<p>Your question is one that bothered cosmologists for a long time, how do you get clumps of matter out of a smooth initial condition, in the time allowed?  It was long understood that a perfectly smooth matter distribution could not be maintained forever, because gravity creates instabilities-- the smallest overdensity in one region would tend, over time, to pull in more matter and increase the overdensity.  But this takes time, and the CMB measurements were coming in surprisingly smooth-- so how did galaxies and stars form in less than a billion years, given that very uniform initial density?</p>

<p>The answer seems to be dark matter.  Dark matter increases the mass and therefore the gravity, so it reduces the time it takes for instabilities to develop and overdensities to grow.  So the simple answer to your question is that even though cosmology assumes a smooth initial density, it does not say the density is <em>exactly</em> smooth, and indeed the CMB tells us there are tiny density variations (you can look at the tiny variations seen by the PLANCK satellite at <a href="https://en.wikipedia.org/wiki/Planck_(spacecraft)#/media/File:Ilc_9yr_moll4096.png" rel="nofollow noreferrer">https://en.wikipedia.org/wiki/Planck_(spacecraft)#/media/File:Ilc_9yr_moll4096.png</a>, for example-- the scale shows variations of 1 part in ten thousand).  Then gravitational instability increases the overdensity, but needs stronger gravity to do this in the time allowed, so that appears to be provided by dark matter.</p>

CC BY-SA 4.0
2019-06-21T10:49:51.750Z
This is disturbing because the homogeneity issue seems often used in two opposite way. Sometimes "how is possible that far away opposite regions of the CMB know of each other" and sometimes "how it is that differentiation took place".Perhaps one should face quantitative aspects to get a real sense of this. Not a critic to the answer, by the way. Just puzzled, or better, perplexed.
Alchimista
CC BY-SA 4.0
2019-06-21T11:03:43.950Z
This is the Q. It was closed, even :)) https://physics.stackexchange.com/q/483541/162193
Alchimista
CC BY-SA 4.0
2019-06-19T23:19:49.557Z
2019-06-19T23:25:10.370Z
2019-06-19T23:25:10.370Z
Ken G
<p>Was the expansion of space time during big bang uniform if so how come the stars and planets be formed because and matter as well as gravity would have been same everywhere</p>

CC BY-SA 4.0
2019-06-19T23:32:21.920Z
Related?: [inflation fluctuations](https://astronomy.stackexchange.com/search?q=inflation+fluctuations)
Keith McClary
<early-universe>
Question about expansion of space time
Ginormousbeet
# Question about expansion of space time

(Asked by: username_0 on 2019-06-19)

Was the expansion of space time during big bang uniform if so how come the stars and planets be formed because and matter as well as gravity would have been same everywhere

**Tags:** early-universe
> **Comments:**
> **username_1**: Related?: [inflation fluctuations](https://astronomy.stackexchange.com/search?q=inflation+fluctuations)  

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## 1 answers

### Answer by username_2 on 2019-06-19. Score: 3 | Accepted answer

Your question is one that bothered cosmologists for a long time, how do you get clumps of matter out of a smooth initial condition, in the time allowed? It was long understood that a perfectly smooth matter distribution could not be maintained forever, because gravity creates instabilities-- the smallest overdensity in one region would tend, over time, to pull in more matter and increase the overdensity. But this takes time, and the CMB measurements were coming in surprisingly smooth-- so how did galaxies and stars form in less than a billion years, given that very uniform initial density?

The answer seems to be dark matter. Dark matter increases the mass and therefore the gravity, so it reduces the time it takes for instabilities to develop and overdensities to grow. So the simple answer to your question is that even though cosmology assumes a smooth initial density, it does not say the density is *exactly* smooth, and indeed the CMB tells us there are tiny density variations (you can look at the tiny variations seen by the PLANCK satellite at <https://en.wikipedia.org/wiki/Planck_(spacecraft)#/media/File:Ilc_9yr_moll4096.png>, for example-- the scale shows variations of 1 part in ten thousand). Then gravitational instability increases the overdensity, but needs stronger gravity to do this in the time allowed, so that appears to be provided by dark matter.

> **Comments:**
> **username_3**: This is disturbing because the homogeneity issue seems often used in two opposite way. Sometimes "how is possible that far away opposite regions of the CMB know of each other" and sometimes "how it is that differentiation took place".Perhaps one should face quantitative aspects to get a real sense of this. Not a critic to the answer, by the way. Just puzzled, or better, perplexed.  
> **username_3**: This is the Q. It was closed, even :)) https://physics.stackexchange.com/q/483541/162193  

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