# The first second

## The Planck era

What is the smallest possible mass for a black hole, compatible with quantum mechanics ? The answer is 20 microgrammes, and the general relativity indicates us that its size will be 10-33 cm. This lenght is known as Planck's length.
Yes, but according to the Heisenberg uncertainty principle, this mass can't be localized in a distance shorter than 10-33 cm !
So there is a conflict between quantum mechanics and general relativity. For lack of a better theory, which would be able to reconcile these two points of view (this is the aim of superstring theory, but this one is only at its beginning), we are forced to admit that during the Planck's era, which corresponds to a time of 10-43 second, our knowledge collapses.

In fact, during the Planck's era, pairs of Planck's particles and antiparticles, with a mass of 20 microgrammes, i.e. an energy of 1028 eV, or a temperature of 1032 K (so called Planck's temperature), constantly appear and disappear, bending the space-time fabric.

So, we can't say anything about the very first moments of the universe. In fact, according to our physical theories, the space and the time appear only at the end of the Planck's era.
We can notice that, during this time, the four forces of the nature can have been unified.

## The loss of the symetries

After this time of 10-43 s, the universe is really born : it begins to inflate, and hence to cool down.
The universe is only a plasma of elementary particles, quarks and gluons, electrons and positrons.

At the end of a time of 10-35 s, the temperature has cooled down to 1028 K : the strong nuclear force differs from the electroweak interaction.

This differentiation of the forces corresponds to a change of phase of the universe, and a break in symmetry .
Let us draw the parallel - very simplifying however - with the water : when it is liquid, water is perfectly symmetric : whatever the direction in which we look at it, it has the same aspect.
When the temperature cools down, it transforms into ice. This is called a change of phase. But the crystals of ice are not fully symmetric, they do not present the same aspect when you change the direction of observation.
This change of phase comes along with a loss of symmetry, and the situation is similar for the universe when the fundamental forces separate.
We will see the impact of these changes of phase on the evolution of the universe in the next chapter.

The cooling goes on until the temperature reaches 1015 K : at this moment, the electromagnetic and weak forces are separating.

Some time later, when the temperature will be about 1012 K, the quarks are becoming enclosed, hence forming hadrons, and allowing the appearance of nucleons, protons and neutrons.

At the temperature of 1010 K, the universe is one second old : it is made of one nucleon for three billion photons, and of one electron for one proton.

## Where does the matter come from ?

We have just seen the scenario which led to the creation of nucleons, which are the basis of the matter as we know it.

Yes, but... according to the standard model, there should be as many particles as antiparticles formed in this process. And the word which surrounds us is only made of matter, and all the observations prompt us that the whole universe is made of matter.

In fact, some theoretical considerations let us think that particles and antiparticles do not strictly have the same behaviour, and so the matter can become slightly dominating over anti-matter : typically in a ratio of a billion and one quark for a billion of anti-quarks.
This tiny supplement has been enough to build our word.