Dark matter is slowing the spin of the Milky Way’s galactic bar

From University College London

The spin of the Milky Way’s galactic bar, which is made up of billions of clustered stars, has slowed by about a quarter since its formation, according to a new study by UCL and University of Oxford researchers.

artist's conception of the Milky Way
Artist’s conception of the Milky Way. Source: Wikimedia Commons. Credit: Pablo Carlos Budassi.

For 30 years, astrophysicists have predicted such a slowdown, but this is the first time it has been measured.

The researchers say it gives a new type of insight into the nature of dark matter, which acts like a counterweight slowing the spin.

In the study, published in the Monthly Notices of the Royal Astronomical Society, researchers analysed Gaia space telescope observations of a large group of stars, the Hercules stream, which are in resonance with the bar – that is, they revolve around the galaxy at the same rate as the bar’s spin.

These stars are gravitationally trapped by the spinning bar. The same phenomenon occurs with Jupiter’s Trojan and Greek asteroids, which orbit Jupiter’s Lagrange points (ahead and behind Jupiter). If the bar’s spin slows down, these stars would be expected to move further out in the galaxy, keeping their orbital period matched to that of the bar’s spin.

The researchers found that the stars in the stream carry a chemical fingerprint – they are richer in heavier elements (called metals in astronomy), proving that they have travelled away from the galactic centre, where stars and star-forming gas are about 10 times as rich in metals compared to the outer galaxy.

Using this data, the team inferred that the bar – made up of billions of stars and trillions of solar masses – had slowed down its spin by at least 24% since it first formed.

Co-author Dr Ralph Schoenrich (UCL Mullard Space Science Laboratory) said: “Astrophysicists have long suspected that the spinning bar at the centre of our galaxy is slowing down, but we have found the first evidence of this happening.

“The counterweight slowing this spin must be dark matter. Until now, we have only been able to infer dark matter by mapping the gravitational potential of galaxies and subtracting the contribution from visible matter.

“Our research provides a new type of measurement of dark matter – not of its gravitational energy, but of its inertial mass (the dynamical response), which slows the bar’s spin.”

Co-author and PhD student Rimpei Chiba, of the University of Oxford, said: “Our finding offers a fascinating perspective for constraining the nature of dark matter, as different models will change this inertial pull on the galactic bar.

“Our finding also poses a major problem for alternative gravity theories – as they lack dark matter in the halo, they predict no, or significantly too little slowing of the bar.”

The Milky Way, like other galaxies, is thought to be embedded in a ‘halo’ of dark matter that extends well beyond its visible edge.

Dark matter is invisible and its nature is unknown, but its existence is inferred from galaxies behaving as if they were shrouded in significantly more mass than we can see. There is thought to be about five times as much dark matter in the Universe as ordinary, visible matter.

Alternative gravity theories such as modified Newtonian dynamics reject the idea of dark matter, instead seeking to explain discrepancies by tweaking Einstein’s theory of general relativity.

The Milky Way is a barred spiral galaxy, with a thick bar of stars in the middle and spiral arms extending through the disc outside the bar. The bar rotates in the same direction as the galaxy.

The research received support from the Royal Society, the Takenaka Scholarship Foundation, and the DiRAC supercomputing facility of the Science and Technology Facilities Council (STFC).

Via EurekAlert!

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June 17, 2021 2:09 am

The counterweight slowing this spin MUST be dark matter….” 
Translation: Nobody knows what’s going on so we’ll blame it on something we don’t understand. We don’t know if it’s dark, we don’t know if it’s matter, we can’t measure it, we can’t see it, but that won’t stop us from saying we have the answer.

Joseph Zorzin
Reply to  CRISP
June 17, 2021 2:38 am

at least they’re not blaming in on evil, dark, CO2

but, you’re right- they have no clue- it’s one thing to suggest what it might be but not to say it “MUST be”

Hal McCombs
Reply to  Joseph Zorzin
June 17, 2021 9:13 pm

Dark matter is made of CO2.

Reply to  Hal McCombs
June 19, 2021 5:59 am

Problematic – atmospheric gasses don’t interact with gravity, remember? Dark matter on the other hand is only known from its gravitational effects.

Jay Willis
Reply to  CRISP
June 17, 2021 3:09 am

We do have the answer. Dark matter. It takes the shape of one, or more, giant scarab beetles which drag the sun and stars around in the firmament. Just get over it, and get with the program.

Reply to  Jay Willis
June 17, 2021 3:40 am

No. Surely its turtles!

Reply to  Disputin
June 19, 2021 6:02 am

Au contraire – the scarabs are settled

Reply to  CRISP
June 17, 2021 4:05 am

I will resist the severe temptation to engage in a formidable and excessive use of puns in re: this subject matter.

Reply to  CRISP
June 17, 2021 4:30 am

It’s no longer a black hole… black wh…e h/t NAACP but I digress… but rather a gray hole, which is infilled with brown matter a la climate science.

We infer images, and from them objects, from signals of unknown, and, in fact unknowable fidelity, from outside of a limited frame of reference (i.e. scientific logical domain).

Ignore the media - Think for yourself
Reply to  CRISP
June 17, 2021 4:52 am

I’m not very smart, but I’ve been suspicious of dark matter from my awareness of ‘it.’ I think ‘it’s a huge HBquark!

Reply to  Ignore the media - Think for yourself
June 18, 2021 1:24 am

Every time people start talking about dark matter, I start to smell epicycles.

Reply to  CRISP
June 17, 2021 7:25 am

‘Dark matter’ is misnomer, dark refers to an object ability to absorb and reflect light i.e, a dark object is absorbing most of incident light but not reflecting much; when in shade a dark object is approaching black but it is not black. Since what they term ‘dark matter’ is not reflecting any light it should be referred to as ‘Black matter’, but that would not be politicaly correct, and in no circumstances should be confused with ‘black lives matter’

mario lento
Reply to  Vuk
June 17, 2021 8:42 am

That was funny Vuk… The dead pan humor made my day.

Reply to  Vuk
June 19, 2021 6:04 am

Black matter lives!

Reply to  CRISP
June 17, 2021 7:53 am

They should ask Greta. She can probably see it, smell it and taste it.

Reply to  CRISP
June 17, 2021 11:50 am

Co-author and PhD student Rimpei Chiba, of the University of Oxford, said: “Our finding offers a fascinating perspective for constraining the nature of dark matter, as different models will change this inertial pull on the galactic bar.”

It’s a model using assumptions and bias.

as different models will change this inertial pull on the galactic bar.”

Of course they give different results!
Beliefs, assumed inputs, selected outputs, programmer’s choice of coding calculations and order.

June 17, 2021 2:19 am

Problem is we have no Quantum Gravity theory despite 40 years of amazing efforts :
From someone who has worked across these efforts :
Lee Smolin: Galaxy rotation curves: missing matter, or missing physics? – YouTube

Ignore the media - Think for yourself
Reply to  bonbon
June 17, 2021 5:47 am

Thanks for the link; a WONDERFUL lecture from someone who has a grasp of the problem!

Michael S. Kelly
Reply to  Ignore the media - Think for yourself
June 17, 2021 12:09 pm

He’s way too hyper for my taste. I am worn out just listening to him…

(Kind of the Steven Wright of physics.)

Last edited 1 year ago by Michael S. Kelly
Reply to  bonbon
June 19, 2021 6:15 am

I’m a big fan of Lee Smolin.
Here’s another great YouTube interview between Smolin and Brian Keating:


Looking at this conversation between a younger and a senior quantum cosmologist, one sees the tremendous respect and affection of Keating for the knowledge, intellect and integrity of his mentor. One wonders where one would find such a thing in climate science.

June 17, 2021 2:59 am

is invisible and its nature is unknown

So, it’s thought to be.

Peta of Newark
June 17, 2021 3:05 am

What is this ‘counterweight’? What is it ‘countering’

C’mon Shirley lets be ‘avin you, if The Bar is slowing, isn’t just conserving angular momentum – because it’s getting bigger/longer

Ain’t that what happens with Earth’s (gravitationally locked) moon, gradually slowing and increasing the size of its orbit
(As we’re oft told: Total eclipses will be things of the past before long)

Is that not ‘just what happens’
With planets & moon, stars & planets or galactic centres & ‘galaxy’ or even Universes in fact, they must all ‘do something’
i.e. Expand or Collapse.
IOW Evolve and Change

Hello hello, we’re up against the same mentality as Climate Change – the thinking that ‘climate’ must somehow be constant.
(there’s a cliche there, you all know it)

Seriously, I wonder if this will affect The Climate in Australia.

If sufficient folks do latch on, It Certainly Will – a firestorm of Honesty and Integrity may overtake the place.
been there done that got the T-shirt 🙂

June 17, 2021 3:42 am

There is a plausible hypothesis to explain many apparent anomalies.
It could be that there was no big bang and instead we are living in a universe that has ‘bubbles’ of light and matter moving away from a centre of gravity and other ‘bubbles’ moving back towards a centre of gravity.
Those moving away would be expanding and those moving back to the centre would be contracting.
Over time, expanding regions would slow down as they moved away from the centre and eventually fell back whilst contracting regions would do the opposite.
Features that we see as ‘black holes’ would be an observable interface between expanding and contracting regions.
Isaac Asimov once described the visible phenomena within a region of space that was contracting and that first directed me to the concept of a convecting universe.
The concepts of dark matter and dark energy would then just be a conceptual ‘sticking plaster’ for our flawed concept of the entire universe expanding at the same time.

Moderately Cross of East Anglia
Reply to  Stephen Wilde
June 17, 2021 5:27 am

Paging Halton Arp…hello…Mr Arp, you’re needed urgently…

Reply to  Stephen Wilde
June 17, 2021 5:38 am

So you’re saying that the universe is a lava lamp sitting on a book case in God’s living room.

Reply to  Stephen Wilde
June 17, 2021 7:45 am

The redshift could be a result of a non-homogenous cosmos. There is much more to that story.

June 17, 2021 4:47 am

The universe is expanding therefore the rotation of the Milky Way should slow its rotation in the same way figure skaters slow their rotation by spreading their arms.

Reply to  commieBob
June 17, 2021 8:54 am


If the ‘ice rink’ were contracting then the ice skater would need to spread their arms to keep from speeding up.

If the ‘ice’ were expanding, then the skater would slow down.

If the skater is expanding then she would slow down all by herself without sticking her arms out.

If all and everything is expanding (instantaneously) … how would we know?

(And, in the big picture, what is the difference between the discounted Aether & said Dark Matter?)

Reply to  DonM
June 17, 2021 4:20 pm

Aether was posited as the medium in which light waves travelled much in the way that sound waves travel through air. When they discovered that light waves travel at the same speed in any direction, it made it illogical that light needed some medium through which it could propagate.

Dark matter was posited as the answer to a different question.

Reply to  commieBob
June 18, 2021 9:16 am

Hey Bob,

that’s not my minus….

my point … both concepts seem to have started out as place savers for unknown future corrections; then they morphed into real & accepted (by some) things.

(I still like the idea of a magic universal aether & that gravity is an associated repulsive force rather than a magically instantaneous attractive force)

Joao Martins
June 17, 2021 5:48 am

Galactic Emergency!

We have less than 10 years to stop galactic slowing!

Save the galaxy, send money!

June 17, 2021 5:59 am

You do not need “dark matter” to explain the apparent lack of gravity. If you understand Mach’s principle, which for large parts was the inspiration to Einstein’s theory of relativity, than a simple, much more profound explanation is obvious.

June 17, 2021 6:01 am

There is some force stopping our galaxy and others from collapsing in on their cores as the spiral arms wind tighter and tighter as Newtonian physics indicates should happen. Dark matter is the current popular answer as to why this is not happening.

I will not mock the efforts to try and figure out the answer. Tough question to answer and even this truck driver understands that if and when they do find the answer it will be a momentous accomplishment having potentially far reaching effects.

Jeff in Calgary
Reply to  rah
June 18, 2021 10:35 am

Blaming everything on Dark Matter doesn’t get us any closer to the real answer. It retards actual research.

Right-Handed Shark
June 17, 2021 6:14 am

Please wait for the galaxy to come to a complete stop before getting off.

Last edited 1 year ago by Right-Handed Shark
Reply to  Right-Handed Shark
June 17, 2021 9:25 am

And those that don’t listen will be barred.

June 17, 2021 7:08 am

If dark mater does exist, then it is doing its job slowing down the angular momentum of he galaxy, as per expected. But for dark matter to exist, then that implies dark energy exists and that force is repulsive to gravity and inflating space making everything appear further apart and ‘flying’ apart from everything else. If this is the way it all ends, then it is the heat death of the universe while everything retreats past the event horizon and everything will be lights out and cold forever after tens of trillions of years, also implying this universe was one off event. But intuition tells me this couldn’t be true and the universe must be recycled eternally and infinitely, therefore making dark matter and dark energy just concepts that aren’t the full story yet. I think we are still trying to figure out how this this amazing universe actually works.

Reply to  Earthling2
June 17, 2021 9:04 am

So the universe is a perpetual motion machine? It is bounded by zero entropy on one end and zero Gibbs Free Energy on the other.

Reply to  JamesD
June 17, 2021 9:16 am

Energy and matter cannot be destroyed, just exchange states, so must hold true for infinity. The universe has to be a perpetual motion ‘machine’, otherwise it is tits up, long term. The alternative is depressing, in that the lights go out, and that was it for this universe forever. Well, maybe then a multiverse, but that is even more conjuncture than DM and DE. Maybe we never know the unknowable.

Reply to  Earthling2
June 17, 2021 11:53 am

The limiting factor is not energy, but entropy. That only goes one way until all potential is used up. On the flip side, entropy is zero bound, showing a beginning. Entropy is not conserved, but increases.

Reply to  JamesD
June 17, 2021 4:01 pm

Yes, you right about entropy. The dishes don’t do themselves, and the house doesn’t get cleaner with time. The arrow of time is also forward, although with no future reference if there is no mater because everything goes through the Big Rip, then time doesn’t exist either. This is as much philosophy as it is science. The more we learn, the bigger the mystery, at least for me. I guess this is why ‘they’ invented religion. A lot of this is pure faith based, depending what camp you are in. Just like climate AGW.

Reply to  Earthling2
June 18, 2021 8:30 am

Nobody invented religion. 2500 years ago the Greek philosophers derived what was necessary. The Necessary Being is a statement on entropy. Potential being reduced to Actual is also a statement on entropy. Superposition is limited by potential. Entanglement is the entanglement of information (Greek Forms), not physical entanglement. The importance of Information traces back to Greek Forms. And these weren’t the Greeks of Zeus and Apollo, but the Greeks of First Cause, Necessary Being, Pure Act. And then you get to explain perception, qualia, and intentionality. Have fun.

John Tillman
Reply to  Earthling2
June 17, 2021 10:42 am

The concept of dark matter dates at least as far back as Lord Kelvin’s 1884 suggestion. Dark energy in its present form dates from 1998. Similar ideas arose after Einstein’s proposed cosmological constant, but current dark energy thought owes to the apparent observation of accelerating expansion of the universe, near the end of the last century.

Hal McCombs
Reply to  Earthling2
June 17, 2021 9:24 pm

I love dark mater samiches.

June 17, 2021 7:20 am

If there is a force out there and especially here we don’t know about, best to find and define it. Could be dangerous or helpful.

June 17, 2021 8:35 am

If you add peanuts to the Milky Way Bar, it becomes a Snickers.

Reply to  zmdavid
June 17, 2021 9:23 am

A Snickers w/a dark-chocolate center.

June 17, 2021 8:55 am

Consider that all of our astronomical observations are essentially 2-D. So how can you account for all the visible matter in the Milky Way? The tiny change in viewing angle as the Earth orbits the sun doesn’t cut it.

Reply to  JamesD
June 17, 2021 10:52 am

There is at least one other “D” to astronomical observations which does give radial information, when coupled with a form of Gauss’s law (specialization of the divergence theorem). Indeed parallax measurements, given the radius of the earth’s orbit, can only get radial distance measures out to about 100 parsecs for optical telescopes and about an order of magnitude more for radio telescopes. Given that the milky way is about 10^5 parsecs across, that hardly gets us a small distance within our own galaxy. In fact, in the 1920s there was a huge debate about whether so called “nebula” were outside of the milky way or not. This was more or less equivalent to the earlier debate about the earth-centered solar system (the Universe of the 1500s). Hubble’s other, less widely known cool thing was using the observation that Cepheid variables empirically demonstrated a strong correlation between the observed period and the absolute magnitude, enabled him to measure a Cepheid variable period of stars within nebula. Then using the correlation with absolute brightness, he showed that for the observed brightness to be reduced by the one over r^2, from Gauss’s law, the distance to the stars had to be so large as to put it well outside the milky way, leading to recognition of “nebula” as actually other galaxies outside our own, out to distances of order 20 million light years which includes the Council of Giants and a number of other galactic structures. Hubble’s other cool thing is his well known red shift-distance correlation, which can now get us out to previously unimaginable distances. Recently, gravitational wave observations, coupled with gamma ray observations of the same events is giving us confirmation of a whole new population of objects at measurable distances, not to mention finally answering a longstanding question on whether gravitational waves and light travel at the same speed. The 2015 merging black hole event put out one of my favorite amounts of power, since it uses the largest power of 10 abbreviation in common usage, putting out about 40 yotta-yotta watts. So if someone asks how much power merging black holes can put out, you can seriously say yotta-yotta watts. For the brief instant it radiated, that power exceeds the power of all of the other stars in the universe.

All in all, there are a number of observational methods that give us a 3-D picture of the cosmos.

June 17, 2021 9:13 am

In some galaxies, the bar becomes its most prominent feature.

Last edited 1 year ago by beng135
June 17, 2021 12:32 pm

Dark matter is slowing the spin of the Milky Way’s galactic bar…
I did hear that the further you go out there, the worse the service gets. 😏

Ray in SC
June 17, 2021 1:38 pm

Doesn’t the entire galaxy rotate at the same rate as the galactic bar at the center? If so, are they saying that the entire galaxy has slowed or are they implying that only the center has slowed?

Shanghai Dan
June 17, 2021 3:28 pm

Proof that as we hit middle age, we slow down…

Chris G
June 17, 2021 3:39 pm

Dark matter is something that was invented to make theories work properly. Funnily enough it always exists in exactly the right amount everywhere you look to do this despite being completely undetectable. Anyone would be forgiven that it looks a bit like an excuse for math that is just plain wrong.

Alternatively look up MIHSC and the excellent “Physics from the edge” blog.
A theory that explains what is going on without undetectable invisible stuff.

June 17, 2021 7:58 pm

Are we doomed?

What can we do to stop this slow-down?

Reply to  RoHa
June 18, 2021 7:46 am

I’ve been in more spinning bars than I can remember.
When they start to slow down, it means trouble ahead.

June 17, 2021 9:06 pm

It’s worse than we thought…

Walter Sobchak
June 17, 2021 9:36 pm

A faith in things unseen.

June 18, 2021 1:38 am

“The counterweight slowing this spin must be dark matter … Dark matter is invisible and its nature is unknown”

I’m sorry, but is this scientific thought? There are a lot of things I don’t understand in physics. Like what exactly is the Universe expanding into? Is String Theory for real? Will everything eventually degrade into a photon? And so on.

In fact, I find the Universe confusing most of the time, but I have learned to say ‘I don’t know’ when asked to explain something that can’t be explained. Isn’t this allowed in Physics?

June 19, 2021 6:35 am

This paper from University College London comes in parallel with another, Korean study which also finds indirect gravitational evidence for dark matter:


Sungwook Hong and colleagues from the University of Seoul applied deep learning to the local galactic map to infer the 3D architecture of dark matter from its gravitational signature.

They found a compellingly realistic filamentous architecture which gave the result the status of evidence for the existence of dark matter. This could be a significant scientific achievement of deep learning methodology.

Both the UCL study inferring dark matter from galactic rotation slowing, and the Korean study reconstructing dark matter by deep learning are examples of mathematical inverse problems. The foundation of the mathematics of inverse problems was the paper by Alexander Tikhonov, “Об устойчивости обратных задач” (“On the stability of inverse problems”).

David Blenkinsop
Reply to  Hatter Eggburn
June 20, 2021 1:18 am

How about sparse galaxies whose rotation is quite slow to begin with? This slower motion of the thinly distributed stars is taken as indicating that there’s no dark matter there to drive a faster galactic rotation.


Reply to  David Blenkinsop
June 20, 2021 2:10 am

Indeed here’s Matt O’Dowd saying the same thing on PBS Spacetime – that in a way finding 2 galaxies devoid of dark matter proves dark matter’s existence. More inverse inferences.


Reply to  Hatter Eggburn
June 20, 2021 2:13 am

This for me is kind of disappointing since I was attracted to the ideas of modified gravity and other alternatives to darkness, especially Eric Verlinde’s emergent quantum gravity. But these findings make it seem more likely that Fritz Zwicky’s “dunkel materie” is really a thing.

Last edited 1 year ago by Hatter Eggburn
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