If we go back in time from today, we shall find a more and more
dense and hot universe. The best way to see the past of the universe is to look
The speed of light is finite, so looking far in space is also looking far in the past.
If we travel back in time, about 14 billions years in the past,
we arrive at a point were the temperature of the universe is 3000 K.
Above this temperature, the atoms of hydrogen which form the major part of the matter in the universe are separated : there is a plasma, a mixture of protons and free electrons. And such a plasma is optically opaque : the photons can not travel through it.
So, 3000 K is the temperature of the freedom of the photons, that's
also the starting point of the fossil radiation.
This fact means that the image of the sky seen by the COBE satellite is the image of the universe when its temperature was 3000 K, about 14 billions years ago. And whatever the wavelenght of scrutation may be, we cannot see farther.
This temperature is called the electromagnetic decoupling temperature.
We know that the various elements of the universe are formed inside the stars, in particular at the end of their life. But some light elements cannot result from stars, especially deuterium, made up of one proton and one neutron. Deuterium can simply not support the high temperatures of the stars.
Just as the abundance of the helium in the universe, the proportion of lithium cannot be explained only by its production in the core of the stars.
Again, let's go back in time, and let's assume that the temperature
reached 10 billions degrees : 1010 K.
At this time, the atomic nuclei simply don't exist : the universe is only a warm soup, mainly made up of protons, neutrons, electrons and neutrinos.
According to a reference clock, the age of the universe is now one second.
When the temperature drops down below 1010
K, the energy of the neutrinos decreases, preventing them from interacting with
nucleons. They become then free
to circulate. This temperature plays the same role towards the neutrinos as
the 3000 K towards photons : it is called the weak decoupling temperature.
If we were able to build a telescope which could detect neutrinos in place of photons, we could see the universe as it was at this temperature. And hence, we could travel back in the past for another one million years, which is the time required for the universe to cool down from 1010 K to 3000 K.
The more we travel back in time, the shorter are the proper times. High temperatures accelerate the rythm of physical phenomena.
Towards one billion degrees, i.e. a hundred or so seconds after
the decoupling of the neutrinos, neutrons are able to merge with protons to
form nuclei of deuterium , and go on merging with other nucleons
to create nuclei of helium-3, helium-4 and lithium-7.
These nuclear reactions are well known and reproduced in laboratories. So, we can theoretically calculate the respective abundances of these elements : the theory is quite in good agreement with the measurements of the surface of the stars and of the interstellar medium.
Neutrons slowly disappear further to the creation of light elements because they are unstable particles.
This phenomenon, called primordial nucleosynthesis, is the second proof of the Big Bang : no other theory is able to explain these abundances of light elements in the current universe.
So, the universe needed a hundred of seconds to make up all the light elements that we can always find today.
Summary of the evolution of the universe since the first second.
When the universe is one billion years old, galaxies begin to form by gathering of stars and gas, and quasars will appear towards 3 billions years.
Since 14 billions years, matter is gathering to form galaxies.
How is it possible, within an expanding universe?
Gravity is the only force that is able to gather matter. The most likely scenario uses "germs", microscopic areas denser than their neighbourhood, which can attract the surrounding matter. This matter goes and increases the local overdensity, and so on in a snowball effect.
But this scenario asks us two questions :
We have already noticed that the rotation of the spiral galaxies indicates the existence of a great amount of undetected masses in their halo. These masses could be made of so called "dark" matter, very weakly interacting with the light, what explains why we do not discover it.
If this dark matter is so little active, it was able to begin to
concentrate before the electromagnetic decoupling, at a temperature greater
than 3,000 K, where the atoms begin to exist. All the problem is to detect this
dark matter in order to prove its existence.
A variation of this dark matter, the WIMPs (Weakly Interactive Massive Particle), are foreseen by the supersymetrical theories, which come as a supplement to the standard model. But these particles still remain to discover experimentally.
We must recognize that the Big Bang theory doesn't supply a very clear explanation for the forming of the galaxies. Numerous searches, theoretical and experimental, still remain to lead.