Gravity rules the cosmos. You can’t come to terms with the origins, evolutions and ultimate fates associated with cosmology or astrophysics without understanding gravity and the theory of relativity. Quantum theory also has to apply to cosmology (and astrophysics) anytime you run across micro phenomena where quantum effects need to be considered and where they apply. Unfortunately, there are circumstances where both gravity and quantum physics need to be simultaneously considered - singularities. Unfortunately gravity and quantum physics aren’t compatible.
The Alpha: In the beginning was the Big Bang event origin of our Universe – 13.7 billion years ago. The origin of the Universe (the Big Bang) was a quantum event because the initial size of our Universe was such that quantum effects dominated. At least that’s what the standard model dictates. It was also a time of extreme gravity, since all the mass of the cosmos was situated at the same time and place. But the relevant and separate equations of relativity and quantum mechanics break down as one approachs such extremes as would of applied at the Big Bang Alpha, giving rise for the necessity of a new theory of quantum gravity in order to come to terms with the Alpha object.
There are two main pillars of modern physics – relativity (part of classical physics) and quantum mechanics or quantum physics. Alas, the two pillars aren’t compatible, and thus, a Holy Grail for physicists is to find a ‘Theory of Everything’ (TOE) that merges the two. Now in the day-to-day life of physicists, a TOE isn’t essential, because relativity deals with the very big (the macro-universe) and quantum mechanics the very small (the micro-universe), and rarely do the twains meet. But, meet the two do in exceptional circumstances. Relativity deals with gravity (in the main), and on quantum scales, gravity is so weak that gravity can safely be ignored. But, there are objects that are very small, yet very dense – that is, tiny objects that have high gravity. There are basically two such objects – the Alpha Big Bang object or singularity and Black Hole singularities, or, to be honest, singularities in general regardless of where or when. And thus, to come to terms with the physics of singularities (immense gravity; micro size), the relativity and quantum worlds need to combine. So, TOE is basically a search for a theory of quantum gravity, and there are various highly complex and theoretical scenarios that fit the bill (though not yet even remotely experimentally confirmed).
Now while theories of everything or theories of quantum gravity are, in the final analysis, necessary (it just doesn’t wash that relativity and quantum mechanics can’t be made compatible – you can’t have two separate software packages governing the overall Universe), it is my opinion that they aren’t necessary to come to terms with singularities, which are usually described as an object of zero (point) dimensions and infinite density.
However, it is my opinion that it is absurd, in the extreme; to even slightly entertain the idea that a (Big Bang or Black Hole) singularity even remotely approaches such limits, far less acquires them. One cannot have a zero (point) dimensional object; one cannot have an object of infinite density. A singularity must have some sort of volume, and must have a finite density, even if the volume is very tiny, and the density is extremely extreme.
The basic logic is that a singularity has a finite volume and finite density. As you add more stuff to the singularity, the volume might remain the same but the density increases. However, as more and more stuff gets added, ultimately the density reaches the maximum possible, and from that point onwards, the volume of the singularity increases, finally increasing beyond the point where quantum mechanics can play any useful role, and gravity alone is the lone player left standing in the game.
Thus, a singularity could be large enough in volume that relativity theory alone can deal with the extreme gravitational conditions. The Big Bang object, containing the mass of the entire Universe, would be (the ultimate as) such a singularity. Massive (Galactic) Black Hole singularities, ditto. Singularities aren’t quantum objects. If you continue to add mass to a Black Hole, it gets bigger; the singularity at the centre gets bigger. To believe otherwise is, IMHO, entering the realm of scientific fantasy.
The upshot off all this is that the Big Bang was not a quantum event, nor would a future Omega Big Crunch be, and likewise, Black Holes are not quantum objects.
The Omega: In the beginning was the Big Bang event origin of our Universe – 13.7 billion years ago. Now what? It’s taken 13.7 billion years to get to ‘now’; what’s the state of play in another 13.7 billion years, or even 137 billion years hence?
What’s the ultimate fate of our Universe? Our Universe is currently expanding post Big Bang – ever increasing in volume like a balloon blowing up and up. Now either our expanding Universe will one day cease to expand as gravity slows things down to a crawl, then a stop, then a reversal – a contracting Universe, or our Universe will keep on expanding forever and ever and a day, ultimately terminating in a Heat Death. A Heat Death is when the temperature of the entire Universe becomes uniform. Everyplace has the same temperature, and that’s going to be cold – as close to absolute zero as makes no odds. Thus the Heat Death is the death of heat. That’s the Universe ending not with a bang (or even a very Big Crunch) but with a whimper. I really don’t like that ending at all.
Assuming that the Universe will ultimately contract into a Big Crunch, what will happen? Well, as one gets ever closer to the Big Crunch, density increases (but will not, can not, become infinite) and temperature increases (but again, not infinitely so) and the volume of space decreases (but will never become infinitely tiny) and time just keeps ticking on. Further, we know there are lots of Black Holes out in the cosmos; both small and massive (such as exists at the centre of our own galaxy). As the Universe contracts, these Black Holes will get closer and closer, not only to each other, but to the rest of non-Black Hole stuff as well. Ultimately, all the non-Black Hole stuff will get sucked into existing Black Holes as the Universe shrinks and matter’s density increases. Of course large Black Holes will also suck in smaller Black Holes, until ultimately, at the time approaching Big Crunch; there will be one ultimate/universal Black Hole left containing all that was.
Then what happens? The conditions inside a Black Hole are still unknown, beyond the equations of current physics, but whatever parameters are present, infinities aren’t among them (which might put me at odds with most astrophysicists). My reasoning is that no matter what, there’s only a finite amount of stuff comprising the universal Black Hole. Squeezed into a tiny area, the density will be extreme, but not infinitely so. The volume will be tiny, but not infinitely so. That is because there is an ultimate limit to how small length (hence volume) can get. The smallest possible length is known as Planck length and anything less than that space ceases to exist. Planck length is 1.6 x 10 to the minus 35 meters. Gravity might be so intense that not even light can escape, but it doesn’t take an infinite amount of gravity to do that. And there can’t be a time equals zero, either at the beginning (Big Bang) or at the end (Big Crunch). Because time exists in discrete quantum units (Planck-Wheeler time units), one must go from a minus one (contracting phase) time unit directly to a plus one (expanding) time unit, as there can be no time unit where time equals zero. In other words, you go from a Big Crunch directly to a Big Bang, contraction to expansion, endlessly cycling or recycling. Or, the universal Black Hole sucking in all matter and energy (approaching the Big Crunch) turns inside out and becomes a universal White Hole (the Big Bang) spewing out stuff (matter and energy).
That’s sort of akin to having four cars approach an intersection, on each from the north, south, east and west. If each car is one kilometre from the intersection, and each car is travelling at say 50 kilometres per hour, then it is clear this contraction of automobiles will result in a Big Crunch. However, it might be difficult to then go to an automobile expansion as the cars will be a wreck and in no condition to go anywhere! That’s one possibility.
The other possibility is that it might be unrealistic to expect in a contracting Universe that each and every bit and piece will meet at exactly the same point in time and space. Using our car analogy, what if each car was one kilometre away from the intersection, but say the north car was going 46 kilometres per hour, the east car 48 kilometres per hour, the south car 50 kilometres per hour, and the west car 52 kilometres per hour. Then, we can go directly from automobile contraction to automobile expansion as each car passes through the intersection while only having near misses with the other vehicles.
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