Results for "Roy Maartens"

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Brane-world cosmological perturbationsFeb 20 2004Brane-world models provide a phenomenology that allows us to explore the cosmological consequences of some M theory ideas, and at the same time to use precision cosmology as a test of these ideas. In order to achieve this, we need to understand how brane-world ... More
Causal Thermodynamics in RelativitySep 18 1996I review the causal relativistic thermodynamics developed by Israel and Stewart, and discuss some applications in cosmology and astrophysics. The lectures begin with an overview of relativistic fluid dynamics (in a covariant formalism) and equilibrium ... More
Dark Energy from Brane-world GravityFeb 18 2006Recent observations provide strong evidence that the universe is accelerating. This confronts theory with a severe challenge. Explanations of the acceleration within the framework of general relativity are plagued by difficulties. General relativistic ... More
Geometry and dynamics of the brane-worldJan 16 2001Apr 17 2001Recent developments in string theory have led to 5-dimensional warped spacetime models in which standard-model fields are confined to a 3-brane (the observed universe), while gravity can propagate in the fifth dimension. Gravity is localized near the ... More
Cosmological dynamics on the braneApr 24 2000Aug 08 2000In Randall-Sundrum-type brane-world cosmologies, the dynamical equations on the three-brane differ from the general relativity equations by terms that carry the effects of imbedding and of the free gravitational field in the five-dimensional bulk. We ... More
Nonlinear Effects in the Cosmic Microwave BackgroundJul 21 1999Major advances in the observation and theory of cosmic microwave background anisotropies have opened up a new era in cosmology. This has encouraged the hope that the fundamental parameters of cosmology will be determined to high accuracy in the near future. ... More
Density perturbations with relativistic thermodynamicsJul 08 1997Sep 12 1997We investigate cosmological density perturbations in a covariant and gauge-invariant formalism, incorporating relativistic causal thermodynamics to give a self-consistent description. The gradient of density inhomogeneities splits covariantly into a scalar ... More
Magnetized cosmological perturbationsApr 28 1999Mar 31 2000A large-scale cosmic magnetic field affects not only the growth of density perturbations, but also rotational instabilities and anisotropic deformation in the density distribution. We give a fully relativistic treatment of all these effects, incorporating ... More
Measuring ISW with next-generation radio surveysDec 04 2018Feb 13 2019The late-time integrated Sachs-Wolfe (ISW) signal in the CMB temperature anisotropies is an important probe of dark energy when it can be detected by cross-correlation with large-scale structure surveys. Because of their huge sky area, surveys in the ... More
Quadrupole anisotropy from photon quantum effectsNov 15 1995We consider quantum effects of an electromagnetic field in a radiation-dominated almost FRW spacetime. The dominant non-local quantum correction to the photon distribution is a quadrupole moment, corresponding to an effective anisotropic pressure in the ... More
Brillouin scattering and the CMBMay 18 2001Jan 13 2005Brillouin scattering of photons off the density fluctuations in a fluid is potentially important for cosmology. We derive the Brillouin spectral distortion of blackbody radiation, and discuss the possible implications for the cosmic microwave background. ... More
Cosmic microwave background anisotropies: Nonlinear dynamicsAug 16 1998Feb 18 1999We develop a new approach to local nonlinear effects in cosmic microwave background anisotropies, and discuss the qualitative features of these effects. New couplings of the baryonic velocity to radiation multipoles are found, arising from nonlinear Thomson ... More
Synergies between intensity maps of hydrogen linesMar 19 2018Jun 26 2018We study synergies between HI 21cm and H$\alpha$ intensity map observations, focusing on SKA1-like and SPHEREx-like surveys. We forecast how well such a combination can measure features in the angular power spectrum on the largest scales, that arise from ... More
Probing primordial non-Gaussianity with SKA galaxy redshift surveys: a fully relativistic analysisSep 29 2014Dec 02 2016The Square Kilometre Array (SKA) will produce spectroscopic surveys of tens to hundreds of millions of HI galaxies, eventually covering 30,000 sq. deg. and reaching out to redshift z~2. The huge volumes probed by the SKA will allow for some of the best ... More
Nonlinear relativistic corrections to cosmological distances, redshift and gravitational lensing magnification. II - DerivationFeb 09 2014Sep 22 2014We present a derivation of the cosmological distance-redshift relation up to second order in perturbation theory. In addition, we find the observed redshift and the lensing magnification to second order. We do not require that the density contrast is ... More
Nonlinear relativistic corrections to cosmological distances, redshift and gravitational lensing magnification. I - Key resultsJul 09 2012Sep 22 2014The next generation of telescopes will usher in an era of precision cosmology, capable of determining the cosmological model to beyond the percent level. For this to be effective, the theoretical model must be understood to at least the same level of ... More
Nonlinear modulation of the HI power spectrum on ultra-large scales. ISep 12 2015Apr 04 2016Intensity mapping of the neutral hydrogen brightness temperature promises to provide a three-dimensional view of the universe on very large scales. Nonlinear effects are typically thought to alter only the small-scale power, but we show how they may bias ... More
Gravitational waves from inflation on the braneJun 01 2000Aug 31 2000We discuss the evolution of gravitational waves in a brane-world cosmology embedded in five-dimensional anti-de Sitter spacetime. We show that during slow-roll inflation, modelled as a period of quasi-de Sitter expansion on the brane, there is a discrete ... More
Degeneracy between primordial non-Gaussianity and interaction in the dark sectorSep 17 2014Oct 24 2014If dark energy and dark matter interact via exchange of energy and momentum, then this may affect the galaxy power spectrum on large scales. When this happens, it may be degenerate with the signal from primordial non-Gaussianity via scale-dependent bias. ... More
Inflation driven by causal heat fluxOct 31 1997We find a simple inflationary solution in an inhomogeneous spacetime with heat flux. The heat flux obeys a causal transport equation, and counteracts the inflationary decrease of energy density. At late times, the heat flux tends to zero and the fluid ... More
Brane-world gravityDec 10 2003Apr 29 2004The observable universe could be a 1+3-surface (the "brane") embedded in a 1+3+d-dimensional spacetime (the "bulk"), with standard-model particles and fields trapped on the brane while gravity is free to access the bulk. At least one of the d extra spatial ... More
Testing the Cosmological Principle in the radio skyMay 29 2019Aug 26 2019The Cosmological Principle states that the Universe is statistically isotropic and homogeneous on large scales. In particular, this implies statistical isotropy in the galaxy distribution, after removal of a dipole anisotropy due to the observer's motion. ... More
Cosmological magnetic fieldsJul 24 2000Magnetic fields are observed not only in stars, but in galaxies, clusters, and even high redshift Lyman-alpha systems. In principle, these fields could play an important role in structure formation and also affect the anisotropies in the cosmic microwave ... More
Restoring the sting to metric preheatingSep 22 1999Feb 04 2000The relative growth of field and metric perturbations during preheating is sensitive to initial conditions set in the preceding inflationary phase. Recent work suggests this may protect super-Hubble metric perturbations from resonant amplification during ... More
Dynamics of interacting dark energyDec 10 2008Mar 20 2009Dark energy and dark matter are only indirectly measured via their gravitational effects. It is possible that there is an exchange of energy within the dark sector, and this offers an interesting alternative approach to the coincidence problem. We consider ... More
The cosmological background of vector modesDec 07 2008Feb 19 2009We investigate the spectrum of vector modes today which is generated at second order by density perturbations. The vector mode background that is generated by structure formation is small but in principle it contributes to the integrated Sachs-Wolfe effect, ... More
LIMITS ON ANISOTROPY AND INHOMOGENEITY FROM THE COSMIC BACKGROUND RADIATION,Jan 06 1995We consider directly the equations by which matter imposes anisotropies on freely propagating background radiation, leading to a new way of using anisotropy measurements to limit the deviations of the Universe from a Friedmann-Robertson-Walker (FRW) geometry. ... More
Covariant velocity and density perturbations in quasi-Newtonian cosmologiesAug 21 1998Recently a covariant approach to cold matter universes in the zero-shear hypersurfaces (or longitudinal) gauge has been developed. This approach reveals the existence of an integrability condition, which does not appear in standard non-covariant treatments. ... More
Is the Universe homogeneous?Apr 07 2011Oct 06 2011The standard model of cosmology is based on the existence of homogeneous surfaces as the background arena for structure formation. Homogeneity underpins both general relativistic and modified gravity models and is central to the way in which we interpret ... More
Brane-world cosmological perturbations: a covariant approachApr 23 2003The standard cosmological model, based on general relativity with an inflationary era, is very effective in accounting for a broad range of observed features of the universe. However, the ongoing puzzles about the nature of dark matter and dark energy, ... More
Linearisation instability of gravity waves?Sep 30 1996Gravity waves in irrotational dust spacetimes are characterised by nonzero magnetic Weyl tensor $H_{ab}$. In the linearised theory, the divergence of $H_{ab}$ is set to zero. Recently Lesame et al. [Phys. Rev. D {\bf 53}, 738 (1996)] presented an argument ... More
Exact non-equilibrium solutions of the Einstein-Boltzmann equations. IINov 14 1996We find exact solutions of the Einstein-Boltzmann equations with relaxational collision term in FRW and Bianchi I spacetimes. The kinematic and thermodynamic properties of the solutions are investigated. We give an exact expression for the bulk viscous ... More
Brane-World GravityApr 22 2010Sep 09 2010The observable universe could be a 1+3-surface (the "brane") embedded in a 1+3+\textit{d}-dimensional spacetime (the "bulk"), with Standard Model particles and fields trapped on the brane while gravity is free to access the bulk. At least one of the \textit{d} ... More
Gravitational waves in preheatingFeb 25 2000We study the evolution of gravitational waves through the preheating era that follows inflation. The oscillating inflaton drives parametric resonant growth of scalar field fluctuations, and although super-Hubble tensor modes are not strongly amplified, ... More
Anisotropic universes with conformal motionFeb 09 1996By imposing natural geometrical and kinematical conditions on a conformal Killing vector in Bianchi I spacetime, we show that a class of axisymmetric metrics admits a conformal motion. This class contains new exact solutions of Einstein's equations, including ... More
Vorticity affects the stability of neutron starsSep 22 1999Dec 14 1999The spin rate \Omega of neutron stars at a given temperature T is constrained by the interplay between gravitational-radiation instabilities and viscous damping. Navier-Stokes theory has been used to calculate the viscous damping timescales and produce ... More
Imprints of local lightcone projection effects on the galaxy bispectrum IV: Second-order vector and tensor contributionsNov 13 2018Feb 19 2019The galaxy bispectrum on scales around and above the equality scale receives contributions from relativistic effects. Some of these arise from lightcone deformation effects, which come from local and line-of-sight integrated contributions. Here we calculate ... More
Cosmological perturbations in Horava-Lifshitz theory without detailed balanceJul 10 2009Jan 14 2010In the Horava-Lifshitz theory of quantum gravity, two conditions -- detailed balance and projectability -- are usually assumed. The breaking of projectability simplifies the theory, but it leads to serious problems with the theory. The breaking of detailed ... More
Structure formation in the DGP cosmological modelNov 22 2005The DGP brane-world model provides an alternative to the standard LCDM cosmology, in which the late universe accelerates due to a modification of gravity rather than vacuum energy. The cosmological constant $\Lambda$ in LCDM is replaced by a single parameter, ... More
Perturbations in inflationary cosmologies with smooth exitJul 27 1998We give a comprehensive analysis of how scalar and tensor perturbations evolve in cosmologies with a smooth transition from power-law-like and de Sitter-like inflation to a radiation era. Analytic forms for the super-horizon and sub-horizon perturbations ... More
Exact Perturbations for inflation with smooth exitJul 15 1997Apr 09 1998Toy models for the Hubble rate or the scalar field potential have been used to analyze the amplification of scalar perturbations through a smooth transition from inflation to the radiation era. We use a Hubble rate that arises consistently from a decaying ... More
Stars in the braneworldJul 02 2001Oct 18 2001We show that in a Randall-Sundrum II type braneworld, the vacuum exterior of a spherical star is not in general a Schwarzschild spacetime, but has radiative-type stresses induced by 5-dimensional graviton effects. Standard matching conditions do not lead ... More
Anisotropic stresses in inhomogeneous universesAug 24 1998Anisotropic stress contributions to the gravitational field can arise from magnetic fields, collisionless relativistic particles, hydrodynamic shear viscosity, gravitational waves, skew axion fields in low-energy string cosmologies, or topological defects. ... More
Dark Energy and Modified GravityNov 25 2008Explanations of the late-time cosmic acceleration within the framework of general relativity are plagued by difficulties. General relativistic models are mostly based on a dark energy field with fine-tuned, unnatural properties. There is a great variety ... More
Magnetic Bianchi I Universe in Loop Quantum CosmologyDec 10 2008We examine the dynamical consequences of homogeneous cosmological magnetic fields in the framework of loop quantum cosmology. We show that a big-bounce occurs in a collapsing magnetized Bianchi I universe, thus extending the known cases of singularity-avoidance. ... More
Asymmetric brane-worlds with induced gravityNov 19 2004Jan 21 2005The Randall-Sundrum scenario, with a 1+3-dimensional brane in a 5-dimensional bulk spacetime, can be generalized in various ways. We consider the case where the Z2-symmetry at the brane is relaxed, and in addition the gravitational action is generalized ... More
Quintessence with quadratic coupling to dark matterNov 16 2009We introduce a new form of coupling between dark energy and dark matter that is quadratic in their energy densities. Then we investigate the background dynamics when dark energy is in the form of exponential quintessence. The three types of quadratic ... More
The Emergent Universe: inflationary cosmology with no singularityNov 25 2002Oct 25 2003Observations indicate that the universe is effectively flat, but they do not rule out a closed universe. The role of positive curvature is negligible at late times, but can be crucial in the early universe. In particular, positive curvature allows for ... More
Acoustic oscillations and viscosityFeb 06 1998Jul 08 1998Using a simple thermo-hydrodynamic model that respects relativistic causality, we revisit the analysis of qualitative features of acoustic oscillations in the photon-baryon fluid. The growing photon mean free path introduces transient effects that can ... More
Relativistic corrections and non-Gaussianity in radio continuum surveysJun 04 2012Jan 24 2013Forthcoming radio continuum surveys will cover large volumes of the observable Universe and will reach to high redshifts, making them potentially powerful probes of dark energy, modified gravity and non-Gaussianity. Here we extend recent works by analyzing ... More
Cosmology with Doppler LensingJan 15 2014Jul 08 2014Doppler lensing is the apparent change in object size and magnitude due to peculiar velocities. Objects falling into an overdensity appear larger on its near side, and smaller on its far side, than typical objects at the same redshifts. This effect dominates ... More
Gravity-wave detectors as probes of extra dimensionsMay 13 2005If string theory is correct, then our observable Universe may be a 3-dimensional "brane" embedded in a higher-dimensional spacetime. This theoretical scenario should be tested via the state-of-the-art in gravitational experiments -- the current and upcoming ... More
Cosmological perturbations on a magnetised Bianchi I backgroundDec 10 1999May 24 2000Motivated by the isotropy of the CMB spectrum, all existing studies of magnetised cosmological perturbations employ FRW backgrounds. However, it is important, to know the limits of this approximation and the effects one loses by neglecting the anisotropy ... More
Brane-world generalizations of the Einstein static universeMay 17 2001Dec 05 2001A static Friedmann brane in a 5-dimensional bulk (Randall-Sundrum type scenario) can have a very different relation between the density, pressure, curvature and cosmological constant than in the case of the general relativistic Einstein static universe. ... More
Symmetries of particle motionDec 11 1997We define affine transport lifts on the tangent bundle by associating a transport rule for tangent vectors with a vector field on the base manifold. The aim is to develop tools for the study of kinetic/ dynamical symmetries in relativistic particle motion. ... More
Observational constraints on self-accelerating cosmologyMar 14 2006Oct 07 2006The DGP brane-world model provides a simple alternative to the standard LCDM cosmology, with the same number of parameters. There is no dark energy - the late universe self-accelerates due to an infrared modification of gravity. We compute the joint constraints ... More
Backreaction effects of dissipation in neutrino decouplingJan 15 1999Oct 07 2000Dissipative effects during neutrino decoupling in the early universe create a small backreaction on the Hubble rate, and lead to a small rise in temperature and entropy. We use a simplified thermo-hydrodynamic model, which provides a causal approximation ... More
Ultra large-scale cosmology in next-generation experiments with single tracersMay 28 2015Dec 07 2015Future surveys of large-scale structure will be able to measure perturbations on the scale of the cosmological horizon, and so could potentially probe a number of novel relativistic effects that are negligibly small on sub-horizon scales. These effects ... More
Inhomogeneity and the foundations of concordance cosmologyMay 12 2010Jun 01 2010The apparent accelerating expansion of the Universe is forcing us to examine the foundational aspects of the standard model of cosmology -- in particular, the fact that dark energy is a direct consequence of the homogeneity assumption. We discuss the ... More
Dark Energy and Dark GravityNov 01 2007Dec 05 2007Observations provide increasingly strong evidence that the universe is accelerating. This revolutionary advance in cosmological observations confronts theoretical cosmology with a tremendous challenge, which it has so far failed to meet. Explanations ... More
Density perturbations in the brane-worldSep 01 2000Nov 24 2000In Randall-Sundrum-type brane-world cosmologies, density perturbations generate Weyl curvature in the bulk, which in turn backreacts on the brane via stress-energy perturbations. On large scales, the perturbation equations contain a closed system on the ... More
Nonlinear bulk viscosity and inflationNov 25 1996We develop a nonlinear generalisation of the causal linear thermodynamics of bulk viscosity, incorporating positivity of the entropy production rate and the effective specific entropy. The theory is applied to viscous fluid inflation (which is necessarily ... More
Testing General Relativity with the Doppler magnification effectOct 30 2018Jul 09 2019The apparent sizes and brightnesses of galaxies are correlated in a dipolar pattern around matter overdensities in redshift space, appearing larger on their near side and smaller on their far side. The opposite effect occurs for galaxies around an underdense ... More
Testing the standard model of cosmology with the SKA: the cosmic radio dipoleOct 11 2018Mar 21 2019The dipole anisotropy seen in the {cosmic microwave background radiation} is interpreted as due to our peculiar motion. The Cosmological Principle implies that this cosmic dipole signal should also be present, with the same direction, in the large-scale ... More
Gravito-electromagnetismApr 22 1997Jan 28 1998We develop and apply a fully covariant 1+3 electromagnetic analogy for gravity. The free gravitational field is covariantly characterized by the Weyl gravito-electric and gravito-magnetic spatial tensor fields, whose dynamical equations are the Bianchi ... More
Almost-homogeneity of the universe in higher-order gravityNov 15 1995In the $R+\alpha R^2$ gravity theory, we show that if freely propagating massless particles have an almost isotropic distribution, then the spacetime is almost Friedmann-Robertson-Walker (FRW). This extends the result proved recently in general relativity ... More
Kaluza-Klein anisotropy in the CMBAug 29 2001Jan 12 2002We show that 5-dimensional Kaluza-Klein graviton stresses can slow the decay of shear anisotropy on the brane to observable levels, and we use cosmic microwave background anisotropies to place limits on the initial anisotropy induced by these stresses. ... More
On the phase diagram of quenched QCD with Wilson fermionsOct 20 2003In this talk, I reported on recent work on the Aoki phase diagram for quenched QCD with two flavors of Wilson fermions. Part of this work was done in collaboration with Yigal Shamir, and a shorter account of this part appeared recently. In this write-up, ... More
Lattice Chiral Gauge TheoriesNov 03 2000I review the substantial progress which has been made recently with the non-perturbative construction of chiral gauge theories on the lattice. In particular, I discuss three different approaches: a gauge invariant method using fermions which satisfy the ... More
On tadpole improvement for staggered fermionsSep 16 1998An explanation is proposed for the fact that Lepage--Mackenzie tadpole improvement does not work well for staggered fermions. The idea appears to work for all renormalization constants which appear in the staggered fermion self-energy. Wilson fermions ... More
Luminosities and Space Densities of Gamma-Ray BurstsAug 18 1999We use a homogeneous sample of gamma-ray bursts (GRB) extracted from 5.9 years of BATSE DISCLA data (astro-ph/9908190) and a variety of broken power-law luminosity functions to derive GRB luminosities and space densities. Luminosity functions that are ... More
Chiral Perturbation Theory, Non-leptonic Kaon Decays, and the LatticeNov 06 2000In this talk, I first motivate the use of Chiral Perturbation Theory in the context of Lattice QCD. In particular, I explain how partially quenched QCD, which has, in general, unequal valence- and sea-quark masses, can be used to obtain real-world (i.e. ... More
Quark pair simulation near threshold : Reweighting and hadronizationNov 16 2001Methods are presented allowing a realistic, experiment-based simulation of low-mass quark pairs coupling to a virtual photon. Differential cross-section reweighting factors allow to reproduce the resonant structure observed near quark pair production ... More
$H_D$-Quantum Vertex AlgebrasJun 16 2008We discuss a class of quantum vertex algebras where not only the commutativity of the vertex algebra is broken by a braiding map $S^{(\tau)}$, but also the translation covariance is broken by a translation map $S^{(\gamma)}$. The new class of quantum ... More
Topological K-theory of affine Hecke algebrasOct 21 2016Let H(R,q) be an affine Hecke algebra with a positive parameter function q. We are interested in the topological K-theory of H(R,q), that is, the K-theory of its C*-completion C*_r (R,q). We will prove that $K_* (C*_r (R,q))$ does not depend on the parameter ... More
Ekedahl-Oort strata in the supersingular locusFeb 27 2008Aug 19 2008We give a description of the individual Ekedahl-Oort strata contained in the supersingular locus in terms of Deligne-Lusztig varieties, refining a result of Harashita.
Parabolically induced representations of graded Hecke algebrasApr 02 2008Oct 16 2009We study the representation theory of graded Hecke algebras, starting from scratch and focusing on representations that are obtained with induction from a discrete series representation of a parabolic subalgebra. We determine all intertwining operators ... More
Exact potential-density pairs for flattened dark haloesOct 30 2008Cosmological simulations suggest that dark matter haloes are not spherical, but typically moderately to strongly triaxial systems. We investigate methods to convert spherical potential-density pairs into axisymmetric ones, in which the basic characteristics ... More
Black hole mass measurements using ionized gas discs: systematic dust effectsSep 08 2008Using detailed Monte Carlo radiative transfer simulations in realistic models for galactic nuclei, we investigate the influence of interstellar dust in ionized gas discs on the rotation curves and the resulting black hole mass measurements. We find that ... More
Gamma-Ray Burst Luminosity Functions Based On a Newly Discovered Correlation Between Peak Spectral Energy and V/V_maxMay 18 2009Aug 06 2009We have discovered a correlation between the observed peak spectral energy E_pk,obs and the average Euclidean value of V/V_max of gamma-ray bursts (GRBs). We present the evidence for the correlation in the GUSBAD catalog and use it to derive the luminosity ... More
Are Durations of Weak Gamma-Ray Bursts Reliable?Aug 16 2005Simulations in the GUSBAD Catalog of gamma-ray bursts suggest that the apparent duration of a burst decreases as its amplitude is decreased. We see no evidence for this effect in the BATSE catalog. We show that for a burst at the detection limit, the ... More
Hochschild homology of affine Hecke algebrasAug 26 2011Nov 13 2012Let H = H (R,q) be an affine Hecke algebra with complex, possibly unequal parameters q, which are not roots of unity. We compute the Hochschild and the cyclic homology of H. It turns out that these are independent of q and that they admit an easy description ... More
Magnetized gravitational wavesJan 10 2001May 19 2001We investigate the influence of cosmic magnetic fields on gravitational wave perturbations, and find exact solutions on large scales. We show that a large-scale magnetic field can generate large-scale non-decaying gravitational waves. In the general case ... More
Gravitational waves from brane-world inflation with induced gravityJul 19 2004We calculate the amplitude of gravitational waves produced by inflation on a de Sitter brane embedded in five-dimensional anti-de Sitter bulk spacetime, extending previous calculations in Randall-Sundrum type cosmology to include the effect of induced ... More
Holonomy and gravitomagnetismApr 17 2001Nov 27 2001We analyze parallel transport of a vector field around an equatorial orbit in Kerr and stationary axisymmetric spacetimes that are reflection symmetric about their equatorial planes. As in Schwarzschild spacetime, there is a band structure of holonomy ... More
Anisotropic Observations in Universes with Nonlinear InhomogeneityFeb 07 1996We calculate the off--center observational relations in a spherically symmetric dust universe that is inhomogeneous at small redshifts. In contrast to the usual model, in which the CMBR dipole is interpreted as a Doppler effect due to peculiar velocity, ... More
Delocalization of brane gravity by a bulk black holeApr 06 2005We investigate the analogue of the Randall-Sundrum brane-world in the case when the bulk contains a black hole. Instead of the static vacuum Minkowski brane of the RS model, we have an Einstein static vacuum brane. We find that the presence of the bulk ... More
Detecting extra dimensions with gravity wave spectroscopy: the black string brane-worldAug 11 2004Feb 28 2005Using the black string between two branes as a model of a brane-world black hole, we compute the gravity wave perturbations and identify the features arising from the additional polarizations of the graviton. The standard four-dimensional gravitational ... More
Optimal constraints on the growth rate using the observed galaxy power spectrumJul 05 2019The large-scale structure growth index $\gamma$ provides a consistency test of the standard cosmology and is a potential indicator of modified gravity. We investigate the constraints on $\gamma$ from next-generation spectroscopic surveys, using the power ... More
Can primordial magnetic fields be the origin of the BICEP2 data?Mar 26 2014Apr 28 2014If the B-mode signal in the CMB polarization seen by the BICEP2 experiment is confirmed, it has dramatic implications for models of inflation. The result is also in tension with Planck limits on standard inflationary models. It is therefore important ... More
A causal model of radiating stellar collapseOct 31 1997We find a simple exact model of radiating stellar collapse, with a shear-free and non-accelerating interior matched to a Vaidya exterior. The heat flux is subject to causal thermodynamics, leading to self-consistent determination of the temperature $T$. ... More
Causality and the speed of soundMar 25 2007May 29 2007A usual causal requirement on a viable theory of matter is that the speed of sound be at most the speed of light. In view of various recent papers querying this limit, the question is revisited here. We point to various issues confronting theories that ... More
Exact isotropic cosmologies with local fractal number countsApr 08 1998Jul 30 1998We construct an exact relativistic cosmology in which an inhomogeneous but isotropic local region has fractal number counts and matches to a homogeneous background at a scale of the order of $10^2$ Mpc. We show that Einstein's equations and the matching ... More
Anisotropy dissipation in brane-world inflationNov 29 2000Feb 06 2001We examine the behavior of an anisotropic brane-world in the presence of inflationary scalar fields. We show that, contrary to naive expectations, a large anisotropy does not adversely affect inflation. On the contrary, a large initial anisotropy introduces ... More
Probing primordial non-Gaussianity with SKA galaxy redshift surveys: a fully relativistic analysisSep 29 2014Apr 23 2015The Square Kilometre Array (SKA) will produce spectroscopic surveys of tens to hundreds of millions of HI galaxies, eventually covering 30,000 sq. deg. and reaching out to redshift z~2. The huge volumes probed by the SKA will allow for some of the best ... More
Clustering of quintessence on horizon scales and its imprint on HI intensity mappingMay 20 2013May 05 2014Quintessence can cluster only on horizon scales. What is the effect on the observed matter distribution? To answer this, we need a relativistic approach that goes beyond the standard Newtonian calculation and deals properly with large scales. Such an ... More
Probing the imprint of interacting dark energy on very large scalesFeb 23 2015Mar 07 2015The observed galaxy power spectrum acquires relativistic corrections from lightcone effects, and these corrections grow on very large scales. Future galaxy surveys in optical, infrared and radio bands will probe increasingly large wavelength modes and ... More
Non-commutative inflation and the CMBAug 11 2003Sep 08 2003Non-commutative inflation is a modification of standard general relativity inflation which takes into account some effects of the space-time uncertainty principle motivated by ideas from string theory. The corrections to the primordial power spectrum ... More
Gravitational collapse on the brane: a no-go theoremAug 03 2001Oct 26 2001We investigate how braneworld gravity affects gravitational collapse and black hole formation by studying Oppenheimer-Snyder-like collapse on a Randall-Sundrum type brane. Without making any assumptions about the bulk, we prove a no-go theorem: the exterior ... More
Scalar field perturbations in Horava-Lifshitz cosmologySep 28 2009Apr 23 2010We study perturbations of a scalar field cosmology in Horava-Lifshitz gravity, adopting the most general setup without detailed balance but with the projectability condition. We derive the generalized Klein-Gordon equation, which is sixth-order in spatial ... More