[bouncycastle]

However, after the universe finishes expanding, wouldn't it start contracting again?

Btw, Andromeda will collide with the Milky Way in about 4 billion years, so at least we won't need ten times the mass of Mt. Everest to get there...

[throwaway201103]

If you subscribe to Roger Penrose's theory, at the point of infinite expansion, the current "Aeon" will end and a new big bang will happen. As best I understand it, at the point of infinite expansion, there are only photons. Mass no longer exists therefore scale no longer exists. So an infinitely huge universe becomes indistinguishable from an infinitely small universe.

[tokamak-teapot]

What causes the Big Bang in this scenario?

[odorousrex]

Basically, the way I understand it, is the probability of a Big Bang spontaneously occurring (due to the quantum fluctuations) is some godawful infinitesimally low number 0.00000...1 that for all intents and purposes is 0, but is NOT zero.

So given ungodly eons of time (googleplex ^ googleplex * graham's number, etc. of years) something with a very, very, very low probability becomes inevitable.

[macintux]

> However, after the universe finishes expanding, wouldn't it start contracting again?

Unless the answer has changed in the last several years, no, the consensus is that it's expanding too fast and has too little mass (and thus gravity) to collapse again.

(Disclaimer: not even an amateur at this, at least not for 40 years.)