$\star$ Universe: The laboratory to study black energy.

Black energy is intrinsically very diffuse, of very weak energy and thus impossible to reproduce out of accelerator, inobservable in the clusters or the galaxies. The only laboratory is the whole universe.

The first observable influence of black energy is that it fixes the growth rate which assigns the distance r(z) to objects observed with a redshift given Z:

$$H^2(z) = H_0^2 (1+z)^2 \left[\Omega_{\rm M}+ \Omega_{\rm X}(1+z)^ {3\omega}\right], \ \ \ r(z) = \int_0^z \frac{du}{H(u)}$$ (1.37)

It was supposed here that the universe is flat. Measurements of distances are thus the perfect tool to force the parameter $\omega$. We will see in the following chapter in detail how they are estimated and how one can extract some from cosmological information.