Observational constraints on secret neutrino interactions from big bang nucleosynthesis

We investigate possible interactions between neutrinos and massive scalar bosons via g(phi)(nu) over bar nu phi (or massive vector bosons via g(V)(nu) over bar gamma(mu)nu V-mu) and explore the allowed parameter space of the coupling constant g phi (or g(V)) and the scalar (or vector) boson mass m(phi) (or m(V)) by requiring that these secret neutrino interactions (SNIs) should not spoil the success of big bang nucleosynthesis (BBN). Incorporating the SNIs into the evolution of the early Universe in the BBN era, we numerically solve the Boltzmann equations and compare the predictions for the abundances of light elements with observations. It turns out that the constraint on g(phi) and m(phi) in the scalar-boson case is rather weak, due to a small number of degrees of freedom (d.o.f.). However, in the vector-boson case, the most stringent bound on the coupling g(V) less than or similar to 6 x 10(-10) at 95% confidence level is obtained for m(V) similar or equal to 1 MeV, while the bound becomes much weaker g(V) less than or similar to 8 x 10(-6) for smaller masses m(V) less than or similar to 10(-4) MeV. Moreover, we discuss in some detail how the SNIs affect the cosmological evolution and the abundances of the lightest elements.