How Many Dimensions Are There?

[septimus]

@Pasta

From a particle physics perspective, the required properties of dark matter particle(s) are quite sensible and not weird at all.

Pasta, will you indulge me and let me re-ask one of my favorite questions?

How do “we” know that dark matter does not interact with photons? Chronos has answered this question for me at least twice before — and his answers seem better than the answers at Physics.Stackexchange, but I didn’t fully understand them.

@Chronos

The laws of thermodynamics absolutely require that if something be able to absorb radiation, it also be able to re-emit it. We can never be absolutely certain, of course, that everything in the Universe actually follows the laws of thermodynamics, but then, the same is true of literally everything else we know.

It seems to me that ordinary stars emit a lot more light than they absorb. Surely, at least in a thought-experiment à la the demons of Maxwell and Boltzman, we could imagine an object which absorbs a lot more light than it emits.

@Chronos

[Quoth septimus:

Is that because there are no dark blotches in the sky? What if the dark matter is in very tiny clumps of high density?]
If it were in tiny clumps, you’d need a lot of them, and they’d form a sort of gray fog across the sky. If instead they were in large clumps (called “MACHOs”, for "MAssive Compact Halo Objects), they’d be much harder to notice, at least via their overall fogging effect. But you could still detect them through what’s called microlensing: Every so often, one such clump would pass between us and a star, and its gravitational field would slightly bend light around it, causing a focusing effect which would make the star appear to momentarily brighten in a particular way. Microlensing searches have been done, and have concluded that MACHOs can account for only a small fraction of the dark matter.

But there’s another problem with MACHOs: What are they made of? We have very good bounds on the total amount of baryons (protons and neutrons) in the Universe, based on the relative abundances of isotopes of the lightweight elements. And that’s also far short of the dark matter total. Since our familiar sort of matter gets almost all of its mass from protons and neutrons, there must be some other sort of matter making up the dark matter. And if that nonbaryonic dark matter stuff, whatever it is, interacts with the electromagnetic force, then why can’t we detect any of it in our particle-accelerator experiments?

@Chronos

Our models of Big Bang nucleosynthesis are quite strong, and hence our confidence in the amount of baryonic matter is quite high. No dark matter model is anywhere near strong enough to significantly shake that confidence.

Black holes might be some or all of the dark matter, but then you have to ask where they came from. The baryonic-matter limits still apply, so either the holes would have to have been formed from something other than baryonic matter, or they would have to have been formed before the era of nucleosynthesis, very early in the Universe’s history. And if primordial, low-mass black holes are really so abundant, then you also have to ask why we’ve never detected any of them. This is a problem in general with the more exotic hypotheses for dark matter: The more exotic something is, the more likely it should be that we would have detected it in some other way.

And the microlensing MACHO searches are good at least as far down as planetary masses (and would also, incidentally, detect black holes in that mass range). I don’t know how large dark-matter objects could be without producing the fog effect.

Put it all together, and we don’t know that the bulk of the dark matter is made up of stuff that doesn’t interact electromagnetically at all, but it seems by far the safest way to bet. It’s really not so surprising, after all, that there would be some such particles, and if they exist, there would be a good reason why we don’t easily detect them.