Uniformity versus time

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PhilKravitz

Main Question or Discussion Point

How does the uniformity of the universe change over time? Or maybe I should say inhomogeneity versus time.
 

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  • #2
marcus
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How does the uniformity of the universe change over time? Or maybe I should say inhomogeneity versus time.
I hope several people will respond to this. It is such a general question that it seems to me quite a challenge just to get a grip on it.

The concept of entropy is in the background. As long as gravity is attractive and matter dominates, matter will tend to clump.
A smooth uniform grav. field is the low entropy condition and a clumpy field is high entropy.

But radiation behaves differently. It doesn't clump. It sort of spreads out randomly and uniformly. So I would say in the most general terms that

1. you start out with a uniform grav field and uniform matter soup

2. and that curdles and clumps and gets more and more pimply, develops black holes etc.

3. but then in the very long term maybe matter decays and black holes evaporate (we don't really know if that's true but suppose) so then you have radiation

4. the radiation spreads out and randomizes and gets uniform.

This is quite puzzling to me. There seems to be no simple verdict about inhomogeneity.
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And then in LQG at very high density there are quantum corrections to gravity which dominate and make it repel instead of attract. So that reverses the sign of entropy. It is the clumpy matter with clumpy grav. field that is low entropy, and the tendency is for things to fly apart and spread out uniformly. Uniform matter and grav field is then the high entropy case.

But then as soon as some expansion has occurred and gravity settles down to be an attractive force again, that high entropy condition is redefined as low entropy. And stuff begins to coagulate and curdle once more.

This is definitely puzzling. I have not heard a good clear straightforward discussion of it yet.
 
  • #3
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It seems that entropy can reduce temporarily in some small areas but overall it never stops increasing. How big can these "small areas" be?
There are probably various levels of homogenity/inhomogeniety depending on the scale and type of observation.
 
  • #4
PhilKravitz
Marcus, thank you for your thoughtful answer. It is points 1 and 2 that I am most interested in. More exactly when can the first black hole form? It has taken me a long time to understand that in the early universe (say at 10 nanoseconds) there are no black holes because of the uniformity. My original thinking was gee sky high densities there much be black holes all over the place. But of course the error is a sphere may have at it's surface an escape velocity greater than c due to the gravitational force of all the stuff inside it but there is all the stuff outside that cancels the force leaving a net force of near zero (i.e. no black hole).

This seem like an ideal case for some computer simulation to gauge (common usage) the time scale on which a near uniform distribution clumps. Maybe someone has already done this.

Its almost a phase change kind of thing.
 
  • #5
bapowell
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Great answer Marcus. I think it's important to emphasize that in order to go from 1) to 2) one needs seed perturbations. In inflationary cosmology, these perturbations are generated during the primordial inflationary era. It is possible that these perturbations be large enough on certain scales that the perturbations quickly collapse into primordial black holes. Gravitationally bound structures will not form until matter has dominated the energy density of the universe because, as Marcus says, a radiation dominated energy density doesn't clump -- it oscillates (these are the famous acoustic oscillations seen the cosmic microwave background!)
 
  • #6
PhilKravitz
Great answer Marcus. I think it's important to emphasize that in order to go from 1) to 2) one needs seed perturbations. In inflationary cosmology, these perturbations are generated during the primordial inflationary era.
I thought it was just the opposite the inflation flattens any pre-existing lumps. If that were true then lumps must be initial conditions of the universe?

Or if lumps are not initial conditions how does inflation make lumps? And then all the questions like how many, how big, at what time, etc...

Penrose [The Road to Reality] is big on the fact that the initial condition of the universe is very low entropy.
 
  • #7
bapowell
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I thought it was just the opposite the inflation flattens any pre-existing lumps. If that were true then lumps must be initial conditions of the universe?
You're quite right Phil. Inflation does smooth out initial inhomogeneities leading to a flat universe. This was initially viewed as bittersweet: it gives us a homogeneous and isotropic universe (that we observe on large scales), but seems to simultaneously preclude the formation of structure and local inhomogeneity we see around us.
Or if lumps are not initial conditions how does inflation make lumps? And then all the questions like how many, how big, at what time, etc...
The key point is that inflation leads to a homogeneous universe classically. However, the inflaton field is a quantum field. As such, it is subject to inherent quantum fluctuations. As the universe inflates, these fluctuations are amplified and stretched to large scales (in fact, they are stretched to scales larger than the causal horizon since the inflating spacetime expands faster than the Hubble radius). These quantum fluctuations couple to the background spacetime and generate tiny curvature perturbations. When inflation has completed, one is left with a spectrum of seed perturbations across a continuous range of scales -- from super-horizon down to the galactic. According to inflationary cosmology galaxy clusters, galaxies, and so on, originally arose as a quantum fluctuation in the very early universe. To me, that is the most amazing scientific assertion out there.
 
  • #8
PhilKravitz
To me, that is the most amazing scientific assertion out there.
Thank you bapowell I understand the idea.

I find having to invoke quantum mechanics for the structure of the universe unsatisfying.
 
  • #9
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Phil Kravitz, you are probably think about Occams razor and I sympathize with your view.

http://en.wikipedia.org/wiki/Occam's_razor


A few centuries years from now we may have collected enough observational evidence to come up with a more satisfying, accurate and simple understanding of what happened. Until then we have to be patient and weigh the truth in every proposition. The first ever detection of CMBR was only made in 1940.
 

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