Cosmic Matter Flux May Turn Hawking Radiation Off
  An astrophysical (cosmological) black hole forming in a cosmological context
will be subject to a flux of infalling matter and radiation, which will cause
the outer apparent horizon (a marginal trapping surface) to be spacelike
\cite{ellisetal14}. As a consequence the radiation emitted close to the
apparent horizon no longer arrives at infinity with a diverging redshift.
Standard calculations of the emission of Hawking radiation then indicate that
no blackbody radiation is emitted to infinity by the black hole in these
circumstances, hence there will also then be no black hole evaporation process
due to emission of such radiation as long as the matter flux is significant.
The essential adiabatic condition (eikonal approximation) for black hole
radiation gives a strong limit to the black holes that can emit Hawking
radiation. We give the mass range for the black holes that can radiate,
according to their cosmological redshift, for the special case of the cosmic
blackbody radiation (CBR) influx (which exists everywhere in the universe). At
a very late stage of black hole formation when the CBR influx decays away, the
black hole horizon becomes first a slowly evolving horizon and then an isolated
horizon; at that stage, black hole radiation will start. This study suggests
that the primordial black hole evaporation scenario should be revised to take
these considerations into account.
