If neutrinos dominate the mass density of the Universe, they play a critical role in the gravitational instability theory of galaxy formation. For neutrinos of mass $30m_{30}$ eV, a maximum Jeans mass $M_{\nu m}\approx 4\times {10}^{15}{m_{30}}^{-2\mathrm{}}{M}_{\odot }$ is derived. On smaller scales, neutrino fluctuations are damped, and the growth of baryon fluctuations is greatly inhibited. Structures on scales $>M_{\nu m}$ may collapse, forming galaxies as in Zel'dovich's "pancake" theory.

• Received 9 July 1980

DOI:https://doi.org/10.1103/PhysRevLett.45.1980