A cosmological model

A cosmological model is a mathematical description of the universe, which tries to explain the reasons of its current aspect, and to describe its evolution during time.
Of course, it must account for the observations, and be able to make predictions that later observations will be able to check .

The current models are based on general relativity, because this theory is what produces at present the best agreement for large-scale behavior.

From his theory of general relativity, which is a theory of gravitation, Einstein writes the equations which govern an universe filled with matter. But he thought that the universe had to be static. So, he introduced a term, called the cosmological constant, into his equations, in order to obtain this result.
Afterward, in view of the results of Hubble, he will return on this idea, and will admit that universe can effectively be expanding.

Soon after him, the Dutch De Sitter, the Russian Friedmann and the Belgian Lemaître introduce non-static universes as solutions for the Einstein equations of relativity.
If the universe of De Sitter corresponds to an empty universe, the Friedmann model is dependent on the density of matter inside the universe. This model is always the basis of current cosmological models.

The so-called Friedman-Lemaître-Robertson-Walker metric (FLRW) which rules the evolution of the universe in this model is expressed in the form :
where are the polar coordinates, R(t) the scale factor (positive), and k is +1, 0 or -1 depending upon the geometry of the universe.

The basic hypothesis

The first hypothesis of a cosmological model rests on the cosmological principle  : the Earth has no reason for standing at the center of the Universe, or in any privileged area. According to this hypothesis, the universe is considered as :

• homogeneous, i.e. it presents the same properties everywhere on a cosmological scale - of course, on a smaller scale there are different situations, if we look at the solar system or outside the Galaxy, for example.
• isotropic, i.e. it always has the same properties, in every direction where we could look. Especially, it is not "flattened" in one direction.

The second necessary hypothesis is the universality of the laws of physics. These laws are the same, in any place and any time.

Considering the content of the universe as a perfect fluid is another hypothesis. The characteristic dimensions of its constituents are negligible in front of the distances which separate them .

The parameters of the model

In order to completely describe a model, in accordance with these previous hypothesis, the Friedmann-Lemaître models use three parameters which totally characterize the evolution of the universe :

• The Hubble constant, which characterizes the rate of expansion of the universe,
• The mass density parameter, noted Ω, which measures the ratio between the density ρ of the studied universe and a particular density, called the critical density ρ_c, linked with the Hubble constant.
  The current value of this parameter is noted Ωo.
• The cosmological constant, noted Λ , which represents a force opposing to the gravitation.

The density of matter within the universe is the key parameter for the foreseeing of its evolution : if it is very dense (Ωo > 1), gravity will be able to win against expansion, and the universe will go back to its initial state.
In the opposite case, the expansion will continue forever.

The value of the critical density ρ_c is about 6 10-27 kg/m3, that is two atoms of hydrogen per cubic meter.
This very low figure explains why the models which suppose an empty universe are not so bad !

Closed or open universe ?

The density of matter inside the universe determines its geometry : for a high density we will obtain a closed universe with a positive curvature, but with a density lower than the critical density, we will obtain an open universe.

Let us note that a closed universe is necessarily of finished size, whereas a flat or open universe can be finite or infinite.

All the measurements until now did not allow to put in evidence a curvature of the universe.

Measurements of the fossil radiation by the Boomerang baloon tend once again to accredit the hypothesis of a flat universe.

The measurements made by the WMAP satellite, and the Planck satellite confirm this hypothesis.

So, the universe would be flat. But this fact puts us two questions:

• If it is flat, it means that the matter density is equal to the critical density Ωo = 1. But, at the very most, the visible matter in the universe accounts for only 5% of this density. Where is the rest ?
  In the same way as the birth of the galaxies, we once again have to appeal to the dark matter.

• Why is it flat ? This is a very special situation. We shall try to find an answer to this question a bit later.

The age of the universe

We can show that the age of the universe is proportional to the inverse of the Hubble constant.
So, an accurate determination of this constant is a crucial problem for the cosmology. Recent measurements indicate a value included between 50 and 100 km/s/MPc, i.e. the universe would be between 7 and 20 billions years old.
But the universe must necessarily be older than its oldest stars. These are estimated between 13 and 16 billions years old.

The last results supplied by the Hubble and Spitzer satellites conclude with a value of H=67.15 ±1.2 km/sec./Mpc, hence the universe would be 13,8 billions years old.