It is true that the universe is expanding. How can we prove it? We can use the priniciple of Doppler effect to prove it. According to the Big Band Theory, the universe is expanding and all stars are moving from us. By examining the light reaching us from all stars, we do find that the wavelengths of hte spectral lines to be lengthened - Red Shifts (non-relativistic)
Expansion of the universe is being slowed by gravitational force. If it has enough energy, the universe will continue to expand forever. But if not, the expansion will eventually stop. Gravitational force will begin to draw the universe back again into highly compressed state. If this second alternative applies, the fireball will be recreated and the whole process will repeat.
I hope the above explanation will help you a bit. Feel free to contact me at any time.