Fate of gravitational collapse in semiclassical gravity (0712.1130v1)

Abstract: While the outcome of gravitational collapse in classical general relativity is unquestionably a black hole, up to now no full and complete semiclassical description of black hole formation has been thoroughly investigated. Here we revisit the standard scenario for this process. By analyzing how semiclassical collapse proceeds we show that the very formation of a trapping horizon can be seriously questioned for a large set of, possibly realistic, scenarios. We emphasise that in principle the theoretical framework of semiclassical gravity certainly allows the formation of trapping horizons. What we are questioning here is the more subtle point of whether or not the standard black hole picture is appropriate for describing the end point of realistic collapse. Indeed if semiclassical physics were in some cases to prevent formation of the trapping horizon, then this suggests the possibility of new collapsed objects which can be much less problematic, making it unnecessary to confront the information paradox or the run-away end point problem.

Overview of "Fate of Gravitational Collapse in Semiclassical Gravity"

The research paper "Fate of Gravitational Collapse in Semiclassical Gravity," authored by Carlos Barceló, Stefano Liberati, Sebastiano Sonego, and Matt Visser, critically examines the conventional understanding of black hole formation within the framework of semiclassical gravity. Traditional models in general relativity predict the formation of black holes with event horizons following the gravitational collapse of massive astrophysical objects. However, these models do not fully integrate semiclassical aspects and often lead to unresolved theoretical issues, such as the information-loss paradox associated with Hawking radiation.

Central to the authors' discourse is the contention that the assumed inevitability of trapping horizon formation — a precursor to black holes — is not as certain when viewed through the lens of semiclassical gravity. They challenge the conventional model by positing that under specific scenarios, the formation of trapping horizons might be circumvented, leading to alternative astrophysical objects that are less problematic, particularly concerning the information paradox and the singularity at the core of classical black holes.

Key Points and Analyses

1. Classical vs. Semiclassical Collapse:
   • The classical account of gravitational collapse in general relativity unilaterally leads to the formation of black holes. These are characterized by event horizons from which no information can escape, thereby contributing to the information paradox when considered in light of quantum mechanics.
   • The exploration of semiclassical scenarios reveals potential deviations from this narrative, with quantum effects possibly inhibiting complete horizon formation. Such deviations become significant during the final stages of collapse.
2. Hawking Radiation without Event Horizons:
   • The authors discuss a scenario where Hawking-like radiation could still be observed even if event horizons do not form. This suggests that the radiation originates from a surface-like structure that asymptotically approaches typical horizon characteristics.
   • This formulation allows for a reconciliation of unitarity with the evaporation process, as information is not irrevocably lost — it could, in principle, be retrieved from the correlation of the emitted radiation.
3. Quantum Effects and Trans-Planckian Issues:
   • A critical examination is provided on the trans-Planckian problem, where semiclassical theory presupposes modes of arbitrarily high frequency near horizons — a postulate that is unsustainable physically and calls for new physics at high energies.
   • Assumptions made about cutting-off these high frequencies influence predictions about trapping horizon formation, bringing to fore the interplay between trans-Planckian considerations and the prevention of event horizon formation.
4. Dynamic Scenarios - Black Stars and Quasi-black Holes:
   • The paper proposes alternative end states for gravitational collapse, termed "black stars" and "quasi-black holes," as substitutes for traditional black holes.
   • Black stars are envisioned as compact horizonless objects where quantum pressures support the structure, whereas quasi-black holes mimic black holes for extended periods yet avoid full horizon formation.
5. Implications and Future Directions:
   • The proposed constructs of black stars and quasi-black holes hold significant implications for interpreting astronomical observations of compact objects conventionally identified as black holes.
   • Further exploration of the semiclassical and quantum gravitational contributions is warranted to fully understand how these affect realistic models of collapse, necessitating high energy astrophysical observations and simulations.

In conclusion, the paper scrutinizes the established paradigm of black hole formation under semiclassical considerations, proposing scenarios which avert the creation of traditional event horizons without dismissing the credible predictions of Hawking radiation. These findings stimulate a reevaluation of theoretical frameworks concerning black hole physics and point towards new avenues for research that incorporate the subtleties of quantum and semiclassical effects within gravitational collapse.