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In several classes of particle physics models -- ranging from the classicalMajoron models, to the more recent scenarios of late neutrino masses orMass-Varying Neutrinos -- one or more of the neutrinos are postulated to coupleto a new light scalar field. As a result of this coupling, neutrinos in theearly universe instead of streaming freely could form a self-coupled fluid,with potentially observable signatures in the Cosmic Microwave Background andthe large scale structure of the universe. We re-examine the constraints onthis scenario from the presently available cosmological data and investigatethe sensitivity expected from the Planck satellite. In the first case, we findthat the sensitivity strongly depends on which piece of data is used. The SDSSMain sample data, combined with WMAP and other data, disfavors the scenario ofthree coupled neutrinos at about the 3.5$\sigma$ confidence level, but alsofavors a high number of freely streaming neutrinos, with the best fit at 5.2.If the matter power spectrum is instead taken from the SDSS Large Red Galaxysample, best fit point has 2.5 freely streaming neutrinos, but the scenariowith three coupled neutrinos becomes allowed at $2\sigma$. In contrast, Planckalone will exclude even a single self-coupled neutrino at the $4.2\sigma$confidence level, and will determine the total radiation at CMB epoch to$\Delta N_\nu^{eff} = ^{+0.5}_{-0.3}$ ($1\sigma$ errors). We investigate therobustness of this result with respect to the details of Planck's detector.This sensitivity to neutrino free-streaming implies that Planck will be capableof probing a large region of the Mass-Varying Neutrino parameter space. Planckmay also be sensitive to a scale of neutrino mass generation as high as 1 TeV.

Constraining Models of Neutrino Mass and Neutrino Interactions with the Planck Satellite