Dummies on dark matter 2: there are problems…
from Forbes website, see below
Canto: So there are candidates for dark matter, and there are also those who think that, though there is a serious problem in cosmology, to do with mass and energy, ‘dark matter’ won’t be the fix.
Jacinta: By the way, I was chatting with another dummy on this topic recently, who had the excuse of being much much younger than myself, and she asked if dark matter had anything to do with black holes. I wasn’t able to give a very effective answer, but Sabine Hossenfelder, one of our heroes, says in a video linked below that black holes and brown dwarfs (whatever they are), and other such exotic objects ‘would make too many gravitational lenses, which have not been seen’, to be candidates. Also black holes are so called because they ‘swallow’ light, that’s to say, light-emitting particles. So they really are black, in a sense, whereas ‘dark’ matter is more transparent than anything, according to Hossenfelder.
Canto: Well, getting back to Peter Fisher’s Royal Institution talk, he talks about the 1980s as a time of confusion and excitement in theoretical physics and cosmology when so many things weren’t adding up. At the same time a concept called super-symmetry was being mooted. It ‘predicted all kinds of heavier particles that we wouldn’t have observed in accelerators because they weren’t powerful enough’, according to Fisher. He also presented, what I’ve heard before, a conjecture that there must be this ginormous halo of dark matter surrounding galaxies to make up the missing mass and to account for the behaviour of visible matter at the edge of galaxies. In other words, this dark matter must have a gravitational effect on the outer arms of these galaxies.
Jacinta: I know that Hossenfelder is no great fan of bigger and more expensive accelerator-colliders in the hope of discovering more teensy-tiny but ultra-ultra numerous particles to fit the dark matter bill, but Fisher also goes on to talk about the Standard Model and how effective it has been, without dark matter screwing it up…
Canto: Yes it’s been very effective for accounting for some 4% of the mass-energy of the universe. Anyway Fisher helped to debunk a theory regarding ‘heavy neutrinos’ as a candidate for dark matter in the late eighties, which seems to this dilettante like an absurdity – neutrinos being near-massless, which presumably helps them to pass through planets as if they’re not there.
Jacinta: I think this heavy neutrino thing might’ve morphed, in theory, into the idea of weakly interacting massive particles, or WIMPs, and they’re still looking for em, for example on the International Space Station. They’re also looking for theoretical particles called axions, using special detectors. No luck so far for WIMPs or axions.
Canto: Fisher describes another source, black holes, via work done by Stephen Hawking, but I found it difficult to follow, so I’ll try roughly quoting:
In the beginning of the universe there was all this mass around, it’s very dense, and in a regime where quantum mechanics is very important, so the density is… fluctuating and changing, and [Hawking] thought, could it be that the density would be high enough to form a black hole? He did some rough calculations and found that, yes they could collapse into a black hole, and there’d be a lot of black holes, but there must be a way of getting rid of them, because we don’t see them. Over time, he invented a mechanism by which black holes radiate light at a very low level, a concept now called Hawking radiation, a remarkable notion, as it suggests the only connection we currently have between gravity and quantum mechanics.
The connection with dark matter is that there still may be ‘primordial’ black holes with a lot of mass but tiny in size. No luck in finding them either, needless to say.
Jacinta: So now let’s focus on Sabine Hossenfelder’s RI talk. It seems to me she goes into a lot more detail about the anomalies in what we observe, ascribed to the missing matter. For example, structure formation in the universe – which I remember being fascinated by when Carl Sagan presented it image-wise in his Cosmos series. Here are two points she makes on structure formation:
- Dark matter cannot build up radiation pressure and therefore starts forming structures sooner than normal matter
- normal matter on its own does not produce sufficient structures on short scales to be compatible with observation
And I have no idea what they mean. She mentions things that we can see, such as ‘galactic filaments, and so on and forth’. So, thinks me, wtf are galactic filaments? Well, Wikipedia calls them galaxy filaments, and they’re the largest known structures in the universe…
Canto: This is actually exciting – how could I have lived so long without knowing about these things?
Jacinta: Haha well Wikipedia is pretty good on this stuff:
In cosmology, galaxy filaments are the largest known structures in the universe, consisting of walls of galactic superclusters. These massive, thread-like formations can commonly reach 50/h to 80/h Megaparsecs (160 to 260 megalight-years) — with the largest found to date being the Hercules-Corona Borealis Great Wall at around 3 gigaparsecs (9.8 Gly) in length — and form the boundaries between voids. Due to the accelerating expansion of the universe, the individual clusters of gravitationally bound galaxies that make up galaxy filaments are moving away from each other at an accelerated rate; in the far future they will dissolve.
Galaxy filaments form the cosmic web and define the overall structure of the observable universe.
Canto: Great. What’s a void?
Jacinta: Cosmic voids, doncha know, are those vast spaces between filaments that contain few or no galaxies. But to return to Hossenfelder, who’s a theoretical physicist, and a lot more proficient in maths than we are, as we’ll see. She’s also something of a dark matter skeptic, it seems. She highlights four problems that dark matter doesn’t solve, and we should try to understand them:
- the brightness of galaxies is strongly correlated with the (asymptotic) rotational velocity (‘Tully-Fisher Law’). Dark matter doesn’t explain this
- dark matter leads to density peaks in galactic centres which badly fits with observations (‘galaxy cusps’)
- dark matter predicts too many dwarf galaxies
Canto: Okay let’s start with asymptotic rotational velocity. Asymptotic analysis, in mathematics, is about describing behaviour of functions as they approach a limit, such as infinity. So galactic velocity presumably has some sort of limit, which can be calculated mathematically. The Tully-Fisher Law, or Relation, from Wikipedia:
is a widely verified empirical relationship between the mass or intrinsic luminosity of a spiral galaxy and its asymptotic rotation velocity or emission line width. Since luminosity is distance-dependent, the relationship can be used to estimate distances to galaxies from measurements of their rotational velocity.
So the point Hossenfelder makes here, I think, is that rotational velocity correlates well with brightness, which correlates with distance, as measured from Earth. Dark matter appears to be irrelevant to these calculations. Of course I may be getting this all wrong.
Jacinta: I wouldn’t know. But it seems that dark matter and its supposed halo should be interfering with orbital velocities and so interfering with calculations, but it isn’t?
Canto: Hmmm.. Anyway, second point. Galaxy cusps takes me to the ‘cuspy halo problem’, which I’ll try to explain in my own words. It’s also called the core-cusp problem, which, broadly speaking, is a discrepancy found in small, low-mass galaxies, according to different measurement systems or predictions. ‘Cuspy’ represents an energy-mass distribution which is denser at small radii, whereas most dwarf galaxies have a more flat ‘core’ distribution.
Jacinta: These are all just the beginnings of our explorations of this topic. In a hundred years or so we’ll be fully conversant with the issues. As to dark matter predicting too many dwarf galaxies, aka the dwarf galaxy problem, apparently we’ve observed and identified some 38 of these dwarf galaxies in our Local Group (a dumbbell-shaped group of galaxies with the Milky Way and Andromeda forming the two lobes), instead of the 500 or so predicted by dark matter simulations, and that’s just around the Milky Way.
Canto: Okay, we’re doing well, sort of. Final problem – the alignment of satellite galaxies. Essentially, they form a disk rather than a halo. Perhaps surprisingly, the Forbes website has what seems to me an excellent article on the subject. Dark matter simulations produce halos merging together in spiral formations surrounded by sub-halos in a variety of orientations. But that’s not what we see – we see satellite galaxies in the same orientation as their ‘hosts’, and co-rotating with them. This has been observed for the Milky Way, Andromeda and most satellite galaxies observed, such as Centaurus A. So what accounts for these discrepancies?
Jacinta: You don’t know? Well, we’ll have to look at that next time, or not. I suspect there might end up being hundreds of these dark matter posts. We might even have to learn some maths….
References
Is Dark Matter Real? – with Sabine Hossenfelder (Royal Institution video)
What is dark matter? – with Peter Fisher (Royal Institution video)
https://en.wikipedia.org/wiki/Galaxy_filament
https://en.wikipedia.org/wiki/Void_(astronomy)
https://en.wikipedia.org/wiki/Tully–Fisher_relation
https://en.wikipedia.org/wiki/Cuspy_halo_problem
https://en.wikipedia.org/wiki/Local_Group
Journey into dark matter , iTelescope Webinars, with Dr Maggie Lieu
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