Big bang nucleosynthesis ( BBN ) and the cosmic microwave background ( CMB ) are two major pillars of cosmology .
Standard BBN accurately predicts the primordial light element abundances ( ^ { \mathrm { 4 } } He , D , ^ { \mathrm { 3 } } He and ^ { \mathrm { 7 } } Li ) , depending on one parameter , the baryon density .
Light element observations are used as a baryometer .
The CMB anisotropies also contain information about the content of the universe which allows an important consistency check on the Big Bang model .
In addition CMB observations now have sufficient accuracy to not only determine the total baryon density , but also resolve its principal constituents , H and ^ { \mathrm { 4 } } He .
We present a global analysis of all recent CMB data , with special emphasis on the concordance with BBN theory and light element observations .
We find \Omega _ { B } h ^ { 2 } = 0.0250 ^ { +0.0019 } _ { -0.0026 } and Y _ { p } = 0.250 ^ { +0.010 } _ { -0.014 } ( fraction of baryon mass as ^ { \mathrm { 4 } } He ) using CMB data alone , in agreement with ^ { \mathrm { 4 } } \mathrm { He } Â abundance observations .
The determination of Y _ { p } allows us to constrain the relativistic degrees of freedom during BBN , measured through the effective number of light neutrino species , N _ { \nu,eff } = 3.02 ^ { +0.85 } _ { -0.79 } , in accord with the Standard Model of Particle physics .
With this concordance established we show that the inclusion of standard , N _ { \nu,eff } \equiv 3 , BBN theory priors significantly reduces the volume of parameter space .
In this case , we find \Omega _ { B } h ^ { 2 } = 0.0245 ^ { +0.0015 } _ { -0.0028 } and Y _ { p } = 0.2493 ^ { +0.0007 } _ { -0.0010 } .
We also find that the inclusion of deuterium abundance observations reduces the Y _ { p } and \Omega _ { B } h ^ { 2 } 
ranges by a factor of \sim 2 .
Further light element observations and CMB anisotropy experiments will refine this concordance and sharpen BBN and the CMB as tools for precision cosmology .