Stars as Laboratories for Fundamental Physics - MPP Theory Group

Nonstandard Neutrinos 259
Fig. 7.2. Cosmological bounds on neutrino masses and lifetimes as described
in the text. The experimental limits are shown above the main panel. If the
dominant decay channel is the majoron mode ν → ν ′ χ the BBN-excluded
range extends between the dashed lines. The dotted line is τ ν |U e3 | 2 for
standard-model decays ν 3 → ν 1 according to Eq. (7.17).
and lifetimes according to Dicus, Kolb, and Teplitz (1977) 42 is shown
in Fig. 7.2 as a shaded area marked “Mass Density.”
Decaying neutrinos would cause a second cosmic epoch of radiation
domination, suppressing the growth of density fluctuations and thus
the formation of structure (Steigman and Turner 1985; Krauss 1991;
Bond and Efstathiou 1991). Somewhat schematically, the area above
the shaded band in Fig. 7.2 marked “Structure Formation” is excluded
by this more model-dependent argument. For masses and lifetimes on
this band, neutrinos would actually have the beneficial effect of modifying
the primordial spectrum of density fluctuations such as to avoid the
problem of too much small-scale power in cold dark matter universes
(Bardeen, Bond, and Efstathiou 1987; Bond and Efstathiou 1991; Dodelson,
Gyuk, and Turner 1994; White, Gelmini, and Silk 1995).
42 Note that the corresponding limits discussed in the book by Kolb and Turner
(1990) are somewhat less restrictive because their treatment does not seem to be
entirely self-consistent (G. Gelmini, private communication).