Big bang singularity

Nstraw

I am a bit confused about big bang. Is the big bang referred to as the expansion of universe from singularity or not. Sorry if i have posted it in wrong section

bapowell

The Big Bang model refers to the expansion of the universe from a young, hot, dense state. This model does not address the initial singularity -- it is understood to signal a breakdown of the physical theory. So when people refer to the Big Bang in mainstream cosmology, they aren't talking about the actual bang itself, but the model of an expanding universe emerging from a much denser initial state.

Nstraw

Ok so why did this expansion occured

bapowell

There is no direct cause. It might be unsatisfying, but the universe seems to be expanding simply because it can. Given the exact same distribution of energy, the universe could just as easily be contracting.

The correct way to think of the initial expansion of the universe is as a boundary condition on the dynamics.

Nstraw

So if i am correct than the main question arises why universe was in the hot and dense state in the first place right?

bapowell

That, sir, is an excellent question.

Nstraw

So are there any theories yet which suggest why

Whovian

There is one, but all it says is basically, if I remember correctly, something like "something called dark energy is causing this expansion and we know nothing about dark energy or why it causes the Universe to expand."

I'm sure it's a lot more precise, but the gist of it is that, if there is a cause, then it's something we don't know anything about.

(Please don't hold me to this, I could be quite wrong.)

bapowell

I think the OP is asking more about the Big Bang itself, rather than theories of present-day expansion.

The Big Bang process, more specifically, the exceptional low entropy initial conditions of the universe, remain a major unsolved problem in modern cosmology -- nobody understands why the early universe had the conditions it had (see the Boltzmann brain problem.)

There are other ideas regarding the actual bang itself, or more correctly, what physical process was taking place at t=0. Hawking's proposal of a quantum origin for the universe -- that it was the result of a zero-energy quantum fluctuation -- is one popular example.

Nstraw

Thank you both so we will have to wait now