Evolution of dark matter velocity dispersion

Understand

Cosmological perturbation theory for the late Universe dominated by dark matter is extended beyond the perfect fluid approximation by taking the dark matter velocity dispersion tensor as an additional field into account.

• A proper tensor decomposition of the latter leads to two additional scalar fields, as well as a vector and a tensor field.
• Most importantly, the trace of the velocity dispersion tensor can have a spatially homogeneous and isotropic expectation value.
• While it decays at early times, we show that a back-reaction effect quadratic in perturbations makes it grow strongly at late times.