Presence Of Event Horizon Research Articles

Effective photon mass from black-hole formation

We compute the value of effective photon mass mγ at one-loop level in QED in the background of small (1010 g≲M≪1016 g) spherically symmetric black hole in asymptotically flat spacetime. This effect is associated with the modification of electron/positron propagator in presence of event horizon. Physical manifestations of black-hole environment are compared with those of hot neutral plasma. We estimate the distance to the nearest black hole from the upper bound on mγ obtained in the Coulomb-law test. We also find that corrections to electron mass me and fine structure constant α at one-loop level in QED are negligible in the weak gravity regime.

SECOND-ORDER INVARIANTS AND HOLOGRAPHY

Motivated by recent works on the role of the holographic principle in cosmology, we relate a class of second-order Ricci invariants to the IR cutoff characterizing the holographic dark energy density. The choice of second-order invariants provides an invariant way to account the problem of causality for the correct cosmological cutoff, since the presence of event horizons is not an a priori assumption. We find that these models work fairly well, by fitting the observational data, through a combined cosmological test with the use of SNeIa, BAO and CMB. This class of models is also able to overcome the fine-tuning and coincidence problems. Finally, to make a comparison with other recent models, we adopt the statistical tests AIC and BIC.

Unveiling quantum entanglement degradation near a Schwarzschild black hole

We analyze the entanglement degradation provoked by the Hawking effect in a bipartite system Alice-Rob when Rob is in the proximities of a Schwarzschild black hole while Alice is free falling into it. We will obtain the limit in which the tools imported from the Unruh entanglement degradation phenomenon can be used properly, keeping control on the approximation. As a result, we will be able to determine the degree of entanglement as a function of the distance of Rob to the event horizon, the mass of the black hole, and the frequency of Rob's entangled modes. By means of this analysis we will show that all the interesting phenomena occur in the vicinity of the event horizon and that the presence of event horizons do not effectively degrade the entanglement when Rob is far off the black hole. The universality of the phenomenon is presented: There are not fundamental differences for different masses when working in the natural unit system adapted to each black hole. We also discuss some aspects of the localization of Alice and Rob states. All this study is done without using the single mode approximation.

Probes and Tests of Strong-Field Gravity with Observations in the Electromagnetic Spectrum.

Neutron stars and black holes are the astrophysical systems with the strongest gravitational fields in the universe. In this article, I review the prospect of using observations of such compact objects to probe some of the most intriguing general relativistic predictions in the strong-field regime: the absence of stable circular orbits near a compact object and the presence of event horizons around black-hole singularities. I discuss the need for a theoretical framework, within which future experiments will provide detailed, quantitative tests of gravity theories. Finally, I summarize the constraints imposed by current observations of neutron stars on potential deviations from general relativity.

Very long time scales and black hole thermal equilibrium

We estimate the very long time behaviour of correlation functions in the presence of eternal black holes. It was pointed out by Maldacena (hep-th 0106112) that their vanishing would lead to a violation of a unitarity-based bound. The value of the bound is obtained from the holographic dual field theory. The correlators indeed vanish in a semiclassical bulk approximation. We trace the origin of their vanishing to the continuum energy spectrum in the presence of event horizons. We elaborate on the two very long time scales involved: one associated with the black hole and the other with a thermal gas in the vacuum background. We find that assigning a role to the thermal gas background, as suggested in the above work, does restore the compliance with a time-averaged unitarity bound. We also find that additional configurations are needed to explain the expected time dependence of the Poincar\'e recurrences and their magnitude. It is suggested that, while a semiclassical black hole does reproduce faithfully ``coarse grained'' properties of the system, additional dynamical features of the horizon may be necessary to resolve a finer grained information-loss problem. In particular, an effectively formed stretched horizon could yield the desired results.

A Bisognano - Wichmann-like theorem in a certain case of a non-bifurcate event horizon related to an extreme Reissner - Nordström black hole

Thermal Wightman functions of a massless scalar field are studied within the framework of a `near horizon' static background model of an extremal R - N black hole. This model is built up by using global Carter-like coordinates over an infinite set of Bertotti - Robinson submanifolds glued together. The analytical extendibility beyond the horizon is imposed as constraints on (thermal) Wightman's functions defined on a Bertotti - Robinson submanifold. It turns out that only the Bertotti - Robinson vacuum state, i.e. T = 0, satisfies the above requirement. Furthermore, the extension of this state onto the whole manifold is proved to coincide exactly with the vacuum state in the global Carter-like coordinates. Hence a theorem similar to the Bisognano - Wichmann theorem for the Minkowski spacetime in terms of Wightman functions holds with vanishing `Unruh - Rindler temperature'. Furthermore, the Carter-like vacuum restricted to a Bertotti - Robinson region, resulting in a pure state there, has vanishing entropy despite the presence of event horizons. Some comments on the real extremal R - N black hole are given.

Horizon divergences of fields and strings in black hole backgrounds.

General arguments based on curved space-time thermodynamics show that any extensive quantity, like the free energy or the entropy of thermal matter, always has a divergent boundary contribution in the presence of event horizons, and this boundary term comes with the Hawking-Bekenstein form. Although the coefficients depend on the particular geometry we show that intensive quantities, like the free energy density are universal in the vicinity of the horizon. {} From the point of view of the matter degrees of freedom this divergence is of infrared type rather than ultraviolet, and we use this remark to speculate about the fate of these pathologies in String Theory. Finally we interpret them as instabilities of the Canonical Ensemble with respect to gravitational collapse via the Jeans mechanism.

Entropy generation in quantum gravity

The area entropy and the related Hawking temperature in the presence of event horizons are rederived from the Wheeler-De Witt equation in a formulation of quantum physics which does not require the notion of time. The entropy arises from a tunneling of the metric field and quantifies the information lost in its quantum fluctuations.

Thermal and nonthermal particle production without event horizons

Usually, particle production in accelerated frames is discussed in connection with the presence of event horizons and with a planckian spectrum. We construct accelerated frames without event horizons where particle production takes place with thermal as well as nonthermal distributions.