Results for "Valerio Bozza"

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Secondary caustics in close multiple lensesFeb 14 2000May 16 2000We investigate the caustic structure of a lens composed by a discrete number of point-masses, having mutual distances smaller than the Einstein radius of the total mass of the system. Along with the main critical curve, it is known that the lens map is ... More
Signs of the cusps in binary lensesJun 15 2000The cusps of the caustics of any gravitational lens model can be classified into positive and negative ones. This distinction lies on the parity of the images involved in the creation/destruction of pairs occurring when a source crosses a caustic in a ... More
O(d,d)-invariant collapse/inflation from colliding superstring wavesJul 20 2000Generalizing previous work, we study the collision of massless superstring plane waves in D space-time dimensions within an explicitly O(D-2,D-2)-invariant set of field equations. We discuss some general properties of the solutions, showing in particular ... More
Gravitational lensing of stars orbiting the Massive Black Hole in the Galactic CenterDec 19 2008The existence of a massive black hole in the center of the Milky Way, coinciding with the radio source Sgr A*, is being established on more and more solid ground. In principle, this black hole, acting as a gravitational lens, is able to bend the light ... More
Optical caustics of Kerr spacetime: the full structureJun 25 2008Oct 08 2008We present an exhaustive numerical investigation of the optical caustics in gravitational lensing by a spinning black hole for an observer at infinity. Besides the primary caustic, we examine higher order caustics, formed by photons performing one or ... More
The complete catalogue of light curves in equal-mass binary microlensingJan 09 2015Mar 31 2015The light curves observed in microlensing events due to binary lenses span an extremely wide variety of forms, characterised by U-shaped caustic crossings and/or additional smoother peaks. However, all peaks of the binary-lens light curve can be traced ... More
XV International Conference on Gravitational Microlensing: Conference BookFeb 02 2011Microlensing is a mature and established tool of research over a broad range of astrophysical issues, from dark matter searches to the detection of new extrasolar planets of very low mass, down to Earth-size. This volume collects the abstracts in extended ... More
Gravitational Lensing of stars in the central arcsecond of our GalaxyMar 30 2005In the neighborhood of Sgr A*, several stars (S2, S12, S14, S1, S8, S13) enjoy an accurate determination of their orbital parameters. General Relativity predicts that the central black hole acts as a gravitational lens on these stars, generating a secondary ... More
Constraints on LISA Pathfinder's self-gravity: design requirements, estimates and testing proceduresApr 28 2016LISA Pathfinder satellite has been launched on 3th December 2015 toward the Sun-Earth first Lagrangian point (L1) where the LISA Technology Package (LTP), which is the main science payload, will be tested. With its cutting-edge technology, the LTP will ... More
Is the Hawking quasilocal energy "Newtonian"?Oct 13 2015The Misner-Sharp-Hernandez mass defined in general relativity and in spherical symmetry has been recognized as having a Newtonian character in previous literature. In order to better understand this aspect we relax spherical symmetry and we study the ... More
Quasilocal energy in modified gravityAug 27 2015Feb 09 2016A new generalization of the Hawking-Hayward quasilocal energy to scalar-tensor gravity is proposed without assuming symmetries, asymptotic flatness, or special spacetime metrics. The procedure followed is simple but powerful and consists of writing the ... More
Reply to `A comment on `The Cauchy problem of f(R) gravity''Jun 29 2009We reply to a comment by Capozziello and Vignolo about the Cauchy problem of Palatini f(R) gravity.
Modified gravity and the stability of de Sitter spaceSep 01 2005Within the context of modified gravity and dark energy scenarios of the accelerating universe, we study the stability of de Sitter space with respect to inhomogeneous perturbations using a gauge-independent formalism. In modified gravity the stability ... More
Solving for the dynamics of the universeJan 08 1999A new method of solving the Einstein-Friedmann dynamical equations of a spatially homogeneous and isotropic universe is presented. The method is applicable when the equation of state of the material content assumes the form P=(g -1) rho, g = constant. ... More
On the rotation of polarization by a gravitational lensNov 17 1992It is proved that the field of a gravitational lens induces no rotation in the polarization vector of electromagnetic radiation, in agreement with the previous literature, but with a different approach. The result is generalized to the case of less conventional ... More
A symmetry of the spatially flat Friedmann equations with barotropic fluidAug 10 2011A string-inspired duality symmetry of the spatially flat Friedmann equations of general-relativistic cosmology is discussed and generalized, providing a map between exact solutions corresponding to different values of the barotropic index.
Black hole entropy in scalar-tensor and f(R) gravity: an overviewMay 13 2010A short overview of black hole entropy in alternative gravitational theories is presented. Motivated by the recent attempts to explain the cosmic acceleration without dark energy, we focus on metric and Palatini f(R) gravity and on scalar-tensor theories. ... More
Extension of the EGS theorem to metric and Palatini f(R) gravityNov 12 2008By using the equivalence between metric and Palatini f(R) (or "modified") gravities with omega=0, -3/2 Brans-Dicke theories, it is shown that the Ehlers-Geren-Sachs theorem of general relativity is extended to modified gravity. In the case of metric f(R) ... More
Generalized slow-roll inflationApr 03 2000Apr 17 2000The slow-roll approximation to inflation is ultimately justified by the presence of inflationary attractors for the orbits of the solutions of the dynamical equations in phase space. There are many indications that the inflaton field couples nonminimally ... More
Modelling Complexity: the case of Climate ScienceJun 07 2011We briefly review some of the scientific challenges and epistemological issues related to climate science. We discuss the formulation and testing of theories and numerical models, which, given the presence of unavoidable uncertainties in observational ... More
Stochastic perturbations to dynamical systems: a response theory approachMar 01 2011Sep 12 2011We study the impact of stochastic perturbations to deterministic dynamical systems using the formalism of the Ruelle response theory and explore how stochastic noise can be used to explore the properties of the underlying deterministic dynamics of a system. ... More
Light amplification by gravitational waves in scalar-tensor theories of gravityFeb 29 1996It is shown that the amplification of a light beam by gravitational waves in scalar-tensor theories of gravity is a first order effect in the wave amplitudes. In general relativity, instead, the effect is only of second order.
The creation of multiple images by a gravitational waveOct 21 1996We describe gravitational lensing by a gravitational wave, in the regime in which multiple images of a light source are created. We adapt the vector formalism employed for ordinary gravitational lenses to the case of a non-stationary spacetime, and we ... More
Non-minimal coupling of the scalar field and inflationFeb 21 1996We study the prescriptions for the coupling constant of a scalar field to the Ricci curvature of spacetime in specific gravity and scalar field theories. The results are applied to the most popular inflationary scenarios of the universe; their theoretical ... More
Amenability, locally finite spaces, and bi-lipschitz embeddingsMay 02 2012Sep 08 2012We define the isoperimetric constant for any locally finite metric space and we study the property of having isoperimetric constant equal to zero. This property, called Small Neighborhood property, clearly extends amenability to any locally finite space. ... More
Topology on locally finite metric spacesNov 01 2011Nov 09 2011The necessity of a theory of General Topology and, most of all, of Algebraic Topology on locally finite metric spaces comes from many areas of research in both Applied and Pure Mathematics: Molecular Biology, Mathematical Chemistry, Computer Science, ... More
Gender differences in lying in sender-receiver games: A meta-analysisMar 10 2017Jul 16 2018Whether there are gender differences in lying has been largely debated in the past decade. Previous studies found mixed results. To shed light on this topic, here I report a meta-analysis of 8,728 distinct observations, collected in 65 Sender-Receiver ... More
Gender differences in the equity-efficiency trade-offMay 10 2019Jun 03 2019Gender differences in human behaviour have attracted generations of social scientists, who have explored whether males and females act differently in domains involving competition, risk taking, cooperation, altruism, honesty, as well as many others. Yet, ... More
Regular two-component bouncing cosmologies and perturbations thereinJun 07 2005Sep 08 2005We present a full investigation of scalar perturbations in a rather generic model for a regular bouncing universe, where the bounce is triggered by an effective perfect fluid with negative energy density. Long before and after the bounce the universe ... More
Time Delay in Black Hole Gravitational Lensing as a Distance EstimatorMay 02 2003Sep 23 2003We calculate the time delay between different relativistic images formed by black hole gravitational lensing in the strong field limit. For spherically symmetric black holes, it turns out that the time delay between the first two images is proportional ... More
Conformally coupled inflationSep 19 2013A massive scalar field in a curved spacetime can propagate along the light cone, a causal pathology, which can, in principle, be eliminated only if the scalar couples conformally to the Ricci curvature of spacetime. This property mandates conformal coupling ... More
Gauge-invariant gravitational wave modes in pre-big bang cosmologySep 03 2010The t<0 branch of pre-big bang cosmological scenarios is subject to a gravitational wave instability. The unstable behaviour of tensor perturbations is derived in a very simple way in Hwang's covariant and gauge-invariant formalism developed for extended ... More
The Hawking temperature of expanding cosmological black holesOct 10 2007Nov 13 2007In the context of a debate on the correct expression of the Hawking temperature of an expanding cosmological black hole, we show that the correct expression in terms of the Hawking-Hayward quasi-local energy m of the hole is T=1/(8\pi m(t)). This expression ... More
A common misconception about LIGO detectors of gravitational wavesFeb 14 2007A common misconception about laser interferometric detectors of gravitational waves purports that, because the wavelength of laser light and the length of an interferometer's arm are both stretched by a gravitational wave, no effect should be visible, ... More
Solar System experiments do not yet veto modified gravity modelsJul 05 2006The dynamical equivalence between modified and scalar-tensor gravity theories is revisited and it is concluded that it breaks down in the limit to general relativity. A gauge-independent analysis of cosmological perturbations in both classes of theories ... More
Negative energy and stability in scalar-tensor gravityAug 20 2004Linearized gravitational waves in Brans-Dicke and scalar-tensor theories carry negative energy. A gauge-invariant analysis shows that the background Minkowski space is stable at the classical level with respect to linear scalar and tensor inhomogeneous ... More
Evolving black hole horizons in General Relativity and alternative gravitySep 19 2013From the microscopic point of view, realistic black holes are time-dependent and the teleological concept of event horizon fails. At present, the apparent or the trapping horizon seem its best replacements in various areas of black hole physics. We discuss ... More
No "big trips" for the universeFeb 27 2007It has been claimed in several papers that a phantom energy-dominated universe can undergo a ``big trip'', i.e., tunneling through a wormhole that grows faster than the cosmic substratum due to the accretion of phantom energy, and will reappear on the ... More
Phase space geometry in scalar-tensor cosmologyFeb 03 2005We study the phase space of spatially homogeneous and isotropic cosmology in general scalar-tensor theories. A reduction to a two-dimensional phase space is performed when possible-in these situations the phase space is usually a two-dimensional curved ... More
Evidence of dispersion relations for the nonlinear response of the Lorenz 63 systemSep 01 2008Along the lines of the nonlinear response theory developed by Ruelle, in a previous paper we have proved under rather general conditions that Kramers-Kronig dispersion relations and sum rules apply for a class of susceptibilities describing at any order ... More
Response Theory for Equilibrium and Non-Equilibrium Statistical Mechanics: Causality and Generalized Kramers-Kronig relationsOct 04 2007We consider the general response theory proposed by Ruelle for describing the impact of small perturbations to the non-equilibrium steady states resulting from Axiom A dynamical systems. We show that the causality of the response functions allows for ... More
A back-reaction approach to dark energyMar 21 2008Apr 23 2008This thesis is mainly about how to set up and carry out in a physically meaningful way the idea of back-reaction, according to which dark energy could be an effective source. There are, broadly speaking, two distinct approaches. One is focused on how ... More
On the local and non-local content of bipartite qubit and qutrit correlationsDec 14 2007Apr 08 2008The local and non-local contents of non-local probability distributions are studied using the approach of Elitzur, Popescu and Rohrlich [Phys. Lett. A \textbf{162}, 25 (1992)]. This work focuses on distributions that can be obtained by single-copy von ... More
Classical and quantum: some mutual clarificationsSep 15 2003This paper presents two unconventional links between quantum and classical physics. The first link appears in the study of quantum cryptography. In the presence of a spy, the quantum correlations shared by Alice and Bob are imperfect. One can either process ... More
The emergence of hyper-altruistic behaviour in conflictual situationsOct 06 2014Dec 22 2014Situations where people have to decide between hurting themselves or another person are at the core of many individual and global conflicts. Yet little is known about how people behave when facing these situations in the lab. Here we report a large experiment ... More
Women are slightly more cooperative than men (in one-shot Prisoner's dilemma games played online)May 21 2018Differences between men and women have intrigued generations of social scientists, who have found that the two sexes behave differently in settings requiring competition, risk taking, altruism, honesty, as well as many others. Yet, little is known about ... More
Halfspaces minimise nonlocal perimeter: a proof via calibrationsMay 02 2019We consider a nonlocal functional $J_K$ that may be regarded as a nonlocal version of the total variation. More precisely, for any measurable function $u\colon \mathbb{R}^d \to \mathbb{R}$, we define $J_K(u)$ as the integral of weighted differences of ... More
Quantum Dynamical Entropies and Complexity in Dynamical SystemsMar 18 2004We analyze the behaviour of two quantum dynamical entropies in connection with the classical limit. Using strongly chaotic classical dynamical systems as models (Arnold Cat Maps and Sawtooth Maps), we also propose a discretization procedure that resembles ... More
The Jebsen-Birkhoff theorem in alternative gravityJan 13 2010We discuss the validity, or lack thereof, of the Jebsen-Birkhoff theorem in scalar-tensor theories by generalizing it and regarding the Brans-Dicke-like scalar as effective matter. Both the Jordan and Einstein frames are discussed and an apparent contradiction ... More
Clifton's spherical solution in f(R) vacuo harbours a naked singularitySep 02 2009Sep 14 2009Clifton's exact solution of f(R)=R^(1+\delta) gravity describing a dynamical spherical metric which is asymptotically Friedmann-Lemaitre-Robertson-Walker is studied. It is shown that it harbours a strong spacetime singularity and that this singularity ... More
Palatini f(R) gravity as a fixed pointJun 04 2008In the context of modified gravity, we point out how the Palatini version of these theories is singled out as a very special case corresponding to the unique fixed point of a transformation involving a special conformal rescaling of the metric. This mathematical ... More
de Sitter space and the equivalence between f(R) and scalar-tensor gravityMar 06 2007Mar 26 2007It is shown that, when f'' is non-vanishing, metric f(R) gravity is completely equivalent to a scalar-tensor theory (with zero Brans-Dicke parameter) with respect to perturbations of de Sitter space, contrary to previous expectations. Moreover, the stability ... More
Nonstationary Gravitational Lenses and the Fermat PrincipleMay 01 1992We apply Perlick's (1990a) rigorous formulation of the Fermat principle in arbitrary spacetimes to prove the correctness of the description of gravitational lensing by gravitational waves, given in the literature using the scalar and vector formalisms. ... More
Horizons and singularity in Clifton's spherical solution of f(R) vacuumMay 28 2010Due to the failure of Birkhoff's theorem, black holes in f(R) gravity theories in which an effective time-varying cosmological "constant" is present are, in general, dynamical. Clifton's exact spherical solution of R^(1+\delta) gravity, which is dynamical ... More
A viability criterion for modified gravity with an extra forceOct 05 2007A recently proposed theory of modified gravity with an explicit ``anomalous'' coupling of the Ricci curvature to matter is discussed, and an inequality is derived which expresses a necessary and sufficient condition to avoid the notorius Dolgov-Kawasaki ... More
Matter instability in modified gravityOct 25 2006Nov 10 2006The Dolgov-Kawasaki instability discovered in the matter sector of the modified gravity scenario incorporating a 1/R correction to Einstein gravity is studied in general f(R) theories. A stability condition is found in the metric version of these theories ... More
Big smash of the universeJul 18 2003The occurrence of a big smash singularity which ends the universe in a finite time in the future is investigated in the context of superquintessence, i.e. dark energy with effective equation of state parameter w<-1. The simplest model of superquintessence ... More
A crucial ingredient of inflationSep 08 2000Nonminimal coupling of the inflaton field to the Ricci curvature of spacetime is generally unavoidable, and the paradigm of inflation should be generalized by including the corresponding term in the Lagrangian of the inflationary theory. This paper reports ... More
Architecture of the LHCb muon Frontend control system upgradeDec 08 2015The LHCb experiment(Fig. 1), that is presently taking data at CERN (European Center for Nuclear Research) Large Hadron Collider (LHC), aims at the study of CP violation in the B meson sector. Its key elements is the Muon detector [1], which allows triggering, ... More
The redshift periodicity of galaxies as a probe of the correctness of general relativityAug 28 1996Recent theoretical work determines the correct coupling constant of a scalar field to the Ricci curvature of spacetime in general relativity. The periodicity in the redshift distribution of galaxies observed by Broadhurst {\em et al.}, if genuine, determines ... More
Limiting the complexity of quantum states: a toy theoryMar 30 2015This paper discusses a restriction of quantum theory, in which very complex states would be excluded. The toy theory is phrased in the language of the circuit model for quantum computing, its key ingredient being a limitation on the number of interactions ... More
A solution concept for games with altruism and cooperationFeb 16 2013Sep 10 2013Over the years, numerous experiments have been accumulated to show that cooperation is not casual and depends on the payoffs of the game. These findings suggest that humans have attitude to cooperation by nature and the same person may act more or less ... More
Does the truth come naturally? Time pressure and low cognitive reflection promote honesty in deception gamesJun 15 2016Many interactions require people to act quickly, and are characterized by asymmetric information. Since asymmetric information makes people tempted to misreport their private information for their own benefit, it is important to understand whether time ... More
A Model of Human Cooperation in Social DilemmasJul 16 2013Sep 17 2013Social dilemmas are situations in which collective interests are at odds with private interests: pollution, depletion of natural resources, and intergroup conflicts, are at their core social dilemmas. Because of their multidisciplinarity and their importance, ... More
Lemaitre model and cosmic massJun 21 2015The mass of a sphere of simmetry in the Lemaitre universe is discussed using the Hawking-Hayward quasi-local energy and clarifying existing ambiguities. A covariantly conserved current introduced by Cahill and McVittie is shown to be a multiple of the ... More
Gender differences in the equity-efficiency trade-offMay 10 2019Gender differences in human behaviour have attracted generations of social scientists, who have explored whether males and females act differently in domains involving competition, risk taking, cooperation, altruism, honesty, as well as many others. Yet, ... More
A notion of continuity in discrete spaces and applicationsOct 08 2012Sep 17 2013We propose a notion of continuous path for locally finite metric spaces, taking inspiration from the recent development of A-theory for locally finite connected graphs. We use this notion of continuity to derive an analogue in Z^2 of the Jordan curve ... More
A Survey on Connes' Embedding ConjectureMar 10 2010In a very celebrated paper A. Connes has formulated a conjecture which is now one of the most important open problem in Operator Algebras. This importance comes from the works of many mathematicians who have found some unexpected equivalent statements ... More
Scalar field mass in generalized gravityJun 10 2009Jun 19 2009The notions of mass and range of a Brans-Dicke-like scalar field in scalar-tensor and f(R) gravity are subject to an ambiguity that hides a potential trap. We spell out this ambiguity and identify a physically meaningful and practical definition for these ... More
Does the non-minimal coupling of the scalar field improve or destroy inflation ?Jul 23 1998The non-minimal coupling of a scalar field to the Ricci curvature in a curved spacetime is unavoidable according to several authors. The coupling constant is not a free parameter: the prescriptions for the value of the coupling constant in specific scalar ... More
Cosmological apparent and trapping horizonsJun 22 2011The dynamics of particle, event, and apparent horizons in FLRW space are discussed. The apparent horizon is trapping when the Ricci curvature is positive. This simple criterion coincides with the condition for the Kodama-Hayward apparent horizon temperature ... More
f(R) gravity: successes and challengesOct 15 2008We review the state of the art of f(R) theories of gravity (in their various formulations), which have been proposed as an explanation of the cosmic acceleration alternative to dark energy. The successes of f(R) gravity are discussed, together with the ... More
Singularities in scalar-tensor gravityMar 03 2004The analysis of certain singularities in scalar-tensor gravity contained in a recent paper is completed, and situations are pointed out in which these singularities cannot occur.
SuperquintessenceOct 02 2001There is marginal evidence that the quintessential form of matter responsible for the acceleration of the universe observed today has ratio between pressure and energy density $w<-1$. Such a regime, called superacceleration, cannot be achieved with conventional ... More
Inflation and quintessence with nonminimal couplingFeb 26 2000Apr 17 2000The nonminimal coupling (NMC) of the scalar field to the Ricci curvature is unavoidable in many cosmological scenarios. Inflation and quintessence models based on nonminimally coupled scalar fields are studied, with particular attention to the balance ... More
From symmetry break to Poisson point process in 2D Voronoi tessellations: the generic nature of hexagonsAug 07 2007We bridge the properties of the regular square and honeycomb Voronoi tessellations of the plane to those of the Poisson-Voronoi case, thus analyzing in a common framework symmetry-break processes and the approach to uniformly random distributions of tessellation-generating ... More
Towards a definition of climate scienceAug 08 2004The intrinsic difficulties in building realistic climate models and in providing complete, reliable and meaningful observational datasets, and the conceptual impossibility of testing theories against data imply that the usual Galilean scientific validation ... More
On cosmological observables in a swiss-cheese universe (Conference proceeding)May 27 2008By means of a toy Swiss-cheese cosmological model we will discuss how to set up and carry out in a physically meaningful way the idea of back-reaction, according to which dark energy could be an effective source. We will follow two distinct approaches. ... More
Multiple imaging by gravitational wavesJul 21 1997Gravitational waves act like lenses for the light propagating through them. This phenomenon is described using the vector formalism employed for ordinary gravitational lenses, which was proved to be applicable also to a non-stationary spacetime, with ... More
The device-independent outlook on quantum physics (lecture notes on the power of Bell's theorem)Mar 13 2013Jun 18 2015This text is an introduction to an operational outlook on Bell inequalities, which has been very fruitful in the past few years. It has lead to the recognition that Bell tests have their own place in applied quantum technologies, because they quantify ... More
Quantum ComputingApr 20 1998Oct 06 1998The main features of quantum computing are described in the framework of spin resonance methods. Stress is put on the fact that quantum computing is in itself nothing but a re-interpretation (fruitful indeed) of well-known concepts. The role of the two ... More
A Survey on the Classical Limit of Quantum Dynamical EntropiesAug 21 2007Dec 13 2007We analyze the behavior of quantum dynamical entropies production from sequences of quantum approximants approaching their (chaotic) classical limit. The model of the quantized hyperbolic automorphisms of the 2-torus is examined in detail and a semi-classical ... More
A note on homology for Smale spacesJul 18 2018Jan 30 2019We collect three observations on the homology for Smale spaces defined by Putnam. The definition of such homology groups involves four complexes. It is shown here that a simple convergence theorem for spectral sequences can be used to prove that all complexes ... More
Entanglement and Irreversibility in the Approach to Thermal EquilibriumJul 06 2007When a physical system is put in contact with a very large thermal bath, it undergoes a dissipative (i.e., an apparently irreversible) process that leads to thermal equilibrium. This dynamical process can be described fully within quantum physics, involving ... More
Feats, Features and Failures of the PR-boxMar 02 2006Mar 11 2006One of the most intriguing features of quantum physics is the non-locality of correlations that can be obtained by measuring entangled particles. Recently, it has been noticed that non-locality can be studied without reference to the Hilbert space formalism. ... More
Quantum information: primitive notions and quantum correlationsOct 22 2009This series of introductory lectures consists of two parts. In the first part, I rapidly review the basic notions of quantum physics and many primitives of quantum information (i.e. notions that one must be somehow familiar with in the field, like cloning, ... More
The universe would not be perfect without randomness: a quantum physicist's reading of AquinasJan 05 2015Randomness is an unavoidable notion in discussing quantum physics, and this may trigger the curiosity to know more of its cultural history. This text is an invitation to explore the position on the matter of Thomas Aquinas, one of the most prominent philosophers ... More
Solution of Wald's game using loadings and allowed strategiesJun 18 2011Mar 11 2012We propose a new interpretation of the strange phenomena that some authors have observed about the Wald game. This interpretation is possible thanks to the new language of \emph{loadings} that Morrison and the author have introduced in a previous work. ... More
The omega-tends-to-infinity limit of Brans-Dicke theoryMay 14 1998The standard tenet that Brans-Dicke theory reduces to general relativity in the omega-tends-to-infinity limit has been shown to be false when the trace of the matter energy-momentum tensor vanishes. The issue is clarified in a new approach and the asymptotic ... More
Turnaround radius in modified gravityJul 31 2015Nov 04 2015In an accelerating universe in General Relativity there is a maximum radius above which a shell of test particles cannot collapse, but is dispersed by the cosmic expansion. This radius could be used in conjunction with observations of large structures ... More
The Lagrangian description of perfect fluids and modified gravity with an extra forceDec 07 2009Dec 18 2009We revisit the issue of the correct Lagrangian description of a perfect fluid (pressure versus minus energy density) in relation with modified gravity theories in which galactic luminous matter couples nonminimally to the Ricci scalar. These Lagrangians ... More
The rotation of polarization by gravitational wavesSep 04 2007There are conflicting statements in the literature about the gravitational Faraday rotation of the plane of polarization of polarized electromagnetic radiation travelling through a gravitational wave. This issue is reconsidered using a simple formalism ... More
Phantom cosmology with general potentialsJun 17 2005We present a unified treatment of the phase space of a spatially flat homogeneous and isotropic universe dominated by a phantom field. Results on the dynamics and the late time attractors (Big Rip, de Sitter, etc.) are derived without specifying the form ... More
Coupled oscillators as models of phantom and scalar field cosmologiesApr 17 2004We study a toy model for phantom cosmology recently introduced in the literature and consisting of two oscillators, one of which carries negative kinetic energy. The results are compared with the exact phase space picture obtained for similar dynamical ... More
A new solution for inflationJun 10 2000Many pedagogical introductions of cosmological inflation are effective due to the simplicity of the relevant equations. Here an analytic solution of the cosmological equations is presented and used as an example to discuss fundamental aspects of the inflationary ... More
The correspondence between a scalar field and an effective perfect fluidJan 06 2012It is widely acknowledged that, for formal purposes, a minimally coupled scalar field is equivalent to an effective perfect fluid with equation of state determined by the scalar potential. This correspondence is not complete because the Lagrangian densities ... More
R^n gravity and the chameleonJun 01 2011The Solar System bounds on R^n gravity are often ignored in the literature by invoking the chameleon mechanism. We show that in order for the latter to work, the exponent n must be ridicolously close to unity and, therefore, these theories are severely ... More
Harrison's interpretation of the cosmological redshift revisitedAug 24 2009Harrison's argument against the interpretation of the cosmological redshift as a Doppler effect is revisited, exaggerated, and discussed. The context, purpose, and limitations of the interpretations of this phenomenon are clarified.
An analysis of the Sultana-Dyer cosmological black hole solution of the Einstein equationsJul 26 2009The Sultana-Dyer solution of general relativity representing a black hole embedded in a special cosmological background is analysed. We find an expanding (weak) spacetime singularity instead of the reported conformal Killing horizon, which is covered ... More
de Sitter attractors in generalized gravityJul 06 2004We obtain conditions for the existence and stability of de Sitter attractors in the phase space of spatially homogeneous and isotropic cosmology in generalized theories of gravity (including non-linear and scalar-tensor theories). These conditions are ... More