At one moment,there was nothing and the next,it was BOOM! This was the way ,everything got started.Or we can say it was the moment the Time started.Before this ,there was nothing.
Big Bang :
This boom is called “The Big Bang”,the moment when everything exploded from a point of infinite density and temperature(also called Singularity) at a finite time in the past.The big bang is also called the birth of the universe and the point after which everything can be verified through our existing physical laws(General Relativity and The Standard Model of Particle Physics) as laws of physics won’t work for anything before the big bang.
WHAT HAPPENS AFTER THE BIG BANG:
Since the big bang ,the universe has been expanding at an accelerating rate ,thanks to the dark energy.The dark energy has been playing a pivotal role in the gradual exansion of the cosmos.The dark enrgy is believed to be homogenous and not very dense and it doesn’t interact with any of the four fundamental forces except gravity.It’s also believed to be the reason behind the accelerating expansion of the universe in its last half life time.But one day this expansion may cause the cosmos its doomsday.This depends on a particular type of dark energy called Phantom Energy.The universe would be expanded to such an extent that the matter of the cosmos,from stars and galaxies to atoms and subatomic particles and the spacetime itself would tear away from each other causing “The Big Rip”.
WHAT IS THE BIG RIP:
The relative expansion of the cosmos or the cosmic scale factor is accelerating ,which is a key parameter of the Friedmann equations who through his field equations showed mathematically that the universe is not static,further proved by American astrophysicist Edwin Hubble.During the Big Rip the Hubble constant H which is a proportionality between recessional velocity of and distance to distant objects ,would increase to infinity which would result in weakening of gravity to such an extent that it’d be too weak to hold the milky way ,also the solar system;as a consequence of which all the planets will go far away from their star’s orbit .They won’t be able to get the fuel from their stars which will result in their destruction.
It would also destroy the atoms as the expansion of the cosmos would make the fabric of space-time move faster than the speed of light which means ,for example,two galaxies moving far away from each other won’t be able to receive the light from the other as the space-time in which they are ,is itself moving away faster than light.Hence the rate at which spacetime expands would be way more powerful than the strong nuclear force which is the reason behind existence of atom ,resulting in destruction of atoms ,ultimately the cosmos.
But relax , don’t worry about this as this “Big Rip “ is estimated to occur some 22million years from now, when we even can’t say whether our beloved earth would be existing or not.