Black holes preserve information about the stuff that falls into them, according to Prof Stephen Hawking. Physicists have long argued about what happens to information about the physical state of things that are swallowed up by black holes. Read more

Title: Backreaction of Hawking Radiation on a Gravitationally Collapsing Star I: Black Holes? Author: Laura Mersini-Houghton

Particle creation leading to Hawking radiation is produced by the changing gravitational field of the collapsing star. The two main initial conditions in the far past placed on the quantum field from which particles arise, are the Hartle Hawking vacuum and the Unruh vacuum. The former leads to a time symmetric thermal bath of radiation, while the latter to a flux of radiation coming out of the collapsing star. The energy of Hawking radiation in the interior of the collapsing star is negative and equal in magnitude to its value at future infinity. This work investigates the backreaction of Hawking radiation on the interior of a gravitationally collapsing star, in a Hartle-Hawking initial vacuum. It shows that due to the negative energy Hawking radiation in the interior, the collapse of the star stops at a finite radius, before the singularity and the event horizon of a black hole have a chance to form. That is, the star bounces instead of collapsing to a black hole. A trapped surface near the last stage of the star's collapse to its minimum size may still exist temporarily. Its formation depends on the details of collapse. Results for the case of Hawking flux of radiation with the Unruh initial state, will be given in a companion paper II.

Title: Blocking the Hawking Radiation Author: Martin Autzen, Chris Kouvaris

Some severe constraints on asymmetric dark matter are based on the scenario that certain types of WIMPs can form mini-black holes inside neutron stars that can lead to their destruction. A crucial element for the realization of this scenario is that the black hole grows after its formation (and eventually destroys the star) instead of evaporating. The fate of the black hole is dictated by the two opposite mechanics i.e. accretion of nuclear matter from the center of the star and Hawking radiation that tends to decrease the mass of the black hole. We study how the assumptions for the accretion rate can in fact affect the critical mass beyond which a black hole always grows. We also study to what extent degenerate nuclear matter can impede Hawking radiation due to the fact that emitted particles can be Pauli blocked at the core of the star.

Title: Fading Hawking Radiation Author: I. Sakalli, M. Halilsoy, H. Pasaoglu

In this study, we explore a particular type Hawking radiation which ends with zero temperature and entropy. The appropriate black holes for this purpose are the linear dilaton black holes. In addition to the black hole choice, a recent formalism in which the Parikh-Wilczek's tunneling formalism amalgamated with quantum corrections to all orders in \hbar is considered. The adjustment of the coefficients of the quantum corrections plays a crucial role on this particular Hawking radiation. The obtained tunneling rate indicates that the radiation is not pure thermal anymore, and hence correlations of outgoing quanta are capable of carrying away information encoded within them. Finally, we show in detail that when the linear dilaton black hole completely evaporates through such a particular radiation, entropy of the radiation becomes identical with the entropy of the black hole, which corresponds to "no information loss".

Title: Hawking Radiation from Higher-dimensional Black Holes Author: Panagiota Kanti, Elizabeth Winstanley

We review the quantum field theory description of Hawking radiation from evaporating black holes and summarize what is known about Hawking radiation from black holes in more than four space-time dimensions. In the context of the Large Extra Dimensions scenario, we present the theoretical formalism for all types of emitted fields and a selection of results on the radiation spectra. A detailed analysis of the Hawking fluxes in this case is essential for modelling the evaporation of higher-dimensional black holes at the LHC, whose creation is predicted by low-energy models of quantum gravity. We discuss the status of the quest for black-hole solutions in the context of the Randall-Sundrum brane-world model and, in the absence of an exact metric, we review what is known about Hawking radiation from such black holes.

Orbiting moon gives chilling clue to black hole heat

To expand Hawking's theory to moving black holes, Samuel Gralla and Alexandre Le Tiec at the University of Maryland in College Park modelled a scenario in which a moon is orbiting a black hole at the same speed that the hole rotates, so that the moon seems to hover in place. The team found that a partner would cause the black hole to wobble, lowering surface gravity and cooling it - although not by much. A lone black hole about five times the mass of the sun and 30 kilometres wide would be about 10 billionths of a degree above absolute zero. Adding a moon with a mass of 1000 tonnes would reduce this by just 10-35 kelvin. Read more

Orbiting moon gives chilling clue to black hole heat

To expand Hawking's theory to moving black holes, Samuel Gralla and Alexandre Le Tiec at the University of Maryland in College Park modelled a scenario in which a moon is orbiting a black hole at the same speed that the hole rotates, so that the moon seems to hover in place. The team found that a partner would cause the black hole to wobble, lowering surface gravity and cooling it - although not by much. A lone black hole about five times the mass of the sun and 30 kilometres wide would be about 10 billionths of a degree above absolute zero. Adding a moon with a mass of 1000 tonnes would reduce this by just 10-35 kelvin. Read more

Title: Effective Conformal Descriptions of Black Hole Entropy: A Review Authors: S. Carlip

Black holes behave as thermodynamic objects, and it is natural to ask for an underlying "statistical mechanical" explanation in terms of microscopic degrees of freedom. I summarise attempts to describe these degrees of freedom in terms of a dual two-dimensional conformal field theory, emphasising the generality of the Cardy formula and the consequent universal nature of the conformal description.

Title: Black Hole's Quantum N-Portrait Authors: Gia Dvali, Cesar Gomez

We establish a quantum measure of classicality in the form of the occupation number, N, of gravitons in a gravitational field. This allows us to view classical background geometries as quantum Bose-condensates with large occupation numbers of soft gravitons. We show that among all possible sources of a given physical length, N is maximised by the black hole and coincides with its entropy. The emerging quantum mechanical picture of a black hole is surprisingly simple and fully parameterised by N. The black hole is a leaky bound-state in form of a cold Bose-condensate of N weakly-interacting soft gravitons of wave-length \sqrt{N} times the Planck length and of quantum interaction strength 1/N. Such a bound-state exists for an arbitrary N. This picture provides a simple quantum description of the phenomena of Hawking radiation, Bekenstein entropy as well as of non-Wilsonian UV-self-completion of Einstein gravity. We show that Hawking radiation is nothing but a quantum depletion of the graviton Bose-condensate, which despite the zero temperature of the condensate produces a thermal spectrum of temperature T \, = \, 1/\sqrt{N}. The Bekenstein entropy originates from the exponentially growing with N number of quantum states. Finally, our quantum picture allows to understand classicalisation of deep-UV gravitational scattering as 2 ightarrow N transition. We point out some fundamental similarities between the black holes and solitons, such as a t'Hooft-Polyakov monopole. Both objects represent Bose-condensates of N soft bosons of wavelength \sqrt{N} and interaction strength 1/N. In short, the semi-classical black hole physics is 1/N-coupled large-N quantum physics.

Title: Effective temperature, Hawking radiation and quasinormal modes Authors: Christian Corda

Parikh and Wilczek have shown that Hawking radiation's spectrum cannot be strictly thermal. Such a non-strictly thermal character implies that the spectrum is also not strictly continuous and thus generates a natural correspondence between Hawking radiation and black hole's quasinormal modes. This issue endorses the idea that, in an underlying unitary quantum gravity theory, black holes result highly excited states. We use this key point to re-analyse the spectrum of black hole's quasinormal modes by introducing a black hole's effective temperature. Our analysis changes the physical understanding of such a spectrum and enables a re-examination of various results in the literature which realises important modifies on quantum physics of black holes. In particular, the formula of the horizon's area quantization and the number of quanta of area are modified becoming functions of the quantum "overtone" number n. Consequently, Bekenstein-Hawking entropy, its sub-leading corrections and the number of microstates, i.e. quantities which are fundamental to realise unitary quantum gravity theory, are also modified. They become functions of the quantum overtone number too. Previous results in the literature are re-obtained in the very large n limit.