Ultra-cold WIMPs: relics of non-standard pre-BBN cosmologies

Weakly interacting massive particles (WIMPs) are one of very few probes of cosmology before Big Bang nucleosynthesis (BBN). We point out that in scenarios in which the Universe evolves in a non-standard manner during and after WIMP kinetic decoupling, the horizon mass scale at decoupling can be smaller and the dark matter WIMPs can be colder than in standard cosmology. This would lead to much smaller first objects in hierarchical structure formation. In low reheating temperature scenarios the effect may be large enough as to noticeably enhance indirect detection signals in GLAST and other detectors, by up to two orders of magnitude.