We have just described the most complete and most finished theory at the moment : the hot Big Bang with inflation.
This theory explains three observed phenomena :
Combined with the inflation, it also explains the flatness of the universe. On the other hand, some questions always stay without answer, especially the origin of the galaxies.
You should not forget that the Big Bang is only a model that the
physicists try to make as realistic as possible.
But it's only a model with all its defects.
Of course, other theories do exist in order to try to explain the universe. Among these theories, let us quote the pseudo-stationnary universe, which implies a continuous creation of matter.
Intuitively, we could think that the expansion of the universe,
triggered by the Big Bang, must go slowing down, as the gravity must act against
the initial effect of this "explosion".
In fact, the last measurements seems to show that it does not slow down. It even seems to increase ...
In a cosmological model as the one
that we study, it is easy to give an account of it : you just have to modify
the cosmological constant to obtain this result. But, in the reality, what is
the physical meaning of this fact, and what is the cause of this repulsive energy
Some ideas are considered :
One of the big problems to come is the determination of the nature of this "black" energy.
According to all these data, our universe should be flat, and in
an infinite expansion.
This doesn't necessarily mean that its size is infinite.
In order to determine it, we must interest ourselves in the topology of the universe, but this problem is seldom tackled nowadays.
But the arguments for a finite or infinite universe are up to now of a more philosophical than scientific order.
At all events, we are quite sure that the universe will not end in a "Big Crunch", a massive collapse which will bring it back to the initial state of a tiny sphere of energy.