The possibility that the so-called ‘ ‘ lithium problem ’ ’ , i.e. , the disagreement between the theoretical abundance predicted for primordial ^ { 7 } \mathrm { Li } assuming standard nucleosynthesis and the value inferred from astrophysical measurements , can be solved through a non-thermal Big Bang Nucleosynthesis ( BBN ) mechanism has been investigated by several authors .
In particular , it has been shown that the decay of a MeV-mass particle , like , e.g. , a sterile neutrino , decaying after BBN not only solves the lithium problem , but also satisfies cosmological and laboratory bounds , making such a scenario worth to be investigated in further detail .
In this paper , we constrain the parameters of the model with the combination of current data , including Planck 2015 measurements of temperature and polarization anisotropies of the Cosmic Microwave Background ( CMB ) , FIRAS limits on CMB spectral distortions , astrophysical measurements of primordial abundances and laboratory constraints .
We find that a sterile neutrino with mass M _ { S } = 4.35 _ { -0.17 } ^ { +0.13 } \text { MeV } ( at 95 \% c.l .
) , a decay time \tau _ { S } = 1.8 _ { -1.3 } ^ { +2.5 } \cdot 10 ^ { 5 } \text { s } ( at 95 \% c.l . )
and an initial density \bar { n } _ { S } / \bar { n } _ { \text { cmb } } = 1.7 _ { -0.6 } ^ { +3.5 } \cdot 10 ^ { -4 } ( at 95 \% c.l . )
in units of the number density of CMB photons , perfectly accounts for the difference between predicted and observed ^ { 7 } \text { Li } primordial abundance .
This model also predicts an increase of the effective number of relativistic degrees of freedom at the time of CMB decoupling \Delta N _ { \text { eff } } ^ { \text { cmb } } \equiv N _ { \text { eff } } ^ { \text { cmb } } -3.046 = 0.3 % 4 _ { -0.14 } ^ { +0.16 } at 95 \% c.l..
The required abundance of sterile neutrinos is incompatible with the standard thermal history of the Universe , but could be realized in a low reheating temperature scenario .
We also provide forecasts for future experiments finding that the combination of measurements from the COrE+ and PIXIE missions will allow to significantly reduce the permitted region for the sterile lifetime and density .