New precision Cosmic Microwave Background Radiation ( CMBR ) anisotropy data are beginning to constrain physics beyond the standard model , for example in the form of additional light particle species .
These constraints are complementary to what can be obtained from big bang nucleosynthesis ( BBN ) considerations because they apply to much later times .
We derive a constraint on the equivalent number of neutrino species , N _ { \nu } , from the presently available data .
Specifically we analyse two different CMBR data sets to test the robustness of our results .
Analyzing only CMBR data yields an upper bound of N _ { \nu } \lesssim 17 ( 95 % confidence ) .
Adding large scale structure ( LSS ) data from the PSC-z survey tightens the upper bound slightly .
However , the addition of LSS data gives a non-trivial lower bound of N _ { \nu } \geq 1.5 / 2.5 ( 95 % confidence ) for the two data sets .
This is the first independent indication of the presence of the cosmological neutrino background which is predicted by the standard model , and seen in big bang nucleosynthesis .
The value N _ { \nu } = 0 is disfavoured at 3 \sigma and 4 \sigma for the two data sets respectively .