Hot Big Bang model
Before the twentieth century it was believed that the universe had existed forever , or that it was created at time , to look like it does today. Einstein's general theory of relativity predicted that space time with the Big Bang singularity , and would come to an end at the Big Crunch singularity , if the whole universe should recollapse , or at a singularity inside a black hole , if a star , or other local region were to collapse.
The big question is weather the universe has a beginning or end , and if so , what would it be like. It is important that one should understand the "Hot Big Bang Model". The universe began very hot , and expanded. The temperature falls by half , once the universe doubles in size. It is thought that at the "Big Bang" the universe had zero size , and so to have been infinitely hot.
There was a perfect balance between matter and anti – matter. Thus , all particles had matching anti – particles. The temperature was to high for the particles to bond into stable matter. From there on the universe began to cool down.
At ten to the minus forty seconds after the Big Bang took place , the temperature dropped to ten to the twenty seventh degrees Kelvin. This caused for a soup of particles and anti – particles to freeze out of the energized void , with each particle and anti – particle annihilating each other.
At ten to the minus ten seconds after the Big Bang , the first anti – particle failed to annihilate its matching particles. This caused a state which had the potential for matter to appear. Neutrons and Protons sprang into being.
One second after the Big Bang the temperature would have fallen to about ten thousand million degrees. This is about a thousand times the temperature at the center of the sun.
Protons And neutrons now no longer have sufficient energy to escape the attraction of the strong nuclear force , and combined to produce the nuclei of atoms of deuterium.
For the next million years or so after that , nothing much happened.
After ten million years , giant clouds of gasses were formed , which gradually contract and spin under the force of their own gravity , forming galaxies.
The idea of the universe starting out very hot and cooling down as it expanded , is in agreement with all the observational evidence we have today , but a few question are still left unanswered :
· Why was the early universe so hot ??
· Why does the universe look the same at all points of space , and in all directions ??
· Why is the temperature background radiation very nearly the same when one look in different directions ??
In the Hot Big Bang Model , there was not enough time for heat to have flown from one region to another. Thus , the initial stage of our universe would have to have had the exact same temperature , everywhere, to explain the fact that that microwave background has the same temperature in all the directions.
The initial rate of expansion would have had to be chosen very carefully , for the rate of expansion to still be so close to the critical rate needed to avoid recollapse.
· Why does the universe contain local irregularities such as stars and galaxies . despite its large scale uniformity??