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PetTastic
2015-Jul-22, 09:39 AM
Is there thought to be any trends relating to the amount of dark matter in the universe and how it is distributed?

Is the total amount of DM increasing or decreasing?

Do observations show ancient galaxies had more or less DM than modern galaxies?

antoniseb
2015-Jul-22, 11:33 AM
Is there thought to be any trends relating to the amount of dark matter in the universe and how it is distributed?

Is the total amount of DM increasing or decreasing?

Do observations show ancient galaxies had more or less DM than modern galaxies?

You asked three questions:
1. Dark Matter distribution trends: Yes, computer models show it becoming somewhat more concentrated in cosmic filaments and clusters over time, but not as rapidly as reactive matter gets concentrated.
2. Some models have the amount of Dark Matter very slowly decreasing as is self-interacts and annihilates, other models have it not changing total quantity.
3. Current observations do not show anything with statistical meaning about the ratio of dark matter in ancient galaxies compared to now. Telescopes available in the 2020s might be able to get mild hints about this.

Cougar
2015-Jul-23, 02:31 AM
Do observations show ancient galaxies had more or less DM than modern galaxies?

To add to antoniseb's excellent answers, early structure formation has long been full of unsolved problems. Progress has been made, though, much of which I'm sure I am unaware. However, I have read, and wiki rather confirms as a mainstream finding, that the third peak of the cosmic microwave background acoustic oscillations power spectrum (https://en.wikipedia.org/wiki/Cosmic_microwave_background#Primary_anisotropy) provides information about the dark matter density*. The locations of the several peaks also give important information about the nature of the primordial density perturbations.

Dark matter was certainy present then, and its gravitational potential would have been unhindered by the barrage of photons (https://en.wikipedia.org/wiki/Cosmic_microwave_background#Primary_anisotropy) that kept banging into the H and He nuclei, keeping their gravitational potential at bay until recombination..... at which point the newly formed atoms of H and He gravitated toward the nearest overdensity of dark matter, which had a head start in its formation. Of course, the overdensities were varied, as were the results. :D


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* The amount of DM is not thought to change (how could it?), except for annihilations, as antoniseb mentioned, if they occur.

BigDon
2015-Jul-24, 03:42 PM
One of the better threads I've read in a long time.

PetTastic
2015-Jul-25, 10:38 PM
The amount of DM is not thought to change (how could it?), except for annihilations, as antoniseb mentioned, if they occur.

At the risk of spoiling a good thread. :)

I was thinking in terms of processes capable of creating cosmic-rays that stand a good chance of creating very exotic forms of matter.
Claims about Sterile neutrinos etc.
Or super massive blackholes consuming the high densities of DM thought to exist at the centre of galaxies.

Estimates of DM in the current universe seem to be increasing ( half the stars now thought to exist outside previously detected galaxies, findings that some galaxies are almost entirely DM & galaxies in voids etc)
These all seem to increase the DM and energy density of the modern universe by a quite big factor beyond what was originally expect in big bang models and acoustic osc. etc.

I was questioning how mainstream thinking fits the two together.
DM changing over time or update the old BB models to fit all the newly found DM?

Cougar
2015-Jul-30, 12:14 PM
Estimates of DM in the current universe seem to be increasing ( half the stars now thought to exist outside previously detected galaxies, findings that some galaxies are almost entirely DM & galaxies in voids etc)....

I'm not sure that's the case. Yes, some galaxies have more, but also some have less. I don't see how galaxies in voids or stars outside galaxies leads to an overall increased DM estimate....

PetTastic
2015-Jul-31, 02:11 AM
I'm not sure that's the case. Yes, some galaxies have more, but also some have less. I don't see how galaxies in voids or stars outside galaxies leads to an overall increased DM estimate....

I can't find the two papers I was reading.
I think the claims for stars between galaxies it based on near infra-red observations from rockets, but somewhere I was reading the colour of the background light is more blue than expected for old stars striped from galaxies and could suggest the stars are in undetected galaxies. (more blue than red and dead galaxies) That would mean DM assocaited with those galaxies.
The other was on galaxies in voids, suggesting they are aligned to DM filiments 42 million lightyears out into the void.

Edit---
Found second paper only seconds after I posted reply http://iopscience.iop.org/1538-3881/145/5/120/

Cougar
2015-Jul-31, 10:14 PM
Edit---
Found second paper only seconds after I posted reply http://iopscience.iop.org/1538-3881/145/5/120/

Wow, a "Void Galaxy Survey." Interesting stuff. The filament they actually observed is baryonic though. The three galaxies are embedded in an elongated common HI envelope out there in a void somewhere. They rather presume that there is an underlying dark matter filament, but they don't have a weak lensing study to verify that (which may very well be impossible, given the location). Simulations predict there would be such DM filaments, which would seem reasonable if an early configuration of DM overdensities looked like A B C . Where does B go? I imagine it gets stretched between A and C.

Cougar
2015-Jul-31, 10:26 PM
Of course, the overdensities were varied, as were the results. :D

I keep coming back to this. The gravitational interaction history of the Universe must look like this very large, utterly chaotic movie, especially if you only see a part of it. But it's actually pretty close to deterministic using Newtonian mechanics.