Measurements of the cosmic background radiation give the cooling time of the universe since the Big Bang, of cosmological time.This model is well understood theoretically and strongly supported by recent high-precision astronomical observations such as WMAP.
Today this is largely carried out in the context of the ΛCDM model, where the universe is assumed to contain normal (baryonic) matter, cold dark matter, radiation (including both photons and neutrinos), and a cosmological constant.
The fractional contribution of each to the current energy density of the universe is given by the density parameters Ω, are the most important.
The light travel time to this surface (depending on the geometry used) yields a reliable age for the universe.
Assuming the validity of the models used to determine this age, the residual accuracy yields a margin of error near one percent.
Assuming an extra background of relativistic particles, for example, can enlarge the error bars of the WMAP constraint by one order of magnitude.