To PHYSICISTS, Space expansion; does this question make sense?

Question

Just as a balloon expands and contracts because of the air pressure difference in and outside of it, could the (area, dimension, space) that the universe is expanding in to have a negative or positive (pressure) that controls the universe's expansion instead of gravity????

Answer 1

There is no outside "space" that the universe is expanding into. So there no concept of pressure outside of the universe (there is no outside). All that exists is the universe itself. So whatever is making it expand and accelerate must be present within our 4 dimensional space-time. We know that gravity is present. This will slow the expansion. But we do not know what is making it accelerate!

Answer 2

That's basically one way to (figuratively) look at the cosmological constant (as long as you are aware that there is no inside/outside). As of late the term "dark energy" has been coined for a somewhat deeper approach to the physical roots of this phenomenon.

Both, gravity and "dark energy" or cosmological constant, act together, of course. So one can construct universes in which one or the other dominates or both are in perfect equilibrium.

As of now it looks as if the universe will continue to expand and even accelerate in its rate of expansion.

Growing up at a time where the majority opinion on this was that the universe was matter/gravity dominated and would collapse again (see Weinberg's "The First Three Minutes", which is still an excellent book on the topic) and having seen the supernova results that demonstrated acceleration, I have become somewhat of an agnostic on this topic. I want to see more and independent ways to measure both the total mass in the universe and the rate of acceleration of the expansion (but I expect the acceleration to stay as the ultimate result).

Now, the key question, of course becomes, what creates this enormous pressure? As far as I know, we just don't know. There seems to be no killer hypothesis around that could be tested experimentally or even be made watertight theoretically. But then, I am not following cosmology day t day, so I might already have missed something in the field that does just that.

Answer 3

No, the entropy of the universe is constantly increasing. Therefore the universe is constantly expanding.

There is no outer environment (as there is with your balloon) to compress the universe.