Common sense really. Space is full of CBR (not to mention other radiation and matter). They keep being fed. The surrounding CBR is warmer than the Hawking temperature of the hole and in effect blocks the Hawking radiation process. Ellis et al even calculate far into the future when the CBR is much cooler---even then given the positive constant Lambda of the standard cosmic model they find the BH is unable to completely evaporate http://inspirehep.net/record/1306275 Astrophysical Black Hole horizons in a cosmological context: Nature and possible consequences on Hawking Radiation George F R Ellis, Rituparno Goswami, Aymen I. M. Hamid, Sunil D. Maharaj This paper considers the nature of apparent horizons for astrophysical black hole situated in a realistic cosmological context. Using semi-tetrad covariant methods we study the local evolutions of the boundaries of the trapped region in the spacetime. For a collapsing massive star immersed in a cosmology with Cosmic Background Radiation (CBR), we show that the initial 2 dimensional marginally trapped surface bifurcates into inner and outer horizons. The inner horizon is timelike while the continuous CBR influx into the black hole makes the outer horizon spacelike. We discuss the possible consequences of these features for Hawking radiation in realistic astrophysical contexts. 13 pages, 4 figures, Jul 14, 2014 http://arxiv.org/abs/1407.3577 This has consequences for the expected prevalence of primordial BH explosions according to a followup paper: http://inspirehep.net/record/1309567 Cosmic Matter Flux May Turn Hawking Radiation Off Javad T. Firouzjaee, George F. R. Ellis (Submitted on 4 Aug 2014) 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 . 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. 21 pages, 6 figures, http://arxiv.org/abs/1408.0778 ==quote Ellis and Firouzjaaee, conclusions== • Application of this constraint to primordial black hole evaporation modeling may bring in a correction to their abundance in the cosmos. Specifically, primordial black holes are candidate progenitors of unidentified Gamma-Ray Bursts (GRBs) that are supposed to detect by the Fermi Gamma-ray Space Telescope observatory. Their abundance might be lowered when the above considerations are taken into account. This is all in accord with the discussions in [5, 10], and leads to the conclusion that in a realistic cosmological context, a black hole forming from the collapse of a star in a universe permeated by CBR and matter will not emit Hawking radiation in the past or at the present, and so emission of such radiation from them, or evaporation of such black holes in an explosion, will not occur in the visible universe. To what degree this affects primordial black holes will be very context dependent and will need detailed modeling. ==endquote== In one sense it should make PBH MORE ABUNDANT because some of the small ones which WOULD have already evaporated will have been fed enough to survive. But it is not clear whether it would make visible GRB more or less abundant. The population of PBH might be larger but fewer might be exploding in our neighborhood. According to the Hawking model, you don't get an explosion until the BH has evaporated down to nearly nothing. there is a bright flash at the very end. This Ellis et al paper would have different implications for the Planck star model. It will be interesting to see if any revised expectations come out of it. In the Planck star BH model the lifespan is much shorter. The GRB is much brighter (so more likely to be visible) and it occurs when the BH has only gone through a third or so of its Hawking lifetime. The bounce explosion s not determined by the BH evaporating down to some critical size, it is determined by a time-dilated rebound which proceeds on its own schedule.