Recent updates to the observational determinations of the primordial abundances of helium ( ^ { 4 } He ) and deuterium are compared to the predictions of BBN to infer the universal ratio of baryons to photons , \eta _ { 10 } \equiv 10 ^ { 10 } ( n _ { B } / n _ { \gamma } ) _ { 0 } ( or , the present Universe baryon mass density parameter , \Omega _ { B } h ^ { 2 } = \eta _ { 10 } / 273.9 ) as well as to constrain the effective number of neutrinos ( { N } _ { eff } ) and the number of equivalent neutrinos ( \Delta { N } _ { \nu } ) .
These BBN results are compared to those derived independently from the Planck CMB data .
In the absence of a light WIMP ( m _ { \chi } \ga 20 { MeV } ) , { N } _ { eff } = 3.05 ( 1 + \Delta { N } _ { \nu } / 3 ) .
In this case , there is excellent agreement between BBN and the CMB but , the joint fit reveals that \Delta { N } _ { \nu } = 0.40 \pm 0.17 , disfavoring standard big bang nucleosynthesis ( SBBN ) ( \Delta { N } _ { \nu } = 0 ) at \sim 2.4 \sigma , as well as a sterile neutrino ( \Delta { N } _ { \nu } = 1 ) at \sim 3.5 \sigma .
In the presence of a light WIMP ( m _ { \chi } \la 20 { MeV } ) , the relation between { N } _ { eff } and \Delta { N } _ { \nu } depends on the WIMP mass , leading to degeneracies among { N } _ { eff } , \Delta { N } _ { \nu } , and m _ { \chi } .
The complementary and independent BBN and CMB data can break some of these degeneracies .
Depending on the nature of the light WIMP ( Majorana or Dirac fermion , real or complex scalar ) the joint BBN + CMB analyses set a lower bound to m _ { \chi } in the range 0.5 - 5 { MeV } ( m _ { \chi } / m _ { e } \ga 1 - 10 ) and , they identify best fit values for m _ { \chi } in the range 5 - 10 { MeV } .
The joint BBN + CMB analyses find a best fit value for the number of equivalent neutrinos , \Delta { N } _ { \nu } \approx 0.65 , nearly independent of the nature of the WIMP .
The best fit still disfavors the absence of dark radiation ( \Delta { N } _ { \nu } = 0 at \sim 95 \% confidence ) , while allowing for the presence of a sterile neutrino ( \Delta { N } _ { \nu } = 1 at \la 1 \sigma ) .
For all cases considered here , the lithium problem persists .
These results , presented at the Rencontres de l ’ Observatoire de Paris 2013 - ESO Workshop and summarized in these proceedings , are based on Nollett & Steigman ( 9 ) .