Does the Big Bang need a cause? [duplicate]

Question

Possible Duplicate:
on causality and The Big Bang Theory

Asking here in layman's terms..

When theoretical physicsists discuss the origin of our Universe, the wider consensus appears to be that it originates from a singularity; a position that rests on observations about the apparent expansion of our Universe.

However, the question why singularity itself came into being, still remains.

But this question takes a different angle: Does the Universe as a whole need a cause to exist at all?

If the law of conservation of energy is universally valid, then questions about a beginning become irrelevant since there can't be any by definition and everything boils down to dynamics of interaction. Thoughts?

Answer 1

I don't think I agree with your first sentence. Our simplest theoretical models, based on classical general relativity, say that there was a singularity in the past, but few if any cosmologists take that as a reason to believe that there actually was such a singularity. Rather, the most likely possibility is that those classical models are wrong at early times.

The truth is that we have no idea what happened "at the Big Bang," or even if that phrase is meaningful. There are some theories in which the Universe has existed for infinite time (Google "eternal inflation" for instance), and others in which time started at a finite point in the past.

Conservation of energy arguments don't really help here. For one thing, conservation of energy in the expanding Universe is more complicated than you might initially expect -- there's really no such thing as the total energy of the Universe. For another thing, conservation of energy, all by itself, wouldn't answer the question of whether there was a beginning. (Global) conservation of energy says that the energy at any one time equals the energy at any other time. It doesn't say anything about whether there was an initial time.

Answer 2

The only well tested theory of gravity we have right now is general relativity (GR). In models based on GR, time and space only exist for $t>0$. In relativity, we use the term "event" to mean a certain position in space at a certain time. The big bang is not an event, because there is no time $t=0$. If you want to find a cause for some event happening at a given time $t>0$, there is always some earlier $t'$, with $0<t'<t$, that can supply that cause. So in this sense, no, the big bang doesn't require a cause, because only events require causes, and GR doesn't describe the big bang as an event.

We also have fundamental reasons to believe that GR lacks self-consistency under the very dense and hot conditions at $t \lesssim 10^{-43}$ s (known as the Planck time), because of quantum-mechanical effects. If we had a theory of quantum gravity that worked under those conditions, then it might turn out that the singularity at $t=0$ was not real, and events at $t>0$ could be explained in terms of causes at $t<0$. This is what seems to happen, for example, in loop quantum cosmology. However, nobody has a theory of quantum gravity that works and has been tested against experiment, so we don't really know.