  Speaker 1: And this gives astronomers another way to investigate it. Instead
• 43:36
  Speaker 1: of looking for the dark matter itself, You can watch
• 43:39
  Speaker 1: the stars. The stars become tracers. Their movements become clues.
• 43:45
  Speaker 1: Their speeds tell you something about the invisible mass surrounding them.
• 43:50
  Speaker 1: This is where the Gaia mission becomes especially important. Gaia
• 43:54
  Speaker 1: is designed to measure the positions and motions of enormous
• 43:57
  Speaker 1: numbers of stars throughout the Milky Way. You can imagine
• 44:01
  Speaker 1: it almost as watching an entire galaxy in slow motion.
• 44:05
  Speaker 1: Not every star is moving in exactly the same way.
• 44:08
  Speaker 1: Some are close to the centre, some are far away.
• 44:12
  Speaker 1: Some move through the spiral arms. Others travel through the
• 44:15
  Speaker 1: quieter regions between them. Together, their movements create a kind
• 44:20
  Speaker 1: of enormous gravitational map. The interesting part is what happens
• 44:25
  Speaker 1: as you travel farther from the centre. You might expect
• 44:28
  Speaker 1: the outer regions of the galaxy to continue rotating in
• 44:31
  Speaker 1: a simple way, but observations reveal a more subtle pattern.
• 44:36
  Speaker 1: The outer parts show a decline in rotational speed. This
• 44:40
  Speaker 1: is sometimes described as a Keplerian decline. The phrase sounds technical,
• 44:46
  Speaker 1: but the basic picture is gentle. As you move farther
• 44:49
  Speaker 1: outward from the centre, the stars begin to move more slowly,
• 44:53
  Speaker 1: and this behaviour gives you another clue about how matter
• 44:56
  Speaker 1: is distributed through the Milky Way. The observation is especially
• 45:01
  Speaker 1: interesting when you return to the idea of fermionic dark matter.
• 45:06
  Speaker 1: Remember the quiet quantum rule from earlier. Identical fermions cannot
• 45:11
  Speaker 1: all occupy exactly the same quantum state. That rule does
• 45:15
  Speaker 1: not disappear simply because you have moved from the tiny
• 45:18
  Speaker 1: world of particles to the enormous scale of a galaxy.
• 45:22
  Speaker 1: If dark matter were made of fermions, their quantum nature
• 45:26
  Speaker 1: could influence how densely they gather. Instead of forming an
• 45:29
  Speaker 1: entirely smooth, unlimited cloud extending outward without any clear structure,
• 45:35
  Speaker 1: the fermionic particles could develop a more defined distribution. The
• 45:40
  Speaker 1: halo could have a sharper outer boundary, not a hard wall,
• 45:45
  Speaker 1: not a visible edge, just a region where the density
• 45:48
  Speaker 1: gradually changes more noticeably, and that changing density could influence
• 45:53
  Speaker 1: the motion of stars near the outer parts of the galaxy.
• 45:56
  Speaker 1: This is one reason the Gaia observations are interesting in
• 46:00
  Speaker 1: the context of the fermionic model. The model can produce
• 46:03
  Speaker 1: a halo that whose structure is more clearly defined than
• 46:07
  Speaker 1: some other possibilities. And a more defined halo can be
• 46:11
  Speaker 1: associated with a gradual change in the gravitational influence felt
• 46:15
  Speaker 1: by stars as they move farther from the centre. So
• 46:18
  Speaker 1: when Gaia observes the motion of those distant stars, it
• 46:22
  Speaker 1: is doing something more profound than simply drawing a map
• 46:26
  Speaker 1: of where the stars are. It is tracing the distribution
• 46:29
  Speaker 1: of gravity across the Milky Way. Every orbit is a
• 46:33
  Speaker 1: small clue, Every velocity is another clue. Every tiny change
• 46:38
  Speaker 1: in motion tells you something about the invisible structure surrounding
• 46:42
  Speaker 1: the visible galaxy. You can imagine standing far above the
• 46:46
  Speaker 1: Milky Way now. The galaxy below you is glowing softly.
• 46:51
  Speaker 1: The spiral arms stretch outward. The centre is bright with
• 46:54
  Speaker 1: countless stars. And around the entire disk, there is a
• 46:58
  Speaker 1: vast region you cannot see. If the fermionic model is correct,
• 47:03
  Speaker 1: This invisible region is not simply an empty background. It
• 47:07
  Speaker 1: is filled with dark fermions. They are spread across an
• 47:11
  Speaker 1: enormous volume. Far from the centre, they remain diffuse. Closer
• 47:16
  Speaker 1: to the centre, their density increases, and deep inside, they
• 47:21
  Speaker 1: may become concentrated enough to form the compact core we
• 47:25
  Speaker 1: have been considering. The picture begins to feel surprisingly continuous.
• 47:30
  Speaker 1: You no longer have to imagine a black hole sitting
• 47:33
  Speaker 1: in the middle of one completely separate dark matter halo. Instead,
• 47:38
  Speaker 1: you can imagine a single hypothetical structure, a diffuse outer region,
• 47:43
  Speaker 1: a denser inner region, and an extraordinarily concentrated centre, the
• 47:49
  Speaker 1: same invisible substance appearing at different densities across different distances.
• 47:54
  Speaker 1: It is a simple picture in a universe that often
• 47:56
  Speaker 1: feels complicated, but simplicity is not the same thing as proof.
• 48:01
  Speaker 1: The decline in the outer rotation of the Milky Way
• 48:04
  Speaker 1: does not demonstrate that dark matter is made of fermions.
• 48:07
  Speaker 1: Gaia did not discover a fermionic halo. The observation is
• 48:11
  Speaker 1: simply one more piece of evidence that different models must
• 48:15
  Speaker 1: attempt to reproduce. And this distinction matters. A scientific model
• 48:20
  Speaker 1: becomes interesting when it can explain observations. It becomes stronger
• 48:25
  Speaker 1: when it can explain many different observations at once. but
• 48:29
  Speaker 1: it becomes established only when evidence continues to support it,
• 48:33
  Speaker 1: while competing explanations fail. For now, that final step has
• 48:38
  Speaker 1: not happened. The standard dark matter picture remains an important
• 48:43
  Speaker 1: part of modern cosmology. And the idea that Sagittarius A-star
• 48:48
  Speaker 1: is a supermassive black hole remains the leading interpretation of
• 48:52
  Speaker 1: the galactic centre. The fermionic model is something more tentative,
• 48:57
  Speaker 1: an alternative. a possibility that seems able to connect several
• 49:02
  Speaker 1: observations in an unexpectedly coherent way. And perhaps that is
• 49:07
  Speaker 1: why the idea is worth following a little farther. Because
• 49:10
  Speaker 1: if the same hypothetical particles can describe the central object
• 49:14
  Speaker 1: and the outer halo, then the Milky Way itself begins
• 49:18
  Speaker 1: to look different. The centre and the edge are no
• 49:21
  Speaker 1: longer completely separate stories. The dense core is connected to
• 49:25
  Speaker 1: the diffuse halo. The fast-moving stars near Sagittarius A star
• 49:31
  Speaker 1: are connected, through gravity, to the slower-moving stars much farther out,
• 49:36
  Speaker 1: and the tiny quantum rule governing invisible particles may somehow
• 49:40
  Speaker 1: echo across tens of thousands of light-years. You can let
• 49:44
  Speaker 1: your view become wider still. The Milky Way turns beneath you.
• 49:49
  Speaker 1: The visible stars seem like a thin layer of light.
• 49:52
  Speaker 1: The invisible halo extends much farther. And somewhere in that
• 49:56
  Speaker 1: enormous darkness, the same hypothetical substance could be quietly changing
• 50:01
  Speaker 1: density from one region to another. At the very centre,
• 50:05
  Speaker 1: perhaps a dense core. Farther away, perhaps a broad and
• 50:10
  Speaker 1: gentle halo, and between them nothing sudden, just a gradual transition,
• 50:16
  Speaker 1: a quiet structure stretching across the galaxy. But there is
• 50:19
  Speaker 1: another way to look at Sagittarius A star. So far
• 50:23
  Speaker 1: you have been listening to what gravity tells you through motion.
• 50:27
  Speaker 1: Now you can begin to wonder what light might reveal.
• 50:31
  Speaker 1: Because near the centre of the Milky Way, light itself
• 50:34
  Speaker 1: is forced to take unusual paths. and when light bends
• 50:38
  Speaker 1: around something this massive, even the darkness can begin to
• 50:42
  Speaker 1: acquire a shape. The Milky Way continues to turn quietly
• 50:48
  Speaker 1: beneath you. From this great distance, the galaxy looks almost peaceful,
• 50:53
  Speaker 1: a broad river of starlight moving through an even greater darkness.
• 50:58
  Speaker 1: But somewhere deep within that glowing spiral is the small
• 51:01
  Speaker 1: region you have been approaching all this time. Sagittarius, a star.
• 51:07
  Speaker 1: You cannot see it with your eyes from here. The
• 51:09
  Speaker 1: centre of the galaxy is hidden behind enormous amounts of dust,
• 51:14
  Speaker 1: and the object itself gives you no ordinary surface to
• 51:17
  Speaker 1: look at. So astronomers have learned to observe it in
• 51:20
  Speaker 1: another way. They listen to the light around it. They
• 51:24
  Speaker 1: watch the material moving nearby, and eventually they built something remarkable,
• 51:30
  Speaker 1: an instrument that allowed them to see the region surrounding
• 51:33
  Speaker 1: Sagittarius A star with extraordinary detail. It is called the
• 51:39
  Speaker 1: Event Horizon Telescope. Despite its name, it is not a
• 51:43
  Speaker 1: single telescope sitting somewhere on Earth. It is a collection
• 51:47
  Speaker 1: of radio observatories working together, using the separation between them
• 51:52
  Speaker 1: to create an observing system with an effective size comparable
• 51:57
  Speaker 1: to the Earth itself. You can imagine several distant observatories
• 52:01
  Speaker 1: quietly looking toward the same tiny region of sky, their
• 52:06
  Speaker 1: measurements combined into one much sharper view. And in 2022,
• 52:11
  Speaker 1: this effort produced the first image of Sagittarius A star.
• 52:15
  Speaker 1: The image is not what you might imagine when you
• 52:17
  Speaker 1: hear the word photograph. There is no bright black sphere
• 52:21
  Speaker 1: floating against a perfectly empty background. Instead, you see a
• 52:26
  Speaker 1: glowing ring-like structure surrounding a darker centre. The glowing material
• 52:31
  Speaker 1: is hot, very hot. Gas and plasma are are moving
• 52:36
  Speaker 1: in the extreme gravitational environment around the central object, releasing
• 52:40
  Speaker 1: radio waves that can eventually be detected here on Earth.
• 52:44
  Speaker 1: The central darkness is sometimes described as the shadow of
• 52:48
  Speaker 1: Sagittarius A star, and when you look at it, the
• 52:51
  Speaker 1: familiar black hole picture becomes very easy to imagine. A
• 52:55
  Speaker 1: dark centre, bright material around it, light bending through an
• 53:00
  Speaker 1: extraordinarily strong gravitational field. everything seems to point toward a
• 53:05
  Speaker 1: black hole. And indeed, the image is entirely consistent with
• 53:09
  Speaker 1: that interpretation. But there is another subtle question. What exactly
• 53:14
  Speaker 1: does the image prove? The image does not simply show
• 53:17
  Speaker 1: you an event horizon, as though you were looking at
• 53:20
  Speaker 1: the surface of a dark planet. The event horizon itself
• 53:25
  Speaker 1: is a boundary. It is the region beyond which light
• 53:28
  Speaker 1: cannot return. What you are seeing is the behaviour of
• 53:32
  Speaker 1: light and glowing material around the compact object. Gravity bends
• 53:36
  Speaker 1: the paths of that light. The hot material shines around
• 53:40
  Speaker 1: the central darkness. Together, these effects create the appearance captured
• 53:45
  Speaker 1: by the Event Horizon Telescope. And this is where the
• 53:49
  Speaker 1: alternative model becomes interesting again. If Sagittarius Estar were a
• 53:54
  Speaker 1: dense fermionic dark matter core rather than a black hole,
• 53:58
  Speaker 1: there would be no Event Horizon. there would be no
• 54:01
  Speaker 1: one-way boundary swallowing every ray of light that crossed it.
• 54:05
  Speaker 1: And yet, if the core were sufficiently dense, its gravity
• 54:09
  Speaker 1: could still bend light very strongly. The surrounding gas could
• 54:13
  Speaker 1: still become extremely hot. The light could still be redirected
• 54:17
  Speaker 1: around the central object. And from far enough away, the
• 54:21
  Speaker 1: resulting image could look surprisingly similar. Researchers have explored whether
• 54:26
  Speaker 1: this kind of dense fermionic core could reproduce the broad
• 54:30
  Speaker 1: appearance of the observed Sagittarius A star image? The answer,
• 54:35
  Speaker 1: within the models studied, is that a similar dark central
• 54:39
  Speaker 1: region surrounded by glowing material can be possible. Again, this
• 54:44
  Speaker 1: does not mean the Event Horizon Telescope discovered a dark
• 54:47
  Speaker 1: matter core. It means the image alone may not provide
• 54:51
  Speaker 1: every detail needed to distinguish the two possibilities. You can
• 54:55
  Speaker 1: think of it as watching two different landscapes from very
• 54:59
  Speaker 1: far away. From a great distance, they might have the
• 55:02
  Speaker 1: same outline. Only when you move closer do the small
• 55:06
  Speaker 1: differences become visible. And perhaps the same is true here.
• 55:10
  Speaker 1: A black hole and a dense fermionic core could create
• 55:14
  Speaker 1: similar large-scale patterns of light. But deep inside those patterns,
• 55:19
  Speaker 1: their physics would be different. One would contain an event horizon,
• 55:23
  Speaker 1: the other would not. That difference may eventually reveal itself
• 55:27
  Speaker 1: through much finer observations. There is another reason the fermionic
• 55:32
  Speaker 1: core idea attracts attention. It is not limited to Sagittarius
• 55:37
  Speaker 1: A star. If dark fermions can gather into dense cores
• 55:40
  Speaker 1: in galaxies today, perhaps similar structures could have formed much
• 55:45
  Speaker 1: earlier in cosmic history. So now, gently, you can allow
• 55:50
  Speaker 1: your view to travel backward through time, away from the
• 55:53
  Speaker 1: Milky Way. away from our galaxy, away from the familiar stars.
• 55:59
  Speaker 1: Back toward the early universe. The James Webb Space Telescope
• 56:03
  Speaker 1: has allowed astronomers to look extraordinarily far into the past.
• 56:07
  Speaker 1: Because light takes time to travel, distant galaxies are also
• 56:11
  Speaker 1: ancient views. When you look at a galaxy billions of
• 56:15
  Speaker 1: light years away, you are not seeing what it looks
• 56:18
  Speaker 1: like today. You are seeing light that began its journey
• 56:22
  Speaker 1: billions of years ago. And among the discoveries made with
• 56:25
  Speaker 1: this remarkable view are unusual compact objects, sometimes called little
• 56:30
  Speaker 1: red dots. They appear small. They appear distant. They appear
• 56:35
  Speaker 1: red because of the immense expansion of the universe and
• 56:38
  Speaker 1: the long journey their light has taken. And some of
• 56:41
  Speaker 1: them appear to contain enormous amounts of mass at surprisingly
• 56:45
  Speaker 1: early times. This creates another gentle puzzle. How did such
• 56:50
  Speaker 1: massive compact objects appear so quickly after the beginning of
• 56:54
  Speaker 1: the universe? The universe was still young. There had not
• 56:58
  Speaker 1: been billions of years for ordinary structures to slowly grow
• 57:01
  Speaker 1: in the familiar way. Yet these distant objects seemed to
• 57:05
  Speaker 1: suggest that something massive had already formed. The conventional black
• 57:10
  Speaker 1: hole picture has possible ways to address this. Black holes
• 57:14
  Speaker 1: can grow. They can gather matter. They can merge. they
• 57:18
  Speaker 1: can become larger over time. But the speed at which
• 57:21
  Speaker 1: some early massive objects appear has encouraged astronomers to consider
• 57:25
  Speaker 1: other possibilities as well. And here, once again, a dense
• 57:30
  Speaker 1: fermionic dark matter core offers an intriguing thought. If dark
• 57:36
  Speaker 1: matter were made from fermions, dense structures might have formed
• 57:40
  Speaker 1: under the conditions of the early universe. The same quantum
• 57:44
  Speaker 1: rules that could help create a compact core today might
• 57:47
  Speaker 1: have operated when the universe was much younger. Perhaps, in
• 57:52
  Speaker 1: some circumstances, dense dark matter structures could appear before conventional
• 57:57
  Speaker 1: supermassive black holes had enough time to grow to enormous sizes.
• 58:02
  Speaker 1: It is an appealing possibility, but it remains a possibility.
• 58:08
  Speaker 1: The existence of little red dots does not prove that
• 58:11
  Speaker 1: fermionic dark matter created them. and it does not disprove
• 58:15
  Speaker 1: the ordinary black hole explanation. Instead, these ancient objects add
• 58:21
  Speaker 1: another place where scientists can ask the same question. How
• 58:26
  Speaker 1: quickly can massive compact structures form? What kind of matter
• 58:30
  Speaker 1: can gather into them? And what traces would those structures
• 58:33
  Speaker 1: leave behind? You can let the question remain open. The
• 58:37
  Speaker 1: light from those distant objects has travelled for billions of years,
• 58:42
  Speaker 1: just to reach the instruments observing it now. Meanwhile, Sagittarius
• 58:47
  Speaker 1: Astar remains only 27,000 light-years away by comparison, quietly sitting
• 58:53
  Speaker 1: at the centre of your own galaxy. Two very different
• 58:57
  Speaker 1: views of the universe, one nearby, one ancient, one at
• 59:02
  Speaker 1: the heart of the Milky Way, one appearing from a
• 59:05
  Speaker 1: time when the universe was still young. And strangely, both
• 59:09
  Speaker 1: can lead you back toward the same quiet question what
• 59:13
  Speaker 1: kind of object can become incredibly massive incredibly compact and
• 59:18
  Speaker 1: yet reveal its true nature only through the subtle way
• 59:22
  Speaker 1: it affects everything around it perhaps the answer is already
• 59:26
  Speaker 1: hidden in the light Perhaps it is hidden in the
• 59:29
  Speaker 1: motion of the stars, or perhaps the difference is so
• 59:32
  Speaker 1: delicate that you simply have not yet looked closely enough.
• 59:36
  Speaker 1: For now, the glowing ring around Sagittarius A star continues
• 59:41
  Speaker 1: to float quietly in the darkness, and beyond it, the
• 59:45
  Speaker 1: ancient red light of the early universe continues its long
• 59:49
  Speaker 1: journey toward you. For a while, you can simply remain
• 59:55
  Speaker 1: near the centre of the Milky Way, There is no
• 59:58
  Speaker 1: need to hurry away. The stars continue their quiet journeys.
• 1:00:03
  Speaker 1: S2 follows its long, curved path. The G objects drift
• 1:00:08
  Speaker 1: through the same deep gravitational environment, and beneath them, Sagittarius
• 1:00:14
  Speaker 1: A star remains almost completely invisible. You have now seen
• 1:00:19
  Speaker 1: two possibilities. The first is familiar, a supermassive black hole
• 1:00:24
  Speaker 1: containing millions of suns within an extraordinarily compact region, surrounded
• 1:00:30
  Speaker 1: by an event horizon that marks a boundary beyond which
• 1:00:33
  Speaker 1: light cannot return. The second is stranger. A dense core
• 1:00:37
  Speaker 1: made from hypothetical fermionic dark matter, compact enough to reproduce
• 1:00:43
  Speaker 1: much of the same gravitational behaviour, yet without an event horizon.
• 1:00:48
  Speaker 1: From far away, they can look remarkably similar. The stars
• 1:00:52
  Speaker 1: may follow almost the same paths, The surrounding gas may
• 1:00:56
  Speaker 1: glow in almost the same region. Even the broad shape
• 1:00:59
  Speaker 1: of the darkness seen by the Event Horizon Telescope may
• 1:01:03
  Speaker 1: be similar. And so the question becomes very quiet. If
• 1:01:07
  Speaker 1: the two objects can imitate each other so well, how
• 1:01:10
  Speaker 1: could you ever tell them apart? The answer may be
• 1:01:13
  Speaker 1: hidden in the details. Imagine light moving around Sagittarius A star.
• 1:01:19
  Speaker 1: Light normally travels along the straightest possible path through empty space.
• 1:01:24
  Speaker 1: but near an enormous concentration of mass, space itself is
• 1:01:28
  Speaker 1: strongly curved. The path of light bends, some rays move away,
• 1:01:34
  Speaker 1: some pass around the object, and some can travel along
• 1:01:38
  Speaker 1: remarkably curved paths, circling the central region before eventually escaping
• 1:01:43
  Speaker 1: toward you. If the central object is a black hole,
• 1:01:47
  Speaker 1: there are special paths where light can circle around the
• 1:01:50
  Speaker 1: black hole for a short time before continuing outward, or
• 1:01:53
  Speaker 1: falling inward. From far away, these repeated paths can contribute
• 1:01:59
  Speaker 1: to a thin, bright feature surrounding the darker central region.
• 1:02:03
  Speaker 1: This is related to what astronomers call a photon ring.
• 1:02:07
  Speaker 1: You do not need to remember the name. Just imagine
• 1:02:10
  Speaker 1: a faint circle of light, created because some light has
• 1:02:13
  Speaker 1: taken an unusually long and curved journey around the central darkness.
• 1:02:18
  Speaker 1: The important part is that a true black hole has
• 1:02:22
  Speaker 1: an event horizon. Light can approach the black hole, circle
• 1:02:26
  Speaker 1: near it, and in some cases disappear beyond that boundary.
• 1:02:30
  Speaker 1: A dense fermionic core would be different. There would be
• 1:02:34
  Speaker 1: no event horizon waiting at the centre. Light could interact
• 1:02:38
  Speaker 1: with the compact matter in ways that a black hole
• 1:02:41
  Speaker 1: would not allow. The detailed pattern of light could therefore
• 1:02:45
  Speaker 1: be subtly different. Perhaps the difference would be tiny. Perhaps
• 1:02:49
  Speaker 1: it would only become visible with much more precise observations.
• 1:02:53
  Speaker 1: But tiny differences are exactly what astronomers can search for
• 1:02:57
  Speaker 1: when they observe the same object again and again. The
• 1:03:01
  Speaker 1: Event Horizon Telescope may one day become even more powerful.
• 1:03:05
  Speaker 1: Future observations could provide sharper information about the glowing material
• 1:03:10
  Speaker 1: surrounding Sagittarius A star. Instead of seeing only the broad shape,
• 1:03:15
  Speaker 1: astronomers may be able to study the fine structure of
• 1:03:18
  Speaker 1: the light. They may be able to watch how bright
• 1:03:21
  Speaker 1: regions move. They may be able to identify repeating patterns.
• 1:03:26
  Speaker 1: They may be able to examine the faint features produced
• 1:03:29
  Speaker 1: by light travelling around the central object. And each improvement
• 1:03:33
  Speaker 1: could make the distinction between the two models slightly clearer.
• 1:03:37
  Speaker 1: There is another path toward the answer. You can return
• 1:03:41
  Speaker 1: to S2. For years, astronomers have watched this star move
• 1:03:46
  Speaker 1: around Sagittarius A star. Every measurement adds another tiny piece
• 1:03:51
  Speaker 1: of information. If the central object is exactly what the
• 1:03:54
  Speaker 1: black hole model predicts, the orbit should follow one precise pattern.
• 1:03:59
  Speaker 1: If the object is instead an extended fermionic core, its
• 1:04:03
  Speaker 1: gravitational field may differ very slightly at particular distances. The
• 1:04:08
  Speaker 1: difference could be almost impossible to notice today. But as
• 1:04:12
  Speaker 1: measurements become more accurate, even a very small deviation could
• 1:04:16
  Speaker 1: become meaningful. A tiny shift in the orbit, a subtle
• 1:04:19
  Speaker 1: change in the timing. a small difference in the way
• 1:04:22
  Speaker 1: the star moves during its closest approach. None of these
• 1:04:26
  Speaker 1: would look dramatic. There would be no sudden change in
• 1:04:30
  Speaker 1: the sky. The star would simply be a little farther
• 1:04:33
  Speaker 1: from where one model expects it to be, and that
• 1:04:36
  Speaker 1: small difference could matter. This is one of the beautiful
• 1:04:39
  Speaker 1: things about precision astronomy. A measurement that seems almost insignificant
• 1:04:44
  Speaker 1: can become important when it is repeated carefully. One observation
• 1:04:49
  Speaker 1: becomes two, two become many, the uncertainty becomes smaller, and
• 1:04:55
  Speaker 1: eventually a faint difference may begin to emerge. The same
• 1:04:59
  Speaker 1: principle applies to the G objects. Their paths can be followed,
• 1:05:04
  Speaker 1: their shapes can be measured, their movements can be compared
• 1:05:07
  Speaker 1: with increasingly detailed models. Perhaps one day the observations will
• 1:05:12
  Speaker 1: clearly favour the black hole description, or perhaps they will
• 1:05:16
  Speaker 1: reveal a small gravitational signature that is difficult to explain
• 1:05:20
  Speaker 1: with a simple black hole. For now, the available tests
• 1:05:24
  Speaker 1: do not provide that decisive separation. And so the uncertainty remains.
• 1:05:31
  Speaker 1: There is also another way to approach the mystery. Instead
• 1:05:34
  Speaker 1: of looking toward the galactic centre, you could search for
• 1:05:37
  Speaker 1: the particles themselves. If dark matter is made from a
• 1:05:42
  Speaker 1: new kind of particle, perhaps those particles could eventually be
• 1:05:45
  Speaker 1: detected directly. Laboratories on Earth are searching for faint signals
• 1:05:50
  Speaker 1: from dark matter passing through ordinary matter. These experiments are difficult.
• 1:05:56
  Speaker 1: The expected interactions are extremely weak. The particles, if they
• 1:06:00
  Speaker 1: exist in the required form, may pass through ordinary material
• 1:06:05
  Speaker 1: almost without notice. But even a tiny interaction could eventually
• 1:06:09
  Speaker 1: reveal what dark matter is made of. And if a
• 1:06:13
  Speaker 1: fermionic dark matter particle were discovered, the idea of a
• 1:06:17
  Speaker 1: fermionic core would become much more physically grounded. It would
• 1:06:22
  Speaker 1: not prove that Sagittarius a star is such a core,
• 1:06:25
  Speaker 1: but it would transform the hypothesis. Instead of being built
• 1:06:29
  Speaker 1: from an unknown possibility, the model could be based on
• 1:06:32
  Speaker 1: a known particle. The pieces could begin to fit together
• 1:06:36
  Speaker 1: more tightly. A particle. A halo. A dense core. A galaxy.
• 1:06:42
  Speaker 1: And perhaps, at the centre … an object that has
• 1:06:46
  Speaker 1: been mistaken for a black hole because its gravity looks
• 1:06:49
  Speaker 1: so similar. Or perhaps the direct dark matter experiments will
• 1:06:54
  Speaker 1: find nothing of this kind. Perhaps increasingly precise observations will
• 1:06:59
  Speaker 1: continue to favour the black hole model. That outcome would
• 1:07:02
  Speaker 1: also be valuable. Science does not need every alternative idea
• 1:07:07
  Speaker 1: to be correct. An alternative hypothesis can still be useful
• 1:07:11
  Speaker 1: because it tells you what to look for It gives
• 1:07:14
  Speaker 1: you another prediction, another comparison, another way to test what
• 1:07:18
  Speaker 1: you think you already know. And Sagittarius A Star is
• 1:07:22
  Speaker 1: an especially beautiful laboratory for this kind of patients. It
• 1:07:26
  Speaker 1: has been observed for decades. Its stars have been followed
• 1:07:30
  Speaker 1: across years. Its surrounding light has been measured. Its gravity
• 1:07:35
  Speaker 1: has been mapped. And still, the deepest question remains quietly open.
• 1:07:41
  Speaker 1: Perhaps it is a black hole. At present, that remains
• 1:07:44
  Speaker 1: the prevailing interpretation, and the evidence for a supermassive black
• 1:07:48
  Speaker 1: hole is extremely strong. But the possibility of a dense
• 1:07:53
  Speaker 1: fermionic core reminds you that even an extraordinarily successful explanation
• 1:07:59
  Speaker 1: can still be examined from another angle. The universe does
• 1:08:03
  Speaker 1: not become less beautiful when an answer is uncertain. Sometimes,
• 1:08:07
  Speaker 1: uncertainty simply means that the story is not finished. You
• 1:08:11
  Speaker 1: can let the Milky Way continue turning around you. The
• 1:08:15
  Speaker 1: stars continue their slow movements. The invisible halo remains spread
• 1:08:19
  Speaker 1: across the galaxy. The central object remains hidden, and somewhere
• 1:08:24
  Speaker 1: between the motion of a distant star, the curve of
• 1:08:27
  Speaker 1: a beam of light, and the faintest signal in a
• 1:08:30
  Speaker 1: quiet laboratory, there may be enough information to finally tell
• 1:08:34
  Speaker 1: the two possibilities apart. Not tonight. There is nothing you
• 1:08:39
  Speaker 1: need to solve, you can simply rest with the question,
• 1:08:42
  Speaker 1: a black hole with its quiet boundary, or a dense
• 1:08:46
  Speaker 1: core of invisible fermions without one, two possibilities, one centre,
• 1:08:52
  Speaker 1: and an enormous universe patiently waiting for us to look
• 1:08:56
  Speaker 1: a little more closely. And so our journey drifts quietly
• 1:09:02
  Speaker 1: to its end. You have travelled slowly toward the centre
• 1:09:06
  Speaker 1: of the Milky Way, through the distant starlight, through the
• 1:09:10
  Speaker 1: strange movements of S2, through the quiet mystery of Sagittarius A-star,
• 1:09:16
  Speaker 1: and into the much larger darkness surrounding the galaxy. You
• 1:09:20
  Speaker 1: have seen how the movements of stars can reveal something
• 1:09:23
  Speaker 1: that cannot be seen directly. You have followed the idea
• 1:09:27
  Speaker 1: of a supermassive black hole, an object so compact and
• 1:09:31
  Speaker 1: massive that light itself cannot escape from beyond its event horizon.
• 1:09:37
  Speaker 1: And you have also allowed yourself to imagine another possibility,
• 1:09:40
  Speaker 1: something stranger and softer, a dense core made from invisible
• 1:09:45
  Speaker 1: fermionic dark matter. surrounded by a much larger halo of
• 1:09:49
  Speaker 1: the same mysterious substance. Perhaps that picture is wrong. Perhaps
• 1:09:54
  Speaker 1: Sagittarius A-star truly is a black hole, just as the
• 1:09:58
  Speaker 1: evidence currently suggests. Or perhaps, hidden beneath the familiar interpretation,
• 1:10:05
  Speaker 1: there is something we have not yet recognised. For now,
• 1:10:09
  Speaker 1: you do not need to choose. You do not need
• 1:10:11
  Speaker 1: to solve the mystery. You can simply let the question
• 1:10:15
  Speaker 1: become quiet. the stars will continue to move. S2 will
• 1:10:20
  Speaker 1: continue along its patient orbit. The Milky Way will continue
• 1:10:24
  Speaker 1: turning slowly through space. The invisible dark matter halo, whatever
• 1:10:29
  Speaker 1: its true nature, will remain spread across the galaxy. And
• 1:10:33
  Speaker 1: Sagittarius A star will remain there at the centre, silent
• 1:10:37
  Speaker 1: and distant, holding millions of suns worth of mass within
• 1:10:42
  Speaker 1: an extraordinarily compact region. Perhaps one day, a more precise
• 1:10:47
  Speaker 1: observation of a star will reveal a tiny difference. Perhaps
• 1:10:51
  Speaker 1: a future image of the galactic centre will show another
• 1:10:54
  Speaker 1: delicate pattern in the light. Perhaps a dark matter experiment
• 1:10:59
  Speaker 1: on Earth will finally detect the particles that make up
• 1:11:02
  Speaker 1: the invisible matter surrounding galaxies. Or perhaps every new observation
• 1:11:07
  Speaker 1: will continue pointing toward the same familiar conclusion, that Sagittarius
• 1:11:13
  Speaker 1: A-star is is indeed a supermassive black hole. Either way,
• 1:11:18
  Speaker 1: the universe does not need you to know tonight. There
• 1:11:21
  Speaker 1: is nothing to do. You are safe. You can simply
• 1:11:24
  Speaker 1: listen and drift. Somewhere far beyond your room, light is
• 1:11:29
  Speaker 1: still travelling through space. Some of it has come from
• 1:11:32
  Speaker 1: the centre of the Milky Way. Some of it began
• 1:11:35
  Speaker 1: its journey before human beings existed. Some of it has
• 1:11:39
  Speaker 1: been travelling for billions of years. carrying faint traces of
• 1:11:43
  Speaker 1: ancient stars, ancient galaxies, and ancient moments in the history
• 1:11:48
  Speaker 1: of the universe. And now, for a brief moment, some
• 1:11:52
  Speaker 1: of that light reaches you. You can let yourself become
• 1:11:55
  Speaker 1: still beneath it. The darkness around Sagittarius A star remains dark.
• 1:12:02
  Speaker 1: The stars continue their gentle paths. The galaxy continues turning.
• 1:12:07
  Speaker 1: and the enormous universe continues unfolding without hurry. Whether Sagittarius
• 1:12:13
  Speaker 1: a star is a black hole, or whether some deeper
• 1:12:16
  Speaker 1: and stranger structure is hiding there, the mystery can remain
• 1:12:20
  Speaker 1: where it has always been, quietly waiting in the centre
• 1:12:23
  Speaker 1: of the galaxy. You can leave it there for tonight.
• 1:12:27
  Speaker 1: You do not need to follow it any farther. Just rest.
• 1:12:30
  Speaker 1: Let the distant stars become softer. Let the movement of
• 1:12:34
  Speaker 1: the galaxy become slower in your imagination. Let the darkness
• 1:12:38
  Speaker 1: become peaceful. And if you are still awake, you can
• 1:12:42
  Speaker 1: simply keep listening, without needing to remember every detail, without
• 1:12:47
  Speaker 1: needing to understand every possibility. The universe will keep asking
• 1:12:51
  Speaker 1: its questions long after you have fallen asleep. The stars
• 1:12:56
  Speaker 1: will keep moving. The planets will keep turning. The invisible
• 1:12:59
  Speaker 1: matter will keep drifting through the darkness. And somewhere, 27,000
• 1:13:03
  Speaker 1: light years away, The quiet centre of the Milky Way
• 1:13:07
  Speaker 1: will remain exactly where it has been, surrounded by stars,
• 1:13:12
  Speaker 1: surrounded by darkness, surrounded by questions. For now, rest well.
• 1:13:18
  Speaker 1: There is nothing to solve. There is nowhere to go.
• 1:13:21
  Speaker 1: You are safe. Just listen and drift. And perhaps, somewhere
• 1:13:26
  Speaker 1: in the quiet darkness between one thought and the next,
• 1:13:29
  Speaker 1: you may begin to notice something even softer. The universe
• 1:13:33
  Speaker 1: is still here. and you can simply sleep.