Big Bang not really the beginning of a completely new universe?

Question

From what I know about the origins of the universe and the big bang, it is stated that it all started from an intensely hot and dense mass. This sounds like a singularity to me, which means a black hole must have created it right? But then where did this black hole come from? Wouldn't it have to had sucked in a lot of matter to create a singularity this dense? If so, does this mean that there was a universe that existed before this, and thus maybe support the theory that the universe's expansion is slowing down and will begin to contract, thus creating the black hole for the next universe in the cycle? I know this question has many parts in it, and it really should be split up, but I see them as all being related, any explanations would be much appreciated.

Answer 1

The statement that at the beginning of the universe energy/mass was concentrated in a single region under conditions of extreme temperature and density is usually extrapolated from experimental data on the energy/mass content of the universe and its expansion, which is then analyzed through the classical (i.e. non-quantum) theories of general relativity and thermodynamics. From this perspective, the initial state of the universe not only is indeed a singularity, but it is a spacetime singularity. In particular, this means that no time can exist before this singularity, thus no concept of "creation" (which should happen in time) can be defined as to explain the existence of such a singularity. So no, the existence of a singularity does not imply that a black hole must have created it; in fact, it implies quite the opposite: no thing can have "created" the singularity itself. From a classical perspective, spacetime begins with a singularity, and this is the end of the story. No black holes, no cycles, no creation process. As your assumption is not true, your questions do not make much sense.

But your concerns about the creation of the universe, its cyclicity and so on are indeed meaningful. This is because classical gravity and thermodynamics are not the whole story to our understanding of the universe, and as such they cannot provide us with a complete understanding of its birth. More elaborate theories should be used in order to understand what happened at the beginning of the universe. First of all, as we know that the universe obeys the laws of quantum physics, any realistic theory of the universe must be a quantum theory. Of course, one could use approximations in which the "objects" that make up the universe behave almost classically or even classically, but as you study smaller and smaller systems (such as the universe in its first stages) these approximations break up, and you must use the full quantum theories in order to get meaningful results. These include quantum field theory and its thermodynamical counterpart (which tell us how matter behaves under conditions of arbitrary temperature), the theory of spontaneously broken symmetries (which tells us what kind of matter can exist under some specified conditions) and quantum gravity (which tells us how spacetime behaves in a more precise way than its classical counterpart). Unfortunately, none of these theories is yet sufficiently developed for us to say what really happened at the beginning of time, or even if there is at all a beginning to time. So we stick to the classical picture in order to be able to say that we know anything at all about the beginning of the universe. But keep in mind that we KNOW that this is the wrong picture.

One thing I can say about the singularity at the beginning of spacetime. It seems that when one studies the problem of the expansion of the universe from a quantum perspective, the singularity disappears. This is (roughly) due to the fact that Heisenberg's uncertainty principle forbids matter to be concentrated in an infinitesimal spot without giving it enough momentum to escape from the spot itself. So the singularity may be one wrong prediction of the (classical) approximation that we are using in order to get around the difficulties that one encounters when using more complete theories.