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Valerio Faraoni

Publications and source records attributed to Valerio Faraoni.

At least 127 records · Page 7Linked to original sources
arXiv2015

Black holes and wormholes subject to conformal mappings

Solutions of the field equations of theories of gravity which admit distinct conformal frame representations can look very different in these frames. We show that Brans class IV solutions describe wormholes in the Jordan frame (in a certain parameter range) but correspond to horizonless geometries in the Einstein frame. The reasons for such a change of behaviour under conformal mappings are elucidated in general, using Brans IV solutions as an example.

gr-qc↗
arXiv2015

Lemaitre model and cosmic mass

The 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 Kodama energy current.

gr-qc↗
arXiv2015

Do Newtonian large-scale structure simulations fail to include relativistic effects?

The Newtonian simulations describing the formation of large-scale structures do not include relativistic effects. A new approach to this problem is proposed, which consists of splitting the Hawking-Hayward quasi-local energy of a closed spacelike 2-surface into a "Newtonian" part due to local perturbations and a "relativistic" part due to the cosmology. It is found that the Newtonian part dominates over the relativistic one as time evolves, lending support to the validity of Newtonian simulations.

gr-qc↗
arXiv2014

Covariantizing the interaction between dark energy and dark matter

Coupling dark energy and dark matter through an effective fluid description is a very common procedure in cosmology, however it always remains in comoving coordinates in the special FLRW space. We construct a consistent, general, and covariant formulation, where the interaction is a natural implication of the imperfectness of the fluids. This imperfectness makes difficult the final step towards a robust formulation of interacting fluids, namely the construction of a Lagrangian whose variation would give rise to the interacting equations. Nevertheless, we present a formal solution to this problem for a single fluid, through the introduction of an effective metric.

gr-qc↗
arXiv2014

The charged McVittie spacetime

The two-parameter charged McVittie solution of the Einstein equations is revisited and its apparent horizons are discussed and located numerically (for the extremal case, analytically). According to the parameter values, this spacetime can be interpreted as a black hole, or a spacelike naked singularity, in a spatially homogeneous and isotropic universe.

gr-qc↗
arXiv2014

Are there really `black holes' in the Atlantic Ocean?

In this letter we point out some interpretational difficulties associated with concepts from General Relativity in a recent article which appeared in J. Fluid Mech. 731 (2013) R4 where a Lorentzian metric was defined for turbulent fluid flow and interpreted as being analogous to a black hole metric. We show that the similarity with black hole geometry is superficial at best while clarifying the nature of the black hole geometry and the work above with some examples.

physics.flu-dyn↗
arXiv2014

Horizon thermodynamics and spacetime mappings

When black holes are dynamical, event horizons are replaced by apparent and trapping horizons. Conformal and Kerr-Schild transformations are widely used in relation with dynamical black holes and we study the behaviour under such transformations of quantities related to the thermodynamics of these horizons, such as the Misner-Sharp-Hernandez mass (internal energy), the Kodama vector, surface gravity, and temperature. The transformation properties are not those expected on the basis of naive arguments.

gr-qc↗
arXiv2013

Conformally coupled inflation

A 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 for the field driving inflation in the early universe. During slow-roll inflation, this coupling can cause super-acceleration and, as a signature, a blue spectrum of primordial gravitational waves.

gr-qc↗
arXiv2013

Evolving black hole horizons in General Relativity and alternative gravity

From 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 the known phenomenology of apparent and trapping horizons for analytical solutions of General Relativity and alternative theories of gravity. These specific examples (we focus on spherically symmetric inhomogeneities in a background cosmological spacetime) may be useful as toy models for research on various aspects of black hole physics.

gr-qc↗
arXiv2013

Are quantization rules for horizon areas universal?

Doubts have been expressed on the universality of holographic/string-inspired quantization rules for the horizon areas of stationary black holes or the products of their radii, already in simple 4-dimensional general relativity. Realistic black holes are not stationary but time-dependent. Using two examples of 4D general-relativistic spacetimes containing dynamical black holes for at least part of the time, it is shown that the quantization rules (even counting virtual horizons) cannot hold, except possibly at isolated instants of time, and do not seem to be universal.

hep-th↗
arXiv2013

Absence of scalar hair in scalar-tensor gravity

Stationary, asymptotically flat black holes in scalar-tensor theories of gravity are studied. It is shown that such black holes have no scalar hair and are the same as in General Relativity.

gr-qc↗
arXiv2013

Apparent horizons in Clifton-Mota-Barrow inhomogeneous universe

We analyze the apparent horizon dynamics in the inhomogeneous Clifton-Mota-Barrow solution of Brans-Dicke theory. This solution models a central matter configuration embedded in a cosmological background. In certain regions of the parameter space we find solutions exhibiting dynamical creation or merging of two horizons.

gr-qc↗
arXiv2012

Interpreting the conformal cousin of the Husain-Martinez-Nunez solution

A 2-parameter inhomogeneous cosmology in Brans-Dicke theory, obtained by conformally transforming the Husain-Martinez-Nunez (HMN) scalar field solution of the Einstein equations is studied and interpreted physically. According to the values of the parameters it describes a wormhole, a naked singularity, or else a spacetime containing apparent black hole horizons which appear/disappear in pairs (as well as a cosmological horizon) as in the original HMN metric. The reasons why there isn't a one-to-one correspondence between conformal copies of this metric are discussed.

gr-qc↗
arXiv2012

Dynamical apparent horizons in inhomogeneous Brans-Dicke universes

The presence and evolution of apparent horizons in a two-parameter family of spherically symmetric, time-dependent solutions of Brans-Dicke gravity are analyzed. These solutions were introduced to model space- and time-varying gravitational couplings and are supposed to represent central objects embedded in a spatially flat universe. We find that the solutions possess multiple evolving apparent horizons, both black hole horizons covering a central singularity and cosmological ones. It is not uncommon for two of these horizons to merge, leaving behind a naked singularity covered only by a cosmological horizon. Two characteristic limits are also explicitly worked out: the limit where the theory reduces to general relativity and the limit where the solutions become static. The physical relevance of this family of solutions is discussed.

gr-qc↗
arXiv2012

The Campanelli-Lousto and veiled spacetimes

The Campanelli-Lousto solutions of Brans-Dicke theory, usually reported as black holes are reconsidered and shown to describe, according to the values of a parameter, wormholes or naked singularities. The veiled Schwarzschild metric recently used as an example to discuss conformal frames and their equivalence corresponds to a special case of the CL metric. The conformal cousins of these solutions, and of the Riegert black hole solution of conformally invariant Weyl theory, are analysed, leading to a word of caution when interpreting physically spacetimes generated via conformal transformations from known seed solutions.

gr-qc↗
arXiv2012

Scalar field cosmology in phase space

Using dynamical systems methods, we describe the evolution of a minimally coupled scalar field and a Friedmann-Lemaitre-Robertson-Walker universe in the context of general relativity, which is relevant for inflation and late-time quintessence eras. Focussing on the spatially flat case, we examine the geometrical structure of the phase space, locate the equilibrium points of the system (de Sitter spaces with a constant scalar field), study their stability through both a third-order perturbation analysis and Lyapunov functions, and discuss the late-time asymptotics. As we do not specify the scalar field's origin or its potential, the results are independent of the high-energy model.

gr-qc↗