in HTRN's thread about the rate of bubble rise and expansion in a fluid, he also saw analogies to cosmological expansion and he speculated:
"Maybe we are on the surface of a bubble, and expansion is a result of external as well as internal forces:
Reduction of outside "pressure"(what ever that is) and internal forces."
This is a common question and surprisingly difficult to answer in a compact fashion, so this will not be short and sweet. It was slightly offtopic in the original thread, hence this Blog entry.
The popular answer is simply that we have no indications that we are on the surface of any 'spacetime bubble', but rather that we are at the center of an expanding observable spacetime bubble. We think that dynamics of this expansion all come from 'forces' inside the observable universe, not from outside.
We know that there must have been some event (or series of events) in the past that caused the matter and radiation of our observable universe to expand (distances and wavelengths are increasing over time). Let's call it the 'initial cause'. The effect of the initial cause is that the universe would keep on expanding, but that the rate will slow down as time goes on. We have actually observed this slowdown in expansion rate over the first 7 or so billion years after the event, because we are 'looking back in time'.
We also know that something is presently causing the rate of this expansion to increase again; let's call that the present cause. Physicists are not sure, but it is possible that the initial and present causes boil doen to the same thing, Einstein's cosmological constant Lambda, just at two different energy levels. Lambda has a peculiar characteristic: it has part normal gravity and part 'antigravity', in that it has negative pressure coupled to positive energy density in a fine balance. Normal matterenergy has both its pressure and energy positive.
Positive pressure contributes significantly to the gravitational force inside a collection of matter, but negative pressure will lessen that gravitational force and may even overwhelm the energy density to give an overall expanding effect. Since Lambda is precisely equivalent to the energy density of empty space, space with no normal matter or radiation has no choice but to expand (or contract) exponentially. The "expand or contract" needs some clarification.
It is possible for any piece of space to be in static equilibrium, with just enough matter and vacuum energy to neither contract (under the energy density), nor expand (under the negative pressure). This is however an unstable condition, since any minute negative volume fluctuation will increase the energy density's gravity, but decrease the negative pressure, resulting in an exponential collapse of that space.* The exact opposite will happen for any minute positive volume fluctuation, resulting in exponential expansion.
The answer to the original question is then clear: space does not need anything 'outside it' in order to contract or expand. It has enough internal mechanisms. We are just not able to characterize those internal mechanisms from first principles yet, because it has a quantumgravitational flavor. And that's still a tough nut to crack in a consistent manner.
There were more questions, but this is already a 'headfull', so I will rather reply to further questions as they arise.
J
* This form of collapse could be the trigger for a cyclic universe, but more about that later.

Re: HTRN's Cosmology Questions