Galaxy formation

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  • #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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  • #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
 
  • #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.
 
  • #4
ChrisVer
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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

Inhomogeneities are there because of fluctuations in the metric during the inflation?
 
  • #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?
 
  • #6
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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?
http://casa.colorado.edu/~bally/ASTR2010_F12/SciAm_Myth_of_Time_Veneziano.pdf [Broken]
 
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  • #7
Drakkith
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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.

Can you elaborate on why dark matter is required for early galaxy formation? Or link something that elaborates?
 
  • #9
CKH
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http://arxiv.org/abs/0909.2021, Dark Matter and Galaxy Formation

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.
 
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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.
 
  • #11
phinds
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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.

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.
 
  • #12
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It's possible that the DM might have kept passing through the nascent BH (the way it passes through the center of galaxie today)

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.
 
  • #13
phinds
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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.

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).
 
  • #14
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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.
Science is not about beliefs. Are there publications supporting this idea?
 
  • #15
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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)

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.
 
  • #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
 
  • #17
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DM appears resistant to capture by black holes
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.
 
  • #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.
 
  • #19
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I would not call something "resistant to capture" if it does fall in, just at a significantly lower rate.
 
  • #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.
 
  • #21
CKH
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Lately papers have been published concerning a resolution of the core/cusp problem raised by DM halos. The solution put forth is that supernova blast waves can drag DM away from the core (through gravitational interaction with the density waves). Perhaps similar processes involving non-uniform accretion of matter around a BH could drag some DM into SMBHs.

Another possibility is "evaporative cooling" that I think may account for the ability of DM to clump on it's own. The particles may exchange momentum through gravitational interaction. Those with the highest velocities escape leaving those that remain a bit cooler. This would likely be a very slow cooling process, nothing comparable to radiating normal matter.

Might there be any special, detectable effect of the consumption of DM by a BH? CDM particles are expected to be heavy.
 
  • #22
TumblingDice
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I would not call something "resistant to capture" if it does fall in, just at a significantly lower rate.
In common terminology, resistant doesn't imply 100% of the time. A water-resistant watch is not water-proof. Wrinkle-resistent clothes, drug resistant virus, etc... The term doesn't mean "always" or "never". In this case, "immune to capture" would imply that DM never falls in.

Dark matter particles can fall into black holes like regular particles. They just don't form an accretion disk.
Using "like" to compare how DM reacts to how 'regular particles' react is a bit of a gloss over. DM would need to be aimed right "on target" (or very close) to the BH or it will slip through any accretion disk and pass right by the BH.
 
  • #23
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Okay I won't argue about words, this does not lead to anything.
Using "like" to compare how DM reacts to how 'regular particles' react is a bit of a gloss over. DM would need to be aimed right "on target" (or very close) to the BH or it will slip through any accretion disk and pass right by the BH.
Its angular momentum has to be smaller than ##x c R_s## where x is a prefactor of the order of 1 (I think 2 for a non-rotating black hole, it gets a function of the orientation for rotating black holes).
If it is cold (cold enough to keep in a galaxy), this corresponds to an impact parameter a thousand times larger than the Schwarzschild radius of the black hole. Still tiny compared to the galaxy, but significantly larger than the black hole.
 
  • #24
Chronos
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One of the newer DM candidates is sterile neutrinos. I find that attractive because it avoids the problems inherent to massive DM particles.
 
  • #25
Jonathan Scott
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One of the newer DM candidates is sterile neutrinos. I find that attractive because it avoids the problems inherent to massive DM particles.
Please clarify what you mean by "massive" here. I thought that DM particles had to have some rest mass, even if very small. If you're suggesting DM could be massless neutrinos, I thought that couldn't work because they couldn't be gravitationally captured.
 
  • #26
Chronos
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Neutrinos are not necessarily massless, as evidenced by flavor change, but, that is unimportant. An unidentified x ray line was discovered earlier this year that is the plausible consequence of decay of a 7.1 Kev sterile neutrino. Rest mass is irrelevant due to the equivalence principle.
 
  • #27
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Neutrinos are not necessarily massless, as evidenced by flavor change, but, that is unimportant. An unidentified x ray line was discovered earlier this year that is the plausible consequence of decay of a 7.1 Kev sterile neutrino. Rest mass is irrelevant due to the equivalence principle.
The mass is relevant - to be cold, the particles have to have mass, and it has to be significantly above their kinetic energy. Otherwise the particles could not clump on the scale of galaxies.
 
  • #28
CKH
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Would sterile neutrinos fall into the category of warm dark matter or even hot dark matter? My understanding was that these cases already have problems. In recent years WDM has been proposed to solve some problems with CDM, but there seems to be some consensus that WDM causes other problems as pointed out by mfb.

Would anyone care to contribute an explanation for the clumping of DM that permits it to accelerate the clumping of normal matter?
 
  • #29
Hi, I just want to share my opinion and what I understand about the topic. Galaxy formation is concerned about the processes on how evolution started, how various galaxies formed and took the many shapes that we knew today.
 
  • #30
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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?

In inflationary cosmology structure formation is seeded during the cold inflation era before the hot big bang era. This is consistent with the Planck data releases and explained by, say, Susskind in his youtube Stanford Lectures in cosmology. (You would want the latest of his several series.)

Inflation is most likely a single scalar field (which type of field we now know there exist more complicated variants of, the Higgs fields). As all fields it will have quantum fluctuations. Those will do several things:

1. The fluctuations in the field will speed up, or slow, inflation locally. The earliest fluctuations will hence set the size of the local universe (which our observable universe will be a minute part of).

2. The fluctuations in the field will translate to energy density fluctuations. These fluctuations are expanded exponentially with the spacetime expansion, while the cosmological horizon (where expansion speed becomes as large as the universal speed limit) remains roughly constant. The result is that fluctiations with wavelength larger than the horizon is "frozen in" as they will become modes of the field rather than local fluctuations.

[ http://ned.ipac.caltech.edu/level5/Sept02/Kinney/Kinney4_5.html ]

Later these fluctuations are overtaken by the horizon.

300px-Horizonte_inflacionario.svg.png

"The physical size of the Hubble radius (solid line) as a function of the linear expansion (scale factor) of the universe. During cosmological inflation, the Hubble radius is constant. The physical wavelength of a perturbation mode (dashed line) is also shown. The plot illustrates how the perturbation mode grows larger than the horizon during cosmological inflation before coming back inside the horizon, which grows rapidly during radiation domination. If cosmological inflation had never happened, and radiation domination continued back until a gravitational singularity, then the mode would never have been outside the horizon in the very early universe, and no causal mechanism could have ensured that the universe was homogeneous on the scale of the perturbation mode."

[ http://en.wikipedia.org/wiki/Inflation_(cosmology) ]

The frozen in fluctuations are then released as energy density fluctuations again. When the hot big bang produces particles from the remaining potential energy of the inflation field, these energy density fluctuations will translate into variations in particle densities and velocities.

As the particles spread due to the variations, they will meet akin to the caustics one can see in the light at the bottom of a swimming pool, where photons likewise has been spread by surface waves. The caustics of dark and ordinary matter that show up as cosmological filaments will gravitate, and that is the seed to galaxies and galaxy clusters - structures.
 
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