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I have read this question:

Why did the universe not collapse to a black hole shortly after the big bang?

where Lubos Motl says:

This matter has no center - it is almost uniform throughout space - and has high enough velocity (away from itself) that the density eventually gets diluted.

Now this and none of the other (there are a lot) answers answer my question specifically. I am not asking about collapse into a black hole. I am asking, right after the Big Bang, the density was extreme, thus gravity and curvature had to be extreme, maybe the escape velocity (meaning in this case the velocity needed for particles to fly apart) could have reached close to or even exceed the speed of light. But that is just gravity. There are the other forces (at that point unified if I understand correctly), that must have been holding particles together. This could involve the photon and the quark epoch as well.

Now first I thought:

  1. Maybe it was not particles flying apart, but simply just space expanding between them. But wait. First of all, space is expanding even now. Everywhere. Contrary to popular belief, space is expanding everywhere. Even here where we are. It is just that here, the other forces are dominating. Us, who are made up of matter, are held together by the other forces, that dominate over space expansion. So space is expanding right here, but the matter we are made up of stays together. No flying apart here. Space was expanding back then too. then how was space expansion able to overcome all the other forces back then but not now?

  2. It might be just a scale issue. Space expansion, some call it dark energy, might just be some kind of force, negative pressure, that is spread over the whole universe. It acts only on large scales. For now. But when the universe was extremely small, the scales were small too, and maybe dark energy was concentrated onto this small region, making it relatively stronger compared to the other forces.

Question:

  1. Right after the Big Bang, how did particles overcome extreme gravity and other forces and manage to fly apart?
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    $\begingroup$ This is similar to asking “How does an apple tossed upward overcome the gravity of the entire Earth for awhile?” Do you find that surprising? $\endgroup$
    – G. Smith
    Commented Oct 25, 2020 at 17:48
  • $\begingroup$ @G.Smith for a while? We are (and everything is) receding from everything for as far as we can model in the future. I am not asking why for a while. I am asking how it happened back then and not now. I am sorry but I really do not see the similarity between your example and my question. Can you please clarify? Space expansion cannot move particles apart now here where we are, but it was able to do it back then, though, the density was even bigger back then? Maybe I will need to clarify so i will edit if it is not clear. $\endgroup$ Commented Oct 25, 2020 at 19:00
  • $\begingroup$ A uniform space expansion doesn't pull matter apart. Local forces hold matter together not against "the force of the space expansion" - no such force exists. Good question anyway +1. $\endgroup$
    – safesphere
    Commented Dec 30, 2020 at 4:12
  • $\begingroup$ @safesphere thank you so much! When you say "no such force exist", do you mean dark energy is not a force? $\endgroup$ Commented Dec 30, 2020 at 5:22
  • $\begingroup$ @ÁrpádSzendrei Forces are not responsible for uniform motion (Newton's First Law), but only for acceleration (Newton's Second Law). "Dark energy" (if exists) can be viewed as a "force" and therefore has nothing to do with the uniform space expansion. No force is causing the expansion and the expansion applies no forces on matter. "Dark energy" would be responsible for acceleration of the expansion, but (even if such acceleration exists) it is so miniscule that can be ignored on the galaxy scale for all practical purposes. $\endgroup$
    – safesphere
    Commented Dec 30, 2020 at 16:04

3 Answers 3

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There are two reasons why there was no gravitational collapse in the very early universe:

  1. The distribution of energy and matter soon after the big bang was very nearly uniform. Because of this symmetry there was no reason for gravitational collapse to happen in one place rather than another - the net gravitational force at each location netted out to something very very close to zero.
  2. The early universe was at a very high temperature, which meant that fundamental particles were moving quickly, and gravity had very little effect on them.

Space was expanding, but this was not "overcoming" gravity. In fact, the expansion of space meant that the universe was cooling down, which assisted gravity. Like a pencil balanced on its point, the universe was in a state of unstable equilibrium, which became more unstable as it cooled.

As the universe expanded and cooled, fundamental particles combined into protons and neutrons, and then into atoms (almost all of which were hydrogen and helium atoms). This took several hundred thousand years. The very small deviations from absolute symmetry were then enough to trigger the collapse of the cooling atoms into gravitationally bound clouds, and then into the first stars and galaxies. But this process was very slow, and the first stars (called Population III stars) took hundreds of millions of years to form.

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  • $\begingroup$ Nice answer. Can you please tell me, are you saying that it was like a uniformly distributed area of particles, where gravity cancelled from all directions, thus having no way to create extreme curvature? So though it was very dense, there was spatial separation between particles, that is, there was some space between them? This could explain gravity. What about the other forces? Why was for example during the quark epoch, the strong (or back then unified) force not holding them together? $\endgroup$ Commented Oct 26, 2020 at 15:56
  • $\begingroup$ @ÁrpádSzendrei Yes, you could say gravity cancelled from all directions. And during the quark epoch the temperature of the universe was so high that even the strong force was not sufficiently strong to bind quarks into protons and neutrons. At this point the universe was filled with a quark-gluon plasma - see en.wikipedia.org/wiki/Quark%E2%80%93gluon_plasma for more details. $\endgroup$
    – gandalf61
    Commented Oct 26, 2020 at 16:26
  • $\begingroup$ "The early universe was at a very high temperature" - How do you know? The entropy increases, so it must be zero at the Big Bang thus implying the absolute zero temperature in the early universe. The Fermi energy may be high due to the Pauli exclusion principle, but only if all matter is produced at the Big Bang rather than after it. We don't know that, but even if so, the Fermi energy can be high at a zero thermodynamical temperature - cold to touch, because this energy cannot be transferred as heat. $\endgroup$
    – safesphere
    Commented Dec 30, 2020 at 4:25
  • $\begingroup$ @safesphere High temperature is compatible with low entropy in the early universe because of gravity. See en.wikipedia.org/wiki/Entropy_(arrow_of_time)#Cosmology : “The universe was in a uniform, high density state at its very early stages ... The hot gas in the early universe was near thermodynamic equilibrium ... in systems where gravitation plays a major role, this is a state of low entropy ...”. $\endgroup$
    – gandalf61
    Commented Dec 30, 2020 at 11:12
  • $\begingroup$ The negative heat capacity you are referring to seems to contradict the statement that the net gravitational force is close to zero. However, this may be a bit beyond the topic of the question, so please never mind. $\endgroup$
    – safesphere
    Commented Dec 30, 2020 at 16:22
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Is it being assumed in the above explanations that a dimensional "space" existing as a certain volume of vacuum (i.e., having or containing a complete absence of matter or energy) must have pre-existed the Big Bang events? That seems like a very large assumption.

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  • $\begingroup$ If you have a new question, please ask it by clicking the Ask Question button. Include a link to this question if it helps provide context. - From Review $\endgroup$
    – Miyase
    Commented Jun 7, 2022 at 7:05
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I think your question is based on a incorrect assumption about the geometry of the universe. You say that you are not asking about a black hole, but my guess is that you see the geometry of the universe a finite collection of matter surrounded by vacuum. If at an early time the temperature of this configuration of matter is not hot enough (or equivalently it does not have enough pressure), then all of the matter would collapse into a black hole.

If I am wrong about your view of the geometry, please let me know, and I will try to answer your question based on the actual geometry you have in mind. Is it finite or infinite? Is it homogeneous or something else? If finite, do you see it as a 3D boundary of a 4D hyper-sphere, or something else? Is dark energy present or not?

BTW: The expansion of space does not cause all matter to move apart, like very distant galaxies move away from the Milky Way. The matter in the Milky Way remains within a range identical to (or almost so) as it is now.

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  • $\begingroup$ I am simply asking, back then, when density was extreme, and gravity was extreme and other forces were acting attracting particles, they were still flying apart, but now, particles are not flying apart anymore, they stay in clumps here where we are for example. $\endgroup$ Commented Oct 25, 2020 at 19:06
  • $\begingroup$ How was space expansion back then able to overcome the other forces but not now> $\endgroup$ Commented Oct 25, 2020 at 19:06
  • $\begingroup$ @Árpád Szendrei Theer is an active concept that space expansion has no effect on matter near each other within a limit. For example, it does not effect the constituents of the Milky Way. However, for matter far enough apart, it has the effect of dominating local gravitational effects. There is a formula used to decide whether the matter is far enough appart to be effected bu universe expansion, I will add that formula later. $\endgroup$
    – Buzz
    Commented Oct 29, 2020 at 22:16
  • $\begingroup$ The formula for escape velocity of a test particle at a distance D from a gravitating body is V_esc=(2GM/D)^(1/2). The formula for expansion is V_exp = HD. D_lim is the value for which V_esc=V_exp. If D>D_lim then the test particle is affected by the expanding universe. If D<D_lim then it isn't. This is understood to be an approximate rule rather than a precise rule. D_lim=(2GM/H^2)^(1/3). $\endgroup$
    – Buzz
    Commented Oct 30, 2020 at 3:28

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