Structure Formation with Generalized Dark Matter

Abstract

The next generation of cosmic microwave background (CMB) experiments, galaxy surveys, and high- redshift observations can potentially determine the nature of the dark matter observationally. With this in mind, we introduce a phenomenological model for a generalized dark matter (GDM) component and discuss its e†ect on large-scale structure and CMB anisotropies. Specifying the gravitational inNuence of the otherwise noninteracting GDM requires not merely a model for its equation of state but one for its full stress tensor. From consideration of symmetries, conservation laws, and gauge invariance, we con- struct a simple but powerful three-component parameterization of these stresses that exposes the new phenomena produced by GDM. Limiting cases include: a particle component (e.g., weakly interacting massive particles, radiation, or massive neutrinos), a cosmological constant, and a scalar -eld com- ponent. Intermediate cases illustrate how the clustering properties of the dark matter can be speci-ed independently of its equation of state. This freedom allows one to alter the amplitude and features in the matter-power spectrum relative to those of the CMB anisotropies while leaving the background cosmol- ogy -xed. Conversely, observational constraints on such phenomena can help determine the nature of the dark matter. Subject headings: cosmic microwave background E cosmology: theory E dark matter E large-scale structure of the universe