- I studied physics at the University of Rome “La Sapienza” where I graduated in 1987, with a thesis on colliding grav... moreI studied physics at the University of Rome “La Sapienza” where I graduated in 1987, with a thesis on colliding gravitational waves, supervised by Valeria Ferrari. I then obtained a studentship from SISSA (Trieste, Italy) where I worked with George Ellis on covariant and gauge-invariant cosmological perturbations, completing my PhD in 1991. After postdocs in QMUL (London), ICTP and SISSA (Trieste), and Cardiff, in 1999 I joined the Relativity and Cosmology Group of the mathematics department at the University of Portsmouth as senior lecturer. In 2002 I became member of the newly formed Institute of Cosmology and Gravitation. In 2005 I was awarded a two year visiting professorship by the Italian government, which I spent at the University of Rome “Tor Vergata”. In 2006 I was promoted to reader. I have lectured undergraduates and postgraduate students in the UK, Italy, Greece and India, I have published over 50 scientific papers in international journals and given talks in four continents (Australians I need an invitation!). I am a keen photographer/traveller/mountaineer.edit
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Research Interests: Physics, Quantum Physics, General Relativity, A Priori Knowledge, Numerical Relativity, and 14 moreNumerical Simulation, Cosmology and Gravitation, Space Time, Physical, Black Hole, Wave propagation, Tetrad, Classical Mechanics, Spacetime, Extraction Method, Test Bed, Perturbation Theory, a priori and a posteriori, and gravitational wave
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Research Interests: Mathematical Physics, Field Theory, Physics, Quantum Physics, Early Universe, and 13 moreAstrophysics, Cosmology, Singularity, Cosmology and Gravitation, Physical, Phase Space, Large Scale, General Relativity and Quantum Cosmology, Initial Condition, Equation of State, Isotropy, attractor, and Gauge invariance
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In General Relativity, the constraint equation relating metric and density perturbations is inherently nonlinear, leading to an effective non-Gaussianity in the dark matter density field on large scales - even if the primordial metric... more
In General Relativity, the constraint equation relating metric and density perturbations is inherently nonlinear, leading to an effective non-Gaussianity in the dark matter density field on large scales - even if the primordial metric perturbation is Gaussian. Intrinsic non-Gaussianity in the large-scale dark matter overdensity in GR is real and physical. However, the variance smoothed on a local physical scale is not correlated with the large-scale curvature perturbation, so that there is no relativistic signature in the galaxy bias when using the simplest model of bias. It is an open question whether the observable mass proxies such as luminosity or weak lensing correspond directly to the physical mass in the simple halo bias model. If not, there may be observables that encode this relativistic signature.
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Research Interests: Physics, Quantum Physics, General Relativity, Numerical Relativity, Numerical Simulation, and 12 moreSpace Time, Physical, Black Hole, Tetrad, Classical Mechanics, Spacetime, Numerical computation, General Relativity and Quantum Cosmology, Test Bed, non perturbative QCd, Perturbation Theory, and gravitational wave
Research Interests: Physics, Theoretical Physics, General Relativity, Quantum Cosmology, Astrophysics, and 11 moreBig Bang, Cosmology, Universe, Loop Quantum Cosmology, Mathematical Sciences, Physical sciences, Cosmological Constant, Einstein, Gravity(classical and Quantum), General Relativity and Quantum Cosmology, and High energy
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ABSTRACT The authors investigate the properties of a specific set of solutions of the Einstein field equations that describe dust (p = 0), with or without a cosmological constant. This class of solutions is specified by: (1) the... more
ABSTRACT The authors investigate the properties of a specific set of solutions of the Einstein field equations that describe dust (p = 0), with or without a cosmological constant. This class of solutions is specified by: (1) the assumption of zero pressure, which implies that the fluid world-lines are geodesic (u˙a = 0); (2) the assumption that the dust is irrotational (ωa = 0); and (3) the assumption that the magnetic Weyl curvature is zero (Hab = 0). In general, however, the solutions admit no symmetries. The dynamical systems approach used here was first systematically applied to these models by Bruni et al. (1995). In analyzing these solutions the authors make use of the evolution equations for the kinematic quantities and the Weyl curvature. The solutions describe the evolution of an expanding or collapsing distribution of dust in which there is no exchange of information between different fluid elements, either by sound waves or by gravitational waves. For this reason the models in this class have been called "silent universes".
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The Teukolsky formalism of black hole perturbation theory describes weak gravitational radiation generated by a mildly dynamical hole near equilibrium. A particular null tetrad of the background Kerr geometry, due to Kinnersley, plays a... more
The Teukolsky formalism of black hole perturbation theory describes weak gravitational radiation generated by a mildly dynamical hole near equilibrium. A particular null tetrad of the background Kerr geometry, due to Kinnersley, plays a singularly important role within this formalism. In order to apply the rich physical intuition of Teukolsky's approach to the results of fully non-linear numerical simulations, one must approximate this Kinnersley tetrad using raw numerical data, with no a priori knowledge of a background. This paper addresses this issue by identifying the directions of the tetrad fields in a quasi-Kinnersley frame. This frame provides a unique, analytic extension of Kinnersley's definition for the Kerr geometry to a much broader class of space-times including not only arbitrary perturbations, but also many examples which differ non-perturbatively from Kerr. This paper establishes concrete limits delineating this class and outlines a scheme to calculate the quasi-Kinnersley frame in numerical codes based on the initial-value formulation of geometrodynamics.
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We investigate the transverse modes of the gravitational and velocity fields in ΛCDM, based on a high-resolution simulation performed using the adaptive-mesh refinement general-relativistic N-body code gramses. We study the generation of... more
We investigate the transverse modes of the gravitational and velocity fields in ΛCDM, based on a high-resolution simulation performed using the adaptive-mesh refinement general-relativistic N-body code gramses. We study the generation of vorticity in the dark matter velocity field at low redshift, providing fits to the shape and evolution of its power spectrum over a range of scales. By analysing the gravitomagnetic vector potential, which is absent in Newtonian simulations, in dark matter haloes with masses ranging from ∼1012.5 h−1M⊙ to ∼1015 h−1M⊙, we find that its magnitude correlates with the halo mass, peaking in the inner regions. Nevertheless, on average, its ratio against the scalar gravitational potential remains fairly constant, below percent level, decreasing roughly linearly with redshift and showing a weak dependence on halo mass. Furthermore, we show that the gravitomagnetic acceleration in haloes peaks towards the core and reaches almost 10−10 h cm/s2 in the most mas...
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We present a new way of deriving gauge transformations in non--linear relativistic perturbation theory. The main ingredient in this formulation is the use of the Baker-Campbell-Hausdorff formula. The associated formal machinery allows us... more
We present a new way of deriving gauge transformations in non--linear relativistic perturbation theory. The main ingredient in this formulation is the use of the Baker-Campbell-Hausdorff formula. The associated formal machinery allows us to generalize one-parameter perturbation theory to an arbitrary number of parameters, and to prove the main results concerning the consistency of the scheme to any order in the perturbations. Gauge transformations at any required order can then be directly derived from a generating exponential formula via a simple Taylor expansion. We outline the relation between our novel formulation and previous results.
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A number of codes for general-relativistic simulations of cosmological structure formation have been developed in recent years. Here we demonstrate that a sample of these codes produce consistent results beyond the Newtonian regime. We... more
A number of codes for general-relativistic simulations of cosmological structure formation have been developed in recent years. Here we demonstrate that a sample of these codes produce consistent results beyond the Newtonian regime. We simulate solutions to Einstein’s equations dominated by gravitomagnetism—a vector-type gravitational field that does not exist in Newtonian gravity and produces frame-dragging, the leading-order post-Newtonian effect. We calculate the coordinate-invariant effect on intersecting null geodesics by performing ray tracing in each independent code. With this observable quantity, we assess and compare each code’s ability to compute relativistic effects.
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Christopher Beetle, 2 Marco Bruni, Lior M. Burko, ∗ and Andrea Nerozzi 4 Department of Physics, Florida Atlantic University, Boca Raton, Florida 33431 Department of Physics, University of Utah, Salt Lake City, Utah 84112 Institute of... more
Christopher Beetle, 2 Marco Bruni, Lior M. Burko, ∗ and Andrea Nerozzi 4 Department of Physics, Florida Atlantic University, Boca Raton, Florida 33431 Department of Physics, University of Utah, Salt Lake City, Utah 84112 Institute of Cosmology and Gravitation, Mercantile House, Hampshire Terrace, PO1 2EG, Portsmouth UK Center for Relativity, Department of Physics, University of Texas at Austin, Austin, Texas 78712-1081 (Dated: February 6, 2008)
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The authors investigate the properties of a specific set of solutions of the Einstein field equations that describe dust (p = 0), with or without a cosmological constant. This class of solutions is specified by: (1) the assumption of zero... more
The authors investigate the properties of a specific set of solutions of the Einstein field equations that describe dust (p = 0), with or without a cosmological constant. This class of solutions is specified by: (1) the assumption of zero pressure, which implies that the fluid world-lines are geodesic (u˙a = 0); (2) the assumption that the dust is irrotational (ωa = 0); and (3) the assumption that the magnetic Weyl curvature is zero (Hab = 0). In general, however, the solutions admit no symmetries. The dynamical systems approach used here was first systematically applied to these models by Bruni et al. (1995). In analyzing these solutions the authors make use of the evolution equations for the kinematic quantities and the Weyl curvature. The solutions describe the evolution of an expanding or collapsing distribution of dust in which there is no exchange of information between different fluid elements, either by sound waves or by gravitational waves. For this reason the models in thi...
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The authors investigate the properties of a specific set of solutions of the Einstein field equations that describe dust (p = 0), with or without a cosmological constant. This class of solutions is specified by: (1) the assumption of zero... more
The authors investigate the properties of a specific set of solutions of the Einstein field equations that describe dust (p = 0), with or without a cosmological constant. This class of solutions is specified by: (1) the assumption of zero pressure, which implies that the fluid world-lines are geodesic (u˙a = 0); (2) the assumption that the dust is irrotational (ωa = 0); and (3) the assumption that the magnetic Weyl curvature is zero (Hab = 0). In general, however, the solutions admit no symmetries. The dynamical systems approach used here was first systematically applied to these models by Bruni et al. (1995). In analyzing these solutions the authors make use of the evolution equations for the kinematic quantities and the Weyl curvature. The solutions describe the evolution of an expanding or collapsing distribution of dust in which there is no exchange of information between different fluid elements, either by sound waves or by gravitational waves. For this reason the models in thi...
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The authors investigate the properties of a specific set of solutions of the Einstein field equations that describe dust (p = 0), with or without a cosmological constant. This class of solutions is specified by: (1) the assumption of zero... more
The authors investigate the properties of a specific set of solutions of the Einstein field equations that describe dust (p = 0), with or without a cosmological constant. This class of solutions is specified by: (1) the assumption of zero pressure, which implies that the fluid world-lines are geodesic (u˙a = 0); (2) the assumption that the dust is irrotational (omegaa = 0); and (3) the assumption that the magnetic Weyl curvature is zero (Hab = 0). In general, however, the solutions admit no symmetries. The dynamical systems approach used here was first systematically applied to these models by Bruni et al. (1995). In analyzing these solutions the authors make use of the evolution equations for the kinematic quantities and the Weyl curvature. The solutions describe the evolution of an expanding or collapsing distribution of dust in which there is no exchange of information between different fluid elements, either by sound waves or by gravitational waves. For this reason the models in this class have been called "silent universes".
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Recent studies of homogeneous anisotropic universe models in the brane world scenario show that the cosmological singularity in this context is isotropic. It has therefore been suggested that this may be a generic feature of singularities... more
Recent studies of homogeneous anisotropic universe models in the brane world scenario show that the cosmological singularity in this context is isotropic. It has therefore been suggested that this may be a generic feature of singularities on the brane, even in the inhomogeneous case. Using a perturbative approach, we show that this is not the case. As in the GR case, the presence of decaying modes in the perturbations signal the instability (in the past) of the isotropic singularity. The brane universe is therefore not born with isotropy built in: as in standard cosmology, the observed large-scale isotropy and homogeneity remains to be explained.
Given that observations seem to favour a density parameter Ω0<1, corresponding to an open universe, we consider gauge-invariant perturbations of nonflat Robertson-Walker universes filled with a general imperfect fluid which can also be... more
Given that observations seem to favour a density parameter Ω0<1, corresponding to an open universe, we consider gauge-invariant perturbations of nonflat Robertson-Walker universes filled with a general imperfect fluid which can also be taken to represent a scalar field. Our aim is to set up the equations that govern the evolution of the density perturbations Δ so that it can be determined through a first order differential equation with a quantity [Formula: see text] which is conserved at any length scale, even in nonflat universe models, acting as a source term. The quantity [Formula: see text] generalizes other variables that are conserved in specific cases (for example at large scales in a flat universe) and is useful to connect different epochs in the evolution of density perturbations via a transfer function. We show that the problem of finding a conserved [Formula: see text] can be reduced to determining two auxiliary variables X and Y, and illustrate the method with two si...
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This paper concerns gauge-invariant perturbations of Robertson-Walker spacetimes, with the aim of (1) giving a complete set of perturbation equations and (2) comparing the coordinate-based method of Bardeen with the covariant approach of... more
This paper concerns gauge-invariant perturbations of Robertson-Walker spacetimes, with the aim of (1) giving a complete set of perturbation equations and (2) comparing the coordinate-based method of Bardeen with the covariant approach of Ellis and Bruni (1989). To this end, we first consider covariantly defined quantities which are gauge-invariant in a perturbed Robertson-Walker universe: for these variables we derive a
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Page 1. Gauge-invariant perturbations in a scalar field dominated universe This article has been downloaded from IOPscience. Please scroll down to see the full text article. 1992 Class. Quantum Grav. 9 921... more
Page 1. Gauge-invariant perturbations in a scalar field dominated universe This article has been downloaded from IOPscience. Please scroll down to see the full text article. 1992 Class. Quantum Grav. 9 921 (http://iopscience.iop.org/0264-9381/9/4/010) ...
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... Printed in USA COVARIANT PERTURBATIONS IN A MULTIFLUID COSMOLOGICAL MEDIUMPETER KS DUNSBY1 School ofMathematical Sciences, Queen Mary and ... the speed oflight to be unit (c = 1). We will assume that Einstein&amp;#x27;s equations... more
... Printed in USA COVARIANT PERTURBATIONS IN A MULTIFLUID COSMOLOGICAL MEDIUMPETER KS DUNSBY1 School ofMathematical Sciences, Queen Mary and ... the speed oflight to be unit (c = 1). We will assume that Einstein&amp;#x27;s equations take the form Gab + Ag~~ = KT ...
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In this paper we introduce a novel class of interacting vacuum models, based on recasting the equation of state originally developed in the context of lattice kinetic theory by Shan & Chen (1993) as the coupling between the vacuum and... more
In this paper we introduce a novel class of interacting vacuum models, based on recasting the equation of state originally developed in the context of lattice kinetic theory by Shan & Chen (1993) as the coupling between the vacuum and cold dark matter (CDM). This coupling allows the vacuum to evolve and is nonlinear around a characteristic energy scale ρ∗, changing into a linear coupling with a typical power law evolution at scales much lower and much higher than ρ∗. Focusing on the simplest sub-class of models where the interaction consists only of an energy exchange and the CDM remains geodesic, we first illustrate the various possible models that can arise from the Shan–Chen coupling, with several different behaviours at both early and late times depending on the values of the model parameters selected. We then place the first observational constraints on this Shan–Chen interacting vacuum scenario, performing an MCMC analysis to find those values of the model and cosmological par...