Speaker
Pierre Sikivie
Description
It has long been accepted that axions produced by vacuum
realignment, and related prccesses, during the QCD phase
transition form a cold degenerate Bose gas and are a
candidate for the dark matter. More recently it was found
that dark matter axions thermalize through their
gravitational self-interactions and form a Bose-Einstein
condensate (BEC). On time scales long compared to their
rethermalization time scale, almost all the axions go to the
lowest energy state available to them. In this behaviour
they differ from the other dark matter candidates. Axions
accreting onto a galactic halo fall in with net overall
rotation because they almost all go to their lowest energy
available state for given total angular momentum. In
contrast, the other proposed forms of dark matter accrete
onto galactic halos with an irrotational velocity field. The
inner caustics are different in the two cases. I'll argue
that the dark matter is axions, at least in part, because
there is observational evidence for the type of inner
caustic produced by axion BEC.
