Non-Schwarzschild Primordial Black Holes as Dark Matter in Quadratic
  Gravity
  One-loop renormalised quantum effective action for gravity contains quadratic
in curvature terms. We have found an approximate analytic black hole solution
in quadratic gravity by keeping only the radial spherically symmetric
fluctuations and dimensionally reducing the 4-dimensional (4D) theory down to
the 2-dimensional (2D) dilaton gravity with a potential. The solution reduces
to the Schwarzschild black hole in the limit of Einstein's gravity, but
otherwise admits non-negative Arnowitt-Deser-Misner (ADM) and positive
quasi-local Misner-Sharp masses that can differ significantly. We then study
the thermodynamics of such quantum corrected black holes and compute their
lifetime under Hawking evaporation. We note that for some range of parameters,
black holes increase in mass while emitting Hawking radiation. This
pathological behaviour is related to the negative energy states that are
present in quadratic gravity. We also find that the micro-lensing of
non-Schwarzschild black holes could significantly deviate from the
micro-lensing of their Schwarzschild counterparts. These findings have
important ramifications for the phenomenology of primordial black holes (PBHs)
as dark matter. In particular, the quoted constraints on PBH dark matter from
micro-lensing data can be completely evaded, thus making PBHs in the mass range
$\sim 10^{-12} - 10~M_{\odot}$ viable dark matter candidates.
