Foiling Black Hole Foils: Revealing Horizon Alternatives with Baryonic Atmospheres

TL;DR

This paper demonstrates that horizonless black hole alternatives, or foils, develop observable baryonic atmospheres that produce thermal emissions, providing a way to distinguish them from true black holes through astrophysical observations.

Contribution

It shows that horizonless black hole models naturally form optically thick baryonic atmospheres, which produce observable thermal emissions, challenging the viability of such models as black hole mimickers.

Findings

Baryonic atmospheres form around horizonless foils in accreting systems.

The emergent luminosity is driven toward equilibrium and weakly depends on redshift.

Atmospheres are convectively stable and form at modest redshift even with extreme surface redshift.

Abstract

Event horizons are a defining feature of black holes. Consequently, there have been many efforts to probe their existence in astrophysical black hole candidates, spanning ten orders of magnitude in mass. Nevertheless, horizons remain an obstacle to unifying general relativity and quantum mechanics, most sharply presented by the information paradox. This has motivated a proliferation of horizonless alternatives (black hole foils) that avoid event horizons and are therefore benign. We show that for typical accreting astronomical targets, largely independent of a foil's underlying microphysics, a horizonless compact surface will generically be ensconced within an optically thick, scattering dominated baryonic settling layer that efficiently reprocesses the kinetic energy of infalling matter into observable thermal emission. The emergent photosphere luminosity is driven toward the accretion-powered equilibrium value and is only weakly sensitive to the foil redshift. These atmospheres are convectively stable and naturally imply that the emitting photosphere forms at modest redshift even when the surface redshift is extreme. Moreover, local gas-surface interaction provides a microphysical lower bound on the effective base temperature, insulating the atmosphere from arbitrarily cold foils. The unknown properties of the foil enter only through local boundary conditions controlling baryon processing and thermal coupling at the surface, making the solutions broadly applicable to horizonless alternatives that do not invoke significant additional nonlocal interactions. Thus, under minimal assumptions (GR exterior and local surface interactions), horizonless foils are generically observationally exposed: the absence of a thermal photosphere directly constrains or rules out broad classes of such models.