Does the big bang theory propose that all the matter and energy in the universe was once compressed into an extremely small volume?

Answer 1

Absolutely not.

This is a common misconception, not helped by popular presentations and by statements that "the Universe" was once the size of a proton.

The problem comes about partially due to the careless failure to distinguish between "the entire Universe" and "that part of the entire Universe that we can presently observe." Many people -- scientists included -- refer to both as simply "the Universe" without making clear that the latter is (at most) an infinitesmal subset of the former.

The present observable Universe is a sphere of radius about 46 billion light years. THAT small fraction of the total Universe was, indeed, the size of a grapefruit at one time. How much larger the ENTIRE Universe was at that time is, at present, impossible to determine -- and may always remain unknowable. The entire Universe may be a "mere" 100,000 times large than the observable Universe, or infinitely large. The only guesstimate ever made was that our entire Universe must be no less than 10^23 times larger than the observable universe.

Thus, what BB cosmology states is that our Universe was once un-imaginably denser than it is now.

Also, please be aware that BB cosmology does NOT state that matter was compressed into a smaller volume. Our Universe is not becoming less dense because matter is spreading out through empty space. It is becoming less dense because SPACE ITSELF is expanding (not exploding) -- matter is simply coming along for the ride.