Whenever a physicist or cosmologist tell people about the fate of our universe, the past and the future of the universe, then some people say how do you know about the future or the past of the universe. People may ask how physicist or cosmologist knows the universe is expanding.
How does physicist or cosmologist know the universe came from the Big Bang? How can they predict the future of the universe?
One way is the physicist or cosmologists do this by looking at the Doppler Shift. Even high school students are familiar with. Doppler shift. Doppler Effect (Doppler Shift) was first proposed by Christian Doppler in 1842 in Prague. Starlight from a star coming towards you turns greenish and bluish when yellow light moves away from you, it turns reddish. When we look in the sky than we actually look at the starlight emitted from distant galaxies. When we observe this light we come to know that this light is more reddish as it was supposed to be. Which means these distant galaxies are moving away from us? From this observation, we conclude that the universe is expanding. Observing this light, scientists calculated the age of the universe which is about 13.7 billion years. So that’s how we know the age of the universe.
Scientists used to think that the universe is slowing down. But they now believe that the universe is actually expanding. If we go forward in time, let say some billion years, then according to current observations, we or our universe may die in a big freeze. If the universe will keep on going to expand then one day we will look in the sky and we will see nothing. That’s because the distant galaxies are so far that light cannot even reach to our telescopes. Nobody is sure about this big freeze will even take place or not. But one thing is sure that if the universe does not stop expanding then after some billion or may be trillion years you can’t be able to find any star in the sky because all the stars will have used their nuclear fuel and the only thing from them will be left behind would be neutron stars or dead black holes. The temperature all across the universe will reach to absolute Zero. At that point, even consciousness or thought cannot exist. In sense laws of physics is a death warrant to all the life in the universe. The most interesting thing at this point is there is a loophole in the laws of physics. There is a term called Planck energy. So if the universe gets cold enough to freeze the entire intelligent life than at that point or may long before it the intelligent life will have mastered the Planck energy. It is the energy of the big bang. This energy is the point where gravity itself begins to break down.
If you crank a nucleus of an atom then you will get plasma of neutron and proton and if you crank it up more then you will get gluon plasma and if you even crank it more, this incredible energy which is about ten to the nineteen billion times electron volts (we are not sure about it), then space itself begins to boils. Space-time becomes unstable and bubbles become to form at this planck energy. These bubbles are gateways to parallel universe (Learn more about parallel universes here). None of this has been proved yet, but there are theories which say that there are parallel universes. Large Hadron Collider will soon give us first experimental evidence of the existence of this parallel universe. So the intelligent life after trillions of years from now may try to enter into the parallel universe.