I've been very curious about this since I starting to believe about the origin of universe. What causes the big bang anyway when there was nothing in existence prior to it?

Great question, we simply don't know.

 

Also one could expect the notion of space and time to be very different to our usual understanding and thus "caused" may not even be the right thing to ask!

 

That said, people via speculative models of quantum gravity do talk about pre-big bang physics. This basically speculates that there was "physics" before the big bang.

The big bang theory describes the evolution of the universe from an early hot dense stage. It doesn't describe the origin and it doesn't say the universe came from nothing. There are all sorts of speculations about the earliest period before our models are valid.

 

The most plausible (to me) explanation is some sort of "big bounce" but, unfortunately, the evidence doesn't currently support that.

 

There is also "eternal inflation" that suggests that new "big bangs" are continuously happening.

 

Then there is Poplawski's idea that a black hole can create a new universe.

 

And on and on ...

Krauss kind of makes an argument that there is no such thing as nothing, in his book ''A Universe from Nothing.''

 

However, I do know that some models exist which describe successfully a universe from nothing. It just involves potentials and statistical probabilities. Hawking for instance, has even proposed that perhaps the creation of one spontaneous creation of a universe is actually one of many spontaneous universes which come into existence, all, if somewhat, mysteriously entangled with our global wave function. (A feature found in the Parallel Universe theory).

 

I also know that it is possible that our universe might behave like a timelike loop. Somehow its beginning and end were inextricably connected as a linked loop in time. It would be analogous to the continuous big bang, big crunch scenario related to Penrose's cyclic universe theory, with the only difference that time vanishes from his universe because it has expanded with not enough gravitational collapse to stop. Then, his universe appears again from an eventual singularity, (which if I understand his theory correctly) makes entropy time-symmetric.

 

 

The Ekpyrotic theory describes our universe like you might find a universe in string theory. It exists as a brane, floating about in a multidimensional sea. (Note thsi is not true for all cases of string theory). But its origin describes a story saying that this universe was once in a frozen state for many many eons.

 

It wasn't until another universe in a high energy state collided with our own, did our universe ''spring into action.'' It is very speculative.

big bang is just a theory that states that universe is created from a bang relasing huge amount of energy from nothing through vaccum fluctuations and rapidly through inflation like a ballon