arXiv subjects

Alex B. Nielsen

Publications and source records attributed to Alex B. Nielsen.

At least 55 records · Page 3Linked to original sources

arXiv2012

Conformally rescaled spacetimes and Hawking radiation

We study various derivations of Hawking radiation in conformally rescaled metrics. We focus on two important properties, the location of the horizon under a conformal transformation and its associated temperature. We find that the production of Hawking radiation cannot be associated in all cases to the trapping horizon because its location is not invariant under a conformal transformation. We also find evidence that the temperature of the Hawking radiation should transform simply under a conformal transformation, being invariant for asymptotic observers in the limit that the conformal transformation factor is unity at their location.

gr-qc↗

arXiv2012

Compact binary coalescence parameter estimations for 2.5 post-Newtonian aligned spinning waveforms

We examine the parameter accuracy that can be achieved by advanced ground-based detectors for binary inspiralling black holes and neutron stars. We use the 2.5 PN spinning waveforms of Arun et al. (2009). Our main result is that the errors are noticeably different from existing 2PN studies for aligned spins. While the masses can be determined more accurately, the individual spins are measured less accurately compared to previous work at lower PN order. We also examine several regions of parameter space relevant to expected sources and the impact of simple priors. A combination of the spins is measurable to higher accuracy and we examine what this can tell us about spinning systems.

gr-qc↗

arXiv2011

Dynamical Surface Gravity in Spherically Symmetric Black Hole Formation

We study dynamical surface gravity in a general spherically symmetric setting using Painlevé-Gullstrand (PG) coordinates. Our analysis includes several definitions that have been proposed in the past as well as two new definitions adapted to PG coordinates. Various properties are considered, including general covariance, value at extremality, locality and static limit. We illustrate with specific examples of "dirty" black holes that even for spacetimes possessing a global timelike Killing vector, local definitions of surface gravity can differ substantially from "non-local" ones that require an asymptotic normalization condition. Finally, we present numerical calculations of dynamical surface gravity for black hole formation via spherically symmetric scalar field collapse. Our results highlight the differences between the various definitions in a dynamical setting and provide further insight into the distinction between local and non-local definitions of surface gravity.

gr-qc↗

arXiv2011

The horizon-entropy increase law for causal and quasi-local horizons and conformal field redefinitions

We explicitly prove the horizon-entropy increase law for both causal and quasi-locally defined horizons in scalar-tensor and $f(R)$ gravity theories. Contrary to causal event horizons, future outer trapping horizons are not conformally invariant and we provide a modification of trapping horizons to complete the proof, using the idea of generalised entropy. This modification means they are no longer foliated by marginally outer trapped surfaces but fixes the location of the horizon under a conformal transformation. We also discuss the behaviour of horizons in "veiled" general relativity and show, using this new definition, how to locate cosmological horizons in flat Minkowski space with varying units, which is physically identified with a spatially flat FLRW spacetime.

gr-qc↗

arXiv2010

The slicing dependence of non-spherically symmetric quasi-local horizons in Vaidya Spacetimes

It is well known that quasi-local black hole horizons depend on the choice of a time coordinate in a spacetime. This has implications for notions such as the surface of the black hole and also on quasi-local physical quantities such as horizon measures of mass and angular momentum. In this paper, we compare different horizons on non-spherically symmetric slicings of Vaidya spacetimes. The spacetimes we investigate include both accreting and evaporating black holes. For some simple choices of the Vaidya mass function function corresponding to collapse of a hollow shell, we compare the area for the numerically found axisymmetric trapping horizons with the area of the spherically symmetric trapping horizon and event horizon. We find that as expected, both the location and area are dependent on the choice of foliation. However, the area variation is not large, of order $0.035\%$ for a slowly evolving horizon with $\dot{m}=0.02$. We also calculate analytically the difference in area between the spherically symmetric quasi-local horizon and event horizon for a slowly accreting black hole. We find that the difference can be many orders of magnitude larger than the Planck area for sufficiently large black holes.

gr-qc↗

arXiv2010

The spatial relation between the event horizon and trapping horizon

The relation between event horizons and trapping horizons is investigated in a number of different situations with emphasis on their role in thermodynamics. A notion of constant change is introduced that in certain situations allows the location of the event horizon to be found locally. When the black hole is accreting matter the difference in area between the two different horizons can be many orders of magnitude larger than the Planck area. When the black hole is evaporating the difference is small on the Planck scale. A model is introduced that shows how trapping horizons can be expected to appear outside the event horizon before the black hole starts to evaporate. Finally a modified definition is introduced to invariantly define the location of the trapping horizon under a conformal transformation. In this case the trapping horizon is not always a marginally outer trapped surface.

gr-qc↗

arXiv2008

Black holes and black hole thermodynamics without event horizons

We investigate whether black holes can be defined without using event horizons. In particular we focus on the thermodynamic properties of event horizons and the alternative, locally defined horizons. We discuss the assumptions and limitations of the proofs of the zeroth, first and second laws of black hole mechanics for both event horizons and trapping horizons. This leads to the possibility that black holes may be more usefully defined in terms of trapping horizons. We also show how Hawking radiation can also be seen to arise from trapping horizons and discuss which horizon area should be associated with the gravitational entropy.

hep-th↗

arXiv2008

Black holes without boundaries

We discuss some of the drawbacks of using event horizons to define black holes and suggest ways in which black holes can be described without event horizons, using trapping horizons. We show that these trapping horizons give rise to thermodynamic behavior and possibly Hawking radiation too. This raises the issue of whether the event horizon or the trapping horizon should be seen as the true boundary of a black hole. This difference is important if we believe that quantum gravity will resolve the central singularity of the black hole and clarifies several of the issues associated with black hole thermodynamics and information loss.

gr-qc↗

arXiv2008

Spherically symmetric trapping horizons, the Misner-Sharp mass and black hole evaporation

Understood in terms of pure states evolving into mixed states, the possibility of information loss in black holes is closely related to the global causal structure of spacetime, as is the existence of event horizons. However, black holes need not be defined by event horizons, and in fact we argue that in order to have a fully unitary evolution for black holes, they should be defined in terms of something else, such as a trapping horizon. The Misner-Sharp mass in spherical symmetry shows very simply how trapping horizons can give rise to black hole thermodynamics, Hawking radiation and singularities. We show how the Misner-Sharp mass can also be used to give insights into the process of collapse and evaporation of locally defined black holes.

gr-qc↗

arXiv2008

Black Holes without Event Horizons

We discuss some of the drawbacks of using event horizons to define black holes. The reasons are both practical, physical and theoretical. We argue that locally defined trapping horizons can remedy many of these drawbacks. We examine of the question of whether black hole thermodynamics should be associated with event horizons or trapping horizons. To this end we discuss what role trapping horizons may play in black hole thermodynamics. In addition, we show how trapping horizons may give rise to Hawking radiation and discuss the issue of gravitational entropy.

gr-qc↗

arXiv2007

Dynamical surface gravity

We discuss how the surface gravity can be classically defined for dynamical black holes. In particular we focus on defining the surface gravity for locally defined horizons and compare a number definitions proposed in the literature. We illustrate the differences between the various proposals in the case of an arbitrary dynamical, spherically symmetric black hole spacetime. We also discuss how the trapping horizon formalism of Hayward can be related to other constructions.

gr-qc↗

arXiv2007

Black holes as local horizons

This talk gives a brief introduction to black hole horizons and their role in black hole thermodynamics. In particular a distinction is made between quasi-locally defined horizons and event horizons. Currently some new techniques have led to interesting developments and the field seems to be growing in two distinct directions. We will show how thermodynamics can equally well be applied to locally defined horizons and discuss some recent results. The emphasis is on giving simple intuitive pictures and mathematical details are largely omitted.

gr-qc↗

arXiv2007

Non-minimally coupled multi-scalar black holes

We study the static, spherically symmetric black hole solutions for a non-minimally coupled multi-scalar theory. We find numerical solutions for values of the scalar fields when a certain constraint on the maximal charge is satisfied. Beyond this constraint no black hole solutions exist. This constraint therefore corresponds to extremal solutions, however, this does not match the κ= 0 constraint which typically indicates extremal solutions in other models. This implies that the set of extremal solutions have non-zero, finite and varying surface gravity. These solutions also violate the no-hair theorems for N>1 scalar fields and have previously been proven to be linearly stable.

gr-qc↗

arXiv2007

Possible polarisation and spin dependent aspects of quantum gravity

We argue that quantum gravity theories that carry a Lie algebraic modification of the Poincare' and Heisenberg algebras inevitably provide inhomogeneities that may serve as seeds for cosmological structure formation. Furthermore, in this class of theories one must expect a strong polarisation and spin dependence of various quantum-gravity effects.

gr-qc↗

arXiv2007

MiniBooNE and a (CP)^2 = -1 sterile neutrino

It has been taken as granted that the observation of two independent mass-squared differences necessarily fixes the number of underlying mass eigenstates as three, and that the addition of a sterile neutrino provides an additional mass-squared difference. The purpose of this Letter is to argue that if one considers a sterile neutrino component that belongs to the (CP)^2 = - 1 sector, then both of the stated claims are false. We also outline how the results reported here, when combined with the forthcoming MiniBooNE data and other experiments, can help settle the issue of the CP properties of the sterile neutrino; if such a component does indeed exist.

hep-ph↗

arXiv2006

Skyrme Black Holes in the Isolated Horizons Formalism

We study static, spherically symmetric, Skyrme black holes in the context of the assumption that they can be viewed as bound states between ordinary bare black holes and solitons. This assumption and results stemming from the isolated horizons formalism lead to several conjectures about the static black hole solutions. These conjectures are tested against the Skyrme black hole solutions. It is shown that, while there is in general good agreement with the conjectures, a crucial aspect seems to violate one of the conjectures.

gr-qc↗

arXiv2006

Hybrid brane worlds in the Salam-Sezgin model

We construct a 6-dimensional warped brane world compactification of the Salam-Sezgin supergravity model by generalizing an earlier hybrid Kaluza-Klein / Randall-Sundrum construction [hep-th/0109099]. In this construction the observed universe is interpreted as a 4-brane in six dimensions, with a Kaluza-Klein spatial direction in addition to the usual three noncompact spatial dimensions. This construction is distinct from other brane world constructions in six dimensions, which introduce the universe as a 3-brane corresponding to a topological defect in six dimensions, or which require a particular configuration of matter fields on the brane. We demonstrate that the model reproduces localized gravity on the brane in the expected form of a Newtonian potential with Yukawa-type corrections. We show that allowed parameter ranges include values which potentially solve the hierarchy problem. An exact nonlinear gravitational wave solution on the background is exhibited. The class of solutions given applies to Ricci-flat geometries in four dimensions, and consequently includes brane world realizations of the Schwarzschild and Kerr black holes as particular examples. Arguments are given which suggest that the hybrid compactification of the Salam-Sezgin model can be extended to reductions to arbitrary Einstein space geometries in four dimensions.

hep-th↗

arXiv2006

Production and decay of evolving horizons

We consider a simple physical model for an evolving horizon that is strongly interacting with its environment, exchanging arbitrarily large quantities of matter with its environment in the form of both infalling material and outgoing Hawking radiation. We permit fluxes of both lightlike and timelike particles to cross the horizon, and ask how the horizon grows and shrinks in response to such flows. We place a premium on providing a clear and straightforward exposition with simple formulae. To be able to handle such a highly dynamical situation in a simple manner we make one significant physical restriction, that of spherical symmetry, and two technical mathematical restrictions: (1) We choose to slice the spacetime in such a way that the space-time foliations (and hence the horizons) are always spherically symmetric. (2) Furthermore we adopt Painleve-Gullstrand coordinates (which are well suited to the problem because they are nonsingular at the horizon) in order to simplify the relevant calculations. We find particularly simple forms for surface gravity, and for the first and second law of black hole thermodynamics, in this general evolving horizon situation. Furthermore we relate our results to Hawking's apparent horizon, Ashtekar et al's isolated and dynamical horizons, and Hayward's trapping horizons. The evolving black hole model discussed here will be of interest, both from an astrophysical viewpoint in terms of discussing growing black holes, and from a purely theoretical viewpoint in discussing black hole evaporation via Hawking radiation.

gr-qc↗