Planck stars

Abstract

A star that collapses gravitationally can reach a further stage of its life, where quantum-gravitational pressure counteracts weight. The duration of this stage is very short in the star proper time, yielding a bounce, but extremely long seen from the outside, because of the huge gravitational time dilation. Since the onset of quantum-gravitational effects is governed by energy density -not by size-the star can be much larger than planckian in this phase. The object emerging at the end of the Hawking evaporation of a black hole can then be larger than planckian by a factor (m/mP ) n , where m is the mass fallen into the hole, mP is the Planck mass, and n is positive. We consider arguments for n = 1/3 and for n = 1. There is no causality violation or faster-than-light propagation. The existence of these objects alleviates the black-hole information paradox. More interestingly, these objects could have astrophysical and cosmological interest: they produce a detectable signal, of quantum gravitational origin, around the 10 −14 cm wavelength.Measuring effects of the quantum nature of gravity is notoriously difficult [1,2], because of the smallness of the Planck scale. Here we suggest that cosmic rays in the GeV range might contain a trace of a quantum gravitational phenomenon. The large gap between this energy and the Planck energy could be bridged by a large multiplicative factor appearing because of the long (cosmological) lifetime of radiating primordial black holes. This could lead to measurements of "quantum gravity in the sky" [3]. This possibility is suggested by the existence of an apparent paradox currently widely discussed in the theoretical literature. Briefly: what is the fate of the information fallen into the hole, after it evaporates via Hawking radiation? Theoretical arguments appear to indicate that the information being carried out by the radiation implies unpalatable phenomena like "firewalls" [4,5]. But if information remains trapped inside, the final stage of the black hole at the end of the evaporation would have to store an amount of information hardly compatible with its expected planckian size [6][7][8]. A possible way out from these unsavory alternatives, suggested by Giddings [9], is that the size of the black hole at the final stage of the evaporation is much larger than planckian (see also [10,11]).Here we observe that this scenario does not require superluminal transfer of information and can follow from the fact that quantum gravitation phenomena become relevant when the matter energy density reaches the Planck scale, and this may happen at length scales much larger than planckian. Quantum gravity may liberate the information stored in the black hole when this is still large compared to the Planck length, implying the existence of a new phase in the life of gravitationally collapsed object [12], which could be short in proper time, but, due to gravitational time dilation, very long for an external observer. This, together with the hypothesis of primordial black holes, opens the possibi...

Quantum-gravity effects outside the horizon spark black to white hole tunneling

“…In quantum gravity, we exit the quantum region when the matter density, or the curvature, reaches the Planck scale (see a full discussion in [23]). This must also be true for black holes [3,9,[11][12][13]. The curvature R is of the order m/r 3 and reaches the Planck value…”

“…A collapsing star might avoid sinking into r = 0 much as a quantum electron in a Coulomb potential does not sink all the way into r = 0. The possibility of such a Planck star phenomenology has been considered by numerous authors [3][4][5][6][7][8][9][10][11][12][13][14][15]. The picture is similar to Giddings's remnant scenario , [16], here with a macroscopic remnant developing into a white hole [17].…”

“…Thus, classical GR is compatible with the possibility that a black hole is a (quantum) bouncing star seen in extreme slow motion. The bounce could lead to observable phenomena [13] whose phenomenology has been investigated in [14].…”

Quantum gravity effects around Sagittarius A*

“…Examples are the quantum description of black holes as Bose-Einstein condensates [4,5], fuzzbals [6], Planck star tunnelling [7][8][9][10][11][12][13], and Giddings's metric fluctuations [14]. Also, one possible interpretation of the firewall theorem [15], which has recently raised a lively discussion in the theoretical world, is as an indication that "something strange" should have to happen at the horizon of a macroscopic black hole, possibly violating the hypotheses of the theorem, which include the validity of local quantum field theory on a given background geometry.…”

Observational properties of rigidly rotating dust configurations

“…Other, more compact, objects have been suggested as well (see for example Ref. [26]) and they may be intrinsically quantum in nature.…”