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 ${10}^{13}$ and ${10}^{17}\sec$ 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 $m_{\nu }\simeq 30\mathrm{eV}$, $\tau \simeq {10}^{13}\sec$, we find an interesting possibility: the integrated Sachs-Wolfe peak produced by the decaying neutrino in low- $\Omega$ models mimics the acoustic peak expected in an $\Omega =1\mathrm{}$ model.