Quintessence with quadratic coupling to dark matter (2009)
Boehmer, Christian G., Caldera-Cabral, Gabriela, Chan, Nyein, Lazkoz, Ruth, Maartens, Roy
We 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...
Scalar field perturbations in Horava-Lifshitz cosmology (2009)
Wang, Anzhong, Wands, David, Maartens, Roy
In this paper we study perturbations of a scalar field cosmology in Horava-Lifshitz gravity, adopting the Sotiriou-Visser-Weifurtner generalization, which is the most general setup without detailed...
Adiabatic initial conditions for perturbations in interacting dark energy models (2009)
Majerotto, Elisabetta, Valiviita, Jussi, Maartens, Roy
We present a new systematic analysis of the early radiation era solution in an interacting dark energy model to find the adiabatic initial conditions for the Boltzmann integration. Adiabatic initial...
Observational constraints on an interacting dark energy model (2009)
Valiviita, Jussi, Maartens, Roy, Majerotto, Elisabetta
We use observations of cosmic microwave background anisotropies, supernova luminosities and the baryon acoustic oscillation signal in the galaxy distribution to constrain the cosmological parameters...
Velocities as a probe of dark sector interactions (2009)
Koyama, Kazuya, Maartens, Roy, Song, Yong-Seon
Dark energy in General Relativity is typically non-interacting with other matter. However, it is possible that the dark energy interacts with the dark matter, and in this case, the dark matter can...
Cosmological perturbations in Horava-Lifshitz theory without detailed balance (2009)
In 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...
The Growth of Structure in Interacting Dark Energy Models (2009)
Caldera-Cabral, Gabriela, Maartens, Roy, Schaefer, Bjoern Malte
If dark energy interacts with dark matter, there is a change in the background evolution of the universe, since the dark matter density no longer evolves as a^{-3}. In addition, the non-gravitational...
Dynamics of interacting dark energy (2008)
Caldera-Cabral, Gabriela, Maartens, Roy, Urena-Lopez, L. Arturo
Dark 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...
Magnetic Bianchi I Universe in Loop Quantum Cosmology (2008)
Maartens, Roy, Vandersloot, Kevin
We 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...
The cosmological background of vector modes (2008)
Lu, Teresa Hui-Ching, Ananda, Kishore, Clarkson, Chris, Maartens, Roy
We 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...
Dark Energy and Modified Gravity (2008)
Explanations 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...
Large-scale instability in interacting dark energy and dark matter fluids (2008)
Valiviita, Jussi, Majerotto, Elisabetta, Maartens, Roy
If dark energy interacts with dark matter, this gives a new approach to the coincidence problem. But interacting dark energy models can suffer from pathologies. We consider the case where the dark...
Schaefer, Bjoern Malte, Caldera-Cabral, Gabriela Alejandra, Maartens, Roy
We consider a phenomenological model for a coupling between the dark matter and dark energy fluids and investigate the sensitivity of a weak lensing measurement for constraining the size of this...
Dynamics of dark energy with a coupling to dark matter (2008)
Boehmer, Christian G., Caldera-Cabral, Gabriela, Lazkoz, Ruth, Maartens, Roy
Dark energy and dark matter are the dominant sources in the evolution of the late universe. They are currently only indirectly detected via their gravitational effects, and there could be a coupling...
Editorial on the GRG special issue on dark energy (2008)
Ellis, George F. R., Nicolai, Hermann, Durrer, Ruth, Maartens, Roy
Challenges for creating magnetic fields by cosmic defects (2007)
Hollenstein, Lukas, Caprini, Chiara, Crittenden, Robert, Maartens, Roy
We analyse the possibility that topological defects can act as a source of magnetic fields through the Harrison mechanism in the radiation era. We give a detailed relativistic derivation of the...
Anisotropic Universes With Conformal Motion (2007)
Roy Maartens And, Preprint Rcg, Roy Maartens, Conrad M. Mellin
. By 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...
Exact Perturbations for Inflation With Smooth Exit (2007)
Toy 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...
Dark Energy and Dark Gravity (2007)
Observations provide increasingly strong evidence that the universe is accelerating. This revolutionary advance in cosmological observations confronts theoretical cosmology with a tremendous...
The Einstein static universe in Loop Quantum Cosmology (2007)
Parisi, Luca, Bruni, Marco, Maartens, Roy, Vandersloot, Kevin
Loop Quantum Cosmology strongly modifies the high-energy dynamics of Friedman-Robertson-Walker models and removes the big-bang singularity. We investigate how LQC corrections affect the stability...
Graceful exit via polymerization of pre-big bang cosmology (2007)
De Risi, Giuseppe, Maartens, Roy, Singh, Parampreet
We consider a phenomenological modification of the Pre Big Bang scenario using ideas from the resolution of curvature singularities in Loop Quantum Cosmology. We show that non-perturbative Loop...
Relativistic cosmology and large-scale structure (2007)
Tsagas, Christos G., Challinor, Anthony, Maartens, Roy
General relativity marked the beginning of modern cosmology and it has since been at the centre of many of the key developments in this field. In the present review, we discuss the...
Causality and the speed of sound (2007)
Ellis, George, Maartens, Roy, MacCallum, Malcolm
A 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....
Cosmological magnetic fields from nonlinear effects (2007)
Kobayashi, Tsutomu, Maartens, Roy, Shiromizu, Tetsuya, Takahashi, Keitaro
In the standard cosmological model, magnetic fields and vorticity are generated during the radiation era via second-order density perturbations. In order to clarify the complicated physics of this...
Magnetic field amplification in cold dark matter anisotropic collapse (2006)
Bruni, Marco, Maartens, Roy, Tsagas, Christos G.
We use the Zel'dovich approximation to analyse the amplification of magnetic fields in gravitational collapse of cold dark matter during the mildly non-linear regime, and identify two key features....
Magnetized gravitational waves (2006)
Maartens, Roy, Tsagas, Christos G., Ungarelli, Carlo
We 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...
Magnetized cosmological perturbations (2006)
Tsagas, Christos G., Maartens, Roy
A 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...
Observational constraints on phantom-like braneworld cosmologies (2006)
Lazkoz, Ruth, Maartens, Roy, Majerotto, Elisabetta
We investigate a simple braneworld model in which the universe contains only cold dark matter and a cosmological constant, but the effective dark energy is phantom-like because of extra-dimensional...
Crossing the phantom divide without phantom matter (2006)
Chimento, Luis P., Lazkoz, Ruth, Maartens, Roy, Quiros, Israel
A class of braneworld models can lead to phantom-like acceleration of the late universe, but without the need for any phantom matter. In the simplest models, the universe contains only cold dark...
Observational constraints on self-accelerating cosmology (2006)
Maartens, Roy, Majerotto, Elisabetta
The 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...
Dark Energy from Brane-world Gravity (2006)
Recent 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...
Structure formation in the DGP cosmological model (2005)
The 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...
A late-accelerating universe with no dark energy - and a finite-temperature big bang (2005)
Brown, Richard A., Maartens, Roy, Papantonopoulos, Eleftherios, Zamarias, Vassilis
Brane-world models offer the possibility of explaining the late acceleration of the universe via infra-red modifications to General Relativity, rather than a dark energy field. However, one also...
Regular collision of dilatonic inflating branes (2005)
Leeper, Emily, Koyama, Kazuya, Maartens, Roy
We demonstrate that a two brane system with a bulk scalar field driving power-law inflation on the branes has an instability in the radion. We solve for the resulting trajectory of the brane, and...
Gravity-wave detectors as probes of extra dimensions (2005)
Clarkson, Chris, Maartens, Roy
If 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...
Delocalization of brane gravity by a bulk black hole (2005)
Seahra, Sanjeev S., Clarkson, Chris, Maartens, Roy
We 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...
A black hole mass threshold from non-singular quantum gravitational collapse (2005)
Bojowald, Martin, Goswami, Rituparno, Maartens, Roy, Singh, Parampreet
Quantum gravity is expected to remove the classical singularity that arises as the end-state of gravitational collapse. To investigate this, we work with a toy model of a collapsing homogeneous...
A black hole mass threshold from non-singular quantum gravitational collapse (2005)
Bojowald,Martin, Goswami,Rituparno, Maartens,Roy, Singh,Parampreet
Quantum gravity is expected to remove the classical singularity that arises as the end-state of gravitational collapse. To investigate this, we work with a simple toy model of a collapsing...
A black hole mass threshold from non-singular quantum gravitational collapse (2005)
Bojowald, Martin, Goswami, Rituparno, Maartens, Roy, Singh, Parampreet
Quantum gravity is expected to remove the classical singularity that arises as the end-state of gravitational collapse. To investigate this, we work with a simple toy model of a collapsing...
Asymmetric brane-worlds with induced gravity (2004)
Gergely, Laszlo, Maartens, Roy
The 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,...
Asymmetric radiating brane-world (2004)
Gergely, Laszlo, Leeper, Emily, Maartens, Roy
At high energies on a cosmological brane of Randall-Sundrum type, particle interactions can produce gravitons that are emitted into the bulk and that can feed a bulk black hole. We generalize...
Detecting extra dimensions with gravity wave spectroscopy: the black string brane-world (2004)
Seahra, Sanjeev S., Clarkson, Chris, Maartens, Roy
Using 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...
Gravitational waves from brane-world inflation with induced gravity (2004)
Bouhmadi-Lopez, Mariam, Maartens, Roy, Wands, David
We 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...
Loop Quantum Gravity and the Cyclic Universe (2004)
Bojowald, Martin, Maartens, Roy, Singh, Parampreet
Loop quantum gravity introduces strong non-perturbative modifications to the dynamical equations in the semi-classical regime, which are responsible for various novel effects, including resolution of...
The 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...
Observational constraints on braneworld inflation: the effect of a Gauss-Bonnet term (2004)
Tsujikawa, Shinji, Sami, M., Maartens, Roy
High-energy modifications to general relativity introduce changes to the perturbations generated during inflation, and the latest high-precision cosmological data can be used to place constraints on...
Brane-world cosmological perturbations (2004)
Brane-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...
Loop Quantum Gravity and the Cyclic Universe (2004)
Bojowald,Martin, Maartens,Roy, Singh,Parampreet
Loop quantum gravity introduces strong non-perturbative modifications to the dynamical equations in the semi-classical regime, which are responsible for various novel effects, including resolution of...
Loop Quantum Gravity and the Cyclic Universe (2004)
Bojowald, Martin, Maartens, Roy, Singh, Parampreet
Loop quantum gravity introduces strong non-perturbative modifications to the dynamical equations in the semi-classical regime, which are responsible for various novel effects, including resolution of...
The 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...
Loop quantum gravity effects on inflation and the CMB (2003)
Tsujikawa, Shinji, Singh, Parampreet, Maartens, Roy
In loop quantum cosmology, the universe avoids a big bang singularity and undergoes an early and short super-inflation phase. During super-inflation, non-perturbative quantum corrections to the...
Dynamics of radiating braneworlds (2003)
Leeper, Emily, Maartens, Roy, Sopuerta, Carlos
If the observable universe is a braneworld of Randall-Sundrum type, then particle interactions at high energies will produce 5-dimensional gravitons that escape into the bulk. As a result, the Weyl...
Braneworld cosmological models with anisotropy (2003)
Campos, Antonio, Maartens, Roy, Matravers, David, Sopuerta, Carlos F.
For a cosmological Randall-Sundrum braneworld with anisotropy, i.e., of Bianchi type, the modified Einstein equations on the brane include components of the five-dimensional Weyl tensor for which...
Evolution of gravitational waves in Randall-Sundrum cosmology (2003)
Easther, Richard, Langlois, David, Maartens, Roy, Wands, David
We investigate the evolution of gravitational wave perturbations about a brane cosmology embedded in a five-dimensional anti-de Sitter bulk. During slow-roll inflation in a Randall-Sundrum...
Non-commutative inflation and the CMB (2003)
Tsujikawa, Shinji, Maartens, Roy, Brandenberger, Robert
Non-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...
Will we observe black holes at LHC? (2003)
Cavaglia, Marco, Das, Saurya, Maartens, Roy
The generalized uncertainty principle, motivated by string theory and non-commutative quantum mechanics, suggests significant modifications to the Hawking temperature and evaporation process of black...
Brane-world cosmological perturbations: a covariant approach (2003)
The 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...
Density perturbations in a brane-world universe with dark radiation (2003)
Gumjudpai, Burin, Maartens, Roy, Gordon, Christopher
We investigate the effects on cosmological density perturbations of dark radiation in a Randall-Sundrum 2 type brane-world. Dark radiation in the background is limited by observational constraints to...
On the Stability of the Einstein Static Universe (2003)
Barrow, John D, Ellis, George, Maartens, Roy, Tsagas, Christos
We show using covariant techniques that the Einstein static universe containing a perfect fluid is always neutrally stable against small inhomogeneous vector and tensor perturbations and neutrally...
The Emergent Universe: inflationary cosmology with no singularity (2002)
Observations 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...
Stacking a 4D geometry into an Einstein-Gauss-Bonnet bulk (2002)
Barcelo, Carlos, Maartens, Roy, Sopuerta, Carlos F., Viniegra, Fermin
In Einstein gravity there is a simple procedure to build D-dimensional spacetimes starting from (D-1)-dimensional ones, by stacking any (D-1)-dimensional Ricci-flat metric into the extra-dimension....
Braneworld Tensor Anisotropies in the CMB (2002)
Leong, Bernard, Challinor, Anthony, Maartens, Roy, Lasenby, Anthony
Cosmic microwave background (CMB) observations provide in principle a high-precision test of models which are motivated by M theory. We set out the framework of a program to compute the tensor...
Gravitational Collapse on the Brane (2002)
The Erwin, Schrödinger International Boltzmanngasse, Marco Bruni, Cristiano Germani, Roy Maartens, Marco Bruni, ...
We 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...
Why do naked singularities form in gravitational collapse? (2001)
Joshi, Pankaj S., Dadhich, Naresh, Maartens, Roy
We investigate what are the key physical features that cause the development of a naked singularity, rather than a black hole, as the end-state of spherical gravitational collapse. We show that...
Kaluza-Klein anisotropy in the CMB (2001)
We 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...
Gravitational collapse on the brane: a no-go theorem (2001)
Bruni, Marco, Germani, Cristiano, Maartens, Roy
We 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...
Stars in the braneworld (2001)
Germani, Cristiano, Maartens, Roy
We 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...
General relativistic analysis of peculiar velocities (2001)
Ellis, George F R, Van Elst, Henk, Maartens, Roy
We give a careful general relativistic and (1+3)-covariant analysis of cosmological peculiar velocities induced by matter density perturbations in the presence of a cosmological constant. In our...
Brillouin scattering and the CMB (2001)
Sandoval-Villalbazo, Alfredo, Maartens, Roy
Brillouin 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...
Brane-world generalizations of the Einstein static universe (2001)
Gergely, Laszlo, Maartens, Roy
A 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...
Holonomy and gravitomagnetism (2001)
Maartens, Roy, Mashhoon, Bahram, Matravers, David
We 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...
Geometry and dynamics of the brane-world (2001)
Recent 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...
Magnetized gravitational waves (2001)
Maartens, Roy, Tsagas, Christos, Ungarelli, Carlo
We 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 cosmological perturbations on the brane (2000)
Langlois, David, Maartens, Roy, Sasaki, Misao, Wands, David
In brane-world cosmologies of Randall-Sundrum type, we show that evolution of large-scale curvature perturbations may be determined on the brane, without solving the bulk perturbation equations. The...
Anisotropy dissipation in brane-world inflation (2000)
Maartens, Roy, Sahni, Varun, Saini, Tarun Deep
We 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...
Adiabatic and entropy perturbations from inflation (2000)
Gordon, Christopher, Wands, David, Bassett, Bruce A., Maartens, Roy
We study adiabatic (curvature) and entropy (isocurvature) perturbations produced during a period of cosmological inflation that is driven by multiple scalar fields with an arbitrary interaction...
Density perturbations in the brane-world (2000)
Gordon, Christopher, Maartens, Roy
In 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...
Cosmological magnetic fields (2000)
Magnetic 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...
Gravitational waves from inflation on the brane (2000)
Langlois, David, Maartens, Roy, Wands, David
We 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...
Gamma-ray bursts as the birth-cries of black holes (2000)
Joshi, Pankaj, Dadhich, Naresh, Maartens, Roy
The origin of cosmic gamma-ray bursts remains one of the most intriguing puzzles in astronomy. We suggest that purely general relativistic effects in the collapse of massive stars could account for...
Cosmological dynamics on the brane (2000)
In 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...
Black holes on the brane (2000)
Dadhich, Naresh, Maartens, Roy, Papadopoulos, Philippos, Rezania, Vahid
We consider exact solutions for static black holes localized on a three-brane in five-dimensional gravity in the Randall-Sundrum scenario. We show that the Reissner-Nordstrom metric is an exact...
Gravitational waves in preheating (2000)
We 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...
Cosmological perturbations on a magnetised Bianchi I background (1999)
Tsagas, Christos, Maartens, Roy
Motivated 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...
Vorticity affects the stability of neutron stars (1999)
The 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...
Restoring the sting to metric preheating (1999)
Bassett, Bruce A., Gordon, Christopher, Maartens, Roy, Kaiser, David I.
The 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...
Nonlinear Effects in the Cosmic Microwave Background (1999)
Major 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...
Magnetized cosmological perturbations (1999)
Tsagas, Christos, Maartens, Roy
A 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...
Backreaction effects of dissipation in neutrino decoupling (1999)
Maartens, Roy, Triginer, Josep
Dissipative 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...
Metric preheating and limitations of linearized gravity (1999)
Bassett, Bruce A., Tamburini, Fabrizio, Kaiser, David I., Maartens, Roy
Recently it has become clear that the resonant amplification of quantum field fluctuations at preheating must be accompanied by resonant amplification of scalar metric perturbations, since the two...
Relaxational effects in radiating stellar collapse (1998)
Govender, Megan, Maartens, Roy, Maharaj, Sunil
Relaxational effects in stellar heat transport can in many cases be significant. Relativistic Fourier-Eckart theory is inherently quasi-stationary, and cannot incorporate these effects. The effects...
Nonperturbative gravito-magnetic fields (1998)
Sopuerta, Carlos, Maartens, Roy, Ellis, George, Lesame, William
In a cold matter universe, the linearized gravito-magnetic tensor field satisfies a transverse condition (vanishing divergence) when it is purely radiative. We show that in the nonlinear theory, it...
General Relativistic effects in preheating (1998)
Bassett, Bruce A., Kaiser, David I., Maartens, Roy
General relativistic effects in the form of metric perturbations are usually neglected in the preheating era that follows inflation. We argue that in realistic multi-field models these effects are in...
Anisotropic stresses in inhomogeneous universes (1998)
Anisotropic stress contributions to the gravitational field can arise from magnetic fields, collisionless relativistic particles, hydrodynamic shear viscosity, gravitational waves, skew axion fields...
Covariant velocity and density perturbations in quasi-Newtonian cosmologies (1998)
Recently 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,...
Cosmic microwave background anisotropies: Nonlinear dynamics (1998)
Maartens, Roy, Gebbie, Tim, Ellis, George
We 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...
Perturbations in inflationary cosmologies with smooth exit (1998)
We 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...
Exact isotropic cosmologies with local fractal number counts (1998)
Humphreys, Neil, Matravers, David, Maartens, Roy
We 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 $10^2$...
Regular spherical dust spacetimes (1998)
Humphreys, Neil, Maartens, Roy, Matravers, David
Physical (and weak) regularity conditions are used to determine and classify all the possible types of spherically symmetric dust spacetimes in general relativity. This work unifies and completes...
Newtonian-like and anti-Newtonian universes (1998)
Maartens, Roy, Lesame, William, Ellis, George
In an irrotational dust universe, the locally free gravitational field is covariantly described by the gravito-electric and gravito-magnetic tensors $E_{ab}$ and $H_{ab}$. In Newtonian theory,...
Acoustic oscillations and viscosity (1998)
Maartens, Roy, Triginer, Josep
Using 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...
Symmetries of particle motion (1997)
We 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...
A causal model of radiating stellar collapse (1997)
Govender, Megan, Maharaj, Sunil, Maartens, Roy
We 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...
Inflation driven by causal heat flux (1997)
Maartens, Roy, Govender, Megan, Maharaj, Sunil
We 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...
Exact Perturbations for inflation with smooth exit (1997)
Toy 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...
Density perturbations with relativistic thermodynamics (1997)
Maartens, Roy, Triginer, Josep
We investigate cosmological density perturbations in a covariant and gauge-invariant formalism, incorporating relativistic causal thermodynamics to give a self-consistent description. The gradient of...
Gravito-electromagnetism (1997)
Maartens, Roy, Bassett, Bruce A.
We 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...
Consistency of dust solutions with div H=0 (1997)
Maartens, Roy, Lesame, William, Ellis, George
One of the necessary covariant conditions for gravitational radiation is the vanishing of the divergence of the magnetic Weyl tensor H_{ab}, while H_{ab} itself is nonzero. We complete a recent...
Nonlinear bulk viscosity and inflation (1996)
We 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...
Reheating and causal thermodynamics (1996)
Zimdahl, Winfried, Pavón, Diego, Maartens, Roy
The reheating process in inflationary universe models is considered as an out-of-equilibrium mixture of two interacting and reacting fluids, and studied within the framework of causal, irreversible...
Exact non-equilibrium solutions of the Einstein-Boltzmann equations. II (1996)
Wolvaardt, Derik, Maartens, Roy
We 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...
Local freedom in the gravitational field (1996)
Maartens, Roy, Ellis, George, Siklos, Stephen
In a cosmological context, the electric and magnetic parts of the Weyl tensor, E_{ab} and H_{ab}, represent the locally free curvature - i.e. they are not pointwise determined by the matter fields....
Linearisation instability of gravity waves? (1996)
Gravity 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....
Causal Thermodynamics in Relativity (1996)
I 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...
Anisotropic universes with conformal motion (1996)
By 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...
Anisotropic Observations in Universes with Nonlinear Inhomogeneity (1996)
Humphreys, Neil, Maartens, Roy, Matravers, David
We 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...
Expanding Spherically Symmetric Models without Shear (1995)
Maharaj, Sunil D, Leach, Peter GL, Maartens, Roy
The integrability properties of the field equation $L_{xx} = F(x)L^2$ of a spherically symmetric shear--free fluid are investigated. A first integral, subject to an integrability condition on $F(x)$,...
Inhomogeneous universes in observational coordinates (1995)
Maartens, Roy, Humphreys, Neil, Matravers, David, Stoeger, Bill
Isotropic inhomogeneous dust universes are analysed via observational coordinates based on the past light cones of the observer's galactic worldline. The field equations are reduced to a single...
Almost-homogeneity of the universe in higher-order gravity (1995)
Taylor, David R, Maartens, Roy
In 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...
Quadrupole anisotropy from photon quantum effects (1995)
Nesteruk, Alexei V, Maartens, Roy
We 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,...
Anisotropy and inhomogeneity of the universe from $\Delta T/T$ (1995)
Maartens, Roy, Ellis, George, Stoeger, William
A recent paper (Martinez--Gonzalez & Sanz 1995) showed that if the universe is homogeneous but anisotropic, then the small quadrupole anisotropy in the cosmic microwave background radiation implies...
LIMITS ON ANISOTROPY AND INHOMOGENEITY FROM THE COSMIC BACKGROUND RADIATION, (1995)
Maartens, Roy, Ellis, George F. R., Stoeger, William R.
We 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...
Idealized observations in relativistic cosmology. (1980)
Thesis (PhD -- Applied Mathematics)--University of Cape Town, 1980.