Why did the universe expand when it was small enough to be gravitationally bound?

[Rolando]

According to concordance cosmology, the universe expands, and its increase in linear size is described by a function a(t). However, any objects that are gravitationally bound (up to the size of galaxy clusters) do not participate in this expansion. They retain their size.

 

Minor question:

How large is the fraction x of the volume of the universe that is at present, i.e., in our cosmic vicinity (up to z = 0.1 or so), occupied by gravitationally bound objects?

 

If a(t) increases with time and gravitationally bound objects retain their size, it is obvious that x must have been larger in the past, so that at an earlier time (not extremely long ago) most or all of the matter in the universe was gravitationally bound and so would never expand.

 

Major question:

How is this to be handled properly in concordance cosmology? What is it that is assumed to have made gravitationally bound objects to expand in the past? Can you point me to a model?

[hoola]

in lieu of a better answer, here is mine.....the expansion in the early universe is thought to be caused by the big bang and the subsequent inflation over-riding the initial gravity forces. Some literature predicts that the expansion did slow down until the dark energy totals began to halt the deceleration and cause a re-acceleration at about the mid-point of the age of the universe....with the dark energies increasing as the universe expands, this leads to an increasing rate of expansion as time goes on....edd

[petrushka.googol]

In entropic terms the preferred state (as it still is) is for entropy to increase.

This seems a natural agent of universal expansion (and precludes implosion).

Edited February 9, 2014 by petrushka.googol

[Rolando]

Thank you, hoola and petrushka.googol, for your responses. I have thought about this myself a little more and I realize that it is difficult to model exactly. This is how one can think about it within the frame of orthodox cosmology:

 

There are gravitationally bound objects up to the size of galaxy clusters. These can be thought of as enveloped by a ”zero-velocity surface” inside which gravitation is strong enough to prevent expansion (see e.g. Karachentsev, 2005, doi:10.1086/426368). However, this is not without exceptions. For a galaxy to escape beyond the zero-velocity surface, it requires in some cases only very little extra acceleration, which can arise by gravitational interaction with another galaxy. There is only a strong tendency for galaxies to remain gravitationally bound to their naighbors. If the universe expands, this tendency may explain why there are so large voids in it. There appears to be no force that counteracts the expansion of voids.

[md65536]

I too was waiting for a better answer.

 

I don't think the universe was ever gravitationally bound. I think there is an error assuming that it must have been, due to its arbitrarily small size early on.

 

If the expansion rate happens faster than c, the expanding stuffs can't be gravitationally bound to each other. Without inflation, it would be a black hole and remain collapsed. There would be an event horizon which nothing inside could pass. However if you have inflation faster than c, as is modelled, then there is an event horizon where stuff is pulled away from other stuff faster than they could possibly affect each other. As long as the things are separated by a horizon, they essentially can't know the other things exist, and so aren't gravitationally bound.

 

http://en.wikipedia.org/wiki/Particle_horizon

 

I guess in the current theory it sounds like the universe actually would have been gravitationally bound *prior* to inflation, but inflation itself would have undone that. http://en.wikipedia.org/wiki/Horizon_problem#Inflation

[Airbrush]

How large was the universe by the time the force of gravity originated? As said above, the expansion was too fast for gravity to get to work.

Edited February 12, 2014 by Airbrush

[Schneibster]

By the time the BB happened only 10^-42 seconds had elapsed, but the universe was a minimum of tens of billions of light years wide; remember that gravity acts at c. The Universe was never gravitationally bound; it might eventually have been had dark energy not surpassed gravity seven billion years ago. But it was inevitable dark energy should do so; it is a property of space, not of energy and matter. It is lambda, the cosmological constant in the Einstein equation. And the more space there is, the more lambda there is. And space is not conserved.