Isolated horizons in higher-dimensional Einstein-Gauss-Bonnet gravity

The isolated horizon framework was introduced in order to provide a local description of black holes that are in equilibrium with their (possibly dynamic) environment. Over the past several years, the framework has been extended to include matter fields (dilaton, Yang-Mills etc) in D=4 dimensions and cosmological constant in $D\geq3$ dimensions. In this article we present a further extension of the framework that includes black holes in higher-dimensional Einstein-Gauss-Bonnet (EGB) gravity. In particular, we construct a covariant phase space for EGB gravity in arbitrary dimensions which allows us to derive the first law. We find that the entropy of a weakly isolated and non-rotating horizon is given by $\mathcal{S}=(1/4G_{D})\oint_{S^{D-2}}\bm{\tilde{\epsilon}}(1+2\alpha\mathcal{R})$. In this expression $S^{D-2}$ is the $(D-2)$-dimensional cross section of the horizon with area form $\bm{\tilde{\epsilon}}$ and Ricci scalar $\mathcal{R}$, $G_{D}$ is the $D$-dimensional Newton constant and $\alpha$ is the Gauss-Bonnet parameter. This expression for the horizon entropy is in agreement with those predicted by the Euclidean and Noether charge methods. Thus we extend the isolated horizon framework beyond Einstein gravity.Comment: 18 pages; 1 figure; v2: 19 pages; 2 references added; v3: 19 pages; minor corrections; 1 reference added; to appear in Classical and Quantum Gravit

Lorentz-violating vs ghost gravitons: the example of Weyl gravity

We show that the ghost degrees of freedom of Einstein gravity with a Weyl term can be eliminated by a simple mechanism that invokes local Lorentz symmetry breaking. We demonstrate how the mechanism works in a cosmological setting. The presence of the Weyl term forces a redefinition of the quantum vacuum state of the tensor perturbations. As a consequence the amplitude of their spectrum blows up when the Lorentz-violating scale becomes comparable to the Hubble radius. Such a behaviour is in sharp contrast to what happens in standard Weyl gravity where the gravitational ghosts smoothly damp out the spectrum of primordial gravitational waves.Comment: 14 pages, 3 figures, REVTeX 4.

Inflation with a Weyl term, or ghosts at work

In order to assess the role of ghosts in cosmology, we study the evolution of linear cosmological perturbations during inflation when a Weyl term is added to the action. Our main result is that vector perturbations can no longer be ignored and that scalar modes diverge in the newtonian gauge but remain bounded in the comoving slicing.Comment: 14 pages, 4 figure

From logic of competition to conflict: understanding the dynamics of EU-Russia relations

To understand the gradual worsening of EU–Russia relations in the decade preceding the Ukraine crisis, it is essential to understand the dynamics of their interaction. This article divides EU–Russia relations into three stages on the basis of changing intergroup dynamics: asymmetrical cooperation (1992–2003), pragmatic but increasing competition (2004–2013) and conflict (2013–present). It draws on the concept of ‘attributional bias’ to explain the escalating logic of competition during the second stage. The EU and Russia started to attribute each other negative geopolitical intentions up to the point where these images became so dominant that they interpreted each other’s behaviour almost exclusively in terms of these images, rather than on the basis of their actual behaviour. With the Ukraine crisis, EU–Russia relations changed from competition over institutional arrangements in the neighbourhood and over normative hegemony to conflict over direct control

Extremal single-charge small black holes: Entropy function analysis

We study stretched horizons of the type AdS_2 x S^8 for certain spherically symmetric extremal small black holes in type IIA carrying only D0-brane charge making use of Sen's entropy function formalism for higher derivative gravity. A scaling argument is given to show that the entropy of this class of black holes for large charge behaves as \sqrt{|q|} where q is the electric charge. The leading order result arises from IIA string loop corrections. We find that for solutions to exist the force on a probe D0-brane has to vanish and we prove that this feature persists to all higher derivative orders. We comment on the nature of the extremum of these solutions and on the sub-leading corrections to the entropy. The entropy of other small black holes related by dualities to our case is also discussed.Comment: 19 pages, v2:typos corrected and references adde

Quasinormal modes for tensor and vector type perturbation of Gauss Bonnet black holes using third order WKB approach

We obtain the quasinormal modes for tensor perturbations of Gauss-Bonnet (GB) black holes in $d=5, 7, 8$ dimensions and vector perturbations in $d = 5, 6, 7$ and 8 dimensions using third order WKB formalism. The tensor perturbation for black holes in $d=6$ is not considered because of the fact that it is unstable to tensor mode perturbations. In the case of uncharged GB black hole, for both tensor and vector perturbations, the real part of the QN frequency increases as the Gauss-Bonnet coupling ($\alpha'$) increases. The imaginary part first decreases upto a certain value of $\alpha'$ and then increases with $\alpha'$ for both tensor and vector perturbations. For larger values of $\alpha'$, the QN frequencies for vector perturbation differs slightly from the QN frequencies for tensorial one. It has also been shown that as $\alpha' \to 0$, the quasinormal mode frequency for tensor and vector perturbation of the Schwarzschild black hole can be obtained. We have also calculated the quasinormal spectrum of the charged GB black hole for tensor perturbations. Here we have found that the real oscillation frequency increases, while the imaginary part of the frequency falls with the increase of the charge. We also show that the quasinormal frequencies for scalar field perturbations and the tensor gravitational perturbations do not match as was claimed in the literature. The difference in the result increases if we increase the GB coupling.Comment: 17 pages, 11 figures, change in title and abstract, new equations and results added for QN frequencies for vector perturbations, new referencees adde

TeV-Scale Black Hole Lifetimes in Extra-Dimensional Lovelock Gravity

We examine the mass loss rates and lifetimes of TeV-scale extra dimensional black holes (BH) in ADD-like models with Lovelock higher-curvature terms present in the action. In particular we focus on the predicted differences between the canonical and microcanonical ensemble statistical mechanics descriptions of the Hawking radiation that results in the decay of these BH. In even numbers of extra dimensions the employment of the microcanonical approach is shown to generally lead to a significant increase in the BH lifetime as in case of the Einstein-Hilbert action. For odd numbers of extra dimensions, stable BH remnants occur when employing either description provided the highest order allowed Lovelock invariant is present. However, in this case, the time dependence of the mass loss rates obtained employing the two approaches will be different. These effects are in principle measurable at future colliders.Comment: 27 pages, 9 figs; Refs. and discussion adde

Infrared effects in inflationary correlation functions

In this article, I briefly review the status of infrared effects which occur when using inflationary models to calculate initial conditions for a subsequent hot, dense plasma phase. Three types of divergence have been identified in the literature: secular, "time-dependent" logarithms, which grow with time spent outside the horizon; "box-cutoff" logarithms, which encode a dependence on the infrared cutoff when calculating in a finite-sized box; and "quantum" logarithms, which depend on the ratio of a scale characterizing new physics to the scale of whatever process is under consideration, and whose interpretation is the same as conventional field theory. I review the calculations in which these divergences appear, and discuss the methods which have been developed to deal with them.Comment: Invited review for focus section of Classical & Quantum Gravity on nonlinear and nongaussian perturbation theory. Some improvements compared to version which will appear in CQG, especially in Sec. 2.3. 30 pages + references

Barbero-Immirzi parameter, manifold invariants and Euclidean path integrals

The Barbero-Immirzi parameter $\gamma$ appears in the \emph{real} connection formulation of gravity in terms of the Ashtekar variables, and gives rise to a one-parameter quantization ambiguity in Loop Quantum Gravity. In this paper we investigate the conditions under which $\gamma$ will have physical effects in Euclidean Quantum Gravity. This is done by constructing a well-defined Euclidean path integral for the Holst action with non-zero cosmological constant on a manifold with boundary. We find that two general conditions must be satisfied by the spacetime manifold in order for the Holst action and its surface integral to be non-zero: (i) the metric has to be non-diagonalizable; (ii) the Pontryagin number of the manifold has to be non-zero. The latter is a strong topological condition, and rules out many of the known solutions to the Einstein field equations. This result leads us to evaluate the on-shell first-order Holst action and corresponding Euclidean partition function on the Taub-NUT-ADS solution. We find that $\gamma$ shows up as a finite rotation of the on-shell partition function which corresponds to shifts in the energy and entropy of the NUT charge. In an appendix we also evaluate the Holst action on the Taub-NUT and Taub-bolt solutions in flat spacetime and find that in that case as well $\gamma$ shows up in the energy and entropy of the NUT and bolt charges. We also present an example whereby the Euler characteristic of the manifold has a non-trivial effect on black-hole mergers.Comment: 18 pages; v2: references added; to appear in Classical and Quantum Gravity; v3: typos corrected; minor revisions to match published versio

Geometrothermodynamics of five dimensional black holes in Einstein-Gauss-Bonnet-theory

We investigate the thermodynamic properties of 5D static and spherically symmetric black holes in (i) Einstein-Maxwell-Gauss-Bonnet theory, (ii) Einstein-Maxwell-Gauss-Bonnet theory with negative cosmological constant, and in (iii) Einstein-Yang-Mills-Gauss-Bonnet theory. To formulate the thermodynamics of these black holes we use the Bekenstein-Hawking entropy relation and, alternatively, a modified entropy formula which follows from the first law of thermodynamics of black holes. The results of both approaches are not equivalent. Using the formalism of geometrothermodynamics, we introduce in the manifold of equilibrium states a Legendre invariant metric for each black hole and for each thermodynamic approach, and show that the thermodynamic curvature diverges at those points where the temperature vanishes and the heat capacity diverges.Comment: New sections added, references adde