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Galaxy formation

  1. Sep 18, 2014 #1

    wolram

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    When trying to work out how galaxies formed, there must have been some seed; some perturbations that started the ball rolling, These perturbations which collapsed gravitationaly to form seed galaxies must have been primordial in origin, where did they come from?
     
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  3. Sep 18, 2014 #2

    phinds

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    Tiny inhomogeneities in the early universe led to small differences in the density of regions of space after the surface of last scattering and that was enough to get the ball rolling. WHY there was any inhomogeneities in the early universe is beyond my knowledge. We see the remnants of it in the CMB, which shows inhomogeneities of 1 part in 100,000
     
  4. Sep 18, 2014 #3

    Chronos

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    Keep in mind there was also a prodigious amount of dark matter in the primordial universe that influenced galaxy formation. Without dark matter, it is nearly impossible to even model early galaxy formation. You might get the odd star here and there, but, not dense populations. It is even considered possible DM fostered the formation of primordial SMBH, which seeded galaxy formation.
     
  5. Sep 18, 2014 #4

    ChrisVer

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    Inhomogeneities are there because of fluctuations in the metric during the inflation?
     
  6. Sep 18, 2014 #5

    wolram

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    Does it make any difference if the seed galaxies were formed from inhomogeneites in a bouncing universe, where deconstruction was not complete?
     
  7. Sep 18, 2014 #6

    mfb

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    http://casa.colorado.edu/~bally/ASTR2010_F12/SciAm_Myth_of_Time_Veneziano.pdf [Broken]
     
    Last edited by a moderator: May 6, 2017
  8. Sep 18, 2014 #7

    Drakkith

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    Can you elaborate on why dark matter is required for early galaxy formation? Or link something that elaborates?
     
  9. Sep 20, 2014 #8

    Chronos

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  10. Sep 21, 2014 #9

    CKH

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    At the time when the CMB originated, how were CDM and baryonic matter distributed?

    The confusion I have with CDM helping to condense baryonic matter is that CDM has a very small ability to condense on it's own because there is no way that it can dissipate energy (it doesn't radiate).

    Oh wait, except perhaps by having high velocity particles escape slightly denser areas, those areas can lose energy and condense. But that leaves all the high velocity CDM particles freely moving through neighboring denser regions and possibly heating those regions causing them to become less dense.

    What happens? It's not obvious. Maybe it's not obvious for baryonic matter either. Radiation will travel and eventually get absorbed.

    Oh wait, perhaps the expansion itself creates space for hot stuff to get lost by cooling.
     
  11. Sep 23, 2014 #10
    Obviously there is no way to know for sure yet, but I believe that the hydrogen gas clouds in the early universe compressed then formed super massive black holes. Then the remaining gas that was orbiting the bh later compressed into stars due to the effects of the bh.
     
  12. Sep 23, 2014 #11

    phinds

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    May well have happened that way, but don't forget, dark matter was interspersed with regular matter as far as is known so the BHs would all be 80% dark matter (not that it matters since once inside the EH, it's irrelevant what kind of matter it was outside) ; I'm just pointing out that the BHs would NOT have formed primarily from hydrogen as you posit but from the same inhomogeneities (in ALL matter) that I mentioned in post #2

    EDIT: It's possible that the DM might have kept passing through the nascent BH (the way it passes through the center of galaxie today) and not be as much of it as I'm thinking.
     
  13. Sep 23, 2014 #12

    PeterDonis

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    DM passes through other matter today because it doesn't interact with it. But passing through a BH would require the DM to travel faster than light (in order to escape the hole's horizon once it was inside). That's not possible.
     
  14. Sep 23, 2014 #13

    phinds

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    Yes, but I didn't SAY it would pass through a BH, 'cause I know better. I said it might pass through the NASCENT black hole (and thus not make up as much of the BH as I first stated it would).
     
  15. Sep 23, 2014 #14

    mfb

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    Science is not about beliefs. Are there publications supporting this idea?
     
  16. Sep 23, 2014 #15

    PeterDonis

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    Ah, ok, so by "NASCENT black hole" you meant "a region where a black hole is going to form but hasn't formed yet", so the DM can still pass through and escape. Sorry for the misunderstanding on my part.
     
  17. Sep 25, 2014 #16

    Chronos

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    DM appears resistant to capture by black holes according to http://arxiv.org/abs/1002.0553, An upper limit to the central density of dark matter haloes from consistency with the presence of massive central black holes
     
  18. Sep 26, 2014 #17

    mfb

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    I don't see where this conclusion comes from. We know upper limits on the amount of dark matter falling into the black holes - so what? Dark matter particles can fall into black holes like regular particles. They just don't form an accretion disk.
     
  19. Sep 28, 2014 #18

    Chronos

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    DM cannot shed kinetic energy so it does not spiral in like baryonic matter. Assuming that is not in dispute, I don't quite grasp your point.
     
  20. Sep 28, 2014 #19

    mfb

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    I would not call something "resistant to capture" if it does fall in, just at a significantly lower rate.
     
  21. Sep 28, 2014 #20

    Chronos

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    The choice of words may be arguable, but, it does contrast the behavior of baryonic matter and DM being attracted by a black hole. BM will shed kinetic energy via friction, whereas DM cannot shed kinetic energy. The slowed BM occupies a lower orbit that continues to decay via friction until it eventually falls out of orbit and is eaten. The faster DM has no means to slow hence is largely immune to being eaten. That is not to say DM can never eaten by a black hole, merely that it is a rare occurrence.
     
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