Can baryonic dark matter be solid hydrogen?

Abstract

Some requirements are discussed for solid hydrogen formation in cold dark dense clouds in galaxies. If temperatures in the clouds are near the microwave background temperature of 2.7 K and molecular hydrogen densities are 3×105 cm−3 or higher, as suggested by recent observations, it may be possible for solid hydrogen objects to form. Comet size hydrogen solids could build from molecular hydrogen condensation on grains and by collisions. Heated primarily by cosmic rays, objects with 100 km radii could last billions of years. The larger objects may be detectable, in the future, by sensitive gravitational lensing or eclipsing observations. Other possibilities are discussed for future detection of the cold dark dense molecular hydrogen regions. In our model, helium is added along with the hydrogen to preserve the primordial helium to hydrogen mass ratio,Y p , of the standard model. In the hot regions of the universe the solid hydrogen objects sublime and melt so our model predictsY p =0.250, the same as other baryonic dark matter models with identical values of ω=0.1,H o =50 and η=6.8×10−10. This value cannot be ruled out at present because of the large systematic uncertainties in the observed value of 0.232. In the cold dark regions where solid hydrogen objects exist, we predict thatY p will be greater than 0.250. Observations are not yet sensitive enough to measure this ratio.