For 50 years, dark matter explained why galaxies spin too fast.
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Dark Matter Might Not Exist. But MOND Might Be Wrong Too.
The INTO THE IMPOSSIBLE Podcast
Dark Matter Might Not Exist. But MOND Might Be Wrong Too.
BK
Speaker
Brian Keating
Astrophysicist Brian Keating discusses new Gaia spacecraft data challenging both dark matter and MOND theories for galaxy rotation curves, revealing unexpected declines in star velocities that question established galactic mass and gravitational models, hinting at exciting potential revisions in our understanding of cosmic structures and gravity.
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Highlights
“The mass term becomes approximately constant, so the predicted velocity falls off like 1 over the square root of radius.”
“The Mystery of Flat Galaxy Rotation Curves: "Flat rotation curves map where the missing gravity appears to live.”
“The Disputed Decline in Galactic Rotation Speed: "In this analysis, the inferred speed would fall by about 30 kilometers per second between 19.5 to 26.5 kiloparsecs from the center. The fitted outer slope is -0.47± 0.15.”
“But notice the phrase I just used— spherical equivalent. The Milky Way contains a disk, gas, a bulge, a warp, and a 3-dimensional halo.”
“Now, at this point, when you start to hear that the paradigm of dark matter may have failed to reproduce the dynamics, you might start to celebrate if you're a MOND advocate like my past guest Stacy McGaugh or the founder of the MOND paradigm, Mordecai Milgrom himself.”
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Full transcript
Its rival, MOND, explained the same thing with no new particles at all. New data from a European spacecraft suggests both of them predicted the wrong curve.
I'm Brian Keating. I build telescopes that map the early universe and monitor distant astronomical objects for signs of cracks in relativity. 400 years ago, Johannes Kepler worked out how fast a planet should orbit. The farther out you go, the slower it moves. Galaxies refused to do that. Dark matter was one answer.
Modifying Newton's equations was the other.
Then Gaia measured a billion stars in our own galaxy, and the number it got back was the one nobody expected.
So, start with Kepler. The solar system gave him the familiar picture. Mercury moves much faster than Earth, and Earth moves much faster than Jupiter. Neptune moves with the urgency of a teenager that you've asked to empty the dishwasher. The farther away you get from the Sun, the slower it orbits— not directly proportional, but actually proportional to 1 over the square root of the radius from the Sun. Good ol' Johannes worked this out over 4 centuries ago. Now Kepler had 3 laws. For a circular orbit, which most of the planets nearly follow, the speed that the planet orbits is determined by the gravitating matter enclosed within that orbit.
In fact, the equation is circular speed squared equals Newton's constant times the enclosed mass divided by radius. Newton actually proved mathematically why Kepler's laws are correct. Measure an orbital speed, and after assuming a geometry— basically circular— you can infer the gravitational force that's pulling on the orbiting planet. If the enclosed mass—within the orbital radius—stops increasing, the radius in the denominator keeps growing, but the numerator stays nearly fixed. Therefore, the speed must fall. Now let's replace the Sun with a galaxy containing only its visible gas and stars. Far outside the bright disk, almost all of that baryonic mass is entirely enclosed. The mass term becomes approximately constant, so the predicted velocity falls off like 1 over the square root of radius.
And we've seen that far, far out in many galaxies. Now here on the slide, the gray curve is the visible matter only prediction. If galaxies behaved like enlarged solar systems with all of the mass essentially being at the center, that would be the end of the story. The universe, though, has other plans, and a larger budget for the invisible accounting that dark matter seems to require. What astronomers actually found, dating back to Vera Rubin and her collaborators, was the blue curve. The outer velocity remains approximately constant. And if circular speed stays constant while radius increases, the enclosed those gravitating masses must keep increasing in roughly proportion to their radius. The luminosity of the galaxy fades away, but the gravitational influence does not.
And that mismatch is one of the clearest reasons dark matter became central to modern astrophysics. It's not the only reason, and it's not merely that galaxies spin too fast— it's that their radial pattern of motion implies more gravitating mass at larger radii than the visible mass can provide. Flat rotation curves map where the missing gravity appears to live. The standard explanation surrounds the visible disk with a much larger dark matter halo. The halo contributes little light, but it keeps adding enclosed mass as we move outward. Combine the disk with the halo and the rotation curve can remain essentially flat. That's what was observed. So where is the extra gravity coming from? These questions are not exactly identical, but there is some commonality between them, and the rotation curve addresses the second one.
The extra gravity. Now measuring our galaxy has one disadvantage: we're inside of it. This beach ball shows the perspective of God outside of it, but we're inside of it looking out. So it's like trying to infer the shape of a football stadium from inside the bleachers. Gaia gave us these exquisite measurements of stellar positions and their motions, but Gaia doesn't provide a button labeled the true Milky Way rotation curve, click here. That was easy. So we have to begin with proxies. We measure stellar positions and their velocities.
We correct for their distances. It's really good that Gaia is capable of doing that. We have to choose a model and correct for the asymmetric drift of stars that add peculiar effects. And we have to assume something about the equilibrium and symmetry of the physics of the problem. And then we can correct for a circular velocity and any biases that we may have induced. The instrument Gaia provides the data, and the pipeline tells us what we think those data points mean. Revolutionary claims need us to keep those chapters together. And one of the most important things is that stars do not travel on perfectly circular tracks.
Just like planets, they're not perfectly ellipticity-free, but they also have wobbles. They wobble radially and vertically, and stellar populations' average azimuthal speed is therefore lower than the circular speed of the gravitational field. That difference is called by professional astronomers asymmetric drift. Recovering the circular velocity requires a model of that random motion. Unfortunately, the correction matters most in the outer galaxies where stars become sparse, and that's the most important part because you have most of the enclosed mass within you the farther out you go. And there is where the disputed decline becomes interesting. In this analysis, the inferred speed would fall by about 30 kilometers per second between 19.5 to 26.5 kiloparsecs from the center. The fitted outer slope is -0.47± 0.15.
Even though it sounds close to -0.5, which would be 1 over square root of radius, a declining outer curve looks increasingly plausible. A precisely Keplerian decline is more debatable. Converting it into an extraordinarily low galactic mass is more model-dependent even still. If the circular velocity really fell as 1 over the square root of radius, then the velocity squared falls as 1 over radius. Now if you insert that into the circular velocity equation and the enclosed spherical equivalent mass becomes You move outward but infer surprisingly little additional gravitating mass, which is awkward for a large extended halo whose enclosed mass should keep growing. And that's the dominant paradigm for the dark matter picture, where galaxies are formed and held together and their rotation is driven by the enclosed mass within their visible light radius. But notice the phrase I just used— spherical equivalent. The Milky Way contains a disk, gas, a bulge, a warp, and a 3-dimensional halo.
Turning one curve into a mass Really requires a lot of geometrical insight. So where do we go next? The dramatic analysis produces an average total mass estimate of about 2.6× 10^11 solar masses. That's a lot— 260 billion equivalent solar masses. So this isn't actual stars, but it's stars, gas, and dust. That's below many independent estimates, which are closer to a trillion solar masses. The number comes from fitting the measured rotation curve and extending a mass model well beyond the region directly constrained by the stars alone. So this value is very interesting, but its precision should not hide the assumptions that produce it, as all good models have to incorporate. The Gaia-based rotation curve is constrained over roughly 9 to 27 kiloparsecs.
A Milky Way halo, though, may extend to something like 200 kiloparsecs. So when data extending nearly 30 kiloparsecs produce a total halo mass, most of the mass is supplied by that fitted model rather than that traced directly by the stars in that curve. Extrapolation is therefore unavoidable. But observed and extrapolated aren't synonyms. Inside, Gaia constrains the dynamics. Outside, we have to make theoretical choices to do most of the work. Now, at this point, when you start to hear that the paradigm of dark matter may have failed to reproduce the dynamics, you might start to celebrate if you're a MOND advocate like my past guest Stacy McGaugh or the founder of the MOND paradigm, Mordecai Milgrom himself. MOND proposes that below a characteristic acceleration, the effective dynamics depart from Newtonian relationships.
In the MOND region, the acceleration is approximately the square root of the Newtonian acceleration times the MOND scale, which is known as a0. For an isolated baryonic mass, this changes the expected orbital behavior without surrounding the galaxy with a conventional particle halo. So MOND has achieved real predictive successes at galactic scales, including the tight relationship between baryonic structure, and the observed acceleration. So if the dark matter paradigm's curve has a problem, does MOND just win? Now this is where it becomes deliciously inconvenient for both scenarios. How does MOND's predictions relate to what Gaia has observed? So in this low acceleration limit required for structure to ever form and not have too high a velocity dispersion, the velocity to the 4th power equals Newton's constant times the baryonic masses times this constant a0, this baseline acceleration. So what's missing from this equation? Radius. There's no radius in there. And that's why MOND produces an asymptotically flat rotation curve for an isolated galaxy.
That was one of the great attractions— no pun intended— that Mordecai and others were drawn in by. But now it's part of its vulnerability. It might be part of its downfall if these data are reproducible. If the Milky Way's outer curve is Keplerian, MOND is also expecting something flatter too, but it didn't find that. Dark matter and MOND arrive at the flat outer rotation curves through completely different physics. Dark matter says there's an additional gravitating mass. MOND says the low acceleration dynamics are different themselves. A robust Keplerian decline would challenge the simplest extended halo expectation and the deep MOND asymptotic behavior.
The question becomes not which of these 2 camps won, but why we're both expecting the wrong curve. So bad news for everybody is Often excellent news for science. I usually say that flaws lead to new laws. That's what I teach my students. When you find a crack, when you find something unexpected, as Einstein did with Newton's gravity, for example, and as MOND may have done with the dark matter paradigm, and as inflation did with the Big Bang paradigm, these are exciting times for scientists. A Keplerian decline would not instantly falsify every version of MOND. MOND is nonlinear, so an external gravitational field can influence a galaxy's internal dynamics and modify the Milky Way's outer behavior. The external field effect can generate a decline.
So the defensible conclusion is not that Gaia killed MOND. A robust Keplerian curve would instead create tension with the isolated prediction and require that the external gravitational field or another refinement do substantial quantitative work. The scientific test is whether the theory fits the measured curve with independent, justified parameters, not whether or not we can tell a cool story after seeing it. Now here's where I have to insert A warning. This is where experimentalists like me get interested, excited, but also a little bit nervous. Distance errors will alter both the star's inferred position and its tangential velocity. Any asymmetric drift corrections depend on the tracer density, the gravitational field, and the velocity dispersion— how much these stars are moving independently of the gravitational force of dark matter or MOND. Selection effects can change which stars enter the sample.
The warp violates simple disk geometry. Sagittarius and the Large Magellanic Cloud also drive non-circular motion— they're like outer gravitating masses. These tracers become sparse at great distances, and you have to question whether or not the stars are at actual equilibrium. Now, in the Gaia analyses, neglected dynamical terms and the systematic error budget grow towards the outermost radii. They increase. It gets harder and harder to do, and you get more and more contamination from external gravitating masses like the LMC. None of this proves the decline is false, by the way. It's a brilliant result.
It means that the blue curve we showed earlier may conceal a messy galaxy. Maybe that teenager's to blame. The more revolutionary the inference, the more carefully we have to distinguish between what Gaia measured from what we, or proponents of MOND or dark matter, would like to interpret.
Now, before you declare a winner, MOND has one more move. It isn't a linear theory, which means a galaxy sitting inside someone else's gravitational field doesn't behave like one sitting by itself. And the Milky Way sure ain't lonely.
As I said, the Milky Way is not some isolated, perfect galaxy that's relaxed in a laboratory just hanging out. The Sagittarius Dwarf Galaxy, nearby but not part of our galaxy, has reportedly crossed and perturbed our Milky Way's disk. The Large Magellanic Cloud is massive, it's nearby, and it's dynamical— it's rotating, it's doing its own thing too. Together with the Milky Way's galactic warp, these interactions can produce ripples, star streams, and north-south asymmetries between the upper and lower halves of the galaxy. These motions are valuable, but they're not necessarily indicative of equilibrium circular motion. So you wouldn't expect Kepler's law to actually hold in that sense. Force a disturbed population into a steady axisymmetric model, and the reconstructed curve will absorb the disturbance and present it as a modification to gravity. Sometimes the galaxy is telling us about dark matter, sometimes it's telling us that it recently had a close encounter of the third kind.
So where does it leave us? At this point, there's 3 possibilities that I would say remain viable. First, the decline is real, but it's moderate. The Milky Way has a lighter or more concentrated halo than some older models suggested. Dark matter and MOND both can adjust their parameters, tune them, and survive. Second, the decline is real, but it's exaggerated. It's affected by systematics and disequilibrium which were implicitly assumed in the models. It's the least glamorous answer, which is why scientists have to take it seriously. Third possibility: the outer disk is sufficiently disturbed that the reconstructed curve can never be assumed to be equilibrium and circular and represent the Keplerian profile at all.
So we have one pattern and 3 possible physical stories that explain it. The evidence that we have doesn't uniquely constrain or select between the 3 of them. So what would actually settle the tie, if you will? But there are different objects we can use. Young stars called Cepheids are dynamically colder, Stellar streams can probe objects farther out, and globular cluster satellites can test at larger radii. That's in fact how we knew the galaxy had a certain size from the beginning with the Shapley debate of the 1920s. Future Gaia releases improve their astrometry, the position and velocity. We'll also get radio astronomical surveys that will supply different tracers, typically of the gas. And better theoretical models can include the warp, the Sagittarius mini dwarf galaxy effect, and the LMC's effects as well explicitly.
If we combine those methods with different assumptions and different systematics, and we recover the same decline, then we'll have to listen. It'll go from 3 sigma to many, many sigma in that case, potentially. But our galaxy may be asking a nastier question. Not did dark matter lose, not did MOND win, but rather, are we expecting the wrong curve? So, what do you think is more preferable given the evidence we've presented today? MOND? dark matter, or something else entirely? Leave your comment below, give the video a thumbs up, exercise your thumb, and don't forget to share this like invisible dark matter throughout your own universe. I'm Brian Keating, Chancellor's Distinguished Professor of Physics at the University of California San Diego, and I'll see you next time on the channel.
And I'd like to conclude this video by thanking my good friend Alessandro Melchiorri and his collaborator Ruchika. They produced the paper that inspired this. It came out in August, it's still a preprint, but it's called The Rotation Curve of the Milky Way: State-of-the-Art The Keplerian Decline Debate and Implications for Dark Matter. It's a brilliant paper and anyone can understand it. They summarize the field, the history, and the controversy, so make sure you check that out. I'll leave a link in the description below. Kepler said that the outer stars should slow down. For 50 years, our galaxy said otherwise, and we invented an invisible halo to explain it.
Now our galaxy may be taking it back. If that changes how you think about what we actually know, subscribe and tell me which one you prefer. And don't forget to watch my interviews with Stacey McGaugh and with Mordecai Milgrom.
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💡 Speaker bios
Brian Keating, fascinated by the wonders of the cosmos, often starts his storytelling with Kepler’s revolutionary insights. He draws vivid connections, explaining how Johannes Kepler’s three laws of planetary motion—developed over four centuries ago—transformed our understanding of the solar system. With humor and clarity, Brian describes how Mercury races around the Sun while Neptune lags behind, their speeds governed by the gravitational pull of the material inside their orbits. Keating’s passion lies in making complex cosmic concepts accessible, blending historical discoveries with engaging analogies to spark curiosity and wonder about the universe.
🔖 Titles
Gaia Data Challenges Dark Matter and MOND—Are We Expecting the Wrong Galactic Rotation Curve?
Keplerian Decline in the Milky Way: Trouble for Both Dark Matter and MOND Theories
Is Dark Matter Failing? Milky Way Rotation Curve Surprises Challenge MOND Too
Milky Way Rotation Mystery: When Both Dark Matter and MOND Predictions Fall Short
What Gaia’s Galaxy Data Means for Dark Matter, MOND, and Kepler’s Laws
Dark Matter or MOND? Surprising New Data Suggests Both May Be Wrong
Unexpected Rotation Curve: Milky Way Data Upends Dark Matter and MOND Models
The Galactic Rotation Curve No One Predicted—What Does It Mean for Dark Matter?
Rethinking Galactic Gravity: Why Gaia Data Complicates Dark Matter and MOND Explanations
Disturbing Results: When Gaia’s Milky Way Data Challenges Everything We Know About Gravity
💬 Keywords
dark matter, MOND, rotation curve, Milky Way, Gaia spacecraft, Kepler's laws, Newton's constant, galaxies, baryonic mass, visible matter, galactic halo, gravitational force, orbital speed, Johannes Kepler, Vera Rubin, circular velocity, stellar positions, asymmetric drift, external field effect, Sagittarius Dwarf Galaxy, Large Magellanic Cloud, mass model, non-circular motion, equilibrium, velocity dispersion, systematic errors, stellar streams, globular clusters, Cepheid stars, astrometry, radio astronomical surveys
💡 Speaker bios
Brian Keating, inspired by the legacy of Johannes Kepler, finds wonder in the motions of our solar system. He reflects on Kepler’s revelation over four centuries ago: the farther a planet is from the Sun, the slower it moves, with its speed determined by the mass enclosed within its orbit according to Kepler’s laws. Through this lens, Keating shares the story of our cosmic clockwork, highlighting not only the beauty of planetary motion, but also the enduring insights of scientific discovery that shape how we understand our universe.
ℹ️ Introduction
Welcome back to the INTO THE IMPOSSIBLE Podcast. For the past 50 years, the mystery of why galaxies spin faster than they should has been attributed to the elusive presence of dark matter. On the other side of the cosmic debate, MOND—Modified Newtonian Dynamics—has offered an alternative, suggesting that no new particles are required and that our understanding of gravity itself might need an upgrade. But what if both ideas are missing the mark? In this episode, Brian Keating explores groundbreaking new data from the European Gaia spacecraft that challenges both the dark matter and MOND paradigms, revealing that the rotation curve of our very own Milky Way doesn’t behave as either theory predicts. Are we overlooking the true nature of our galaxy? Grab your telescopes—and maybe a pinch of skepticism—as we dive into the history, data, and scientific tensions reshaping our understanding of the cosmos.
📚 Timestamped overview
00:00 The section discusses how the discrepancy between observed galactic rotation curves and visible mass distributions supports the existence of dark matter halos surrounding galaxies, which provide additional gravitational influence.
05:08 The text discusses the plausibility of a declining outer curve in circular velocity, the complexity of translating it into galactic mass models, and the challenges it poses to the conventional dark matter paradigm with the Milky Way's varied components.
07:31 In the MOND region, the acceleration is derived from the square root of the Newtonian acceleration times the MOND scale a0, altering expected orbital behavior without needing a particle halo, and producing an asymptotically flat rotation curve for an isolated galaxy, although its compatibility with Gaia's observations and the lack of reliance on radius present challenges for both MOND and the dark matter paradigm.
11:36 The Milky Way's interactions with the Sagittarius Dwarf Galaxy and the Large Magellanic Cloud cause disruptions like ripples and star streams, challenging equilibrium models and potentially indicating either modified gravity or recent galactic encounters.
13:07 The section discusses using young stars, stellar streams, and globular cluster satellites to distinguish between three physical stories explaining a pattern, with future Gaia data and radio surveys offering improved astrometry and new tracers, alongside theoretical models considering additional galactic influences.
📚 Timestamped overview
00:00 Understanding dark matter halos
05:08 Debating Galactic Mass and Structure
07:31 MOND theory and galactic predictions
11:36 Galactic interactions and disturbances
13:07 Galactic structure and measurement methods
❇️ Key topics and bullets
Sequence of Topics Covered
1. Introduction to Galaxy Rotation and Competing Theories
Brian Keating notes dark matter's role in explaining galaxy spin for 50 years 00:00:00
MOND (Modified Newtonian Dynamics) offers an alternative with no new particles 00:00:03
New data suggests neither dark matter nor MOND matches observed rotation curves 00:00:07
2. Historical Context: Kepler, Newton, and Galactic Motion
Brian Keating explains Kepler's laws and expectations for planetary motion 00:00:19
Application of Keplerian expectations to galaxies 00:00:24
Introduction of dark matter and modifying Newton’s equations as proposed solutions 00:00:27
3. Observational Discrepancies and Dark Matter
Summary of expected vs. observed rotation curves ("gray curve" vs. "blue curve") 00:01:52
Historical discovery by Vera Rubin and colleagues 00:02:12
Explanation of why galaxies’ outer rotation curves are flat, not declining 00:02:17
Introduction of the dark matter halo surrounding galaxies 00:02:51
4. Measuring the Milky Way’s Rotation Curve
Challenges in measuring from inside our own galaxy 00:03:18
Gaia mission’s role in providing stellar positions and motions 00:03:32
Corrections needed for models: distance, asymmetric drift, and equilibrium assumptions 00:03:47
Issues with non-circular orbits and stellar wobbles 00:04:19
Asymmetric drift and its significance 00:04:32
Details on the specific Gaia-inferred outer rotation curve decline 00:04:52
5. Interpreting Rotation Curves: Keplerian vs. Dark Matter vs. MOND
Consequences if Milky Way's rotation curve declines Keplerian fashion 00:05:08
Galaxy mass estimates from rotation curves 00:06:08
Limitations due to geometry and model assumptions 00:06:02
The extrapolation from Gaia's direct measurement to larger halo sizes 00:06:45
6. MOND Theory and its Predictions
MOND's different physical assumptions for low acceleration galaxies 00:07:24
Mathematical form of MOND's predictions and key parameters 00:07:31
No radius dependence in deep MOND regime, resulting in flat rotation curves 00:08:25
Dilemma: both dark matter and MOND expect flat outer curves, yet Gaia finds a decline 00:08:39
7. Scientific Implications and Open Questions
A Keplerian decline challenges both dark matter and MOND expectations 00:08:59
The role of external gravitational fields in MOND 00:09:38
The importance of fitting models to data with justified, independent parameters 00:10:02
8. Experimental Challenges and Systematic Errors
Distinction between Gaia’s direct measurements and interpretation 00:10:10
Complications due to asymmetric drift corrections, tracer density, and velocity dispersion 00:10:23
Effects of galaxy warp, Sagittarius, and Large Magellanic Cloud perturbing the Milky Way 00:10:36
Increased systematic uncertainties at greater galactic radii 00:10:57
9. Non-Equilibrium Dynamics in the Milky Way
Milky Way disturbances due to interactions with nearby galaxies 00:11:41
Influence of warp, Sagittarius Dwarf Galaxy, and LMC on stellar motion 00:11:49
Impact of disequilibrium on rotation curve reconstruction 00:12:10
10. Possible Explanations and Future Directions
Three scenarios for the observed rotation curve:
The decline is real and moderate; both theories could be adjusted 00:12:35
The decline is real but exaggerated by systematic effects 00:12:45
The outer disk is too disturbed for equilibrium or circular models to be valid 00:12:53
The need for alternative tracers: Cepheids, stellar streams, globular clusters 00:13:21
Future Gaia releases and improvements in modeling and data combination 00:13:36
11. Concluding Thoughts and Invitation for Further Exploration
Reiteration that evidence does not uniquely select an explanation yet 00:13:16
Importance of combining methods and accounting for systematics 00:13:53
Open questions for the audience on preference for dark matter, MOND, or another theory 00:14:08
12. Acknowledgments and Further Reading
Gratitude to Alessandro Melchiorri and Ruchika for their inspiring paper 00:14:37
Recommendation to read "The Rotation Curve of the Milky Way: State-of-the-Art The Keplerian Decline Debate and Implications for Dark Matter" 00:14:45
Encouragement to continue exploring the topic and related interviews 00:15:16
👩💻 LinkedIn post
🚀 New data is shaking up our understanding of the universe! For decades, dark matter and its rival theory MOND have vied to explain why galaxies spin faster than visible matter allows. But recent results from the Gaia spacecraft suggest that neither theory may have the full story—and the Milky Way might be asking us to rethink what we expect entirely.
🔑 Key Takeaways:
The Gaia mission has measured the motions of a billion stars, revealing a declining Milky Way rotation curve that challenges standard dark matter and MOND expectations.
Both dark matter and MOND predict flatter rotation curves at the galaxy’s outskirts, but the new results hint at a more “Keplerian” (declining) curve, raising deep questions for theorists.
Systematic uncertainties and galactic disturbances (like interactions with nearby galaxies) mean we need multiple lines of evidence before rewriting the textbooks.
The universe continues to surprise us—flaws in our models might just be the doorway to new laws of physics. Are we missing something fundamental? Let’s keep exploring!
#astrophysics #darkmatter #MOND #science #research #Gaia #milkyway
Catch more insights on The INTO THE IMPOSSIBLE Podcast with Brian Keating.
🧵 Tweet thread
🚨 DARK MATTER vs. MOND — BOTH CAUGHT OFF GUARD? 🚨
1️⃣ For 50 years, dark matter explained why galaxies spin way too fast. Its rival, MOND, said: "No new particles needed!" But a bombshell from ESA’s Gaia telescope just turned the tables — both got the Milky Way’s rotation curve wrong! 00:00:03
2️⃣ Quick recap: Kepler (400 years ago!) showed the farther a planet is from the Sun, the slower it moves. If that held for galaxies, outer stars should slow down too, right? Not so fast. Galaxies break all the rules. 00:00:24
3️⃣ The classic “flat rotation curve” problem: Measure real galaxy speeds and the numbers just won’t fall off fast enough. Solution? Dark matter halos — invisible mass, massive influence. 00:02:12
4️⃣ MOND fans (Modified Newtonian Dynamics) counter: Maybe gravity itself changes at huge scales — no extra matter needed! Both dark matter and MOND could fit the old data… but the Gaia spacecraft just rewrote the rulebook. 00:07:24
5️⃣ Gaia tracked a BILLION stars with unprecedented detail. When Brian Keating and others analyzed the results, the Milky Way’s outer stars do slow down — the curve actually drops. Not flat, not fitting either major theory’s expectations! 00:04:52
6️⃣ Why does this matter? If star speeds fall with distance, there’s less unseen gravity out there than dark matter predicts. But MOND’s equations don’t fit either — it expects a flat speed! 00:08:21
7️⃣ It gets even messier: The real Milky Way isn’t a simple disk in equilibrium — it’s being poked, prodded, and warped by neighbors like the Large Magellanic Cloud. That messes with the data and the models. 00:12:10
8️⃣ Brian Keating: Sometimes the galaxy tells us about dark matter, sometimes it’s just “had a close encounter of the third kind.” Maybe we’re all expecting the wrong curve entirely! 00:12:22
9️⃣ Where do we go from here?
Maybe the Milky Way’s dark matter is simply more compact than thought
Maybe the models are misled by a messy, perturbed galaxy
Maybe (gulp) we need a whole new idea 00:13:29
🔟 Future Gaia data + new star tracers will help settle the score. Until then? EVERYONE is uncomfortable — but as Brian Keating says, “Flaws lead to new laws.” This is how real science evolves. 00:09:16
📢 Which do you think wins: dark matter, MOND, or something wild and new? Drop your thoughts and let’s debate the fate of our universe! 👇🪐 #Physics #DarkMatter #MOND #Gaia #MilkyWay
🗞️ Newsletter
INTO THE IMPOSSIBLE Podcast Newsletter
Episode Highlight: "Dark Matter Might Not Exist. But MOND Might Be Wrong Too."
Greetings Explorers of the Impossible,
Our latest episode shakes the foundations of modern astrophysics, questioning everything we thought we knew about dark matter, gravity, and the way galaxies spin.
🌌 FEATURED EPISODE RECAP
For 50 years, dark matter has been the go-to explanation for why galaxies like ours spin at seemingly impossible speeds. Its main competitor, MOND (Modified Newtonian Dynamics), needs no mysterious particles—just a tweak to Newton's laws. But, as Brian Keating reveals in this eye-opening talk, new data from the Gaia spacecraft shows that both theories might be missing the mark altogether 00:00:03.
Key Moments:
Kepler's Legacy vs. Galactic Reality
Brian Keating brings us up to speed on Kepler’s laws, explaining why, based on visible matter, the outer stars in a galaxy should slow down 00:00:24. Yet, observations say otherwise—galaxies maintain unexpectedly high speeds in their outskirts 00:02:17.The Flat Rotation Curve Problem
This mismatch led to the invention of dark matter halos—vast, invisible masses that could account for the missing gravity 00:02:53. MOND, alternatively, suggests the laws of gravity themselves change at low accelerations 00:07:24.Gaia’s Revelations
Gaia mapped a billion stars, allowing researchers to derive our own galaxy’s “rotation curve.” Shockingly, neither dark matter nor MOND gets the curve exactly right according to the latest data 00:00:32.Three Possible Explanations… and More Questions
There's a real, possible decline in orbital speed. Is it a lighter-than-expected halo? Model errors? Or is our galaxy simply too disturbed for classic models to even apply? 00:13:07
🎧 Listen & Reflect
This isn’t just a technical challenge; it’s a deep philosophical one. Are we expecting the wrong answers from our universe? Brian Keating urges us to remain skeptical, hungry for data, and ready to rewrite our textbooks.
What do you think? Is dark matter doomed? Is MOND the answer, or do we need to think even bigger? Share your thoughts—your feedback shapes future episodes!
📚 Further Reading
Inspired by a brilliant preprint from Alessandro Melchiorri and Ruchika, this topic is at the cutting edge of space science. Brian Keating recommends reading “The Rotation Curve of the Milky Way: State-of-the-Art, The Keplerian Decline Debate and Implications for Dark Matter” (link in episode description) 00:14:45.
🔭 Stay Tuned
Next time, we’ll dive deeper into cosmic frontiers—with new guests and more impossible questions.
If this episode changed how you see the universe, subscribe, rate, and share INTO THE IMPOSSIBLE with a friend—or a skeptic!
Clear skies,
The INTO THE IMPOSSIBLE Team
---
Missed earlier interviews? Check out Brian Keating’s discussions with Stacy McGaugh and Mordecai Milgrom 00:15:16.
Share your reflections or questions—we love hearing from our curious community!
❓ Questions
Discussion Questions
How did the original observations of galaxy rotation curves challenge the predictions of Newtonian gravity and Kepler's laws, and what role did this play in motivating the dark matter hypothesis?
In what ways do the predictions made by MOND (Modified Newtonian Dynamics) differ fundamentally from those of the dark matter paradigm, especially regarding the rotation curves of galaxies?
What new findings did the Gaia spacecraft contribute to our understanding of the Milky Way's rotation curve, and why were these findings unexpected by both MOND and dark matter theories?
How does being located inside the Milky Way present unique challenges for accurately determining the galaxy's mass distribution and rotation curve?
What is "asymmetric drift," and why is correcting for this effect particularly important in the outer regions of galaxies?
Why might recent data showing a Keplerian decline in the Milky Way’s rotation curve pose problems for both the standard dark matter halo model and for MOND?
Brian Keating mentions external gravitational fields, such as those from the Large Magellanic Cloud and the Sagittarius Dwarf Galaxy. How can interactions with such neighbors impact our interpretation of galactic dynamics?
The episode suggests that both dark matter and MOND could be "expecting the wrong curve." What does this mean for the process of scientific theory development and revision in astrophysics?
What alternative observations or objects (e.g., Cepheids, stellar streams, globular clusters) does Brian Keating propose to further test these competing theories, and how might they help resolve outstanding uncertainties?
Given the current uncertainties and interpretations, do you find it more plausible that dark matter exists, that MOND is correct, or that a different explanation altogether is needed? Why?
curiosity, value fast, hungry for more
✅ Are we chasing ghosts in the cosmos?
✅ Brian Keating dives into fresh data that challenge BOTH dark matter and MOND theories—what if our galaxy is telling us something new?
✅ The INTO THE IMPOSSIBLE Podcast unpacks a scientific mystery: Gaia’s latest measurements suggest our understanding of galactic rotation might be fundamentally flawed.
✅ Maybe it’s not about picking a side—maybe our entire curve is wrong. Don’t miss this cosmic rethink!
Conversation Starters
Conversation Starters for Facebook Group Discussion
The new Gaia data seems to challenge both dark matter and MOND predictions for our galaxy’s rotation curve. What are your initial thoughts—does this suggest a need for an entirely new theory? Why or why not?
Brian Keating mentions that “flaws lead to new laws” at 09:16. Can you think of historical examples in science where unexpected data led to a major paradigm shift?
After decades of believing in flat rotation curves, how surprising is it to hear the Milky Way may actually show a Keplerian decline at great distances? What implications do you think this could have?
If you had to choose between MOND and dark matter as a preferable explanation based on the current evidence, which would you pick and why?
The episode describes three possible interpretations for the rotation curve data (13:07): a lighter halo, systematic errors, or a fundamentally disturbed galaxy. Which do you find most convincing, and what evidence would sway you?
How do interactions with nearby galaxies (like the Large Magellanic Cloud), as discussed by Brian Keating at 12:10, affect our ability to interpret galactic dynamics? Should we consider the Milky Way a special case?
The Gaia mission provides unprecedented data, but Brian Keating cautions about systematic uncertainties and model dependencies (10:17). How important is it for us to remain skeptical of new results, even when they challenge established ideas?
Beyond the data debate: What do you think is more exciting for physics—a finding that confirms an existing theory, or one that challenges everything we thought we knew?
Has this episode changed your view on the “invisible accounting” of dark matter? Does the idea of modifying gravity (MOND) feel more or less plausible in light of these results?
What experiments or observations would you like to see next to help settle the Milky Way’s rotation curve debate?
🐦 Business Lesson Tweet Thread
1/ The universe just threw astrophysics a curveball. What if the rotation of our own galaxy breaks all the rules?
2/ For 50 years, dark matter was the hero—explaining why galaxies spin so fast.
3/ MOND showed up as the maverick: “Change the rules, forget new particles.”
4/ Gaia satellite mapped a billion stars. The results? Neither theory nailed it.
5/ Kepler’s old law says the farther from the center, the slower you go. But galaxies don’t listen.
6/ Astronomers expected a flat “rotation curve.” That’s where dark matter or MOND took over.
7/ But Gaia says: slow decline. Maybe not flat after all. Maybe even a Keplerian drop-off.
8/ Bad news for both teams. Dark matter expects more mass out there. MOND expects different physics.
9/ If both are wrong, what are we missing? Maybe we’re asking the wrong question.
10/ Lesson: When reality refuses your story, rewrite your questions. That’s where breakthroughs emerge.
✏️ Custom Newsletter
🚀 Into the Impossible: Is Dark Matter or MOND Right… Or Are BOTH Wrong? 🌌
Hey Cosmic Explorers,
We just dropped a galaxy-brain-busting new episode of the INTO THE IMPOSSIBLE Podcast: “Dark Matter Might Not Exist. But MOND Might Be Wrong Too.” Hosted by your friendly neighborhood telescope builder, Brian Keating, this one dives into the big mysteries spinning at the heart of our own Milky Way!
🌟 In This Episode, You’ll Discover:
How Dark Matter and MOND stack up
– 50 years of cosmic detective work and why both these big ideas may have missed the mark according to brand new Milky Way data.What Johannes Kepler figured out 400 years ago
– and how his work still shapes how we think about stars dancing around the galaxy (00:00:19).Why recent Gaia spacecraft data shocked astronomers
– The rotation curve it revealed broke every prediction and left both dark matter and MOND camps scratching their heads (00:00:32).A crash course in the “asymmetric drift” effect
– and why star motions aren’t as tidy as those neat planetary orbits in astronomy textbooks (00:04:24).Why sometimes, when everyone is wrong, science wins
– Bad news for theories is often good news for scientific progress (00:09:12)!
🎲 Fun Fact:
The legendary Kepler’s Laws should predict that outer stars slow down the farther they get from galactic center… just like planets in our solar system. But for 50 years, our galaxy spun a totally different tale, making us invent an invisible “dark matter” halo — and now the story might be flipping again! (00:15:04)
🚦 Outtro
The universe is messy, data is tricky, and it might just be that we’ve been expecting the wrong answers all along. Is the real headline here that BOTH dark matter and MOND have cracks? Or is there something even weirder out there?
👉 Call to Action
Listen now and tell us:
Do you side with Team Dark Matter, Team MOND, or are you secretly rooting for a third cosmic underdog? HIT REPLY with your thoughts, give the episode a thumbs up, and share this newsletter with a friend who loves big questions and even bigger mysteries.
Stay curious,
Brian Keating
Chancellor’s Distinguished Professor of Physics, UC San Diego
P.S. Don’t miss the shout-out to the cutting-edge research paper that inspired this episode — and if you haven’t already, check out our interviews with stars of the MOND and dark matter debates!
👽 Listen here → [Full Episode Link]
🛰️ Sub to the podcast, so you never miss an astronomical update!
🎓 Lessons Learned
1. Kepler’s Laws and Expectations
Kepler’s laws predict orbiting bodies slow down with distance, foundational for understanding rotation in solar systems and galaxies.
2. Galactic Rotation Curve Anomalies
Observed galaxy rotation curves stay flat, not declining as predicted, suggesting additional mass or modified gravity is required.
3. Role of Dark Matter
Dark matter theory explains flat rotation curves by proposing invisible halos surrounding galaxies, contributing extra gravitating mass.
4. MOND as an Alternative
MOND modifies Newton’s laws at low acceleration, eliminating the need for dark matter to explain galactic motion.
5. Gaia’s Surprising Measurements
Data from Gaia spacecraft challenged both dark matter and MOND predictions, showing an unexpected rotation curve in the Milky Way.
6. Modeling and Measurement Challenges
Correcting data for distances, velocities, and stellar motions introduces uncertainties, especially at the galaxy’s outer edges.
7. Implications of Keplerian Decline
If a Keplerian decline is real, both dark matter halos and MOND face difficulties explaining the data without modifications.
8. Systematic Errors and Contamination
Outer galactic measurements are complicated by systematics, warped disks, and nearby galaxies affecting the data’s interpretation.
9. Future Observational Strategies
Other tracers—Cepheid stars, stellar streams, and globular clusters—will help independently verify the Milky Way’s rotation behavior.
10. Open Questions and Scientific Revision
Neither dark matter nor MOND fits perfectly; new laws or refined models may be needed if the current curve holds up.
10 Surprising and Useful Frameworks and Takeaways
Ten Most Surprising and Useful Frameworks & Takeaways
1. Historical Laws Still Frame the Debate
Four centuries ago, Kepler and Newton established laws about planetary motion—planets farther from the Sun should move slower, governed by the enclosed mass. These simple equations fundamentally shaped expectations for galactic motion, but galaxies defy them 00:00:19, 00:01:01.
2. Rotation Curves and the Missing Mass Problem
The observed flat rotation curves of galaxies (stars far out rotate faster than expected) sparked the dark matter hypothesis and the rise of MOND as alternatives. The mismatch between visible mass and gravitational pull is foundational for both camps 00:02:12, 00:02:20.
3. Data Overturning Old Paradigms
New findings from the Gaia spacecraft challenge both dark matter and MOND—neither predicted the observed rotation curve of the Milky Way, suggesting a possible Keplerian decline instead of a flat curve 00:00:32, 00:08:39.
4. Model Dependence and the Challenge of Observation
Recovering galactic rotation curves from within the galaxy (as opposed to viewing it from outside) introduces substantial modeling challenges and systematics: distance errors, equilibrium assumptions, star selection, and asymmetric drift corrections all affect the outcome 00:03:17, 00:04:32.
5. Extrapolation Hazards
The total mass of the Milky Way inferred from data is heavily model-dependent, relying on extrapolations far beyond regions directly measured by Gaia, reminding us "observed" and "extrapolated" are not synonyms 00:06:33, 00:07:01.
6. MOND and Dark Matter: Different Physics, Same Predictions—Now Both Challenged
MOND alters gravitational dynamics at low accelerations, doing away with dark matter, whereas the standard model invokes invisible halos—yet both converge on flat rotation curves, and both now appear to miss the mark with the new data 00:08:53.
7. The External Field Effect in MOND
MOND isn't linear; the presence of external gravitational fields (from the Large Magellanic Cloud, the Sagittarius Dwarf Galaxy) can influence dynamics, complicating predictions and saving the paradigm from being immediately falsified 00:09:38, 00:11:25.
8. Systematic Error Budget Grows with Distance
Measuring the outer galaxy's rotation grows increasingly fraught with uncertainties—tracer star scarcity, departures from equilibrium, and contamination from interactions, demonstrating how error bars can outweigh the data at the limits 00:10:10, 00:10:50.
9. Three Possible Interpretations for the New Curve
The surprising rotation curve demands three broad frameworks:
Decline is real (Milky Way’s halo is lighter or more concentrated; both paradigms survive with parameter tweaking)
Decline is exaggerated (modeling/disequilibrium/systematics inflate the effect)
Outer disk too disturbed—no steady, equilibrium curve is possible 00:12:31.
10. Future-Ready: Cross-Validation and Scientific Humility
Only with multiple, independent tracers (Cepheid stars, stellar streams, satellite clusters, radio surveys) and improved theoretical modeling can the true nature of the rotation curve be revealed. Science advances not by confirmation but by being proven wrong and adapting the models 00:13:36, 00:13:53.
Flaws lead to new laws: Progress comes when neither side "wins," but when both are forced to explain what nature actually reveals 00:09:12, 00:14:08.
Clip Able
Clip 1
Title: Did Dark Matter and MOND Both Get It Wrong?
Timestamps: 00:00:00 – 00:03:16Caption:
For 50 years, scientists have explored why galaxies spin faster than expected. Dark matter and MOND were two top explanations—until new data from the Gaia spacecraft suggested that both may have predicted the wrong rotation curve.
Clip 2
Title: What Gaia Taught Us About Our Galaxy’s True Shape
Timestamps: 00:03:17 – 00:06:01Caption:
Mapping our home galaxy is no easy task—especially when we’re inside it. Brian Keating explains how the Gaia mission measured a billion stars and revealed surprising results about the Milky Way’s mass, leaving both dark matter and MOND theories with big questions.
Clip 3
Title: Why Keplerian Decline Challenges Both Dark Matter and MOND
Timestamps: 00:06:02 – 00:09:06Caption:
A declining outer rotation curve could shake up our understanding of gravity and cosmic structure. Brian Keating walks through why a Keplerian decline in our galaxy’s stars isn’t what either dark matter or MOND would expect, and what that means for the future of astrophysics.
Clip 4
Title: When the Data Stumps Everyone: The Uncomfortable Truth for Scientists
Timestamps: 00:09:06 – 00:12:29Caption:
Instead of picking a winner between dark matter and MOND, the real story might be that both are missing something fundamental. Brian Keating delves into how messy, real-world data forces scientists to reconsider and refine their theories—sometimes in unexpected ways.
Clip 5
Title: Three Possibilities for Our Galaxy—and What Will Settle the Debate
Timestamps: 00:12:29 – 00:15:09Caption:
With new evidence on the table, what’s next? Brian Keating lays out three competing scenarios to explain the Milky Way’s puzzling rotation curve and highlights which astronomical breakthroughs could finally resolve the debate between dark matter, MOND, and something entirely new.
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