keV Neutrino Dark Matter in a Fast Expanding Universe
  We study the possibility of keV neutrino dark matter in the minimal
$U(1)_{B-L}$ gauge extension of the standard model where three right handed
neutrinos are automatically included due to the requirement of anomaly
cancellations. Without considering extra additional particles or symmetries, we
consider the lightest right handed neutrino to be in the keV mass range which
is kinematically long lived. Due to gauge interactions, such a keV neutrino can
be thermally produced in the early Universe followed by decoupling while being
relativistic. The final relic abundance of such keV neutrino typically
overclose the Universe requiring additional mechanism to bring it under
observed limits. We propose a non-standard cosmological history where a scalar
field $\phi$, that redshifts faster than radiation dominates the Universe prior
to the radiation dominated era. We show that such a non-standard phase can keep
the abundance of thermally generated keV neutrino dark matter within observed
relic abundance. We constrain the non-standard phase, $U(1)_{B-L}$ parameters
from these requirements and also briefly comment upon the observational aspects
of such keV neutrino dark matter.
