**1.1. The global description of the Universe**

Ordinarily, the Universe is taken to be a perturbed Robertson-Walker
space-time with dynamics governed by Einstein's equations. This is
described in detail by Olive and Peacock in this volume. Using the
density parameters
_{i} for
the various matter species and
_{} for
the cosmological constant, the Friedmann equation can be written

(1.1) |

where the sum is over all the different species of matter in the
Universe. This equation applies at any epoch, but later in this article
we will use the symbols
_{i} and
_{} to
refer to the present values.
A typical collection would be baryons, photons, neutrinos, and dark
matter (given charge neutrality, the electron density is guaranteed to
be too small to be worth considering separately).

The complete present state of the homogeneous Universe can be described by
giving the present values of all the density parameters and the present
Hubble parameter
*H*_{0} = 100*h* km s^{-1} Mpc^{-1}, and
indeed one of the density parameters can be eliminated using
eq. (1.1). These also allow us to track the history of the Universe
back in time, at least until an epoch where interactions allow
interchanges between the densities of the different species, which is
believed to have last happened at neutrino decoupling shortly before
nucleosynthesis. To probe further back into
the Universe's history requires assumptions about particle interactions, and
perhaps about the nature of physical laws themselves.