Two dimensional concordance plots involving the baryon-to-photon ratio , \eta , and an effective number of light neutrinos , N _ { \nu } , are used to discuss the overall consistency of standard big-bang nucleosynthesis in light of recent determinations of the primordial deuterium ( ^ { 2 } H ) abundance .
Observations of high-redshift Ly- \alpha clouds have provided discordant ^ { 2 } H/H determinations : one cloud with ^ { 2 } H/H high compared with the previously accepted upper limit on ( ^ { 2 } H + ^ { 3 } He ) /H ; and one system with a significantly lower upper bound on ^ { 2 } H/H than those previously obtained .
The high value of ^ { 2 } H/H agrees well with the current observationally-inferred primordial abundances of ^ { 4 } He and ^ { 7 } Li for N _ { \nu } = 3 .
The low value of ^ { 2 } H/H does not fit well with the current observationally-inferred primordial abundance of ^ { 4 } He for N _ { \nu } = 3 .
In addition , if the low value of ^ { 2 } H/H is indicative of the primordial deuterium abundance , then significant depletion of ^ { 7 } Li in old , hot Pop II halo stars is probably required to obtain a concordant range of \eta , for any effective number of neutrino flavors .
The use of conservative ranges for the primordial abundances of ^ { 2 } H , ^ { 4 } He , and ^ { 7 } Li allow success of the standard picture for N _ { \nu } = 3 .