We obtain the photon spectrum induced by a cosmic background of unstable neutrinos. We study the spectrum in a variety of cosmological scenarios and also we allow for the neutrinos having a momentum distribution (only a critical matter-dominated universe and neutrinos at rest have been considered until now). Our results can be helpful when extracting bounds on neutrino electric and magnetic moments from cosmic photon background observations.

We consider lepton-quark contact interactions in models with right-handed neutrinos, and find that observational data from SN1987A restricts the scale of such interactions to be at least $\Lambda>90$ TeV.

We obtain new stringent constraints on a light spinless particle phi coupled only to photons at low energies, considering its effects on the extragalactic photon background, the black-body spectrum of the cosmic microwave background radiation and the cosmological abundance of deuterium.

We show how the $^4$He-abundance in the early Universe can be used to demonstrate that macroscopic samples of neutrinos in thermal equilibrium are indeed distributed according to Fermi-Dirac statistics.

We establish that forces mediated by the exchange of a pair of Majorana neutrinos differ from those due to Dirac neutrino exchange.

The discrepancy between the experimental decay rate of orthopositronium (o-Ps) and the QED theoretical prediction can be solved by invoking decays of o-Ps into exotic particles with branching ratios ∼ 10−3. We show that considerations based on primordial nucleosynthesis and effective Lagrangians place a very stringent upper bound: B ≡ Γ(o-Ps → “exotic” + …)Γ(o-Ps) ≤ 2 × 10−15, ruling out the exotic decay solution to the puzzle.

We use the constraints arising from primordial nucleisynthesis to bound the strength F of non-standard neutrino-neutrino interactions, when the right-handed neutrinos participate in the interaction. We find F<3×10−3GF, which is five orders of magnitude more stringent than the limit obtained using LEP data. We also show that secret interactions of neutrinos mediated by massless particles must have a coupling f less than 2 × 10−5. This also ameliorates previous limits in the literature.