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What prevented all of the hydrogen at the universe's start from coalescing into one gigantic star?

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The "bang" part of the big bang? – Olin Lathrop Jul 17 '13 at 20:16
It is not known. There are only theories stating that it was expanding (postulating expansion without knowing the force behind it). If you will step by one million years later we might know the answer. According to current knowledge nothing can expand after it collapsed into black whole. – Asphir Dom Jul 17 '13 at 20:27
You may be interested to learn that the vast majority of the hydrogen in the universe still hasn't collapsed into stars. Just as steam cannot condense into water droplets above the boiling point, hot gas cannot clump into a star without cooling sufficiently. – Chris White Jul 17 '13 at 22:22
@AsphirDom: It is not known. There are only theories stating that it was expanding (postulating expansion without knowing the force behind it). Not true. No force is required in order to explain or maintain cosmological expansion. According to current knowledge nothing can expand after it collapsed into black whole. This is true in classical GR, but irrelevant, since we're not discussing black holes. See… – Ben Crowell Jul 17 '13 at 22:33

There are two reasons:

First, the expansion of space, which was rapid in the early universe, separated the initial density fluctuations into isolated potential wells. Dark matter and ordinary matter then accumulated into these local potential wells, which eventually become galaxy clusters.

Second, the temperature of the early universe was very high, so that hydrogen and helium atoms were ionized. These ionized atoms and free electrons interacted with photons; basically, the early universe was like a plasma. But these photons exerted a radiation pressure on the matter: as matter falls into the potential wells, it heats up, and the radiation prevents it from compressing further. In fact, the radiation reverses the motion, and the matter recoils.

As the universe expanded, the photons lost energy, and matter could fall back in, etc. This 'tug of war' between matter and radiation caused oscillations, and continued until the universe had cooled sufficiently for the ions and electrons to combine into neutral atoms. The photons then decoupled from the atoms, and we can still observe them today as the Cosmic Microwave Background. The oscillations are imprinted in this CMB as temperature fluctuations.

For a detailed overview of the processes in the early universe, see this source:

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To the first, doesn't this assume that the matter was NOT evenly distributed to begin with? What explains this? – aserwin Jul 17 '13 at 21:50
On large scales, matter was evenly distributed. But locally, there were very small perturbations in the density. The leading theory is that these perturbations were caused by quantum fluctuations at the end of the inflationary era. – Pulsar Jul 17 '13 at 21:56
Yes. Would say it is an additional reason that indeed utter uniformity would be required to create such a single star. That is the least likely of all states given it requires perfect order to impose on a random background. Why would one expect such uniformity, that implies much lower entropy than any other prior state. THere is no reason for that. – user12811 Jul 17 '13 at 22:39
@user12811: You have the entropy argument backwards. In a gravitating system, entropy is maximized by clumping of the matter degrees of freedom. It's essentially the opposite of an ideal gas. The early universe was in fact in a very low-entropy state, because the gravitational-wave degrees of freedom were not activated, and there is currently no explanation for this. – Ben Crowell Jul 17 '13 at 22:53

Pulsar and Chris White have given nice explanations of why the dynamics of the early universe would not lead to the formation of one big starlike object. There is also another, much more generic argument against such a process, which boils down to the existence of cosmological horizons. At any given time in the evolution of the universe, there have been parts of the universe far enough apart that they could never have had any causal relationship -- no signal could have propagated between them, even at the speed of light, even if the signal was emitted immediately after the big bang. If matter in region A and matter in region B are separated in this way, then it's not possible for them to have collapsed into the same object, simply because relativity prevents the matter at A and B from even coming together that fast.

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