Oct 2017 - Sep 2018

Cosmology and Phenomenology of Dark Matter Dilution

by James Unwin

OC218 (IPPP)




The early universe could feature multiple reheating events, leading to 
increases in the visible sector entropy density that dilute both 
particle asymmetries and the number density of frozen-out states. I 
will highlight that an important consequence of late time dilution is 
that a smaller dark matter annihilation cross section is needed to 
obtain the observed relic density, thus weakening experimental bounds 
and also permitting superheavy dark matter. Moreover, I will outline a 
new general dark matter scenario in which freeze-out occurs whilst the 
energy density of the universe is dominated by a decoupled 
non-relativistic species. Notably, decoupling during matter domination 
changes the freeze-out dynamics, since the Hubble rate is 
parametrically different for matter and radiation domination. 
Furthermore, for successful Big Bang Nucleosynthesis the state 
dominating the early universe energy density must decay, this dilutes 
(or repopulates) the dark matter. Finally, I will highlight the 
possibility of superheavy asymmetric dark matter models, made possible 
by a sizeable entropy injection after dark matter freeze-out, which 
have interesting astrophysical consequences. 

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