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Pierre Sikivie
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Bose-Einstein condensation of dark matter axions
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It has long been known that axions produced
by vacuum realignment during the QCD phase
transition in the early universe 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
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 an axion BEC.

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