"Due to its effect on the expansion rate of the Universe during Big Bang nucleosynthesis (BBN), the theoretical expectations for the primordial abundances of light elements depend on Nν. Astrophysical measurements of the primordial He and D abundances lead to a value of Nν =
3.14
−
0.65
+
0.70
{\displaystyle 3.14_{-0.65}^{+0.70}}
at 68% c.l., in very good agreement with the Standard Model expectation."