After Hubble’s Law, it became an established fact that our universe started from singularity. And 13.75 billion years since the beginning, it is expanding at an accelerated rate.
Question to answer now is “How will our universe end”.
There are 2 possible ways for our universe to end itself. But there is a paradox which lies in my previous statement and the paradox is “will universe end itself ?”
One way is that the universe reaches a limit, a threshold and start contracting again into singularity. If this is what it is going to be, then we can easily say that universe undergoes cyclic events which includes a big bang followed by the big crunch (this is something which is part of Hindu philosophy. This is clearly mentioned in holy book of Bhagwat Gita).
Now there comes a twist in the tale. Universe is expanding at an accelerated rate. And there is no way it is going to collapse unless the expansion is retarded. Gravity is the only force which can cause this retardation required to crunch our universe. But gravity fails when it comes slowing things down (reason in my next post but for idea it is because of dark energy). But if gravity fails, then we are left with nothing powerful enough to crunch our universe except for one: THE TIME.
Only time has power to crunch our universe. To understand this we must know few things about arrow of time. Here is a simple representation of the arrow (there can be more complex one, but this one will be sufficient)
Arrow of time describes asymmetrical nature of time.
When we move to future we see our space expanding. Our universe reaches an undetermined threshold, it will reverse the arrow of time in backward direction and our universe will end up again in the state of singularity.
To get more clear picture, imagine you are watching a film. As soon as the film end, you pick your remote and press the playback button. Soon, the movie frames move in reverse direction and you will reach the beginning. Similarly the universe moving in reverse direction of time is what will cause the Big Crunch. Only the god knows, who is handling the remote of this film.