35 research outputs found

    Quantum Ergoregion Instability

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    We have shown that, as in the case of black holes, an ergosphere itself with no event horizon inside can evaporate spontaneously, giving energy radiation to spatial infinity until the ergoregion disappears. However, the feature of his quantum ergoregion instability is very much different from black hole radiation. It is rather analogous to a laser amplification. This analysis is based on the canonical quantization of a neutral scalar field in the presence of unstable modes characterized by complex frequencies in a simple model for a rapidly rotating star.Comment: 10 pages, latex, one epsfig, to appear in the Proceedings of the APCTP Winter School on Duality of String Theory, Korea, Feb. 17-28, 1997; a brief version of gr-qc/9701040 with slightly different presentatio

    Evaporation of large black holes in AdS: greybody factor and decay rate

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    We consider a massless, minimally coupled scalar field propagating through the geometry of a black 3-brane in an asymptotically AdS5×S5AdS_5 \times S^5 space. The wave equation for modes traveling purely in the holographic direction reduces to a Heun equation and the corresponding greybody factor is obtained numerically. Approximations valid in the low- and high-frequency regimes are also obtained analytically. The greybody factor is then used to determine the rate of evaporation of these large black holes in the context of the evaporon model proposed in \cite{Rocha:2008fe}. This setting represents the evolution of a black hole under Hawking evaporation with a known CFT dual description and is therefore unitary. Information must then be preserved under this evaporation process.Comment: 20 pages, 2 figures; v2: added references, published versio

    Towers of Gravitational Theories

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    In this essay we introduce a theoretical framework designed to describe black hole dynamics. The difficulties in understanding such dynamics stems from the proliferation of scales involved when one attempts to simultaneously describe all of the relevant dynamical degrees of freedom. These range from the modes that describe the black hole horizon, which are responsible for dissipative effects, to the long wavelength gravitational radiation that drains mechanical energy from macroscopic black hole bound states. We approach the problem from a Wilsonian point of view, by building a tower of theories of gravity each of which is valid at different scales. The methodology leads to multiple new results in diverse topics including phase transitions of Kaluza-Klein black holes and the interactions of spinning black hole in non-relativistic orbits. Moreover, our methods tie together speculative ideas regarding dualities for black hole horizons to real physical measurements in gravitational wave detectors.Comment: Awarded second prize for 2006 Gravity Research Foundation essay contes

    Charge and mass effects on the evaporation of higher-dimensional rotating black holes

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    To study the dynamics of discharge of a brane black hole in TeV gravity scenarios, we obtain the approximate electromagnetic field due to the charged black hole, by solving Maxwell's equations perturbatively on the brane. In addition, arguments are given for brane metric corrections due to backreaction. We couple brane scalar and brane fermion fields with non-zero mass and charge to the background, and study the Hawking radiation process using well known low energy approximations as well as a WKB approximation in the high energy limit. We argue that contrary to common claims, the initial evaporation is not dominated by fast Schwinger discharge.Comment: Published version. Minor typos corrected. 29 pages, 5 figure

    Damping of Tensor Modes in Cosmology

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    An analytic formula is given for the traceless transverse part of the anisotropic stress tensor due to free streaming neutrinos, and used to derive an integro-differential equation for the propagation of cosmological gravitational waves. The solution shows that anisotropic stress reduces the squared amplitude by 35.6 % for wavelengths that enter the horizon during the radiation-dominated phase, independent of any cosmological parameters. This decreases the tensor temperature and polarization correlation functions for these wavelengths by the same amount. The effect is less for wavelengths that enter the horizon at later times. At the longest wavelengths the decrease in the tensor correlation functions due to neutrino free streaming ranges from 10.7% for ΩMh2=0.1\Omega_Mh^2=0.1 to 9.0% for ΩMh2=0.15\Omega_Mh^2=0.15. An Appendix gives a general proof that tensor as well as scalar modes satisfy a conservation law for perturbations outside the horizon, even when the anisotropic stress tensor is not negligible.Comment: 14 pages. The original version of this paper has been expanded to deal with perturbations of any wavelength. While for wavelengths short enough to enter the horizon during radiation dominance, temperature and polarization correlations are damped by 35.6%, at the longest wavelengths the damping is from 9.0% to 11%. An added Appendix gives a general proof that tensor as well as scalar modes satisfy a conservation law outside the horizon, even during neutrino decoupling. Some references are also adde

    Quantum Vacuum Instability Near Rotating Stars

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    We discuss the Starobinskii-Unruh process for the Kerr black hole. We show how this effect is related to the theory of squeezed states. We then consider a simple model for a highly relativistic rotating star and show that the Starobinskii-Unruh effect is absent.Comment: 17 Pages, (accepted by PRD), (previously incorrect header files have been corrected

    The Primordial Gravitational Wave Background in String Cosmology

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    We find the spectrum P(w)dw of the gravitational wave background produced in the early universe in string theory. We work in the framework of String Driven Cosmology, whose scale factors are computed with the low-energy effective string equations as well as selfconsistent solutions of General Relativity with a gas of strings as source. The scale factor evolution is described by an early string driven inflationary stage with an instantaneous transition to a radiation dominated stage and successive matter dominated stage. This is an expanding string cosmology always running on positive proper cosmic time. A careful treatment of the scale factor evolution and involved transitions is made. A full prediction on the power spectrum of gravitational waves without any free-parameters is given. We study and show explicitly the effect of the dilaton field, characteristic to this kind of cosmologies. We compute the spectrum for the same evolution description with three differents approachs. Some features of gravitational wave spectra, as peaks and asymptotic behaviours, are found direct consequences of the dilaton involved and not only of the scale factor evolution. A comparative analysis of different treatments, solutions and compatibility with observational bounds or detection perspectives is made.Comment: LaTeX, 50 pages with 2 figures. Uses epsfig and psfra

    One loop renormalization of the four-dimensional theory for quantum dilaton gravity.

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    We study the one loop renormalization in the most general metric-dilaton theory with the second derivative terms only. The general theory can be divided into two classes, models of one are equivalent to conformally coupled with gravity scalar field and also to general relativity with cosmological term. The models of second class have one extra degree of freedom which corresponds to dilaton. We calculate the one loop divergences for the models of second class and find that the arbitrary functions of dilaton in the starting action can be fine-tuned in such a manner that all the higher derivative counterterms disappear on shell. The only structures in both classical action and counterterms, which survive on shell, are the potential (cosmological) ones. They can be removed by renormalization of the dilaton field which acquire the nontrivial anomalous dimension, that leads to the effective running of the cosmological constant. For some of the renormalizable solutions of the theory the observable low energy value of the cosmological constant is small as compared with the Newtonian constant. We also discuss another application of our result.Comment: 21 pages, latex, no figures

    CBR Anisotropy and the Running of the Scalar Spectral Index

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    Accurate (â‰Č1%\lesssim 1\% ) predictions for the anisotropy of the Cosmic Background Radiation (CBR) are essential for using future high-resolution (â‰Č1∘\lesssim 1^\circ) CBR maps to test cosmological models. In many inflationary models the variation (``running'') of the spectral index of the spectrum of density perturbations is a significant effect and leads to changes of around 1\% to 10\% in the CBR power spectrum. We propose a general method for taking running into account which uses the derivative of the spectral index (dn/dln⁥kdn/d\ln k). Conversely, high-resolution CBR maps may be able to determine dn/dln⁥kdn/d\ln k, giving important information about the inflationary potential.Comment: Discussion of calculation clarified; error corrected which reduces estimated effect for chaotic inflatio

    Curing singularities in cosmological evolution of F(R) gravity

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    We study F(R)F(R) modified gravity models which are capable of driving the accelerating epoch of the Universe at the present time whilst not destroying the standard Big Bang and inflationary cosmology. Recent studies have shown that a weak curvature singularity with ∣R∣→∞|R|\to\infty can arise generically in viable F(R)F(R) models of present dark energy (DE) signaling an internal incompleteness of these models. In this work we study how this problem is cured by adding a quadratic correction with a sufficiently small coefficient to the F(R)F(R) function at large curvatures. At the same time, this correction eliminates two more serious problems of previously constructed viable F(R)F(R) DE models: unboundedness of the mass of a scalar particle (scalaron) arising in F(R)F(R) gravity and the scalaron overabundance problem. Such carefully constructed models can also yield both an early time inflationary epoch and a late time de Sitter phase with vastly different values of RR. The reheating epoch in these combined models of primordial and present dark energy is completely different from that of the old R+R2/6M2R + R^{2}/6M^{2} inflationary model, mainly due to the fact that values of the effective gravitational constant at low and intermediate curvatures are different for positive and negative RR. This changes the number of e-folds during the observable part of inflation that results in a different value of the primordial power spectrum index.Comment: Discussion expanded, references added, results unchanged, accepted for publication in JCAP. A minor typo in Eq. (2.14) has been correcte
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