# Astronomers Finally Cracked the Mystery of Spiral Galaxies

An episode of [Bedtime Astronomy](https://invisiblenetwork.ai/podcast/1736397134), 11 Aug 2026, 43 min.

Audio: https://dts.podtrac.com/redirect.mp3/api.spreaker.com/download/episode/73142188/galacticp.mp3

Scientists analyzing nearly 400,000 galaxies with the Euclid space telescope have uncovered new clues about why spiral galaxies maintain their iconic shapes. The findings support the density wave theory, revealing that spiral arms behave like cosmic traffic jams rather than permanent structures. Researchers also discovered that a galaxy's spiral arm count is closely linked to its central mass, star formation, and the distribution of dark matter, providing a powerful new window into how galaxies evolve. Thank you for listening to Bedtime Astronomy — your guide to the cosmos. New episodes on space exploration, NASA missions & the latest astronomy breakthroughs. This episode includes AI-generated content.

## Transcript

[0:03] Speaker 1: Welcome to Bedtime Astronomy. Explore the wonders of the cosmos
[0:07] Speaker 1: with our soothing Bedtime Astronomie podcast. Each episode offers a
[0:12] Speaker 1: gentle journey through the stars, planets, and beyond, perfect for
[0:16] Speaker 1: unwinding after a long day. Let's travel through the mysteries
[0:20] Speaker 1: of the universe as you drift off into a peaceful
[0:22] Speaker 1: slumber under the night sky.
[0:27] Speaker 2: If you were to just go out and look up
[0:29] Speaker 2: at the Whirlpool Galaxy right now, assuming you had a
[0:32] Speaker 2: pretty decent telescope, of course, you'd see what looks like
[0:35] Speaker 2: this beautiful spinning pinwheel suspended in the dark.
[0:38] Speaker 3: Yeah. It's really one of the most iconic images in
[0:42] Speaker 3: all of astronomy.
[0:43] Speaker 2: Right. You've got the glowing central core and then those sweeping,
[0:47] Speaker 2: elegant spiral arms just dotted with all that bright blue light.
[0:51] Speaker 3: It's gorgeous, and it gives you this overwhelming sense of
[0:53] Speaker 3: looking at a perfect frozen snapshot.
[0:56] Speaker 2: Exactly. You see it, and your brain instantly goes, Okay,
[0:59] Speaker 2: I understand what this is is. It's a solid structure
[1:02] Speaker 2: slowly spinning in.
[1:03] Speaker 3: Space like a bicycle wheel.
[1:04] Speaker 2: Like a bicycle wheel. Yeah, but here is the massive
[1:08] Speaker 2: secret that astrophysics have been wrestling with for gosh, almost
[1:12] Speaker 2: two centuries now, based on the fundamental laws of physics,
[1:15] Speaker 2: that pin wheel shouldn't exist.
[1:16] Speaker 3: It really shouldn't. It is a massive structural paradox.
[1:19] Speaker 2: It should have wound itself so incredibly tight that it
[1:22] Speaker 2: destroyed its own shape billions of years ago. Yeah, so
[1:25] Speaker 2: what we are looking at is basically a magic trick.
[1:27] Speaker 3: It is the absolute definition of a visual trick, just
[1:30] Speaker 3: on a galactic scale, because, like you said, you look
[1:33] Speaker 3: at that sweeping shape and you intuitively assume you're seeing
[1:36] Speaker 3: a solid object rotating like a hurricane or a pin wheel,
[1:39] Speaker 3: but you aren't. What you're actually witnessing is this highly
[1:43] Speaker 3: complex dynamic engine.
[1:45] Speaker 2: And we are going to completely dismantle how that engine
[1:48] Speaker 2: works today because unraveling why these galaxies look the way
[1:53] Speaker 2: they do is well, it's not just about admiring pretty
[1:55] Speaker 2: space pictures.
[1:56] Speaker 3: No, not at all. It is fundamentally about uncovering the
[2:00] Speaker 3: invisible architecture of the entire universe, including dark matter.
[2:04] Speaker 2: Right when you look at the shape of a spiral arm,
[2:07] Speaker 2: you are actually looking at the gravitational footprint of dark matter,
[2:10] Speaker 2: which is just wild to think about it it's mind blowing.
[2:12] Speaker 2: And to really understand how wild this paradox is, we
[2:15] Speaker 2: have to realize that this mystery actually predates our knowledge
[2:18] Speaker 2: of what a galaxy even is.
[2:20] Speaker 3: Oh yeah, by a long shot.
[2:22] Speaker 2: We have to go way back to eighteen forty five
[2:25] Speaker 2: to this very cold damp field in Ireland, US. Yes,
[2:30] Speaker 2: William Parsons, the third Earl of Ross. This guy built
[2:34] Speaker 2: what was then the absolute largest telescope in the entire world.
[2:38] Speaker 3: The Leviathan of Parsonstown. That name is just incredible.
[2:42] Speaker 2: It's so Victorian. And it was this massive, like fifty
[2:46] Speaker 2: foot long tube just suspended by chains between these thick
[2:50] Speaker 2: stone walls.
[2:51] Speaker 3: A total marvel of engineering for the time. And you
[2:53] Speaker 3: have to remember he was pointing this absolute behemoth into
[2:57] Speaker 3: the sky at a time when we had virtually no
[2:59] Speaker 3: understanding of the cosmos outside our own stellar neighborhood, right.
[3:03] Speaker 2: So he pointed at this fuzzy patch in the sky
[3:05] Speaker 2: known as M fifty one, which is the Whirlpool galaxy
[3:07] Speaker 2: we just talked about. Yeah, and looking through the eyepiece
[3:09] Speaker 2: of Leviathan, he becomes the very first human being to
[3:13] Speaker 2: sketch a spiral structure in the heavens.
[3:15] Speaker 3: He literally drew it by hand.
[3:17] Speaker 2: He did. But here is the kicker. Nobody in eighteen
[3:22] Speaker 2: forty five thought this was a galaxy, that word, that concept,
[3:25] Speaker 2: It just didn't exist in that context.
[3:27] Speaker 3: Now, they called these things spiral nebulae.
[3:29] Speaker 2: Because the scientific consensus back then was so incredibly localized.
[3:32] Speaker 3: Right exactly. The running belief was that the Milky Way
[3:35] Speaker 3: was the entire universe, full stop.
[3:38] Speaker 2: Just one big cluster of stars and that's it, right, So.
[3:41] Speaker 3: These spiral nebulae were assumed to just be a swirling
[3:45] Speaker 3: clouds of gas floating around right inside our own cosmic backyard, like.
[3:50] Speaker 2: Maybe solar systems in the very early stages of forming.
[3:52] Speaker 3: Yeah, exactly. And it wasn't until the nineteen twenties, during
[3:56] Speaker 3: what astronomers literally call the Great Debate, that this entire
[4:00] Speaker 3: paradigm just completely shattered, right.
[4:02] Speaker 2: Because you had Harlow shapely arguing the universe was just
[4:06] Speaker 2: one big Milky Way, and Hebrew Curtis arguing that these
[4:10] Speaker 2: spirals were far too distant that they were actually I
[4:13] Speaker 2: think he called them island universes.
[4:15] Speaker 3: Island universes, yes. And then Edwin Hubble stepped in and
[4:18] Speaker 3: settled it using the hooker telescope in California.
[4:21] Speaker 2: How did he prove it? Because it's a massive leap
[4:24] Speaker 2: to go from that's a gas cloud nearby to that's
[4:27] Speaker 2: a city of billions of stars really far away.
[4:30] Speaker 3: He is something called Cepheid variable stars. He found them
[4:33] Speaker 3: inside the spiral nebulae. And the thing about Cephad's is
[4:36] Speaker 3: that they pulse at a very predictable rate. Okay, and
[4:39] Speaker 3: because of that predictable pulse, we can calculate exactly how
[4:42] Speaker 3: far away they are. It's like a cosmic yardstick. Oh
[4:44] Speaker 3: and Hubble proved beyond a shadow of a doubt that
[4:48] Speaker 3: these spirals were sitting millions of light years away.
[4:51] Speaker 2: So they weren't just small clouds of gas nearby.
[4:54] Speaker 3: Not at all. They were massive, entirely independent galaxies which.
[4:59] Speaker 2: Completely re wornd fires human understanding in an instant. We
[5:02] Speaker 2: go from this cozy, single galaxy universe to realizing the
[5:05] Speaker 2: cosmos is populated by hundreds of thousands, if not billions,
[5:10] Speaker 2: of these massive spiral structures.
[5:12] Speaker 3: It's an astronomical paradigm shift.
[5:14] Speaker 2: But that sheer ubiquity is exactly what creates a psychological
[5:18] Speaker 2: trap for us today. Yeah, because you see a picture
[5:20] Speaker 2: of a spiral galaxy. You see thousands of them, and
[5:22] Speaker 2: they look so elegant, so intuitive, and you.
[5:25] Speaker 3: Just assume their mechanics must be really easy to explain exactly.
[5:28] Speaker 2: You fall for the pinwheel illusion. Yeah, so let's dismantle
[5:31] Speaker 2: that illusion right now. Because if a spiral galaxy were
[5:34] Speaker 2: a solid disk, like say a dinner plate spinning on
[5:37] Speaker 2: your finger, every.
[5:37] Speaker 3: Part of that plate completes a rotation in the exact
[5:39] Speaker 3: same amount of time.
[5:40] Speaker 2: Right, the inder part near your finger and the outer
[5:42] Speaker 2: rim are moving in total walkstep. But galaxies are not
[5:46] Speaker 2: solid plates.
[5:47] Speaker 3: No, they are incredibly fluid agglomerations. It's just stars, gas, dust,
[5:54] Speaker 3: and a whole lot of dark matter.
[5:55] Speaker 2: Which means they have to obey orbital mechanics just like
[5:58] Speaker 2: anything else. The stars in the galaxy are orbiting a
[6:02] Speaker 2: central mass, much like the planets in our Solar system
[6:06] Speaker 2: orbit the Sun.
[6:07] Speaker 3: Exactly, and Johannes Kepler taught us centuries ago that in
[6:10] Speaker 3: a system like that, objects closer to the center of
[6:13] Speaker 3: gravity orbit much faster than objects on the outer edge.
[6:16] Speaker 2: Right, So, mercury zips around the Sun in what eighty
[6:19] Speaker 2: eight days, but Neptune takes something like one hundred and
[6:22] Speaker 2: sixty five years.
[6:23] Speaker 3: That's differential rotation in a nutshell.
[6:25] Speaker 2: Okay, so if you apply that differential rotation to a galaxy,
[6:29] Speaker 2: you hit an immediate brick wall. Logically, yes, what because
[6:32] Speaker 2: if the stars in the inner disc are whipping around
[6:34] Speaker 2: the galactic core at incredible speeds while the stars out
[6:37] Speaker 2: on the outer rim are barely crawling along.
[6:40] Speaker 3: Right, they're taking hundreds of millions of years to make
[6:42] Speaker 3: one lap.
[6:42] Speaker 2: Then just by basic orbital mechanics, shouldn't those distinct spiral
[6:46] Speaker 2: arms just stretch out, wrap around the core and basically
[6:49] Speaker 2: spool up like spaghetti around the spinning fork.
[6:52] Speaker 3: You've just perfectly articulated what astrophysicists call the winding dilemma.
[6:57] Speaker 3: The winding dilemma, Yeah, it is the fundamental problem of
[7:00] Speaker 3: spiral galaxies because of that differential rotation, those sweeping arms
[7:05] Speaker 3: should act exactly like a string wrapping around a spinning top.
[7:10] Speaker 2: But they don't.
[7:10] Speaker 3: No, they don't, and they shouldn't survive very long. If
[7:13] Speaker 3: that were the case. In galactic terms, a single rotation
[7:17] Speaker 3: might take a couple of one hundred million years. Right
[7:20] Speaker 3: after just a few rotations, those distinct arms should be
[7:24] Speaker 3: wound so tightly that the spiral pattern is completely erased.
[7:28] Speaker 2: It would just look like a blurry circle exactly.
[7:30] Speaker 3: The galaxy should just smear out into a smooth, featureless,
[7:34] Speaker 3: glowing disc.
[7:35] Speaker 2: But wait, our own milky way is roughly what thirteen
[7:38] Speaker 2: point six billion years old something like that. Yes, and
[7:40] Speaker 2: the galaxies we see out there have been spinning for
[7:42] Speaker 2: billions of years too, So if the winding dilemma is correct,
[7:46] Speaker 2: all spiral arms should have vanished eons ago.
[7:49] Speaker 3: We shouldn't be seeing any of them today none.
[7:52] Speaker 2: Which means the simple rotation explanation completely.
[7:55] Speaker 3: Fails completely, And that glaring contradiction just sat there honestly
[8:00] Speaker 3: talking astronomers until the nineteen sixties.
[8:02] Speaker 2: When cc lynn and Frank Chu stepped in right.
[8:05] Speaker 3: They proposed what is now the prevailing framework for solving
[8:08] Speaker 3: this whole mess, which is called density wave theory.
[8:11] Speaker 2: I love diving into density wave theory because it's one
[8:14] Speaker 2: of those concepts that sounds super intimidating, but the best
[8:17] Speaker 2: way to visualize it is something everyone listening to this
[8:19] Speaker 2: has unfortunately experienced firsthand.
[8:21] Speaker 3: A traffic jam.
[8:22] Speaker 2: It is at its core, a cosmic traffic jam.
[8:26] Speaker 3: It really is the single most accurate analogy we have
[8:28] Speaker 3: for this.
[8:29] Speaker 2: Okay, So think about a massive multi lane highway and
[8:33] Speaker 2: there's an area where maybe there's a stalled car on
[8:35] Speaker 2: the shoulder, or two lanes merge causing a bottleneck.
[8:39] Speaker 3: So a localized traffic jam forms right there, right now.
[8:42] Speaker 2: Imagine you are in a news helicopter hovering way above
[8:45] Speaker 2: the highway, looking straight down from your perspective, up there
[8:49] Speaker 2: that dense cluster of slow moving cars. The traffic jam
[8:52] Speaker 2: itself stays in the exact same spot on the highway
[8:56] Speaker 2: for hours.
[8:56] Speaker 3: It barely moves forward at.
[8:58] Speaker 2: All, It just sits there. The individual cars making up
[9:01] Speaker 2: that jam are constantly changing exactly. The car zips down
[9:04] Speaker 2: the open highway at seventy miles an hour, it hits
[9:07] Speaker 2: the back of the traffic jam. The driver slams on
[9:09] Speaker 2: the brakes, and the car crawls forward, tightly packed with
[9:13] Speaker 2: all the other cars inching along right. But eventually it
[9:17] Speaker 2: clears the bottleneck, hits the open road again, accelerates and
[9:21] Speaker 2: leaves the jam behind. The cars are always flowing through,
[9:24] Speaker 2: but the traffic jam remains in place.
[9:27] Speaker 3: And Linn and Chu proposed that this is exactly what
[9:30] Speaker 3: is happening in a spiral galaxy.
[9:32] Speaker 2: So the arms aren't physical things.
[9:34] Speaker 3: No, the spiral arms are not fixed structures made of
[9:37] Speaker 3: the same stars forever. The arms are the traffic jams.
[9:40] Speaker 2: So an arm is just a region of space where
[9:42] Speaker 2: things are moving slower.
[9:43] Speaker 3: It is a visible region of higher density, a density
[9:46] Speaker 3: wave sweeping slowly through the galactic disc, and it moves
[9:50] Speaker 3: much much slower than the actual stars and gas.
[9:52] Speaker 2: Okay, So as a star orbits the center of the galaxy,
[9:55] Speaker 2: moving at its faster differential speed, it eventually catches up
[9:59] Speaker 2: to this slow removing density wave from behind.
[10:02] Speaker 3: Yes, and when the star and more importantly, the massive
[10:05] Speaker 3: clouds of interstellar gas enter this wave, the local gravitational
[10:10] Speaker 3: pull is suddenly much stronger. Because everything is bunched up.
[10:13] Speaker 3: The gas hits the brakes, It hits the brakes hard.
[10:16] Speaker 3: The gas clouds compress violently as they bunch up in
[10:19] Speaker 3: this traffic jam. And in astrophysics, when you take a
[10:22] Speaker 3: massive cloud of hydrogen gas and you compress it like that,
[10:26] Speaker 3: you trigger what's called the genes instability.
[10:29] Speaker 2: The genes instability, what does that do?
[10:31] Speaker 3: The gas begins to collapse under its own gravity, which
[10:34] Speaker 3: creates these massive shock waves.
[10:35] Speaker 2: Which is exactly what triggers star formation. Yeah, you are
[10:38] Speaker 2: basically squeezing the cosmic clay until it ignites.
[10:41] Speaker 3: Precisely, this localized compression inside the traffic jam sparks furious
[10:47] Speaker 3: stellar nurseries. You get the birth of these massive, incredibly hot,
[10:51] Speaker 3: short lived stars, the.
[10:52] Speaker 2: O and B type stars, right.
[10:54] Speaker 3: The O and B stars. And these stars burn with
[10:56] Speaker 3: a fierce, brilliant blue light and they ionize the surrounds.
[11:00] Speaker 2: Which creates those glowing red HII regions we see in
[11:04] Speaker 2: the telescope images.
[11:05] Speaker 3: Exactly, this fierce illumination is what suddenly makes the density
[11:09] Speaker 3: wave visible to us. The arm lights up entirely because
[11:12] Speaker 3: of the newborn stars.
[11:14] Speaker 2: That is so cool. And then what happens is the
[11:16] Speaker 2: wave just keeps slowly moving on.
[11:18] Speaker 3: Well, those massive blue stars they live fast and die young.
[11:22] Speaker 3: They often explode a supernovae before they even manage to
[11:25] Speaker 3: exit the traffic jam.
[11:26] Speaker 2: Wow, they don't even make it out.
[11:27] Speaker 3: They don't. But the smaller, longer live stars like our
[11:31] Speaker 3: Sun eventually pass all the way through the density wave.
[11:35] Speaker 3: They exit the front of the traffic jam, speed back up,
[11:38] Speaker 3: and drift out into the darker, quieter regions between the
[11:42] Speaker 3: spiral arms.
[11:43] Speaker 2: Just waiting to lap the next wave exactly. It is
[11:46] Speaker 2: an incredible reframing of reality. We're looking at a process,
[11:50] Speaker 2: not an object.
[11:51] Speaker 3: That's the perfect way to phrase it.
[11:52] Speaker 2: But density wave theory has a major vulnerability, doesn't it.
[11:56] Speaker 2: It does, unfortunately, because it explains why the arms don't
[11:59] Speaker 2: wind themselves into oblivion. But it introduces a whole new
[12:02] Speaker 2: problem regarding how long they can survive at all.
[12:05] Speaker 3: Right, it's the problem of dissipation. Because these density waves
[12:10] Speaker 3: aren't magically exempt from the laws of thermodynamics and fluid dynamics.
[12:14] Speaker 2: They still have to obey the rule.
[12:15] Speaker 3: Exactly over immense stretches of time, physical forces within the
[12:20] Speaker 3: galactic discs, like the friction of all those gas clouds colliding,
[12:23] Speaker 3: and complex gravitational resonances known as linn Blad.
[12:27] Speaker 2: Resonances lind Lead resonances.
[12:29] Speaker 3: Yeah, they're basically gravitational friction points. And all of this
[12:32] Speaker 3: should eventually damp down these waves. The energy of the
[12:36] Speaker 3: traffic jam should dissipate.
[12:38] Speaker 2: Just smoothing out the density wave until the spiral arms
[12:41] Speaker 2: fade away entirely.
[12:42] Speaker 3: The traffic jam should eventually just clear.
[12:44] Speaker 2: But everywhere we look in the universe, spiral galaxies are
[12:48] Speaker 2: just staring back at us. They are incredibly common.
[12:51] Speaker 3: Which means the ultimate mystery remains what sustains these waves
[12:55] Speaker 3: over billions of years? What keeps the traffic jam from
[12:58] Speaker 3: clearing out right?
[13:00] Speaker 2: And to answer a question about longevity, about how something
[13:03] Speaker 2: sustains itself over billions of years, you can't just look
[13:05] Speaker 2: at the universe as it exists today. You have to
[13:07] Speaker 2: look at its history.
[13:08] Speaker 3: You have to trace the evolutionary timeline of these structures.
[13:12] Speaker 2: And that brings us to an absolute marvel of modern engineering,
[13:17] Speaker 2: which is the EUCLID Space Telescope.
[13:19] Speaker 3: EUCLID is a phenomenal observatory. It was launched by the
[13:22] Speaker 3: European Space Agency recently and it's positioned out at lagrange
[13:26] Speaker 3: point two.
[13:27] Speaker 2: Which is a stable orbit about a million miles from
[13:30] Speaker 2: Earth right, and.
[13:31] Speaker 3: Its observational capabilities are just staggering. It was specifically designed
[13:36] Speaker 3: to look at the wide field geometry of the dark universe.
[13:39] Speaker 2: And the scale we are talking about here involves measuring
[13:43] Speaker 2: the red shift of galaxies all the way out to
[13:45] Speaker 2: a value of z equals too. Yes, now, let's make
[13:48] Speaker 2: sure everyone has a really firm grasp on what that
[13:50] Speaker 2: actually means, because it's vital to this whole investigation. We
[13:53] Speaker 2: hear the word redshift thrown around a lot in space news.
[13:57] Speaker 2: What is physically happening to the light that allows us
[14:00] Speaker 2: to use it as a cosmic clock.
[14:02] Speaker 3: It all comes down to the expansion of the universe.
[14:04] Speaker 3: Space itself is stretching, so as light from a distant
[14:07] Speaker 3: galaxy travels across the universe toward our telescopes, the very
[14:11] Speaker 3: fabric of space that is traveling through is expanding, and
[14:15] Speaker 3: this expansion literally stretches the wavelength of the light.
[14:18] Speaker 2: Itself, like stretching a coiled telephone cord.
[14:21] Speaker 3: Exactly like that, And in the electromagnetic spectrum, longer wavelengths
[14:26] Speaker 3: correspond to the red.
[14:28] Speaker 2: End of the spectrum, so it shifts red.
[14:30] Speaker 3: Right, So the longer light has been traveling through expanding space,
[14:33] Speaker 3: the more its wavelength is stretched, and the redder it
[14:36] Speaker 3: appears to us.
[14:37] Speaker 2: That makes total sense. So by measuring exactly how much
[14:40] Speaker 2: the light from a galaxy has been shifted toward the red,
[14:43] Speaker 2: we can calculate exactly how long that light has been
[14:46] Speaker 2: traveling exactly.
[14:46] Speaker 3: So when astronomers say euclid is looking at galaxies at
[14:49] Speaker 3: z equals two, that specific red shift value translates to
[14:54] Speaker 3: a travel time of roughly ten billion years.
[14:56] Speaker 2: Ten billion years. That is just the scale is hard
[14:59] Speaker 2: to wrap your head around. The light hitting EUCLID sensors
[15:01] Speaker 2: left those galaxies ten billion years ago.
[15:04] Speaker 3: We are literally watching a movie of the universe as
[15:07] Speaker 3: it existed ten billion years in.
[15:08] Speaker 2: The past, which is profound, especially when you consider the
[15:11] Speaker 2: universe itself is estimated to be around thirteen point eight
[15:14] Speaker 2: billion years old.
[15:16] Speaker 3: Right, so we're looking back at a relatively young cosmos,
[15:19] Speaker 3: a time when galaxies were still in their really formative,
[15:21] Speaker 3: wildly active stages.
[15:23] Speaker 2: Now I have to assume EUCLID wasn't built for a
[15:25] Speaker 2: billion dollars just to count spiral arms, right.
[15:27] Speaker 3: No, definitely not.
[15:29] Speaker 2: What is its primary mission and how did it get
[15:32] Speaker 2: hijacked for this specific structural mystery.
[15:35] Speaker 3: So euclid's primary mandate is to define the link between
[15:38] Speaker 3: dark energy and the universe's expansion rate. It does this
[15:42] Speaker 3: by measuring red shift to see how fast galaxies are
[15:45] Speaker 3: moving apart, and by measuring the precise shapes of billions
[15:48] Speaker 3: of galaxies to study weak gravitational lensing.
[15:51] Speaker 2: Weak lensing being the way the gravity of invisible dark
[15:54] Speaker 2: matter bends and distorts the light of galaxies sitting behind it. Exactly.
[15:59] Speaker 3: By meticular usly mapping these tiny, tiny distortions in shape,
[16:03] Speaker 3: EUCLID is creating a three D map of the universe's
[16:06] Speaker 3: dark matter scaffolding.
[16:07] Speaker 2: Okay, so where do the spiral arms come in.
[16:10] Speaker 3: Well, here is the brilliant secondary use of all this data.
[16:13] Speaker 3: To measure those tiny distortions caused by lensing, EUCLID has
[16:17] Speaker 3: to take incredibly sharp, high resolution images of galaxies across
[16:22] Speaker 3: vast distances.
[16:24] Speaker 2: Oh I see, and a high resolution image meant to
[16:26] Speaker 2: study tiny distortions is also a perfectly clear picture of
[16:30] Speaker 2: a ten billion year old spiral arm.
[16:33] Speaker 3: Precisely, it allows astrophysicists to perfectly distinguish the spiral arms,
[16:37] Speaker 3: the bulges, and the disks of galaxies sitting billions of
[16:41] Speaker 3: light years away.
[16:42] Speaker 2: It effectively lets us see exactly what spiral galaxies look
[16:45] Speaker 2: like in the ancient universe.
[16:46] Speaker 3: Right, and then we can compare the structural dynamics of
[16:49] Speaker 3: ten billion years ago to the galaxies sitting in our
[16:51] Speaker 3: local neighborhood today.
[16:53] Speaker 2: Which brings us to the massive scale of the research
[16:55] Speaker 2: we are unpacking today. Because this wasn't just a survey
[16:58] Speaker 2: where astronomers picked, you know, a dozen of their favorite
[17:01] Speaker 2: photogenetic galaxies. No.
[17:02] Speaker 3: No.
[17:02] Speaker 2: A team led by A. Beverly Smith, who is a
[17:04] Speaker 2: professor of physics and astronomy at East Tennessee State University,
[17:08] Speaker 2: utilize this EUCLID data to examine the structures of over
[17:11] Speaker 2: three hundred and eighty thousand galaxies.
[17:13] Speaker 3: Three hundred and eighty thousand. It is just difficult to
[17:17] Speaker 3: overstate how revolutionary that number is for observational astronomy.
[17:21] Speaker 2: Because historically studies on galactic morphology might look at a
[17:25] Speaker 2: few hundred, right, Yeah, maybe a couple of thousand galaxies
[17:28] Speaker 2: if they were ambitious, exactly.
[17:30] Speaker 3: But when you're trying to solve a puzzle about the
[17:32] Speaker 3: mechanics of an entire class of celestial objects, scale.
[17:36] Speaker 2: Is everything, Right, If you only look at one hundred galaxies,
[17:39] Speaker 2: you are totally blind to the macro trends. You might
[17:42] Speaker 2: just be looking at a cluster of weird anomalies.
[17:44] Speaker 3: It's like trying to understand global human behavior by interviewing
[17:48] Speaker 3: ten people at a single coffee shop.
[17:50] Speaker 2: You need the Census data. Yeah, and when you study
[17:52] Speaker 2: three hundred and eighty thousand galaxies across ten billion years
[17:55] Speaker 2: of cosmic time. You could finally separate the rule from
[17:58] Speaker 2: the exception get undeniable statistical certainty.
[18:02] Speaker 3: Absolutely, But of course, analyzing three hundred and eighty thousand
[18:05] Speaker 3: highly detailed images presents an immediate massive logistical problem.
[18:09] Speaker 2: Yeah, I'd imagine you cannot.
[18:11] Speaker 3: Have graduate students sitting in a dark room manually counting
[18:14] Speaker 3: the spiral arms on nearly half a million images.
[18:17] Speaker 2: It would take lifetimes, it really would. So how do
[18:19] Speaker 2: they do it? You mentioned they used a tool called
[18:21] Speaker 2: euclid zobot. What exactly is that?
[18:25] Speaker 3: So? Euclid zubot is this highly sophisticated machine learning algorithm.
[18:29] Speaker 3: It's an artificial intelligence specifically trained to recognize and categorize
[18:35] Speaker 3: galactic structures. But what's fascinating is how it actually learned
[18:39] Speaker 3: to do this because it was trained on the backs
[18:41] Speaker 3: of citizen science.
[18:42] Speaker 2: Ah, like the Galaxy Zoo project.
[18:44] Speaker 3: Exactly like Galaxy Zoo. For years, that project allowed hundreds
[18:48] Speaker 3: of thousands of everyday people to just log online, look
[18:51] Speaker 3: at telescope images and answer really simple.
[18:54] Speaker 2: Questions things like is it a spiral, how many arms
[18:56] Speaker 2: does it have? Is that center round or cigar shape?
[18:59] Speaker 3: Exactly? And it turns out out everyday. People are remarkably
[19:01] Speaker 3: good at pattern recognition, often much better than early computer models.
[19:05] Speaker 2: So by feeding millions of these human classified images into
[19:08] Speaker 2: the machine learning model, Zubot learned the subtle visual cues
[19:12] Speaker 2: of galactic morphology. Right.
[19:14] Speaker 3: So, when the EUCLID data came down, the researchers deployed
[19:17] Speaker 3: Zubot to parse the staggering data set, basically standardizing the
[19:21] Speaker 3: classification of three hundred and eighty thousand ancient galaxies in
[19:24] Speaker 3: record time.
[19:25] Speaker 2: And to classify them, they had to force all this
[19:28] Speaker 2: visual chaos into a structured taxonomy. They focused heavily on
[19:33] Speaker 2: the anatomy of the spiral, specifically how many arms does
[19:37] Speaker 2: this thing actually have, and they divided them into three
[19:40] Speaker 2: main macro categories.
[19:41] Speaker 3: The first category is the Grand Design spiral galaxy. These
[19:45] Speaker 3: are the absolute showstoppers, a classic look right. They have
[19:49] Speaker 3: two long, continuous, highly defined symmetrical arms extending outward from
[19:54] Speaker 3: the galactic center M fifty one. The whirlpool galaxy is
[19:58] Speaker 3: the quintusycle example of this.
[20:00] Speaker 2: When you close your eyes and picture a galaxy, you
[20:02] Speaker 2: are picturing a Grand Design exactly. Okay, so grand design
[20:06] Speaker 2: is the classic two armed structure. Now if I'm placing
[20:09] Speaker 2: bets on what happens when a galaxy fails to form
[20:11] Speaker 2: those perfect two arms, I'd assume things just get wildly chaotic.
[20:14] Speaker 3: Because space is a violent place.
[20:16] Speaker 2: Exactly, space is violent, and messy galaxies are colliding, gas
[20:20] Speaker 2: clouds are fragmenting. I'd expect a ton of messy, multi
[20:23] Speaker 2: armed structures to be out there.
[20:25] Speaker 3: That leads directly to the second category, which is the
[20:28] Speaker 3: multile in galaxies. And exactly as it sounds, these are
[20:31] Speaker 3: systems that possess three or more relatively long arms. They
[20:36] Speaker 3: are visually busier, they're more complex, and they completely lack
[20:40] Speaker 3: that perfect bilateral symmetry of the Grand design.
[20:43] Speaker 2: And what happens when a galaxy can't even hold long
[20:46] Speaker 2: arms together all like? It just completely falls apart structurally.
[20:49] Speaker 3: Then you arrive at the third category, which is known
[20:52] Speaker 3: as flocculent galaxies.
[20:54] Speaker 2: Fluculent it sounds like an insult you'd yell at someone
[20:56] Speaker 2: in traffic.
[20:57] Speaker 3: It really does, but it actually means having a loosely
[21:00] Speaker 3: clumped or fluffy texture like tufts of wool oh. In
[21:04] Speaker 3: galactic morphology, Floculent spirals don't have sweeping, continuous highways of
[21:08] Speaker 3: stars instead, their discs are filled with many short, fragmented,
[21:14] Speaker 3: totally disconnected arm pieces, like little fluffy patches. Yeah, they
[21:17] Speaker 3: look like glowing, patchy clouds scattered in a very loose
[21:21] Speaker 3: spiral pattern. It's highly localized instability rather than a galaxy
[21:25] Speaker 3: wide density wave.
[21:26] Speaker 2: All right, So we have our taxonomy grand design, which
[21:29] Speaker 2: is two arms, multi armed which is three or more,
[21:32] Speaker 2: and floculent which is fragmented fluff.
[21:34] Speaker 3: Right.
[21:35] Speaker 2: The researchers run the yuquid zoobot on the three hundred
[21:37] Speaker 2: and eighty thousand galaxies. But first off they have to
[21:40] Speaker 2: deal with the messy reality of data collection, right, because
[21:44] Speaker 2: nothing is ever perfectly clean in astronomy.
[21:46] Speaker 3: Oh absolutely, They had to discard roughly twenty percent of
[21:50] Speaker 3: the sample right out of the gate.
[21:51] Speaker 2: Wow, twenty percent.
[21:53] Speaker 3: Yeah, in astronomy you were always fighting observational hurdles, even
[21:57] Speaker 3: with a telescope as incredibly powerful as euclids. Some galaxies
[22:00] Speaker 3: are just too faint, or they're oriented edge on, so
[22:03] Speaker 3: you can't even see the disc to count the.
[22:05] Speaker 2: Arms, or obscured by foreground dust, I imagine exactly.
[22:08] Speaker 3: Furthermore, they really had to account for something called malm
[22:11] Speaker 3: Quist bias.
[22:12] Speaker 2: Okay, break down mound Quist bias for us because that
[22:15] Speaker 2: sounds important.
[22:16] Speaker 3: It's an inherent selection effect in observational astronomy. Basically, the
[22:20] Speaker 3: further away you look, the harder it is to see
[22:23] Speaker 3: dim objects, right obviously, so at extreme distances, your data
[22:27] Speaker 3: set becomes heavily biased toward only the brightest, most luminous
[22:31] Speaker 3: galaxies because the fainter ones simply don't register on the
[22:34] Speaker 3: sensors anymore.
[22:35] Speaker 2: So you think they are only bright galaxies out there,
[22:37] Speaker 2: but really you're just missing all the dim ones exactly.
[22:40] Speaker 3: The researchers had to rigorously filter the Zubot results to
[22:44] Speaker 3: ensure they weren't just categorizing noise or falling prey to
[22:48] Speaker 3: this bias.
[22:48] Speaker 2: But once they cleaned the data and focused on the
[22:51] Speaker 2: vast majority that remained clearly classifiable, the statistics that fell
[22:55] Speaker 2: out of this ten billion year survey are just staggering.
[22:59] Speaker 3: They really are.
[23:00] Speaker 2: They found that an overwhelming sixty percent to seventy percent
[23:03] Speaker 2: of the spiral galaxies are two armed Grand design spirals.
[23:07] Speaker 3: It is the overwhelmingly dominant standard model of the spiral
[23:10] Speaker 3: universe by a massive, massive margin, which.
[23:13] Speaker 2: Means my assumption about a chaotic, messy universe being the
[23:16] Speaker 2: norm is just completely wrong. Order dominates.
[23:19] Speaker 3: It does order dominates. Following that, about fifteen percent to
[23:22] Speaker 3: twenty percent fall into the three armed multi armed.
[23:24] Speaker 2: Category, which leaves a pretty tiny statistical sliver left over.
[23:28] Speaker 3: And this is where the data through a really fascinating curveball.
[23:31] Speaker 3: Only about one percent of the spirals in the entire
[23:35] Speaker 3: enormous sample are classified as one armed galaxies.
[23:38] Speaker 2: Wait, a one armed galaxy? Logistically, how does that even balance?
[23:43] Speaker 3: It doesn't really.
[23:44] Speaker 2: We just spent all this time talking about density wave theory,
[23:47] Speaker 2: cauplarian dynamics, and the symmetry of gravitational fields. A single
[23:51] Speaker 2: arm extending out from the core seems like a massive contradiction.
[23:55] Speaker 2: It should be entirely lopsided.
[23:57] Speaker 3: It is completely lopsided, which highlight and it's just how
[24:00] Speaker 3: rare and structurally tenuous they really are.
[24:03] Speaker 2: So what's going on with them?
[24:04] Speaker 3: What the EUCLID data shows when you correlate the arm
[24:07] Speaker 3: count with the physical properties of the galaxies is that
[24:10] Speaker 3: these one armed anomalies have uniquely low masses compared to
[24:13] Speaker 3: their two armed cousins.
[24:14] Speaker 2: They are the lightweights of the galactic.
[24:16] Speaker 3: World exactly, and mass dictates gravitational spility. When a galaxy
[24:21] Speaker 3: is a lightweight, its gravitational anchor is extremely weak, its
[24:25] Speaker 3: center of balance is highly susceptible.
[24:26] Speaker 2: To disruption, like a small boat in a storm.
[24:29] Speaker 3: Right, These one armed or lopsided spirals that sometimes called
[24:32] Speaker 3: magellanic spirals. They are almost always the result of a.
[24:35] Speaker 2: Trauma, a trauma like what kind of trauma.
[24:38] Speaker 3: A gravitational interaction with a neighboring dwarf galaxy, or maybe
[24:42] Speaker 3: an asymmetrical infall of cold gas from the intergalactic medium,
[24:46] Speaker 3: or even a localized lopsidedness in their surrounding dark matter halo.
[24:51] Speaker 2: Wow.
[24:51] Speaker 3: Because they don't have the internal mass to stabilize themselves,
[24:55] Speaker 3: a disturbance can trigger a massive density wave on one
[24:58] Speaker 3: side of the galaxy without having the sheer gravitational force
[25:01] Speaker 3: required to form a symmetrical balancing wave on the opposite side.
[25:05] Speaker 2: So it's literally a localized injury in a less anchored system.
[25:09] Speaker 2: Perfectly said, that makes perfect sense. The heavier the ship,
[25:13] Speaker 2: the harder it is to capsize, which brings us perfectly
[25:17] Speaker 2: to the core mechanics of shape. Because finding out that
[25:20] Speaker 2: sixty to seventy percent of galaxies are too armed isn't
[25:23] Speaker 2: just a fun piece of trivia.
[25:24] Speaker 3: No, it is a massive clue.
[25:25] Speaker 2: The number of arms a galaxy has is directly fundamentally
[25:29] Speaker 2: tied to its internal anatomy.
[25:31] Speaker 3: This is where we transition from observing the what to
[25:34] Speaker 3: finally understanding the why of the two armed dominance. The
[25:37] Speaker 3: Beverly Smith study doesn't just count arms. It cross reference
[25:41] Speaker 3: those counts with the structures of the galactic cores, and
[25:43] Speaker 3: they found a rock solid connection between the number of
[25:46] Speaker 3: spiral arms and the mass of the galaxy's central bulge.
[25:50] Speaker 2: Now, the central bulge being that incredibly dense, roughly spherical
[25:54] Speaker 2: concentration of older stars at the very heart of the galaxy.
[25:58] Speaker 2: It's the yolk of the Frida egg.
[25:59] Speaker 3: Right our own Milky Way has a significant central bulge
[26:04] Speaker 3: anchored by a super massive black hole and millions of
[26:07] Speaker 3: densely packed ancient stars.
[26:09] Speaker 2: And what did the EUCLID data reveal about these bulges?
[26:12] Speaker 3: It unequivocally reveals that galaxies possessing large, massive central bulges
[26:17] Speaker 3: almost universally express two dominant spiral arms.
[26:20] Speaker 2: Why what is a physical mechanism that forces a massive
[26:23] Speaker 2: bulge to equal exactly two arms?
[26:26] Speaker 3: It is deeply rooted in the fluid dynamics and the
[26:28] Speaker 3: gravitational rotation curves of the galaxy. Remember via Reuben's pioneering
[26:33] Speaker 3: work on rotation curves.
[26:34] Speaker 2: Right where we found that in many galaxies, thanks to
[26:36] Speaker 2: dark matter halos, the outer stars orbit much faster than
[26:39] Speaker 2: Caplarrian physics alone would predict exactly.
[26:42] Speaker 3: But when a galaxy has a massively dense, heavy central bulge,
[26:46] Speaker 3: the sheer concentration of baryonic mass at the core dominates
[26:50] Speaker 3: the inner gravitational field.
[26:52] Speaker 2: The gravity at the center is overwhelming.
[26:53] Speaker 3: Absolutely overwhelming. This massive central concentration leads to what astrophysicists
[26:59] Speaker 3: call a decline rotation curve in the inner regions of
[27:02] Speaker 3: the disc.
[27:02] Speaker 2: Okay, break that down. What does a declining rotation curve
[27:06] Speaker 2: mean for the actual stars in the disc?
[27:08] Speaker 3: It means the stars in the inner lanes, which are
[27:11] Speaker 3: held in the crushing grip of that massive bulge, are
[27:14] Speaker 3: being whipped around at phenomenally high speeds. Right, But as
[27:18] Speaker 3: you move outward from the bulge, that orbital velocity drops
[27:21] Speaker 3: off severely before eventually leveling out. There is a massive,
[27:26] Speaker 3: sharp difference in velocity between the inner disk and the
[27:28] Speaker 3: outer regions.
[27:29] Speaker 2: The inner lanes of the track are moving at two
[27:31] Speaker 2: hundred miles an hour and the outer lanes are moving
[27:33] Speaker 2: at twenty miles an hour.
[27:34] Speaker 3: Yes, and that extreme velocity differential creates an intense physical
[27:39] Speaker 3: force called a large shear rate.
[27:42] Speaker 2: Now, earlier we talked about a traffic jam, But how
[27:45] Speaker 2: does shear rate apply here? Because if I slide a
[27:48] Speaker 2: book across a table, there's shear. But a galaxy is
[27:52] Speaker 2: a spinning fluid.
[27:53] Speaker 3: Right, Rotational shear in a fluid is entirely different from
[27:55] Speaker 3: linear shear.
[27:57] Speaker 2: Give me an example.
[27:58] Speaker 3: Imagine a cup of black coffee and you pour a
[28:01] Speaker 3: thin ribbon of cream into it from the center out
[28:03] Speaker 3: to the edge.
[28:04] Speaker 2: Okay, got it.
[28:05] Speaker 3: If you stir the center incredibly fast, but the outer
[28:08] Speaker 3: edge barely moves. The liquid in the center is constantly
[28:12] Speaker 3: lapping the liquid on the outside. The friction and the
[28:15] Speaker 3: differential speeds stretch and tear that ribbon of cream. In
[28:19] Speaker 3: a galaxy, this powerful rotational shear acts upon the interstellar
[28:24] Speaker 3: gas and dust in the exact same way.
[28:26] Speaker 2: And how does that destructive tearing force lead to an
[28:29] Speaker 2: elegant two armed grand design?
[28:31] Speaker 3: Seems like it would just make a mess by mercilessly
[28:33] Speaker 3: destroying anything smaller. If a localized, floculent little arm tries
[28:38] Speaker 3: to form in a galaxy with an intense sheer rate,
[28:41] Speaker 3: the extreme velocity differential stretches it and shreds it before
[28:44] Speaker 3: it can ever establish itself.
[28:45] Speaker 2: It just rips it apart exactly.
[28:48] Speaker 3: The math of spiral density waves, specifically those limb lad resonance,
[28:51] Speaker 3: as we mentioned earlier, dictates that in a high sheer environment,
[28:54] Speaker 3: the only density wave mathematically stable enough to survive being
[28:58] Speaker 3: ripped apart is a massive symmetrical two armed waves.
[29:02] Speaker 2: So the sheer literally irons out the wrinkles.
[29:04] Speaker 3: It really does. It forces the chaotic gas into two sweeping,
[29:10] Speaker 3: dominant highways because they are the only structures robust enough
[29:14] Speaker 3: to withstand the extreme physics of the disk.
[29:17] Speaker 2: That is breathtaking. The heavy central bulge is basically acting
[29:21] Speaker 2: as a brutal gravitational metronome, dictating the structural rhythm of
[29:26] Speaker 2: the entire galaxy.
[29:27] Speaker 3: And the inverse is equally true. When you look at
[29:30] Speaker 3: galaxies with smaller bulges, less massive centers, they lack that
[29:34] Speaker 3: severe declining rotation curve.
[29:37] Speaker 2: So they do not have that intense rotational shear. Exactly,
[29:39] Speaker 2: if you don't have that massive anchor. If your center
[29:41] Speaker 2: is lightweight, you don't have the sheer force to iron
[29:44] Speaker 2: out the wrinkles. The galaxy would just fragment into a
[29:47] Speaker 2: bunch of smaller, messy arms.
[29:48] Speaker 3: You've just logically deduced the existence of the multi armed
[29:51] Speaker 3: and floculent galaxies. Without the dominant share of a massive bulge,
[29:56] Speaker 3: localized and stabilities are allowed to thrive, you get three, four,
[30:00] Speaker 3: or dozens of fragmented arms because nothing is strong enough
[30:02] Speaker 3: to rip them apart and force them into a grand design.
[30:06] Speaker 2: So the structure of the entire galaxy is literally a
[30:08] Speaker 2: physical symptom of the mass at its dead center.
[30:11] Speaker 3: It is entirely dictated by it.
[30:12] Speaker 2: And what makes it so incredibly satisfying is that this
[30:15] Speaker 2: massive observational data set from EUCLID finally provides the hard
[30:20] Speaker 2: proof for our theoretical models.
[30:22] Speaker 3: Yes, for decades, astrophysicists have been running n body computer
[30:26] Speaker 3: simulations in supercomputers, basically modeling the behavior of millions of
[30:31] Speaker 3: virtual stars. And in those simulations, if you artificially inject
[30:36] Speaker 3: a massive central bulge, the simulation invariably produces a two
[30:41] Speaker 3: armed spiral.
[30:42] Speaker 2: But a simulation is just math until the universe actually confirms.
[30:45] Speaker 3: It exactly, and Euclid's observation of three hundred and eighty
[30:48] Speaker 3: thousand galaxies across ten million years definitively confirms the theory.
[30:52] Speaker 2: It's beautiful, But the internal mass isn't the only thing
[30:55] Speaker 2: the researchers tracked, is it. They also looked at the
[30:57] Speaker 2: star formation rates or sfrs, basically correlating the structural anatomy
[31:02] Speaker 2: with how aggressively the galaxy is churning out new stars.
[31:05] Speaker 3: This adds a critical temporal layer to our understanding. It
[31:08] Speaker 3: shows us the life cycle and the temperament of these systems, and.
[31:11] Speaker 2: The correlation they found is remarkably stark. The study reveals
[31:15] Speaker 2: that two armed spirals on average have significantly lower star formation.
[31:19] Speaker 3: Rates, which might encounterintuitive at first.
[31:21] Speaker 2: Right now, we should note that within the parameters of
[31:24] Speaker 2: this specific study, the researchers attribute this lower rate to
[31:28] Speaker 2: the fact that the two armed galaxies in their sample
[31:30] Speaker 2: tended to have lower overall disc gas masses available for
[31:34] Speaker 2: star formation despite having those heavy central bulges.
[31:37] Speaker 3: Right, the fuel tank was running a bit low.
[31:39] Speaker 2: Exactly, But the contrast is what really matters here. Contrasts
[31:44] Speaker 2: that low rate with the three armed spirals, which boast
[31:47] Speaker 2: much higher star formation rates.
[31:49] Speaker 3: The structural chaos of a three armed systems is directly
[31:52] Speaker 3: mirrored by their frantic stellar production.
[31:54] Speaker 2: The best way to visualize the difference between these galaxies
[31:57] Speaker 2: is to think about urban development. Armed galaxy, with its
[32:01] Speaker 2: high star formation rate and lack of a stabilizing shear,
[32:05] Speaker 2: is like a booming, chaotic new frontier town.
[32:09] Speaker 3: I love this analogy.
[32:10] Speaker 2: There is raw material everywhere. Construction cranes dominate the skyline.
[32:14] Speaker 2: New neighborhoods are being rapidly built and then torn down.
[32:18] Speaker 2: It is incredibly active. It's churning with energy, but structurally
[32:22] Speaker 2: it's a bit of a mess. There's no master plan.
[32:24] Speaker 3: It's a highly volatile environment exactly.
[32:27] Speaker 2: But a two armed grand design galaxy is like an older,
[32:31] Speaker 2: highly structured, settled metropolis. Think of a city anchored by
[32:35] Speaker 2: a massive, historic, densely packed downtown. That's your central bulge.
[32:41] Speaker 2: Because of that anchor, the major highways and transit lines,
[32:45] Speaker 2: the two spiral arms are rigidly established. There isn't wild,
[32:49] Speaker 2: uncontrolled new construction happening in every single corner. The available
[32:53] Speaker 2: resources are managed. Things have quieted down, and the city
[32:56] Speaker 2: has fallen into a stable, enduring, long term rhythm.
[33:00] Speaker 3: That analogy perfectly captures the internal dynamics. The multi arm
[33:04] Speaker 3: systems are active, chaotic, and just churning through their gas reserves,
[33:07] Speaker 3: while the two arm systems are settled, heavily anchored, and
[33:10] Speaker 3: far more stable in their stellar production.
[33:12] Speaker 2: It's a totally different stage of life.
[33:14] Speaker 3: It really is. But if we pull back even further
[33:17] Speaker 3: away from just the visible stars and the gas, we
[33:19] Speaker 3: arrive at the most profound implication of this entire three
[33:23] Speaker 3: hundred and eighty thousand galaxy survey.
[33:25] Speaker 2: Ah. This is where we must reckon with the invisible
[33:29] Speaker 2: scaffolding of the universe.
[33:30] Speaker 3: We have to talk about dark matter. A galaxy's arms,
[33:34] Speaker 3: its rotation curve, its sheer rate, All of these mechanics
[33:38] Speaker 3: we've been discussing are entirely dictated by gravity. But in
[33:41] Speaker 3: our universe, the visible matter, the stars, the dust, the
[33:45] Speaker 3: glowing gas makes up only a tiny fraction of the
[33:49] Speaker 3: galaxy's total mass.
[33:50] Speaker 2: The vast majority of the matter exerting all that gravitational
[33:53] Speaker 2: force is totally invisible.
[33:55] Speaker 3: Right. Dark matter doesn't interact with electromagnetism at all. It
[33:58] Speaker 3: doesn't emit light, it doesn't reflect light or block light.
[34:00] Speaker 3: You could literally have a clump of dark matter right
[34:02] Speaker 3: in front of your face and you would never see it, but.
[34:05] Speaker 2: It exerts a massive gravitational pull. It is the invisible
[34:08] Speaker 2: skeleton holding the glowing meat of the galaxy together.
[34:11] Speaker 3: What a visual Yes. And because the sheer rate that
[34:14] Speaker 3: shapes the arms is entirely dependent on the total gravitational
[34:18] Speaker 3: potential of the galaxy, the spiral arms are effectively tracing
[34:22] Speaker 3: the gravitational pull of everything in the galaxy, both seen
[34:26] Speaker 3: and unseen.
[34:27] Speaker 2: Now, you might be thinking, if dark matter is completely invisible,
[34:30] Speaker 2: how can looking at a photograph of glowing stars tell
[34:34] Speaker 2: us anything about it?
[34:35] Speaker 3: Because the glowing stars are forced to obey the gravity
[34:38] Speaker 3: of the dark matter. Ah Beverly Smith and her research
[34:41] Speaker 3: team point out a really revolutionary concept here. Analyzing the
[34:45] Speaker 3: arm structure of these galaxies gives astronomers a brand new
[34:48] Speaker 3: proxy to estimate the concentration and distribution of dark matter
[34:52] Speaker 3: deep within galactic centers.
[34:54] Speaker 2: The arms are not just pretty features catching the light.
[34:57] Speaker 2: They are the visible footprints of invisible mass.
[35:00] Speaker 3: Exactly, by reading the number of arms and measuring the
[35:03] Speaker 3: tightness of their winding, we can map the dark mass
[35:06] Speaker 3: that we cannot observe directly.
[35:08] Speaker 2: It's like looking at the ripples on the surface of
[35:10] Speaker 2: a pitch black pond. You can't see what's beneath the water.
[35:13] Speaker 2: But by studying the size, speed, and shape of the
[35:16] Speaker 2: ripples on the surface, you can determine the exact mass
[35:19] Speaker 2: of the boulder that was thrown in. You don't need
[35:21] Speaker 2: to actually see the boulder to know how heavy it is.
[35:24] Speaker 3: That is exactly what this EUCLID data provides, a catalog
[35:27] Speaker 3: of three hundred eighty thousand ripples, allowing us to map
[35:31] Speaker 3: the invisible boulders of the ancient universe.
[35:34] Speaker 2: That is just incredible.
[35:35] Speaker 3: But as with all grand theories in astrophysics, we have
[35:39] Speaker 3: to acknowledge the messy, uncompromising reality of nature.
[35:43] Speaker 2: Because the universe refuses to fit perfectly into our neat
[35:46] Speaker 2: human made taxonomies, never right as satisfying as the grand design,
[35:51] Speaker 2: multi armed and flaculing categories are. A galaxy is a
[35:54] Speaker 2: fluid dynamic system spanning one hundred thousand light years. It's
[35:58] Speaker 2: never truly that simple, is it not? At all?
[36:00] Speaker 3: And the researchers are admirably transparent about this caveat. They
[36:04] Speaker 3: explicitly note that very few galaxies in the universe are
[36:07] Speaker 3: completely two armed or completely three armed in mathematically perfect sense.
[36:11] Speaker 2: There's always some structural noise.
[36:13] Speaker 3: Always there is a continuous spectrum of morphologies. When you
[36:17] Speaker 3: look closely at these high resolution images from EUCLID, even
[36:21] Speaker 3: in a beautiful grand design spiral, you see imperfections.
[36:25] Speaker 2: What kind of imperfection.
[36:26] Speaker 3: You see branches splintering off the main arms. You see
[36:30] Speaker 3: what they call feathers, which are these tiny spurs of
[36:33] Speaker 3: gas and newborn stars bridging the dark gaps between the
[36:37] Speaker 3: major lanes.
[36:37] Speaker 2: So it's a living system, not a perfect geometric drawing exactly.
[36:41] Speaker 3: In fact, within the field of galactic morphology, there is
[36:44] Speaker 3: an entirely different classification system developed by Bruce and debrah
[36:49] Speaker 3: Elmgreen that involves twelve distinct categories.
[36:52] Speaker 2: Twelve categories.
[36:53] Speaker 3: Yeah, it was designed specifically to account for all these
[36:56] Speaker 3: subtle variations, the branching, the spurs, the varying degrees of floculence.
[37:01] Speaker 2: So wait, why didn't the researchers use that highly detailed
[37:04] Speaker 2: twelve category system for this EUCLID study. Why stick to
[37:08] Speaker 2: the simpler three category macro approach.
[37:10] Speaker 3: It basically comes down to computational and practical limits.
[37:13] Speaker 2: Oh, because of the AI right.
[37:15] Speaker 3: Implementing a highly nuanced twelve category system on three hundred
[37:19] Speaker 3: and eighty thousand separate galaxies using current machine learning software
[37:23] Speaker 3: on a data set this vast is simply too difficult
[37:26] Speaker 3: right now.
[37:27] Speaker 2: Because the subtle differences between say, category seven and category
[37:30] Speaker 2: eight in a twelve tier system are incredibly hard for
[37:33] Speaker 2: an AI to parts consistently.
[37:35] Speaker 3: Exactly, especially across fuzzy ten billion year old images. Even
[37:39] Speaker 3: with the mitigation of observational biases. Using the three broader
[37:43] Speaker 3: categories allows astronomers to confidently spot the macro trends.
[37:48] Speaker 2: First, it establishes the baseline, right, the.
[37:50] Speaker 3: Baseline upon which deeper, more granular AI analysis will eventually be.
[37:54] Speaker 2: Built, Which brings us back to the ultimate mystery we
[37:57] Speaker 2: started with the longevity of the traffic jam. If density
[38:00] Speaker 2: wave theory explains how the arms don't wind up, but
[38:03] Speaker 2: struggles to explain why physical forces don't eventually damp the
[38:06] Speaker 2: wave out of existence. What is the grand synthesis of
[38:09] Speaker 2: this new data? How do these arms survive?
[38:11] Speaker 3: The massive scale of the data provides compelling, perhaps definitive hints,
[38:16] Speaker 3: because the two armed grand design spirals utterly dominate the
[38:19] Speaker 3: sample across ten billion years of time. It strongly suggests
[38:23] Speaker 3: that massive subtled discs, the ones with heavy bulges and
[38:27] Speaker 3: dense dark matter halos, are highly likely to maintain stable,
[38:31] Speaker 3: long lived two arm patterns.
[38:33] Speaker 2: The heavy center anchors the wave indefinitely.
[38:35] Speaker 3: Yes, the sheer central mass and the resulting relentless sheer
[38:40] Speaker 3: forces act as a perpetual stabilizing mechanism. It constantly re
[38:44] Speaker 3: energizes the density wave, fighting against the dissipation, keeping the
[38:48] Speaker 3: traffic jam locked in place for billions of years.
[38:50] Speaker 2: Well, what about the multi armed ones.
[38:52] Speaker 3: Well, the multi armed and floculent galaxies are much fewer
[38:55] Speaker 3: in number and are associated with lower central masses and
[38:58] Speaker 3: high chaotic starf formation.
[39:00] Speaker 2: So what does that imply for their longevity? If they
[39:02] Speaker 2: don't have the massive anchor, what happens to their density waves?
[39:06] Speaker 3: It strongly suggests their structures are entirely transient.
[39:09] Speaker 2: Or recurrent, meaning they don't last.
[39:11] Speaker 3: Right, A three armed or fluculent shape is likely not
[39:15] Speaker 3: a permanent state of being for a galaxy. It might
[39:18] Speaker 3: just be a temporary phase it goes through, perhaps lasting
[39:21] Speaker 3: a few hundred million years before the dynamics scramble the
[39:24] Speaker 3: arms entirely or before the galaxy accumulates enough central mass
[39:28] Speaker 3: to finally settle down.
[39:29] Speaker 2: They're continuously tearing themselves apart and reforming.
[39:32] Speaker 3: Yes, it's a temporary burst of chaos versus a permanent
[39:35] Speaker 3: structural equilibrium. But the researchers also introduced a truly fascinating
[39:41] Speaker 3: sub theory about the two armed galaxies themselves, didn't they
[39:44] Speaker 3: They did.
[39:45] Speaker 2: They suggest that we might actually be looking at two
[39:47] Speaker 2: completely different physical phenomena that just happened to produce the
[39:51] Speaker 2: exact same visual.
[39:52] Speaker 3: Shape, which is wild to think about.
[39:54] Speaker 2: They posit there might be two distinct subtypes of two
[39:56] Speaker 2: armed spirals, meaning what are some of them fake?
[40:00] Speaker 3: Two arms not fake, but driven by entirely different mechanics.
[40:04] Speaker 3: Some two armed spirals might be transient, like how For example,
[40:08] Speaker 3: if a galaxy without a massive bulge has a close
[40:11] Speaker 3: gravitational encounter with another passing galaxy, tidal forces can temporarily
[40:15] Speaker 3: whip its disc into a two armed shape, oh, just
[40:18] Speaker 3: from the gravitational pull of the neighbor right, but without
[40:21] Speaker 3: the internal mass to anchor it. That shape will eventually
[40:24] Speaker 3: fade as the wave dissipates.
[40:26] Speaker 2: But the other subtype, the other.
[40:27] Speaker 3: Subtype, the ones possessing the massive central bulges and the
[40:31] Speaker 3: dense dark matter cores are permanent fixtures. They are structurally
[40:35] Speaker 3: locked into that grand design shape for eternity by their
[40:39] Speaker 3: own overwhelming internal gravity.
[40:41] Speaker 2: That is deeply profound. We started this conversation by looking
[40:44] Speaker 2: at a picture of a spiral galaxy as a static,
[40:48] Speaker 2: simple object, a frozen pin wheel in.
[40:51] Speaker 3: The nice sky, and we end up here.
[40:53] Speaker 2: Right through the lens of Lord Rossi, Edwin Hubble, and
[40:56] Speaker 2: now the Euclid telescope, looking back ten billion years, come
[41:00] Speaker 2: to understand them as incredibly dynamic, deeply complex fluid engines.
[41:05] Speaker 3: They are driven by massive central bulges, fierce rotational share,
[41:10] Speaker 3: the frantic explosive churning of stellar nurseries, and the capable
[41:14] Speaker 3: invisible hand of dark matter halos.
[41:16] Speaker 2: And while we haven't completely mapped out every nuance of
[41:19] Speaker 2: how every single density waste sustains itself, we have crossed
[41:23] Speaker 2: a massive threshold.
[41:24] Speaker 3: We really have. We are no longer just catalog in
[41:26] Speaker 3: pretty shapes.
[41:27] Speaker 2: We are literally reading the anatomical structure of a galaxy
[41:31] Speaker 2: to decode the parts of the universe we cannot observe directly.
[41:34] Speaker 2: We are reading the shadows.
[41:35] Speaker 3: That's a beautiful way to put.
[41:36] Speaker 2: It, which leaves us with one final, very provocative thought
[41:39] Speaker 2: to mollover, if multi arms chaotic galaxies are these highly
[41:44] Speaker 2: active transient systems, burning through gas and churning out new stars,
[41:49] Speaker 2: and if massive settled bulges dictate stable, lower activity two
[41:54] Speaker 2: armed patterns, does that mean a two armed grand design
[41:57] Speaker 2: spiral is the ultimate retirement state of a galaxy.
[42:00] Speaker 3: It is a stunning extrapolation. As the universe continues to
[42:03] Speaker 3: expand in age, and as all that dark matter and
[42:06] Speaker 3: intergalactic gas finally settles down, condenses and merges into central
[42:10] Speaker 3: bulges over eons.
[42:11] Speaker 2: Well all the wild, multi armed, fluculent galaxies eventually run
[42:14] Speaker 2: out of chaotic energy.
[42:16] Speaker 3: Well, they all eventually evolve into grand stable, slowly rotating
[42:20] Speaker 3: two armed structures, drifting quietly in the darkening cosmos.
[42:23] Speaker 2: It is an incredible thing to think about. When you
[42:25] Speaker 2: step outside tonight and you look up into the dark sky.
[42:28] Speaker 2: Remember that the shapes you imagine out there are not
[42:30] Speaker 2: just swirling stars caught in the frozen snapshot. They are
[42:33] Speaker 2: a massive, billion year old cosmic traffic jams anchored by
[42:37] Speaker 2: supermassive black holes, locked in an invisible web of dark matter.
[42:41] Speaker 2: Slowly aging in the deep

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