# The Milky Way May Be Filled With Invisible Black Holes

An episode of [Bedtime Astronomy](https://invisiblenetwork.ai/podcast/1736397134), 22 Aug 2026, 46 min.

Audio: https://dts.podtrac.com/redirect.mp3/api.spreaker.com/download/episode/73484489/blackhp.mp3

Most stellar-mass black holes may be completely alone—and virtually invisible. In this episode, we explore new simulations suggesting that over 90% of the Milky Way's black holes have no companion star, making them extremely difficult to detect. Future missions like the Roman Space Telescope could finally uncover this hidden population, transforming our understanding of black holes and the evolution of our galaxy. 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.
[0:26] Speaker 2: I want you to just imagine something for a second.
[0:31] Speaker 2: You are standing outside on a perfectly clear, crisp night.
[0:36] Speaker 2: You're far away from the city lights, you know, maybe
[0:38] Speaker 2: out in the high desert of Chile or up in
[0:40] Speaker 2: the rocky mountains. It's the best spots, really, yeah, exactly.
[0:43] Speaker 2: So you let your eyes adjust to the dark, you
[0:46] Speaker 2: tilt your head back, and you just look up at
[0:48] Speaker 2: the night sky and it is completely overflowing with stars.
[0:52] Speaker 3: It's a staggering view.
[0:54] Speaker 2: It really is. You see the constellations you recognize, and
[0:57] Speaker 2: cutting right across the center of your view is that
[1:00] Speaker 2: that glowing, hazy, complicated structure of the Milky Way. Right
[1:04] Speaker 2: when you stand there taking it all in, it's totally
[1:07] Speaker 2: instinctual to think that you are looking at the universe.
[1:09] Speaker 2: You feel like you're seeing the true sensus of our galaxy.
[1:12] Speaker 3: You know, oh absolutely, we all do that.
[1:14] Speaker 2: But what if I told you that the most massive,
[1:16] Speaker 2: the most dense, and the most gravitationally powerful inhabitants of
[1:19] Speaker 2: that galaxy are entirely invisible to.
[1:22] Speaker 3: You, Yeah, hiding in plain sight?
[1:24] Speaker 2: Right? What if they are wandering silently in the dark,
[1:28] Speaker 2: just weaving through those starfields, completely undetected.
[1:31] Speaker 3: It really forces a total reevaluation of what we consider,
[1:36] Speaker 3: you know, the galaxy to actually be totally because we
[1:39] Speaker 3: are incredibly biased by our own biology. Our eyes evolve
[1:43] Speaker 3: to detect the electromagnetic radiation emitted by our sun, so
[1:47] Speaker 3: we naturally equate the presence of matter with the presence
[1:50] Speaker 3: of light.
[1:50] Speaker 2: If something is there, it shines, right.
[1:53] Speaker 3: We assume it shines, reflects, or glows. But the reality
[1:57] Speaker 3: of the Milky Way's mass distribution is it's much stranger
[2:01] Speaker 3: than that. How So, Well, the most significant stellar remnants,
[2:05] Speaker 3: specifically stellar mass black holes, they are not the ones
[2:08] Speaker 3: emitting blinding radiation.
[2:10] Speaker 2: Right, which is wild to think about.
[2:12] Speaker 3: It is. For over half a century, the entire field
[2:15] Speaker 3: of high energy astrophysics has built its models of black
[2:18] Speaker 3: hole demographics on this highly visible, incredibly loud minority.
[2:23] Speaker 2: Like the ones making all the noise.
[2:24] Speaker 3: Exactly, we've extrapolated our theories from the black holes that
[2:27] Speaker 3: are locked in tight binary systems, the ones interacting violently
[2:30] Speaker 3: with a companion star.
[2:32] Speaker 2: But there's a catch, right.
[2:33] Speaker 3: Yeah, there has always been this underlying mathematical suspicion that
[2:37] Speaker 3: these glowing X ray emitting black holes were the bizarre outliers,
[2:41] Speaker 3: not the standard.
[2:43] Speaker 2: Okay, let's mcact this because this is a fundamental shift
[2:46] Speaker 2: in how we understand the population dynamics of the Milky.
[2:49] Speaker 3: Way, a massive shift.
[2:50] Speaker 2: Today, we are going to explore this groundbreaking realization about
[2:55] Speaker 2: our hidden galactic underworld.
[2:57] Speaker 3: I love that turn galactic underworld.
[2:59] Speaker 2: It fits so well. We're going to look at how
[3:01] Speaker 2: new models, specifically some incredibly detailed computational work coming out
[3:06] Speaker 2: of the Flatteron Institute, are entirely rewriting the biography of
[3:10] Speaker 2: stellar mass black holes.
[3:12] Speaker 3: Right down to the very foundations.
[3:14] Speaker 2: We need to look at how they form, how they
[3:16] Speaker 2: survive the violence of their own birth, and how they
[3:19] Speaker 2: quietly traverse the galaxy without us ever knowing they were there.
[3:22] Speaker 3: To even begin grasping the scale of this hidden population,
[3:26] Speaker 3: we have to completely dismantle our observational.
[3:28] Speaker 2: Biases, which is hard to do.
[3:30] Speaker 3: It is very hard. The reason we are suddenly on
[3:33] Speaker 3: the hunt for isolated black holes requires us to look
[3:36] Speaker 3: at the historical limitations of how we know, as observers
[3:40] Speaker 3: bound to a rocky planet have been forced to study them.
[3:44] Speaker 3: It all comes down to the fundamental physics of the
[3:46] Speaker 3: event horizon and the mechanics of accretion.
[3:49] Speaker 2: Right, because by definition, an isolated stellar mass black hole
[3:53] Speaker 2: is the ultimate cosmic camouflage.
[3:56] Speaker 3: It really is.
[3:57] Speaker 2: We are talking about the collapsed core of a massive star,
[4:01] Speaker 2: pegging anywhere from what five to maybe fifty solar masses
[4:05] Speaker 2: into a sphere with a radius of a few dozen kilometers.
[4:08] Speaker 3: Yeah, incredibly dense.
[4:09] Speaker 2: And the escape velocity at the event horizon exceeds the
[4:12] Speaker 2: speed of light, so they cannot emit light at any
[4:15] Speaker 2: wavelengths none at all, not optical, not infrared, not ultra violet.
[4:19] Speaker 3: And this is exactly where the history of astronomy painted
[4:21] Speaker 3: us into a corner. Before the dawn of space based
[4:24] Speaker 3: X ray telescopes in the nineteen sixties and seventies, black
[4:28] Speaker 3: holes were largely a mathematical.
[4:30] Speaker 2: Curiosity, just equations on a chalkboard.
[4:33] Speaker 3: Exactly. They were a solution to Einstein's field equations of
[4:36] Speaker 3: general relativity that many physicists, including Einstein himself, at times
[4:41] Speaker 3: honestly thought nature would somehow find a way to prevent
[4:44] Speaker 3: from actually forming.
[4:45] Speaker 2: They thought it was too weird to be real, right.
[4:48] Speaker 3: It wasn't until instruments like the Uhuru satellite launched that
[4:52] Speaker 3: we started detecting these blazing points of X ray emission
[4:55] Speaker 3: in the sky like Signus x one and.
[4:58] Speaker 2: Sigmus x one. Wasn't just glowing, right, It was flickering
[5:01] Speaker 2: on these millisecond time scales.
[5:03] Speaker 3: Yes, which is the crucial detail.
[5:06] Speaker 2: Because if it flickers in a thousandth of a second,
[5:09] Speaker 2: the object itself camping much larger than the distance light
[5:12] Speaker 2: travels in a thousandth of a second.
[5:14] Speaker 3: That is the exact logical deduction that proved they existed
[5:17] Speaker 3: in reality.
[5:18] Speaker 2: It's so brilliant.
[5:19] Speaker 3: It really is elegant physics. But notice what we were
[5:21] Speaker 3: detecting there. We weren't detecting the black hole itself.
[5:24] Speaker 2: We were detecting the plasma exactly.
[5:26] Speaker 3: Almost all the stellar mass black holes we have definitively
[5:29] Speaker 3: characterized have been found in X ray binary systems.
[5:33] Speaker 2: So you have a compact object locked in a tight
[5:36] Speaker 2: gravitational dance with a massive main sequence companion star like
[5:41] Speaker 2: a blue supergin.
[5:42] Speaker 3: And that gravitational dance is anything but peaceful.
[5:45] Speaker 2: Oh, I imagine it's incredibly violent. It is.
[5:48] Speaker 3: If that companion star expands as it ages, or if
[5:51] Speaker 3: the orbit is just tight enough, the black hole's gravity
[5:55] Speaker 3: begins to literally strip the outer layers of hydrogen and
[5:58] Speaker 3: helium right.
[5:59] Speaker 2: Off the star, literally tearing its neighbor apart.
[6:01] Speaker 3: Yeah, it's a process called roche lobe overflow.
[6:04] Speaker 2: Rochlobe overflow.
[6:05] Speaker 3: Right, the companion star swells beyond its gravitational boundary, and
[6:10] Speaker 3: the stellar material gets funneled toward the black hole. But
[6:14] Speaker 3: it doesn't fall.
[6:14] Speaker 2: Straight in because everything in space is.
[6:16] Speaker 3: Spinning exactly because the system is rotating, the infalling gas
[6:21] Speaker 3: has angular momentum. It spirals inward, forming this flattened accretion
[6:25] Speaker 3: disc around the equator of the black hole.
[6:28] Speaker 2: I want to spin a second on the physics of
[6:29] Speaker 2: that disc because it's literally the only reason we've been
[6:33] Speaker 2: able to study black holes at all up to this point.
[6:35] Speaker 3: It is the only signal we get, and.
[6:37] Speaker 2: It's not just gas swirling down a drain right. It's
[6:40] Speaker 2: plasma behaving under extreme magneto hydrodynamic forces.
[6:45] Speaker 3: It conditions inside that disc are hellish, I mean truly hellish,
[6:50] Speaker 3: millions of degrees. As the gas spirals closer to the
[6:53] Speaker 3: event horizon, the orbital velocities approach a significant fraction of
[6:57] Speaker 3: the speed of light.
[6:58] Speaker 2: Wow.
[6:59] Speaker 3: But the inner our layers of the disc are orbiting
[7:01] Speaker 3: much faster than the outer layers, which creates immense shear, so.
[7:06] Speaker 2: They're grinding against each other.
[7:07] Speaker 3: Exactly, you have adjacent layers of plasma rubbing against each
[7:11] Speaker 3: other at near light speed. This viscous friction, which we
[7:15] Speaker 3: now understand is heavily driven by magnetic turbulence. Specifically, something
[7:20] Speaker 3: called the magneturitational instability heats the gas to millions of
[7:23] Speaker 3: degrees kelvin.
[7:24] Speaker 2: So it gets so hot that the thermal radiation peaks
[7:27] Speaker 2: in the X ray part of the electromagnetic spectrum.
[7:29] Speaker 3: Right. It blasts out high energy photons and in many
[7:32] Speaker 3: cases you get these powerful relativistic jets shooting out from
[7:35] Speaker 3: the magnetic pulse.
[7:36] Speaker 2: And those X rays are what hit the detectors on
[7:38] Speaker 2: our satellites YEAP.
[7:40] Speaker 3: Or conversely, we look at the companion star and we
[7:44] Speaker 3: observe its radial velocity.
[7:46] Speaker 2: So we see it wobbling exactly.
[7:48] Speaker 3: We see the spectral lines of the star blue shifting
[7:51] Speaker 3: and red shifting as it is whipped around this invisible
[7:53] Speaker 3: center of mass. We calculate the mass of the unseen
[7:56] Speaker 3: partner using Kepler's laws.
[7:58] Speaker 2: The old school orbital mass.
[7:59] Speaker 3: Right, hundreds of years old, and if the mass is
[8:02] Speaker 3: above the Tolmenoppenheimer vulcof limit, which is the theoretical maximum
[8:07] Speaker 3: mass for a neutron star around two to three solar masses,
[8:11] Speaker 3: we deduce that the invisible partner just has to be
[8:13] Speaker 3: a black hole.
[8:14] Speaker 2: So basically every piece of empirical data we have, the
[8:18] Speaker 2: mass distributions, the spin parameters, the orbital periods, it's all
[8:22] Speaker 2: derived entirely from systems where a black hole is actively
[8:25] Speaker 2: cannibalizing anaghbor. We see the destruction it causes, and we
[8:29] Speaker 2: build our models of galactic astrophysics based solely on the
[8:32] Speaker 2: perpetrators of that destruction.
[8:34] Speaker 3: Which brings us to the core problem of observational bias. Right,
[8:38] Speaker 3: what happens if a black hole does not have a
[8:40] Speaker 3: companion star, What if it is just wandering through the
[8:42] Speaker 3: interstellar medium entirely alone.
[8:45] Speaker 2: Well, the space between the stars is incredibly sparse.
[8:48] Speaker 3: Right it is. We call it a vacuum, but it's
[8:51] Speaker 3: not totally empty. It's filled with tenuous clouds of neutral
[8:54] Speaker 3: hydrogen and ionized gas.
[8:56] Speaker 2: But the density that interstellar medium is what maybe one
[9:00] Speaker 2: per cubic centimeter on average.
[9:02] Speaker 3: Yeah, in the cold neutral medium, maybe tens of atoms
[9:05] Speaker 3: per cubic centimeter, but still functionally a vacuum compared to
[9:09] Speaker 3: anything on Earth. Right, If an isolated black hole, say
[9:13] Speaker 3: ten times the mass of our Sun, moves through that medium,
[9:17] Speaker 3: it does exert a gravitational pull on that sparse gas.
[9:20] Speaker 2: It has, It has gravity, right.
[9:22] Speaker 3: It goes through a process called bondy hoil little tin accretion.
[9:26] Speaker 3: It sweeps up a tiny, tiny fraction of that gas
[9:29] Speaker 3: as it travels.
[9:30] Speaker 2: But compared to ripping entire layers off a supergent star,
[9:33] Speaker 2: sweeping up a few stray hydrogen atoms isn't going to
[9:36] Speaker 2: generate the kind of friction needed to produce.
[9:38] Speaker 3: X rays, not even close. The accretion rate is so
[9:41] Speaker 3: infinitesimally low that the gas barely heats up at all.
[9:44] Speaker 2: It's basically starving.
[9:45] Speaker 3: Yes, it certainly doesn't form a blazing optically thick, geometrically
[9:49] Speaker 3: thin accretion disc. The radiation emitted by a solitary black
[9:54] Speaker 3: hole accreting from the interstellar medium is so faint that
[9:58] Speaker 3: it is utterly undetectable by our current X ray or
[10:01] Speaker 3: radio observatories.
[10:02] Speaker 2: So they produce no glowing debris.
[10:05] Speaker 3: None, They leave no wake that we can see.
[10:07] Speaker 2: Wow, it's such a profound realization when you think about
[10:11] Speaker 2: the history of science.
[10:12] Speaker 3: It really makes you second guess everything, right.
[10:15] Speaker 2: Because our entire foundational knowledge of these cosmic objects, the
[10:20] Speaker 2: formation models we teach in universities, the evolutionary tracks we
[10:24] Speaker 2: assume they follow, is based purely on the messy eaters exactly.
[10:28] Speaker 3: We've been studying the anomalies.
[10:29] Speaker 2: Yeah, the extreme edge cases of stellar remnants, and just
[10:33] Speaker 2: assuming they represented the standard galactic population.
[10:35] Speaker 3: It's a classic selection effect.
[10:37] Speaker 2: Give me an analogy here, because it's so wild to
[10:39] Speaker 2: think about.
[10:40] Speaker 3: Okay, it's akin to trying to understand the demographics, behavior,
[10:43] Speaker 3: and daily habits of all marine life. But your only
[10:46] Speaker 3: method of observation is studying the fish that happen to
[10:48] Speaker 3: jump out of the water and land on the deck
[10:50] Speaker 3: of your boat.
[10:51] Speaker 2: That's hilarious and terrifying for.
[10:53] Speaker 3: Science, right, you are going to come away with a
[10:56] Speaker 3: highly skewed data set. You'll think the ocean is entirely
[10:59] Speaker 3: popular by flying fish.
[11:01] Speaker 2: You'd have no idea sharks or whales even existed.
[11:04] Speaker 3: Exactly, every time astronomy realizes that its view is limited
[11:08] Speaker 3: by its observational tools, the universe suddenly gets much larger
[11:12] Speaker 3: and much more complex. Acknowledging that we have been blinded
[11:15] Speaker 3: by the blazing light of X ray binaries is one
[11:18] Speaker 3: of those monumental paradigm shifts.
[11:20] Speaker 2: So if we accept this premise that there are invisible,
[11:25] Speaker 2: gravitationally immense objects wandering the galaxy completely silently, how do
[11:30] Speaker 2: we study them?
[11:31] Speaker 3: That is the million dollar question.
[11:33] Speaker 2: Right, Because if they don't emit X rays, if they
[11:36] Speaker 2: don't have a companion star to wobble around, we can't
[11:38] Speaker 2: point a telescope at them, we can't take a picture,
[11:41] Speaker 2: we can't use spectroscopy.
[11:42] Speaker 3: Nope, nothing traditional works.
[11:44] Speaker 2: So how do we even begin to figure out how
[11:45] Speaker 2: many of them there are or where they might be hiding.
[11:48] Speaker 3: Well, if you can't observe the current state of the
[11:50] Speaker 3: galaxy directly, you have to build your own galaxy from scratch, right, really. Yeah,
[11:54] Speaker 3: you have to rewind the clock to the formation of
[11:57] Speaker 3: the Milky Way and let the physics play out in
[11:59] Speaker 3: it controlled environment. You simulate the universe exactly until our
[12:04] Speaker 3: observational techniques catch up to our theoretical physics. Population synthesis
[12:09] Speaker 3: simulations are the absolute most powerful tool we have to
[12:12] Speaker 3: uncover this hidden population.
[12:14] Speaker 2: And this is where the new research comes in.
[12:16] Speaker 3: Yes, this is where the work led by Tom wag
[12:19] Speaker 3: and the team at the Flat Airn Institute becomes so critical.
[12:22] Speaker 1: Right.
[12:23] Speaker 3: They recognize this massive blind spot in the empirical data,
[12:27] Speaker 3: the fact that X ray binaries were skewing our understanding
[12:30] Speaker 3: of stellar death. So they decided to tackle the problem computationally. Okay,
[12:35] Speaker 3: they developed an incredibly detailed, massive scale simulation of the
[12:39] Speaker 3: Milky Way's entire evolutionary history.
[12:42] Speaker 2: Let's define what we mean by history here, because we
[12:44] Speaker 2: aren't talking about human time scales or even the life
[12:47] Speaker 2: scan of the Earth, right, We're talking about deep cosmological times.
[12:51] Speaker 3: We are talking about roughly thirteen billion years of galactic evolution.
[12:55] Speaker 2: Thirteen billion years.
[12:57] Speaker 3: Yeah, And to do this, they couldn't just model a
[12:59] Speaker 3: far stars. They had to model the entire life cycle
[13:03] Speaker 3: of a representative galactic population from start to finish. Yes,
[13:07] Speaker 3: they tracked how stars coalesce from giant molecular clouds of
[13:12] Speaker 3: gas and dust. They modeled how those stars evolve over
[13:16] Speaker 3: millions or billions of years, depending on their initial mass,
[13:19] Speaker 3: and crucially, they modeled the exact mechanisms of how those
[13:23] Speaker 3: stars eventually die.
[13:24] Speaker 2: I have to pause and push back on the methodology
[13:26] Speaker 2: here co for it because the immediate skeptical response, I
[13:29] Speaker 2: think for a lot of people is if a simulation is,
[13:33] Speaker 2: at its core just a really complex computer program, how
[13:37] Speaker 2: can we trust it to tell us about a population
[13:39] Speaker 2: of black holes we've literally never seen.
[13:41] Speaker 3: It's a fair question.
[13:42] Speaker 2: Are we in danger of circular reasoning? Like we program
[13:45] Speaker 2: our own assumptions into the code and the computer just
[13:48] Speaker 2: spits our assumptions back out at us.
[13:49] Speaker 3: Right, the garbage in, garbage out problem exactly.
[13:52] Speaker 2: If we don't know the exact star formation rate from
[13:55] Speaker 2: ten billion years ago, how can we trust the output
[13:57] Speaker 2: of the simulation today.
[13:59] Speaker 3: That is the necess serious skepticism that computational astrophysicists live
[14:02] Speaker 3: with every single day. The validation of these models is
[14:06] Speaker 3: the hardest part of the job.
[14:07] Speaker 2: So how do they justify it.
[14:09] Speaker 3: Here's why this isn't just an exercise in circular logic.
[14:12] Speaker 3: We aren't making up the rules of stellar evolution. We
[14:16] Speaker 3: are relying on the hard, empirically proven laws of thermodynamics,
[14:20] Speaker 3: quantum mechanics, and general relativity.
[14:23] Speaker 2: So you're saying the underlying physics engine of the simulation
[14:26] Speaker 2: is locked in by what we can prove in the
[14:28] Speaker 2: lab or observe in the nearby universe right now.
[14:31] Speaker 3: Precisely, we know the nuclear physics of how hydrogen fuses
[14:34] Speaker 3: into helium. We have excellent, rigorously tested models of stellar structure.
[14:40] Speaker 2: Stuff we've known for decades.
[14:41] Speaker 3: Yeah, we know that a star of a certain mass
[14:44] Speaker 3: with a certain metallicity meaning the amount of elements heavier
[14:47] Speaker 3: than helium it contains, will burn its fuel at a
[14:50] Speaker 3: specific rate, swell into a red giant or a wolf
[14:53] Speaker 3: rite star, and eventually undergo core collapse.
[14:56] Speaker 2: But what about the initial conditions? How do you know
[14:58] Speaker 2: what to feed into the simulation at the very beginning
[15:00] Speaker 2: thirteen billion years ago?
[15:02] Speaker 3: That is where things get complex and where we have
[15:04] Speaker 3: to use something called the initial mass function or IMF.
[15:08] Speaker 2: The IMF. Okay, what is that?
[15:09] Speaker 3: The IMF is an empirical rule that describes the distribution
[15:12] Speaker 3: of masses for a newly formed population of stars.
[15:15] Speaker 2: Oh, like a statistical bell.
[15:16] Speaker 3: Curve sort of. Yeah. Decades of observing stellar nurseries like
[15:20] Speaker 3: the Orion nebula have shown us that for every massive
[15:24] Speaker 3: O type starborn, nature produces hundreds of tiny, low mass
[15:29] Speaker 3: M type red dwarfs.
[15:30] Speaker 2: Okay, so a lot of small stars for every big one.
[15:33] Speaker 3: Right, we apply that statistical distribution to our virtual galaxy.
[15:38] Speaker 3: We also input the star formation history. How the rate
[15:40] Speaker 3: of starbirth peaked around what we call cosmic noon about
[15:44] Speaker 3: ten billion years ago and has slowly tapered off since then.
[15:47] Speaker 2: So you take the IMF, the star formation history, the
[15:49] Speaker 2: metallicity gradients across the galactic disk, and those iron cloud
[15:52] Speaker 2: rules of stellar evolution, and you just let the code
[15:55] Speaker 2: run forward.
[15:55] Speaker 3: Yes, and we include one more critical variable, the binary fraction.
[16:00] Speaker 3: We know from observing massive stars today that the vast
[16:04] Speaker 3: majority of them, the O and B type stars that
[16:07] Speaker 3: eventually become black holes, are not born alone. They are
[16:10] Speaker 3: born in binary or even triple.
[16:12] Speaker 2: Systems, which is so different from our Sun.
[16:15] Speaker 3: Very different. So the simulation has to track the gravitational interactions,
[16:19] Speaker 3: the mass transfer, and the tidal forces between these virtual
[16:22] Speaker 3: stars over millions of years. When you let all those
[16:25] Speaker 3: physical equations interact over cosmic time scales, the simulation isn't guessing.
[16:31] Speaker 3: It is calculating the inevitable mathematical reality of what must
[16:35] Speaker 3: be out there today.
[16:36] Speaker 2: It's essentially incredibly complex forensic accounting. But instead of tracking money,
[16:41] Speaker 2: you are tracking mass and momentum across billions of years.
[16:44] Speaker 3: I love that analogy, and we know the simulation is
[16:46] Speaker 3: largely accurate because we can check its present day output
[16:48] Speaker 3: against the things we can see too. Clever, right, If
[16:51] Speaker 3: the simulation finishes its thirteen billion year run and produces
[16:54] Speaker 3: a population of visible stars, white dwarfs, and X ray
[16:57] Speaker 3: binaries that actually matches the observed Milky.
[16:59] Speaker 2: Way, then you know the math works exactly.
[17:02] Speaker 3: Then we can be highly confident that the invisible population
[17:05] Speaker 3: it produced alongside them is also accurate. The visible universe
[17:09] Speaker 3: acts as a calibration tool for the invisible universe.
[17:12] Speaker 2: The computational scale of what you were describing is just staggering.
[17:17] Speaker 3: It is massive.
[17:18] Speaker 2: You are tracking the berths, the chaotic mass transfer phases,
[17:22] Speaker 2: and the spectacular deaths of millions of representative stellar populations.
[17:27] Speaker 2: You are calculating the gravitational dynamics. You are factoring in
[17:31] Speaker 2: the mass they lose through intense stellar.
[17:34] Speaker 3: Winds, which is a huge factor.
[17:36] Speaker 2: Because massive stars can blow away huge fractions of their
[17:39] Speaker 2: own mass before they even die.
[17:41] Speaker 3: Oh yeah, they shed mass constantly.
[17:43] Speaker 2: And you are doing all of this across a simulated
[17:45] Speaker 2: canvas representing the one hundred thousand light year disc of
[17:49] Speaker 2: the Milky Way.
[17:49] Speaker 3: It requires immen's computing power, often utilizing Monte Carlo methods
[17:53] Speaker 3: to handle the probabilistic nature of stellar evolution.
[17:57] Speaker 2: Rolling virtual dice based on physics.
[17:59] Speaker 3: Yes. And the reason you need that level of granular,
[18:02] Speaker 3: step by step detail is because the birth of a
[18:05] Speaker 3: black hole is not a clean, simple mathematical transition.
[18:08] Speaker 2: It's not a general fade to black No, it is.
[18:11] Speaker 3: One of the most violent, chaotic, and messy processes in
[18:15] Speaker 3: the universe. The simulation has to model the exact physical
[18:19] Speaker 3: mechanics of stellar death because the manner of death dictates
[18:22] Speaker 3: the entire future trajectory of the resulting black.
[18:25] Speaker 2: Hole, which brings us to the core collapse a big moment. Right.
[18:29] Speaker 2: This is where the simulation reveals why so many of
[18:32] Speaker 2: these black holes end up completely alone. Stellar mass. Black
[18:36] Speaker 2: holes are forged in the fires of core collapse supernovae.
[18:40] Speaker 3: Let's break down what actually happens inside the star because
[18:43] Speaker 3: the physics of the explosion are what drive the kinetics
[18:45] Speaker 3: of the black hole.
[18:46] Speaker 2: Okay, walk me through it.
[18:47] Speaker 3: Imagine a star twenty or thirty times the mass of
[18:49] Speaker 3: our Sun. For millions of years, it has maintained hydrostatic equilibrium.
[18:54] Speaker 2: Which means it's balanced right exactly.
[18:57] Speaker 3: The outward pressure of nuclear fusion in the core perfectly
[19:00] Speaker 3: balances the inward crush of its own immense gravity. It
[19:03] Speaker 3: foises hydrogen to helium, helium to carbon, carbon to oxygen
[19:07] Speaker 3: all the way down the periodic table until it builds
[19:10] Speaker 3: up a core of solid iron.
[19:11] Speaker 2: And iron is the ultimate cosmic dead end.
[19:14] Speaker 3: Iron is the ash of the stellar engine. You cannot
[19:18] Speaker 3: extract energy by fusing iron. It actually requires more energy
[19:22] Speaker 3: to fuse iron than the reaction releases.
[19:24] Speaker 2: It's an energy deficit.
[19:26] Speaker 3: Right, So suddenly the outward radiation pressure ceases. The core,
[19:30] Speaker 3: which is roughly the size of the Earth but contains
[19:33] Speaker 3: more mass than our Sun, is left entirely at the
[19:36] Speaker 3: mercy of gravity.
[19:37] Speaker 2: That is terrifying depiction.
[19:39] Speaker 3: It is it is supported only by electron degeneracy pressure,
[19:42] Speaker 3: which is white. It's a quantum mechanical rule that says
[19:45] Speaker 3: electrons resist being squeezed into the exact same quantum state.
[19:49] Speaker 2: But if the iron core exceeds the Chandi secral limit,
[19:52] Speaker 2: which is about one point four solar masses right at
[19:55] Speaker 2: one point four, then even quantum mechanics can't hold up
[19:57] Speaker 2: the weight of the star.
[19:58] Speaker 3: Gravity always wins. In a fraction of a second, the
[20:01] Speaker 3: core collapses inward at a significant fraction of the speed.
[20:05] Speaker 2: Of light a fraction of a second.
[20:07] Speaker 3: Literally milliseconds. The density becomes so extreme that electrons and
[20:11] Speaker 3: protons are crushed together to form neutrons, releasing a massive
[20:14] Speaker 3: burst of neutrios. If the progenitor star is massive enough,
[20:18] Speaker 3: the collax doesn't stop at the neutron star phase.
[20:20] Speaker 2: It just keeps going.
[20:21] Speaker 3: It continues collapsing entirely past its own event horizon, forming
[20:26] Speaker 3: a singularity. The black hole is born.
[20:29] Speaker 2: Meanwhile, the rest of the star, all those outer layers
[20:31] Speaker 2: of hydrogen, helium, and carbon, are suddenly left with no
[20:35] Speaker 2: core supporting them.
[20:36] Speaker 3: Right the floor falls out.
[20:37] Speaker 2: So they come crashing down in free fall. They hit
[20:39] Speaker 2: the unbelievably dense newly formed compact object at the center,
[20:44] Speaker 2: and they bounce.
[20:45] Speaker 3: The physics of that bounce are incredibly complex. The infalling
[20:48] Speaker 3: material hits a rigid wall of nuclear density, A shockwave
[20:52] Speaker 3: forms and begins propagating outward, blasting the star part well eventually,
[20:56] Speaker 3: but the shockwave actually stalls at first. It loses energy
[20:59] Speaker 3: as it tries to push through the immense weight of
[21:01] Speaker 3: the infalling star, and energy is lost through the photodisintegration
[21:05] Speaker 3: of iron nuclei.
[21:06] Speaker 2: So what pushes it over the edge.
[21:08] Speaker 3: What revives the shockwave, What actually powers the supernova explosion
[21:13] Speaker 3: is that initial burst of neutrinos from the core.
[21:15] Speaker 2: Oh wow. The neutrinos get trapped in the extremely dense
[21:19] Speaker 2: matter behind the shockfront, they deposit their kinetic energy, and
[21:23] Speaker 2: they literally blow the outer layers of the star into
[21:25] Speaker 2: space exactly.
[21:27] Speaker 3: The light from that explosion can briefly outshine the entire
[21:30] Speaker 3: host galaxy.
[21:31] Speaker 2: Now, in an idealized, simplified physics textbook, you might picture
[21:35] Speaker 2: that explosion expanding outward perfectly symmetrically right, like a perfect
[21:40] Speaker 2: expanding sphere of glowing plasma.
[21:42] Speaker 3: But nature abhors a perfect sphere, especially in a collapsing star. Right,
[21:46] Speaker 3: real stars are rotating, they have intense twisted magnetic fields,
[21:51] Speaker 3: and crucially, the region behind the stalled shockwave is boiling
[21:54] Speaker 3: with intense convective currents. The neutrino heating.
[21:57] Speaker 2: Is not uniform, so it's turbulent.
[22:00] Speaker 3: Highly turbulent. Astrophysicists model this using something called the standing
[22:04] Speaker 3: accretion shock instability or sassi okay SASA. Basically, the shock
[22:09] Speaker 3: front actually slashes back and forth inside the collapsing star.
[22:12] Speaker 2: Like water in a bathtub.
[22:13] Speaker 3: Yeah, exactly. This means that when the explosion finally breaks out,
[22:17] Speaker 3: it is almost always asymmetrical. The shockwave blasts out significantly
[22:22] Speaker 3: stronger in one direction than another, Plumes of high velocity
[22:25] Speaker 3: eject to shootout asymmetrically, and the neutrino emissions themselves might
[22:29] Speaker 3: be locksided.
[22:30] Speaker 2: And here is where we get to the kinematics that
[22:32] Speaker 2: change everything. Because of the conservation of momentum Isaac Newton's
[22:36] Speaker 2: third law of motion, that asymmetry has a massive physical
[22:40] Speaker 2: consequence for the newly formed black hole sitting at the
[22:44] Speaker 2: center of the blast.
[22:45] Speaker 3: For every action, there is an equal and opposite reaction,
[22:48] Speaker 3: because the supernova blasts more mass and kinetic energy in
[22:52] Speaker 3: one direction. The incredibly dense black hole left behind is
[22:56] Speaker 3: catapulted in the exact opposite direction.
[22:59] Speaker 2: It gets lawun.
[23:00] Speaker 3: Yes, we call it a natal kick or a supernova
[23:03] Speaker 3: recoil kick.
[23:04] Speaker 2: I always try to visualize this with the classic physics
[23:06] Speaker 2: analogy of firing a massive cannon while standing on a
[23:09] Speaker 2: completely frictionless sheet of ice.
[23:11] Speaker 3: It's a great visual right.
[23:13] Speaker 2: When the gunpowder detonates, the cannonball blasts forward, but the
[23:16] Speaker 2: heavy iron cannon itself recoils violently backwards, sliding across the ice.
[23:21] Speaker 2: In this scenario, the star's outer layers are the cannonball
[23:23] Speaker 2: blasting out into the interstellar medium, and the brand new
[23:26] Speaker 2: black hole is the heavy cannon recoiling in the opposite direction.
[23:29] Speaker 3: It's a highly effective analogy, but we have to scale
[23:32] Speaker 3: up the energy by orders of magnitude that are just
[23:35] Speaker 3: hard for the human brain to process. Sure Enough, the
[23:38] Speaker 3: flatter and simulation modeled the forces of these asymmetric supernobate
[23:42] Speaker 3: based on observational data from pulsar velocities, and the resulting
[23:46] Speaker 3: natal kicks are breath taking.
[23:48] Speaker 2: How fast are we talking?
[23:49] Speaker 3: We are talking about black hole's objects with the mass
[23:52] Speaker 3: of ten or twenty sons being instantly accelerated to velocities
[23:57] Speaker 3: of tens and sometimes hundreds of kilometers percent second relative
[24:01] Speaker 3: to their original orbit in the galactic.
[24:03] Speaker 2: Disk one hundred kilometers per second. That's roughly two hundred
[24:06] Speaker 2: and twenty thousand miles per hour, and it's a mass
[24:09] Speaker 2: of ten suns moving that fast. But the simulation revealed
[24:13] Speaker 2: that the high end tail of that velocity distribution is
[24:16] Speaker 2: even more extreme.
[24:17] Speaker 3: This is where it gets crazy.
[24:18] Speaker 2: Yeah, this brings us to the most mind bending statistic
[24:21] Speaker 2: from the entire study. Roughly three percent of all stellar
[24:24] Speaker 2: mass black holes ever formed in the Milky Way pick
[24:27] Speaker 2: up so much speed from their natal kicks that they
[24:30] Speaker 2: achieve escape velocity. Yes, they are kicked so hard they
[24:33] Speaker 2: literally break the gravitational bonds of the galaxy and leave
[24:35] Speaker 2: completely The.
[24:36] Speaker 3: Physics required to achieve galactic escape velocity are staggering. Think
[24:41] Speaker 3: about the gravitational potential well of the Milky Way.
[24:44] Speaker 2: It's huge.
[24:45] Speaker 3: You aren't just escaping the pull of the local star cluster.
[24:49] Speaker 3: You are fighting the combined gravitational mass of hundreds of
[24:52] Speaker 3: billions of stars, the super massive black hole at the
[24:55] Speaker 3: galactic center, and most importantly, the massive invisible halo of
[25:00] Speaker 3: dark matter that envelops the entire galaxy.
[25:03] Speaker 2: Depending on where the star is located in the galactic disc,
[25:06] Speaker 2: what kind of speed do you need to actually achieve
[25:08] Speaker 2: escape velocity?
[25:09] Speaker 3: In the solar neighborhood where we are, you need to
[25:11] Speaker 3: be moving at roughly five hundred and fifty kilometers per
[25:14] Speaker 3: second relative to the galactic center to escape the Milky
[25:17] Speaker 3: Way entirely.
[25:17] Speaker 2: Five hundred fifty kilometers per second, That.
[25:19] Speaker 3: Is over one point two million miles an hour.
[25:22] Speaker 2: Imagine the sheer localized cosmic violence required to generate a
[25:26] Speaker 2: recoil kick that accelerates a ten solar mass black hole
[25:29] Speaker 2: to over a million miles an hour. The energy scales
[25:32] Speaker 2: are terrifying.
[25:32] Speaker 3: It is one of the most energetic kinematic events in
[25:35] Speaker 3: the universe short of a supermassive black hole merger, and
[25:38] Speaker 3: the cosmological implications of that three percent figure are just.
[25:41] Speaker 2: Profound because it means they are out there exactly.
[25:45] Speaker 3: We tend to think of the intergalactic void the space
[25:47] Speaker 3: between galaxies, as entirely empty, just a freezing, radiation filled vacuum.
[25:54] Speaker 3: But if three percent of all stellar mass black holes
[25:57] Speaker 3: ever created in the history of our galaxy have been ejected.
[26:00] Speaker 3: That means the intergalactic medium is not empty at all.
[26:03] Speaker 2: It is populated by millions of these ejected ultrafast, completely
[26:08] Speaker 2: silent wanders.
[26:09] Speaker 3: Millions of them.
[26:10] Speaker 2: There are rogue black holes shooting through the nothingness between
[26:13] Speaker 2: the Milky Way and the Andromeda Galaxy. Right now.
[26:16] Speaker 3: They are essentially ballistic gravitational warheads cruising through the dark,
[26:21] Speaker 3: completely disconnected from any galactic structure. They will never interact
[26:24] Speaker 3: with another star, They will never accrete gas. They will
[26:27] Speaker 3: simply travel through the expanding void of the universe until
[26:30] Speaker 3: they evaporate via Hawking radiation trillions of years from now.
[26:34] Speaker 2: It sounds like the premise of a sci fi horror novel,
[26:36] Speaker 2: but it is just the inevitable mathematical result of momentum conservation.
[26:39] Speaker 3: Math is scary, sometimes it.
[26:41] Speaker 2: Really is okay, But let's reel it back in from
[26:43] Speaker 2: intergalactic space. Three percent are kicked so hard they leave
[26:47] Speaker 2: the galaxy entirely. What happens to the other ninety seven percent,
[26:51] Speaker 2: the ones that get a violent natal kick but not
[26:53] Speaker 2: quite hard enough to climb out of the dark matter halo.
[26:56] Speaker 2: What happens to them inside the Milky Way?
[26:58] Speaker 3: This is where we return to the binary p As
[27:00] Speaker 3: we discussed earlier, the vast majority of the massive zero
[27:04] Speaker 3: n B type stars that produce black holes are born
[27:07] Speaker 3: in binary bears. So imagine two massive stars orbiting each other.
[27:11] Speaker 3: The primary star, the more massive one, ages faster it
[27:14] Speaker 3: goes supernova. Two things happen simultaneously that spell the end
[27:18] Speaker 3: for that binary relationship.
[27:20] Speaker 2: First, there is the mass loss.
[27:21] Speaker 3: Right it's called the blowkick. When the star explodes, it
[27:25] Speaker 3: ejects a huge portion of its mass into space at
[27:28] Speaker 3: a significant fraction of the speed of light. Because gravity
[27:31] Speaker 3: is dependent on mass, the gravitational tether holding the two
[27:34] Speaker 3: stars together suddenly.
[27:36] Speaker 2: Drops because the mass is just gone.
[27:38] Speaker 3: Exactly, if the exploding star loses more than half of
[27:41] Speaker 3: the total mass of the binary system, the system becomes
[27:45] Speaker 3: gravitationally unbound. Immediately, the companion star just flies off in
[27:50] Speaker 3: a straight line, completely untethered.
[27:52] Speaker 2: That explains the runaway stars we sometimes see streaking across
[27:56] Speaker 2: the galaxy producing massive bow shocks in the interstellar medium.
[28:00] Speaker 2: They are the surviving companions of a broken binary.
[28:03] Speaker 3: Exactly, So you have the mass loss unbinding the system.
[28:06] Speaker 3: But then you add the asymmetric natal kick.
[28:08] Speaker 2: We just talked about the million mile an hour cannonball.
[28:11] Speaker 3: Right, the newly born black hole is suddenly rocketing off
[28:14] Speaker 3: at one hundred kilometers per second in a random direction.
[28:18] Speaker 3: Between the sudden loss of mass and the violent kinetic kick,
[28:21] Speaker 3: the overwhelming majority of binary systems do not survive the
[28:24] Speaker 3: supernova of the primary star. The gravitational bond just snaps, and.
[28:28] Speaker 2: This is where the FLATIRN simulation delivers its critical demographic senses.
[28:32] Speaker 2: The big reveal Yeah. By tracking millions of simulated stellar binaries,
[28:37] Speaker 2: calculating the mass loss during the supernova, and computing the
[28:41] Speaker 2: exact trajectories of the natal kicks, the modern reveals that
[28:44] Speaker 2: around ninety one percent of the Milky Way stellar mass
[28:47] Speaker 2: black holes are currently isolated. Ninety one percent, over nine
[28:51] Speaker 2: out of ten stellar mass black holes in our galaxy
[28:54] Speaker 2: are wandering completely alone. Yes, they are not in binaries,
[28:57] Speaker 2: they are not accreting gas from a companion. They are
[29:00] Speaker 2: not emitting X rays.
[29:01] Speaker 3: They are kinematically decoupled from their berth clusters, drifting silently
[29:06] Speaker 3: through the galactic disc and the galactic klo. There is
[29:09] Speaker 3: no glowing accretion disc to give them away. They are
[29:12] Speaker 3: functionally invisible.
[29:13] Speaker 2: This requires a massive cognitive restructuring of how we view
[29:16] Speaker 2: high energy astrophysics.
[29:18] Speaker 3: It does.
[29:19] Speaker 2: Let's trace the logic of what this means for the field.
[29:22] Speaker 2: If ninety one percent of the population is completely isolated
[29:25] Speaker 2: and invisible, then our entire textbook picture of a black hole,
[29:29] Speaker 2: the swirling, blazing discs of superheated plasma, the relativistic jets,
[29:34] Speaker 2: the violent consumption of a neighborstar is actually a bizarre,
[29:38] Speaker 2: rare exception. We have been looking at the surviving nine percent,
[29:42] Speaker 2: the ones that somehow manage to stay gravitationally bound after
[29:45] Speaker 2: the supernova, and assuming they represented the normal state of.
[29:48] Speaker 3: Affairs, the simulation forces us to confront the fact that
[29:51] Speaker 3: a normal stellar mass black hole is not a glowing
[29:55] Speaker 3: engine of cosmic destruction.
[29:57] Speaker 2: Far from it.
[29:58] Speaker 3: Right, the normal baseline state of a stellar remen is
[30:01] Speaker 3: a cold, silent, solitary nomad. It highlights a massive selection
[30:06] Speaker 3: bias that has permeated astronomy for decades.
[30:10] Speaker 2: It's like trying to formulate a comprehensive theory of human sociology,
[30:14] Speaker 2: but your methodology dictates that you are only allowed to
[30:17] Speaker 2: study people who are currently playing the drums in a
[30:19] Speaker 2: heavy metal band on a brightly lit stage.
[30:21] Speaker 3: That is exactly what it's like.
[30:23] Speaker 2: You are going to come away with a very skewed,
[30:25] Speaker 2: highly energetic idea of what the average human being does
[30:29] Speaker 2: on a Tuesday afternoon.
[30:30] Speaker 3: If you only study the heavy metal drummers, you assume
[30:32] Speaker 3: the entire population is loud, energetic, and constantly interacting with
[30:36] Speaker 3: an audience. You completely miss the vast majority of introverts
[30:39] Speaker 3: reading quietly in.
[30:40] Speaker 2: The library, the solid majority.
[30:43] Speaker 3: Exactly in the galactic context, ninety one percent of the
[30:46] Speaker 3: black hole population are extreme introverts.
[30:48] Speaker 2: And this isn't just a trivial demographic correction. This hidden
[30:52] Speaker 2: population fundamentally changes how we understand the mass distribution of
[30:57] Speaker 2: the Milky Way.
[30:58] Speaker 3: It changes everything.
[30:59] Speaker 2: It forces us to rewrite the models of galactic chemical
[31:02] Speaker 2: enrichment because the supernovae that form these black holes distribute
[31:07] Speaker 2: heavy elements differently depending on their kinetic environment.
[31:10] Speaker 3: Right, the metals get spread differently.
[31:12] Speaker 2: It even impacts the cutting edge field of gravitational way
[31:15] Speaker 2: of astronomy.
[31:16] Speaker 3: That is a crucial point. Actually, instruments like Lego and
[31:20] Speaker 3: Virgo detect the ripples in space time caused by two
[31:23] Speaker 3: black holes merging.
[31:24] Speaker 2: The chirp right, the chirp.
[31:25] Speaker 3: But if ninety one percent of black holes are born
[31:28] Speaker 3: single or become single immediately after birth, how are we
[31:31] Speaker 3: detecting so many mergers.
[31:32] Speaker 2: It's a paradox.
[31:33] Speaker 3: It forces theorists to heavily investigate dynamical assembly models, the
[31:38] Speaker 3: idea that these single black holes migrate into dense environments
[31:42] Speaker 3: like globular clusters, where they gravitationally capture each other in
[31:46] Speaker 3: close encounters, rather than evolving from isolated binary stars. You
[31:51] Speaker 3: cannot accurately calculate merger rates without knowing the true isolated
[31:55] Speaker 3: binary fraction. We have to account for the silent majority
[31:59] Speaker 3: to make the math work.
[32:00] Speaker 2: Okay, So this brings us to the ultimate observational challenge.
[32:04] Speaker 2: Thanks to the computational power of the flat iron models,
[32:07] Speaker 2: we now have this incredibly detailed simulated map of the galaxy.
[32:11] Speaker 3: We have the blueprint right.
[32:12] Speaker 2: We know it should be absolutely teeming with invisible solitary
[32:16] Speaker 2: black holes. We know roughly how many there are, in
[32:19] Speaker 2: the tens of millions to one hundred million. We know
[32:21] Speaker 2: where they should be clustered based on historical star formation rates,
[32:24] Speaker 2: and we know the velocity distribution of those natalkicks. We
[32:27] Speaker 2: possess the theoretical map. But how do we actually transition
[32:31] Speaker 2: from theoretical simulation to empirical discovery.
[32:35] Speaker 3: You have to get creative.
[32:36] Speaker 2: How do you hunt an invisible nomad?
[32:38] Speaker 3: You have to fundamentally change your observational paradigm. You have
[32:42] Speaker 3: to stop looking for the black hole itself if you
[32:45] Speaker 3: can't see it exactly. You have to stop looking for
[32:47] Speaker 3: X rays or accretion disks. Instead, you have to look
[32:50] Speaker 3: for the one physical property a black hole cannot hide,
[32:53] Speaker 3: no matter how isolated it is, it's gravity. You have
[32:57] Speaker 3: to look for the distortion of space time itself.
[33:00] Speaker 2: We are diving into the realm of general relativity. We're
[33:03] Speaker 2: talking about gravitational lensing exactly.
[33:05] Speaker 3: Albert Einstein's field equations demonstrated that mass dictates the curvature
[33:10] Speaker 3: of space time. Massive objects warp the fabric of the
[33:13] Speaker 3: universe around them.
[33:14] Speaker 2: Like a bowling ball in a trampoline.
[33:16] Speaker 3: The classic example. Now, light naturally wants to travel in
[33:19] Speaker 3: a straight line along geodesics. But if that light passes
[33:23] Speaker 3: near a massive, dense object like a black hole, the
[33:26] Speaker 3: curve geometry of space time forces the light path to bend.
[33:30] Speaker 2: So the black hole acts as a literal lens.
[33:32] Speaker 3: It operates on the exact same optical principles as a
[33:35] Speaker 3: glass lens, just driven by gravity instead of electromagnetism. Specifically,
[33:40] Speaker 3: astronomers were looking for a phenomenon called gravitational microlensing.
[33:44] Speaker 2: Let's break down the mechanics of microlensing, because the precision
[33:47] Speaker 2: required to detect this is just mind boggling, it really is.
[33:50] Speaker 2: Imagine you were trying to spot a perfectly clear, solid
[33:54] Speaker 2: glass sphere rolling across a forest floor that is covered
[33:58] Speaker 2: in brightly colored autumn.
[34:00] Speaker 3: Okay, great visual.
[34:01] Speaker 2: You cannot see the sphere itself. It is completely transparent,
[34:06] Speaker 2: doesn't emit any light of its own, and it blends
[34:08] Speaker 2: in perfectly with the shadows. But as it rolls directly
[34:13] Speaker 2: in front of a brightly colored red leaf, the curvature
[34:16] Speaker 2: of the glass gathers the light from that leaf and
[34:18] Speaker 2: focuses it. For a brief second, you see the leaf
[34:21] Speaker 2: behind the sphere suddenly look magnified and much brighter, and then,
[34:25] Speaker 2: as the sphere continues rolling, the leaf fades back to
[34:28] Speaker 2: its normal appearance. You spot the invisible sphere entirely by
[34:32] Speaker 2: watching how the background environment briefly distorts and magnifies as
[34:35] Speaker 2: the object passes by.
[34:36] Speaker 3: That is a perfect macroscopic visualization of microlensing. In the
[34:41] Speaker 3: galactic context, we aim our telescopes at a dense field
[34:44] Speaker 3: of distant background stars, usually looking toward the galactic bulge,
[34:47] Speaker 3: where the star density is highest.
[34:48] Speaker 2: Because you want maximum leaves on the floor.
[34:50] Speaker 3: Exactly, you want a busy background. If one of these solitary,
[34:54] Speaker 3: invisible black holes happens to drift directly between our telescope
[34:57] Speaker 3: and one of those distant background stars, the black hole's
[35:00] Speaker 3: intense gravity acts like that rolling glass sphere. It bends
[35:05] Speaker 3: and focuses the starlight toward Earth.
[35:07] Speaker 2: And what does that look like through a telescope.
[35:10] Speaker 3: Well, for a period of a few weeks or months,
[35:12] Speaker 3: depending on the relative velocity of the black hole, that
[35:15] Speaker 3: distant background star will appear to slowly brighten. It forms
[35:19] Speaker 3: a very specific symmetrical light curve called a patchy ski curve,
[35:23] Speaker 3: and then it will gradually fade back to its normal
[35:25] Speaker 3: baseline brightness as the black hole continues on its trajectory.
[35:29] Speaker 2: But here is the major observational hurdle, right, Oh yeah,
[35:32] Speaker 2: normal dim stars also have mass. If a faint ridge
[35:35] Speaker 2: wharf or a brown dwarf passes in front of a
[35:38] Speaker 2: background star, it will also act as a gravitational lens.
[35:41] Speaker 2: It will also cause the background star to temporarily brighten.
[35:45] Speaker 2: So how do we differentiate between a micro lending event
[35:48] Speaker 2: caused by a boring, low mass red dwarf and a
[35:51] Speaker 2: micro lensing event caused by a tense solar mass isolated
[35:54] Speaker 2: black hole.
[35:54] Speaker 3: That is the degeneracy problem in photometric microlensing degeneracy problem.
[36:00] Speaker 3: If you only measure the change in brightness, you cannot
[36:03] Speaker 3: definitively prove the mass of the lensing object. The timescale
[36:07] Speaker 3: of the brightening event is a combination of the object's mass,
[36:10] Speaker 3: its velocity, and its distance from Earth.
[36:13] Speaker 2: So a slow small thing looks like a fast.
[36:16] Speaker 3: Big thing exactly. A slow moving, low mass star could
[36:19] Speaker 3: produce the exact same brightening curve as a fast moving,
[36:23] Speaker 3: high mass black hole. To break that degeneracy, you need
[36:27] Speaker 3: a tremendous leap in observational technology. You need to move
[36:31] Speaker 3: beyond photometric microlensing and achieve astrometric microlensing.
[36:35] Speaker 2: Anstrometric microlensing astrometry is the precise measurement of a star's
[36:39] Speaker 2: position in.
[36:40] Speaker 3: The sky right when a massive black hole passes in
[36:43] Speaker 3: front of a star, it doesn't just magnify the light.
[36:45] Speaker 3: The asymmetrical bending of the light rays actually causes the
[36:48] Speaker 3: apparent position of the background star to shift on the sky.
[36:51] Speaker 3: It moves, the centroid of the light literally moves. It
[36:54] Speaker 3: looks like the background star has been physically dragged a
[36:57] Speaker 3: microscopic distance from its true location, tracing out a tiny
[37:01] Speaker 3: ellipse before turning to its original position.
[37:03] Speaker 2: So the background star gets brighter, and this like it
[37:06] Speaker 2: physically wobbled its position in the sky exactly.
[37:09] Speaker 3: And the size of that astrometric shift is directly proportional
[37:13] Speaker 3: to the mass of the lensing object. A low mass
[37:16] Speaker 3: red dwarf will cause a minuscule positional shift. A ten
[37:20] Speaker 3: solar mass black hole will cause a significantly larger astrometric shift.
[37:25] Speaker 2: That's the smoking gun.
[37:26] Speaker 3: It is if we detect that massive gravitational signature, the
[37:30] Speaker 3: long duration brightening combined with a large astrometric wobble, but
[37:34] Speaker 3: we detect absolutely zero light coming from the lensing object itself.
[37:38] Speaker 3: We know we have caught a ghost. We know we
[37:41] Speaker 3: have definitively found an isolated stellar mass black hole.
[37:45] Speaker 2: But we have to clarify the scale of the wabble.
[37:47] Speaker 2: We are talking about here.
[37:48] Speaker 3: Oh, it's incredibly small, right.
[37:49] Speaker 2: The distances involved are astronomical. We are measuring the positional
[37:53] Speaker 2: shift of a star tens of thousands of light years away,
[37:56] Speaker 2: caused by an invisible object that is also thousands of
[37:59] Speaker 2: light years away.
[38:00] Speaker 3: The required precision is almost unfathomable. We are talking about
[38:03] Speaker 3: measuring an astrometric shift of a few milliarc seconds or
[38:06] Speaker 3: even microorc second second. Yeah. To put that angular resolution
[38:11] Speaker 3: in perspective, a micro arc second is roughly equivalent to
[38:14] Speaker 3: measuring the diameter of a human hair located in Paris
[38:18] Speaker 3: from a telescope sitting in New York City.
[38:20] Speaker 2: That is just I mean, that's absurd.
[38:22] Speaker 3: It's an observational feat that was considered practically impossible until
[38:26] Speaker 3: the last few years. But this is exactly where the
[38:28] Speaker 3: future of astronomy is pivoting. We are currently building and
[38:32] Speaker 3: deploying the exact tools needed to execute this cosmic treasure hunt.
[38:37] Speaker 2: Let's talk about the hardware, starting with the Nancy Grace
[38:39] Speaker 2: Roman Space Telescope.
[38:41] Speaker 3: The Roman Space Telescope is going to revolutionize this specific
[38:45] Speaker 3: field of study. It is an infrared observatory designed with
[38:48] Speaker 3: a massive field of view roughly one hundred times the
[38:51] Speaker 3: field of view of the Hubble Space telescope, but with
[38:54] Speaker 3: the same deep resolution WOW, and because it operates in
[38:57] Speaker 3: the infrared, it can peer straight through the thick clouds
[39:01] Speaker 3: of interstellar dust that block optical.
[39:03] Speaker 2: Light, which is critical because you want to point the
[39:05] Speaker 2: telescope directly at the galactic center where the background stars
[39:08] Speaker 2: are most densely packed, to maximize your chances of seeing
[39:12] Speaker 2: in alignment.
[39:12] Speaker 3: Precisely, ROMAN is going to stare at hundreds of millions
[39:16] Speaker 3: of stars simultaneously in the galactic bulge, running a dedicated
[39:20] Speaker 3: microlensing survey. It will continuously monitor their brightness, watching and
[39:25] Speaker 3: waiting for those specific symmetrical brightening events that indicate a
[39:28] Speaker 3: massive object is passing through the line of sight.
[39:31] Speaker 2: So it's basically a ness Yes.
[39:33] Speaker 3: It is a statistical dragnet designed to catch these events
[39:36] Speaker 3: in bulk, but.
[39:37] Speaker 2: Roman primarily handles the photometric side the brightening. To get
[39:42] Speaker 2: that critical positional shift to measure the micro orc second
[39:45] Speaker 2: wobble that proves the mass of the black hole, we
[39:48] Speaker 2: have to look to Europe. We have to talk about the.
[39:50] Speaker 3: Gaya Mission oh GIA, the European Space Agency's Gaya Observatory
[39:55] Speaker 3: might be the most quietly revolutionary mission in the history
[39:58] Speaker 3: of astronomy.
[39:59] Speaker 2: It doesn't get a lot of press.
[40:00] Speaker 3: It doesn't because it doesn't produce the spectacular, colorful nebula
[40:04] Speaker 3: photos that the James Webspace Telescope does, so it doesn't
[40:06] Speaker 3: get as much public attention, but its scientific output is foundational.
[40:11] Speaker 3: Guya's singular purpose is astrometry.
[40:13] Speaker 2: It is essentially creating a three dimensional, dynamically moving map
[40:17] Speaker 2: of the Milky Way.
[40:18] Speaker 3: Gaya is positioned out at lagrange point two, slowly spinning,
[40:22] Speaker 3: using two telescopes with a fixed angle between them to
[40:25] Speaker 3: continuously scan the entire sky. It measures the exact positions, distances,
[40:31] Speaker 3: and proper motions of over a billion individual stars with
[40:34] Speaker 3: unprecedented micro ARC's second precision. And it does this over
[40:38] Speaker 3: and over again, right exactly as it scans the sky
[40:41] Speaker 3: repeatedly over its mission lifetime. It is perfectly positioned to
[40:44] Speaker 3: detect the minute astrometric shifts caused by invisible black holes
[40:48] Speaker 3: acting as gravitational lenses.
[40:50] Speaker 2: So you combine the deep wide field infrared photometric data
[40:54] Speaker 2: from the Roman Space Telescope with the ultra precise all
[40:58] Speaker 2: sky astrometric data from Gaia, you merge them you cross
[41:02] Speaker 2: reference the brightening events from Roman with the positional wobbles
[41:05] Speaker 2: from Gaia.
[41:06] Speaker 3: And you combine that with large scale spectroscopic surveys from
[41:09] Speaker 3: ground based telescopes which measure the radial velocities and chemical
[41:13] Speaker 3: made up of the background stars to ensure we understand
[41:15] Speaker 3: the baseline properties of the objects being lensed.
[41:18] Speaker 2: It's a team effort, it really is.
[41:20] Speaker 3: We are building a multi layered, multi wavelength observational net
[41:24] Speaker 3: specifically designed to catch these isolated nomads.
[41:27] Speaker 2: Which brings us completely full circle back to the Flatirn
[41:30] Speaker 2: Institute's models. Tom WAGS simulations aren't just an interesting theoretical
[41:35] Speaker 2: exercise in computational astrophysics. They are a literal treasure map.
[41:39] Speaker 3: That is exactly how the observational teams view it. The
[41:42] Speaker 3: simulation tells the Roman and GIA scientists what the expected
[41:46] Speaker 3: yield should be. It predicts exactly how many micro lensing
[41:49] Speaker 3: events they should expect to see per year, what the
[41:52] Speaker 3: duration of those events should be based on the velocity kicks,
[41:55] Speaker 3: and where in the sky these events are statistically most
[41:58] Speaker 3: likely to cluster.
[42:00] Speaker 2: Using this computational map, astronomers are perfectly positioned to test
[42:04] Speaker 2: their theories against the cold hard reality of the.
[42:06] Speaker 3: Universe the ultimate test.
[42:08] Speaker 2: Right, If Roman and guy A run their surveys and
[42:11] Speaker 2: they find the exact frequency and mass distribution of microlensing
[42:14] Speaker 2: events that the FLATIRN simulation predicted, then we have closed
[42:18] Speaker 2: the loop. We know our fundamental understanding of stellar death,
[42:22] Speaker 2: supernova kinematics, and galactic evolution is accurate. We will have
[42:26] Speaker 2: successfully mapped the hidden underworld of the Milky Way.
[42:29] Speaker 3: And scientifically speaking, what if they don't find what the
[42:32] Speaker 3: simulation predicts.
[42:34] Speaker 2: Oh, that's a good point.
[42:35] Speaker 3: What if the yield is far lower, where the mass
[42:38] Speaker 3: distribution is completely different. That outcome is almost more.
[42:41] Speaker 2: Exciting because it means we're wrong.
[42:43] Speaker 3: Yes, it would mean there is a fundamental flaw in
[42:47] Speaker 3: our understanding of core collapse physics, or the initial mass function,
[42:51] Speaker 3: or even gravity itself. It would mean we get to
[42:54] Speaker 3: discover entirely new physics to fix the models. Either way,
[42:58] Speaker 3: whether the models are valid shattered, we are on the
[43:01] Speaker 3: precipice of opening a completely new empirical window into the
[43:05] Speaker 3: dark sector of the galaxy.
[43:07] Speaker 2: It is deeply satisfying to see how modern science operates
[43:10] Speaker 2: at this level. You have theoretical physics dictating the rules
[43:13] Speaker 2: you have supercomputers simulating the macroscale results over billions of years,
[43:18] Speaker 2: and then you build highly specialized space telescopes to verify
[43:22] Speaker 2: the microscopic distortions in reality that the model predicted.
[43:25] Speaker 3: It is the scientific methods scaled up to galactic proportions.
[43:29] Speaker 2: Let's take a step back and just look at the
[43:30] Speaker 2: intellectual journey we've been on today.
[43:32] Speaker 3: It's been quite a ride, it has.
[43:34] Speaker 2: We started with a historical view of the universe that
[43:37] Speaker 2: was inherently biased, severely limited by our biological and technological
[43:42] Speaker 2: need for things to emit electromagnetic radiation in order to
[43:45] Speaker 2: be studied. We accepted a textbook paradigm of black holes
[43:49] Speaker 2: as these bright, ravenous, highly interactive binary systems.
[43:53] Speaker 3: We built our models on the chaotic, glowing nine percent.
[43:56] Speaker 2: Right, and then by tracing the thermodynamic lives of billions
[44:00] Speaker 2: of virtual stars, by factoring in the incredibly violent, asymmetrical
[44:05] Speaker 2: kinetic kick of a supernova core collapse, we arrive at
[44:08] Speaker 2: this stunning paradigm shifting realization. We are surrounded by a massive,
[44:13] Speaker 2: invisible ocean of solitary black holes, just drifting ninety one
[44:17] Speaker 2: percent of the population, tens of millions of them are
[44:19] Speaker 2: just drifting silently through the galactic void, and another three
[44:22] Speaker 2: percent were kicked with such ferocious energy that they've abandoned
[44:25] Speaker 2: the galaxy entirely wandering the freezing intergalactic space forever.
[44:29] Speaker 3: So poetry.
[44:29] Speaker 2: Honestly, we've gone from believing we understood the bright inhabitants
[44:33] Speaker 2: of the galaxy to realizing we are standing in the
[44:35] Speaker 2: middle of a dark, crowded forest filled with unseen leviathans.
[44:39] Speaker 3: This forces an incredibly important philosophical reckoning for the scientific community.
[44:43] Speaker 3: I think, how so well if our entire understanding of
[44:47] Speaker 3: stellar mass black holes, their behavior, their demographics, their formation channels,
[44:52] Speaker 3: their very nature was so thoroughly warped by a highly visible,
[44:56] Speaker 3: incredibly loud minority, what else are we missing?
[45:01] Speaker 2: Oh? Wow, that is a big question.
[45:02] Speaker 3: Right, What other fundamental, cosmic, physical, or even biological truths
[45:07] Speaker 3: are we currently misjudging simply because the actual majority of
[45:11] Speaker 3: a population doesn't draw any attention to itself. In astrophysics,
[45:15] Speaker 3: we are constantly in danger of assuming that the phenomena
[45:18] Speaker 3: which are easiest for our instruments to measure are the
[45:21] Speaker 3: most important forces in the universe. Go Good's all, we
[45:23] Speaker 3: can see exactly this hidden population of ninety one percent
[45:27] Speaker 3: isolated black holes is a humbling, critical reminder that the
[45:30] Speaker 3: universe is vast, it is mostly silent, and it is
[45:33] Speaker 3: largely indifferent to our observational limitations. The most pervasive and
[45:37] Speaker 3: powerful forces in nature are often the ones making the
[45:40] Speaker 3: absolute least amount of noise.
[45:42] Speaker 2: That is such a profound thought to leave on the
[45:45] Speaker 2: invisible majority shapes the gravitational architecture of the universe just
[45:49] Speaker 2: as much, if not more than the bright, explosive anomalies
[45:52] Speaker 2: that catch our eye. So the next time you find
[45:55] Speaker 2: yourself out on a clear night, far from the city lights,
[45:58] Speaker 2: and you tilt your head back to look up at
[46:00] Speaker 2: that glorious glowing band of the Milky Way, I want
[46:03] Speaker 2: you to appreciate the light of those billions of fusion engines.
[46:07] Speaker 2: But more than that, I want you to respect the
[46:09] Speaker 2: dark spaces in between them. Keep looking up, but never
[46:13] Speaker 2: stop thinking about the masses, silent things that you cannot see,
[46:16] Speaker 2: wandering quietly in the dark right above you.

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