First Dark Matter Sub-Halo Found In The Milky Way:
There are plenty of theories about what dark matter is and how it might be gravitationally affecting the universe. However, proving those theories out is hard since it hardly ever interacts with anything, especially on "small" scales like galaxies. So when a research team claims to have found evidence for dark matter in our own galaxy, it's worth taking a look at how. A new paper from Dr. Surkanya Chakrabati and her lab at the University of Alabama at Huntsville (UAH) does just that. They found evidence for a dark matter "sub-halo" in the galactic neighborhood, by looking at signals from binary pulsars.
A sub-halo is a clumping of dark matter that is brought together inside of a larger "halo" that is thought to form the core of galaxies. Since dark matter primarily interacts through gravity, going theory suggests that it should attract "baryonic" (i.e. normal) matter when it clumps together. This clumping is thought to the scaffolding that galaxies are built on.
Sub-halos are even denser groupings of dark matter that coalesce because of their gravitational attraction. Since they are relatively small compared to the big dark matter halos they are contained in, they can be difficult to detect. To do so, cosmologists would have to find a gravitational signal that deviates from what would be expected given the normal matter surrounding the sub-halo. So far, no one has been able to isolate that kind of signal, despite looking throughout our galactic neighborhood.
Enter binary pulsars - these star pairs contain at least one pulsar, a type of neutron star which emits a large amount of energy on a regular cycle (hence their name). These bursts can be measured so accurately they rival atomic clocks in terms of regularity. The researchers had a theory that they could use deviations in that expected cycle to detect the gravitational effects of a dark matter sub-halo, so they began looking at binary pairs in the galaxy to see if they could find any hint of it.
Overall they looked at 27 binary pulsars, and in particular were looking for gravitational changes between two pairs, to increase the chance there was indeed a structure causing the deviation. They found two, called PSR J1640+2224 and PSR J1713+0747, that had the kind of significant correlated gravitational change they were looking for.
To isolate that gravitational change, the researchers had to eliminate other forms of gravitational acceleration that could be caused by things other than dark matter. One is "gravitational radiation", the acceleration caused when the system gives off gravitational waves, and predicted by the theory of general relativity. Another is the Shklovskii Effect, which is an artifact caused by a binary system moving across our line of sight. Thankfully, both of these effects are well understood and can easily be removed from the calculation of the gravitational influence on the binary system.
Some of that gravitational influence can still come from baryonic matter, but in the case of these two binaries there appeared to be a substantial component that couldn't be explained that way. In fact, the statistics of that additional component were so compelling its hard to argue that it was caused by anything other than an unseen gravitational mass.
Defining that mass was the next step. The researchers pinpointed it at about 2,340 light years away, and determined its mass to be around 2.45 x 107 solar masses. An equivalent amount of baryonic matter causing that gravitational change would be 100 times what is observable in that part of the galaxy.
This research represents the first time a dark matter sub-halo has been detected in the general galactic neighborhood, after having been predicted by theory for years. It also offers a technique by which other researchers could do the same with other sets of binary pulsars. Though rare, astronomers are continually collecting new data on them constantly, giving cosmologists even more data to analyze. Likely this won't be the last time we'll hear of this technique being used to find dark matter sub-halos - there are plenty more places to search for them, and likely many more to discover.
arXiv paper: https://doi.org/10.48550/arXiv.2507.16932
Learn More:
UAH - UAH researchers use pulsar accelerations to detect a dark matter sub-halo in the Milky Way for the first time
S. Chakrabarti et al - Constraints on a dark matter sub-halo near the Sun from pulsar timing
UT - Tying Theory To Practice When Searching For Dark Energy
UT - Astronomers Search for Dark Matter Using Far Away Galaxies
(Score: 3, Interesting) by turgid on Wednesday October 08, @07:49PM (6 children)
I've read through that several times and I don't understand it.
First, what is a halo in this context? Is it a sphere, a spherical shell, a torus, a ring or something else? If this dark matter is attracted by (its own) gravity, then it must be either in some equilibrium where it can't coalesce any further or it must be gradually falling down towards some centre?
What is a sub-halo? Is it just a fancy term for a locally more dense clump of dark matter? Or is it some exotic shape?
Regarding the gravitational effect on the binary pulsars, is this a situation where the binary pairs are "near" some dense clump of dark matter so their orbital rotation rate is affected by some sort of relativistic effects? Is the "halo" in the middle of the pair causing some other time dilation effect?
Regarding dark matter on its own, if it were to coalesce under gravity to form dense object analogous to planets and stars, that implies there is energy to be released. Typically, you'd expect heating as the pressure of the "gas" goes up and electromagnetic radiation to be released. Also the rotation rate of the collapsing object would increase. With dark matter, where's this radiation? It's dark, no radiation... See what I'm getting at?
I refuse to engage in a battle of wits with an unarmed opponent [wikipedia.org].
(Score: 3, Insightful) by sgleysti on Wednesday October 08, @11:43PM
To my best understanding, not an expert.
As far as we know, dark matter consists of particles that only interact gravitationally. They don't have electric charge or respond to the strong nuclear force, etc. so they don't collide or glom together like "ordinary" baryonic matter. The particles basically "pass through" each other. They don't bounce or slow down or emit radiation or anything. Imagine some kid coded up a simulator of particles obeying Newton's laws and only implemented the equations for gravitational attraction.
A dark matter halo is a more locally dense clump of dark matter particles like you said, but it remains diffuse. I imagine it as a sphere or squashed sphere that is more dense toward the center. A sub halo would be a more dense pocket within a larger halo.
Based on https://phys.org/news/2025-02-breakthrough-enables-local-dark-density.pdf [phys.org], it looks like the astronomers use changes in pulsar timing to detect local acceleration. In space, this acceleration would be due to gravity. They found more gravitational acceleration of pulsars than can be accounted for by the nearby visible matter, hence thinking there's dark matter. A lot of it, apparently.
(Score: 3, Insightful) by Gaaark on Thursday October 09, @12:10AM (4 children)
They've given DM so many 'rules of behaviour' that it can match anything you want.
When they 'prove' DM exists, THEN i'll eat my shorts. Until then, I'll still go with "Meh... ANOTHER story showing 'proof' DM exists.
Remember when they said there was the galaxy without DM, which PROVES DM exists, then they found it DID have DM, which proves it exists....
Meh.
And if (Dog my memory sucks) 'binary star systems rotating around each other' (what do you call that?) exist, then DM CANNOT exist.
I'll still go with DM is a crutch, not physics. Meh.
--- Please remind me if I haven't been civil to you: I'm channeling MDC. I have always been here. ---Gaaark 2.0 --
(Score: 3, Insightful) by aliks on Thursday October 09, @03:11AM (2 children)
I don't see the article claiming a new proof - they talk about finding evidence.
Most of the confusion about dark matter models comes from weak science journalism using vague language to make current theories accessible.
For comparison, look up the 18th and 19th C "germ theory of disease" and the sceptism about invisible organisms causing diseases. . . .
To err is human, to comment divine
(Score: 3, Insightful) by Gaaark on Thursday October 09, @10:39AM (1 child)
Dark matter was made up in order to save General Relativity, with little 'evidence' being found for its existence.
They created a kludge in order to save Einsteinian physics.
For comparison, look up Galen and how his legacy of 'greatness' kept back REAL medical science for hundreds of years.
--- Please remind me if I haven't been civil to you: I'm channeling MDC. I have always been here. ---Gaaark 2.0 --
(Score: 3, Insightful) by aliks on Thursday October 09, @05:03PM
You may be right, but something new is needed to explain the anomalies . . .
To err is human, to comment divine
(Score: 2) by turgid on Thursday October 09, @09:18PM
I know exactly what you mean. When I was at University being taught about these things a long time ago, Dark Matter was barely mentioned other than as a fudge to account for apparently missing gravity.
It's merely a placeholder which says, "missing part of reality to be discovered." I'm looking forward to the day when someone discovers some of what that missing part is.
And I'm crossing my fingers that it'll help us make interstellar spacecraft so I can go and play pranks on Zeta Reticuli like they do here after the pubs close on a Friday night.
I refuse to engage in a battle of wits with an unarmed opponent [wikipedia.org].