We have seen that the cosmological microwave
radiation is exceedingly homogeneous, in 1 part for 100,000. Why ?
Of course, nothing forbids this uniformity of the cosmos, but to say that it is about a coincidence, it is maybe to pass next to an essential point. In order to get a better understanding, we must begin by defining the notion of causality.
Physics is always based upon the notion of causality : a cause
always precedes the effect. What's more, according to the theory of relativity,
the information cannot move faster than light. Hence, you cannot detect an effect
before a time at best equal to the necessary time for light to travel from the
From this point, we can define the "sphere of causality" of an event : it is the set of places which, at a definite moment, can perceive the event.
Naturally, this sphere is growing with time at the speed of light.
When the fossil radiation was emitted, the universe was one million years old : so, the sphere of causality of the particles which emitted this radiation was one million light years wide.
So, how can they all be at the same temperature ?
The individual spheres of causality don't overlap. In these conditions,
we must deduce that atoms outside of their individual sphere of causality were
at the same temperature. What is at least curious.
This effect is known as the problem of the horizon, where the horizon is the furthest observable distance.
To explain this fact, the American physicist Alan Guth suggested the hypothesis of inflation.
We saw that the separation of the forces during the early times
of the universe was comparable to a -change of phase, in the same way as the
water which becomes ice.
But, even when the temperature is lower than 0°C, water can exist for short times. This phenomenon is called superfusion.
The same phenomenon appears at the time of the differentiation
of the forces. The forces can remain unified even when the temperature has
gone lower than the temperature of separation.
At this moment, the energy of the scalar field associated to the unification of the forces becomes greater than the temperature of the universe. This field becomes equivalent to a repulsive force, which will make the universe to inflate in a huge proportion : at the time of the breaking of great symmetry, around 1028 K, the radius of the universe increases 1050 times.
Its energy is transformed into an expansion of the universe.
This change of phase must have happened three times during the Big Bang :
What are the effects of this sudden increase of size of the universe?
First of all, this inflation allows us to solve the problem of the horizon that we have previously seen : indeed, all the distances are exceedingly increased in a very short time. The spheres of causality of the various points which are emitting the fossil radiation can have previously overlapped, and hence explain the homogeneity of this radiation.
The speed of the increase of size of the universe was far greater than the speed of light : this does not violate the theory of relativity, because no information was moved during the inflation, only a geometrical property.
The second effect of the inflation is the flatness of the universe.
Indeed, this inflation increases all the distances in the universe, including
its radius of curvature, which tends towards infinity.
In a classical scenario, the universe must begin flat in order to remain so. With the inflation, whatever is the curvature of the universe at the beginning, it ends flat.
The third effect of the inflation is the dilution of the elements of the universe. By increasing its size, the density of various "exotic" objects, like primordial black holes or magnetic monopoles, decreases. These elements are foreseen by the theory, but they have never been discovered.
How can we validate this hypothesis ?The theory shows that the explosive expansion of the universe during the inflation period should have created gravitational waves, and for lack of being able to measure them directly (at least for the moment), we could reveal them through their interaction with the microwave cosmic background.
It is this kind of polarization which has been measured at the South Pole BICEP2 observatory, driven by the
National Science Foundation.
Such a measurement gives us a real validation for the hypothesis of the inflation.
We can also notice that this is the first observation of a quantum gravity effect.