I was reading on this site about big bangs, and it got me wondering, about two things.
There’s no exact spot that the Big Bang happened. In fact, the Big Bang happened everywhere in the Universe.
1) if there were multiple big bangs, then is it possible that one galaxy from another explosion is heading our way?
2) If there are multiple explosions, then did those explosions occur at the same time ? or they all exploded within certain time-frame or is it possible that somewhere in the infinite universe, there is still a big bang yet to happen?
PS : not from a physics background, so please go easy on me.
Best Answer
For some background on what physicists mean by the Big Bang have a look at Did the Big Bang happen at a point?
Strictly speaking the term Big Bang refers to the zero time limit of an FLRW universe. However theories of what the universe might be got more complicated when the theory of inflation was proposed, and in particular when eternal inflation was suggested. So these days the term Big Bang is sometimes used to mean the moment when inflation ended and normal spacetime expansion began.
If we consider an FLRW universe the geometry is pretty straightforward. There was only one Big Bang, it happened everywhere at the same time, the universe is the same everywhere and it expands uniformly. So in a universe like this there is no danger that a universe from another Big Bang is heading our way.
By contrast the theory of Eternal Inflation makes life more complicated. In this theory what we see as our universe is just a small pocket in a much larger spacetime undergoing exponential expansion. In this theory we would be just one of many (possibly infinitely many) such pocket universes, and it is possible that two pocket universes could collide.
There has been some interest in looking for evidence of such collisions in the cosmic microwave background, and claims for such evidence have been made though they are not generally accepted. Even if such collisions have occurred they probably occurred in the very early stages of the universe formation and well before any galaxies formed. Any collision zone would also, with a high probability, be a long way from us and the expansion of the universe would be carrying us away from it. So once again the chance of any galaxies from a different universe heading our way is vanishingly small.
I should emphasise that all theories about what happened in the first few femtoseconds of the universe are speculative and unsupported by any experimental evidence. While they are fascinating to think about you should be very cautious about taking any of them seriously.