if each object in the universe attracts every other object through the gravitational force, then why doesn't the entire universe collapse on itself?

Somebody pushed them apart in the past and they fly apart since then.

Even Newton realised that a static universe is impossible. Any small fluctuation from the static configuration would grow and the universe must expand or contract. A static universe is unstable. Einstein postulated his cosmological constant to try to balance a static universe. This would still be unstable and require some fine-tuning.

 

From observation, we see that the universe is expanding. Why is it expanding is a good question. As physics near the big bang is not well understood, I don't think anyone can give a good answer to the mechanism that started the expansion.

 

Physics at and even before the big bang is somewhat speculative at present. People do investigate models of this period. However, as quantum gravity is expected to be very important at this epoch I would be cautious.

 

At present the universe appears to be in a raped state of accelerated expansion. There is a negative pressure coursing the universe to expand faster. This has been dubbed dark energy.

 

There probably has been other epochs of accelerated expansion, the so called inflationary era.

Physics here is better understood that the very close to the big bang. Generic features of inflation and the mechanisms behind it are understood. The details and phenomenology are still under investigation.

 

In short, it is not really well understood as to why the big bang occurred.

my friend say, The answer is simply..orbital motion

 

what do you think about orbital motion?

orbital motion of two bodies is possible and happens indeed but it is a "bound state". There may be unbound (non restricted, more energetic) motion like flying apart.

Edited November 2, 200916 yr by Bob_for_short

if each object in the universe attracts every other object through the gravitational force, then why doesn't the entire universe collapse on itself?

 

Some day it might:

 

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

 

Probably not, though, for a variety of reasons better explained by someone else. The answer is not orbital motion, though. There was an initial expansion that has to be overcome, first of all, and that might just have "escape velocity." (If you throw a ball in the air hard enough, it will never come back down, even though the Earth's gravity never drops to zero no matter how far away you get.) And there's dark energy, which appears to actually be accelerating the expansion, but isn't well understood, least of all by me.

Edited November 2, 200916 yr by Sisyphus