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In the hope that it may provide information on the development/evolution (if any) of dark matter over time, are there any differences (eg. in structure or concentration) in the dark matter at large radial distances from earth compared with closer radial distances.

I learnt from this question How does the amount of dark matter in measured galaxies vary? that “the smaller a galaxy is, the larger its amount of dark matter is”. But this appears to be due to the erosion of gas in the galaxies rather than developmental changes in the dark matter which I’m looking to understand.

Zinn
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1 Answers1

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Unclear, but the operational assumption is that the amount of dark matter does not change.

Modern cosmology is based on the Friedmann equations, for which the energy density of matter, $\Omega_m$, is a key parameter. $\Omega_m$ varies over time as the universe expands, but it could in principle also vary if dark matter decays into other stuff (like radiation, measured by $\Omega_r$). It could also in principle be possible to increase $\Omega_m$ if some non-matter thing forms dark matter.

However, observations indicate that $\Omega_m$ does not change much. The fraction of dark matter that can decay is less than about 2%. But 2% is not 0%, so it is possible, and people are thinking about it. Example.

Finally, it must be the case that the structure of dark matter changes with distance, because like other objects with mass, gravity affects how it moves. Structure formation is one of the lines of evidence for the existence of dark matter.

Allure
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  • While I agree enough with this answer to +1 it, I tend to think that this answer somewhat underrates the overwhelming evidence we have that, even if dark decay be able to decay, it must do so at an extremely low rate; low enough that observations of galactic dynamics, gravitational lensing, large-scale structure, etc. so far are consistent with no decay at all. – pela Feb 06 '24 at 09:55
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    @pela good if you could write an answer that included those constraints. Do we know how well $\Omega_M(t)$ behaves, or is it more that we don't see see the products of dark matter decay? – ProfRob Feb 06 '24 at 10:02
  • Thanks @allure. A couple of follow ups if you don’t mind:

    1. “The energy density of matter, $\Omega_m$, varies over time as the universe expands” - because space is thought to be expanding, normal matter per area is decreasing?

    2. “it might be possible that dark matter is formed from baryonic-matter interactions.” - meaning that dark matter could be accumulating over time, as opposed to decaying?

    3. I’m curious as to how the answer could be ‘unclear’ when we could look at the surveys of dark matter and get a relationship between dark matter amount vs. distance form earth.

     – Zinn Feb 06 '24 at 10:38
  • @ProfRob I was thinking more of the fact that rotation curves and other kinematical models, lensing mappings, N-body simulations etc. work just well in many aspects without accounting for decaying dark matter, than I was thinking of the lack of direct evidence. But I don't think I have a good enough overview of the constraints to write a good answer. – pela Feb 06 '24 at 11:48
  • @pela But I don't think I have a good enough overview of the constraints to write a good answer Maybe, but you likely have a better idea than me, so you can still write a better answer than I can =) – Allure Feb 06 '24 at 13:22
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    @Zinn 1) Yes. The density of dark matter falls as the universe expands, while the density of dark energy stays constant. 2) In theory, yes. 3) What's unclear is whether dark matter decays. We don't know. The operational assumption is that it doesn't, but it could. If it does decay, it cannot be very fast (see pela's first comment). – Allure Feb 06 '24 at 13:24
  • Thanks @allure. Something that interests me now is the fact that the Friedman equations that underpin all of cosmology are based on an uncertain assumption. A topic for another question maybe :) – Zinn Feb 06 '24 at 13:43
  • @Zinn Hmm, I'll see if I can edit the answer to make it clearer what is meant. Friedmann equations does assume some things, but dark matter stability is not one of them. – Allure Feb 06 '24 at 13:47
  • From your example reference it seems that the lower limit to any dark matter decay lifetimes is around $10^{14}$ years, so basically none of it will have decayed in $10^{10}$ years. – ProfRob Feb 06 '24 at 14:07
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    @Allure Thanks, perhaps, but I would still need to dig deep into the literature. I think users like Sten or Kyle Kanos would be better suited… – pela Feb 06 '24 at 15:18
  • @Zinn My understanding is that all of physics is based on "uncertain" assumptions. One of the key things that experimental physicists do try to put ever-tighter bounds on how much uncertainty there are in those assumptions (i.e. that if value X was off by more than a certain amount we should have seen it). – Ben Feb 07 '24 at 02:21
  • @Ben Fair point, but you have to admit there are vastly different levels to this uncertainty. The scientific community seems to still be in the speculation phase with regards to explaining the ‘missing matter’. So the fact that modern cosmology is based on equations involving measurements of matter we have no idea about is startling to me. – Zinn Feb 07 '24 at 05:14