Missing Matter Not Quite As Missing As Before, But Only Very Slightly

I was asked for my comments on this one, which is getting quite a bit of attention.

The basic issue is that the amount of normal matter predicted by the standard model of cosmology is in disagreement with observations. Not only are there large amounts numbers (whatever) of missing galaxies, there's also a large quantity (hah ! take that, grammar Nazis !) of missing matter in general. Where's it gone ? Is it down the back of the sofa ? Under the mattress ? Did the dog eat it ?

One intriguing idea not discussed all that much these days is that maybe it lives in perfectly normal galaxies but is hard to detect. Could be the good ol' MACHOs (MAssive Compact Halo Objects) - asteroids, dead stars, black holes and the like. More radically it could be cold gas, which doesn't radiate all that much. What would be neat about this is that the amount of missing matter is just about enough to explain the flat rotation curves of galaxies, negating the need for dark matter on these scales (but not on the scale of galaxy clusters). Well, I suppose that's neat if you think of dark matter as a problem to be solved rather than an interesting entity to investigate, anyway.

But these ideas have been largely ruled out : gravitational microlensing surveys have turned up negative results for the former, while the latter would require an unknown mechanism to prevent the cold gas from forming stars. Fortunately, though cold material is hard to detect, hot material is also bloomin' difficult (I suppose normal visible matter is in a sort of Goldilocks Detectability Zone) - hot material quickly disperses to very low densities. The two leading candidates for reservoirs of hot material that could account for the missing mass are galactic halos and filaments. Halos are spheroidalish clouds of material that are bound to individual galaxies. Filaments are long streams of material linking many different galaxies in the so-called "cosmic web".

This paper presents evidence of gas found in halos. While it's damned hard to detect this gas by the radiation it emits, it's easier if it blocks the view of a bright background source like a quasar. In that case some of the light can be absorbed, if the material and the source are correct. As I learned in a workshop last week, this depends on some pretty complex physics. For example, X-rays might not directly be absorbed by the gas but can give rise to UV radiation, which can. I'm not going to pretend to even be qualified to attempt a summary of the processes at work, so I'm going to take the author's word for it that they've done everything correctly.

The point though is that the significance of this result seems a bit overstated. They've found pretty good evidence of the missing material, and rule out other explanations like self-absorption because that ought to vary on short timescales, which it didn't. But this is only for a single quasar, i.e. in a very, very small part of the Universe. It's cool and all, but it doesn't really say much about the overall problem.

I found this detection of hot gas in filaments to be much more convincing. For once the statistical nature of that detection is an advantage, because it means that filaments are present around large numbers of galaxies. That makes it much more convincing as a solution to the problem, or at least a big part of one. While there are a handful of other such claims for halo detection (see above link), I'd like to see much greater numbers before concluding that these too are an important part of the solution. What's particularly weird is that this paper doesn't cite the claims of filamentary hot gas discussed above.

I'd bet money on the problem being solved by a combination of filaments and halos. While the halos do need further scrutiny and much better statistics, in my opinion this issue has pretty much been resolved.

https://www.nature.com/articles/s41586-018-0204-1