Probing Unstable Massive Neutrinos with Current Cosmic Microwave Background Observations

The pattern of anisotropies in the cosmic microwave background depends upon the masses and lifetimes of the three neutrino species. A neutrino species of mass greater than 10 eV with lifetime between 10¹³ and 10¹⁷sec leaves a very distinct signature (due to the integrated Sachs-Wolfe effect): the anisotropies at large angles are predicted to be comparable to those on degree scales. Present data exclude such a possibility and hence this region of parameter space. For m_ν30eV, τ10¹³sec, we find an interesting possibility: the integrated Sachs-Wolfe peak produced by the decaying neutrino in low- Ω models mimics the acoustic peak expected in an Ω0ex0ex=0ex0ex1 model.