arXiv subjects

Luciano Gabbanelli

Publications and source records attributed to Luciano Gabbanelli.

9 recordsLinked to original sources

arXiv2021

Cosmological implications of the hydrodynamical phase of group field theory

In this review we focus on the main cosmological implications of the Group Field Theory approach, according to which an effective continuum macroscopic dynamics can be extracted from the underlying formalism for quantum gravity. Within this picture what counts is the collective behaviour of a large number of quanta of geometry. The resulting state is a condensate-like structure made of "pre-geometric" excitations of the Group Field Theory field over a no-space vacuum. Starting from the kinematics and dynamics, we offer an overview of the way in which Group Field Theory condensate cosmology treats solutions for the homogeneous and isotropic universe. These solutions including a bounce, share with other quantum cosmological approaches the resolution of the singularity characterizing general relativity. Contrary to what is usually done in quantum cosmology, in GFT cosmology no preliminary symmetry reduction is needed for this purpose. We conclude with a discussion of the limits and future perspectives of the Group Field Theory approach.

gr-qc↗

arXiv2020

Condensates beyond the horizons

In this work we continue our previous studies concerning the possibility of the existence of a Bose-Einstein condensate in the interior of a static black hole, a possibility first advocated by Dvali and Gómez. We find that the phenomenon seems to be rather generic and it is associated to the presence of an horizon, acting as a confining potential. We extend the previous considerations to a Reissner-Nordström black hole and to the de Sitter cosmological horizon. In the latter case the use of static coordinates is essential to understand the physical picture. In order to see whether a BEC is preferred, we use the Brown-York quasilocal energy, finding that a condensate is energetically favourable in all cases in the classically forbidden region. The Brown-York quasilocal energy also allows us to derive a quasilocal potential, whose consequences we explore. Assuming the validity of this quasilocal potential allows us to suggest a possible mechanism to generate a graviton condensate in black holes. However, this mechanism appears not to be feasible in order to generate a quantum condensate behind the cosmological de Sitter horizon.

hep-th↗

arXiv2020

Analysis of some classical and quantum aspects of black holes

The purpose of this thesis is to provide insights into different aspects of black hole physics, both at a quantum level and from an observational point of view. On the one hand, regarding their fundamental constitutive characteristics, we have followed ideas proposing a bridge between black holes physics and quantum information. In this approach, these objects can be understood as Bose-Einstein condensates of weakly interacting gravitons. We have studied the existence of such solutions considered as bound systems of many constituents, providing well established metrics (Schwarzschild and Reissner-Nordström). It should be noted that all solutions found can be interpreted as mean field wave functions of the condensate and are strongly related to the classical structure of the metric that supports them. On the other hand, it is well known that the acceleration of very massive bodies produces wave-like disturbances in spacetime itself. We focus on the low-frequency range which is expected to be measured by arrays of pulsars employed as galactic clocks (Pulsar Timing Arrays). When waves passes, the very stable ticks are disturbed. However the existence of a dynamical background, mainly composed by a cosmological constant $Λ$, may affect the gravitational wave propagation and would modify the expected signal over the PTA. We have studied the influence of different backgrounds composed by relativistic and non-relativistic matter, together with the cosmological constant. We present a detailed characterization of this effect which at first order depends only on the Hubble constant $H_0$ (at higher orders different contributions can be disentangled). We also discuss how and where this effect can be found. We hope that our results contribute to a definitive detection by PTA. Needless to say, an independent local determination of $H_0$ would be of enormous interest.

gr-qc↗

arXiv2020

Measuring $H_0$ with pulsar timing arrays

Pulsar Timing Arrays have yet to convincingly observe gravitational waves. Some time ago it was pointed out by one of the authors that a dramatic enhancement of the signal would take place for particular values of the angle subtended by the source and the observed pulsar. This enhancement is due to the fact that waves propagate in a Friedmann-Lemaitre-Robertson-Walker metric where, contrary to some wide-spread belief, a simple harmonic function with a red-shifted frequency is not a solution of the equation of motion. At the first non-trivial order, proper solutions have an effective wave number that differs from the frequency. This leads to some interesting effects in Pulsar Timing Arrays whose most visible manifestation is the enhancement of the signal that, all other parameters kept fixed, is related in a simple manner to the value of $H_0$. In this work, we rederive in an alternative way the main results, extend the formalism to a more realistic setting where all components in the cosmological budget are included, investigate in detail the dependence of the signal on the various parameters involved and propose an observational set-up to hopefully detect this very relevant effect.

gr-qc↗

arXiv2019

Minimal Geometric Deformation in a Reissner-Nordström background

This article is devoted to the study of new exact analytical solutions in the background of Reissner-Nordström space-time by using gravitational decoupling via minimal geometric deformation approach. To do so, we impose the most general equation of state, relating the components of the $θ$-sector in order to obtain the new material contributions and the decoupler function $f(r)$. Besides, we obtain the bounds on the free parameters of the extended solution to avoid new singularities. Furthermore, we show the finitude of all thermodynamic parameters of the solution such as the effective density $\tildeρ$, radial $\tilde{p}_{r}$ and tangential $\tilde{p}_{t}$ pressure for different values of parameter $α$ and the total electric charge $Q$. Finally, the behavior of some scalar invariants, namely the Ricci $R$ and Kretshmann $R_{μνωε}R^{μνωε}$ scalars are analyzed. It is also remarkable that, after an appropriate limit, the deformed Schwarzschild black hole solution always can be recovered.

gr-qc↗

arXiv2018

Gravitational decoupled anisotropies in compact stars

Simple generic extensions of isotropic Durgapal--Fuloria stars to the anisotropic domain are presented. These anisotropic solutions are obtained by guided minimal deformations over a self gravitating isotropic system. When the isotropic and the anisotropic sector interacts in a purely gravitational manner, the conditions to decouple both sectors by means of the minimal geometric deformation approach are satisfied. Hence the anisotropic field equations are isolated resulting a more treatable set. The simplicity of the equations allows one to manipulate the anisotropies that can be implemented in a systematic way to obtain different realistic models for anisotropic configurations. Later on, observational effects of such anisotropies when measuring the redshift are discussed. To conclude, the application of the method over anisotropic solutions is generalized. In this manner, different anisotropic sectors can be isolated of each other and modeled in a simple and systematic way. Besides, a generic property of the minimal geometric deformation approach, its noncommutativity, is discussed. This property duplicates the solutions obtained through this approach; anisotropies applied in the reversed order give different physically acceptable configurations.

gr-qc↗

arXiv2017

On the propagation of gravitational waves in a $Λ$CDM universe

We study here how the presence of non-zero matter density and a cosmological constant could affect the observation of gravitational waves in Pulsar Timing Arrays. Conventionally, the effect of matter and cosmological constant is included by considering the redshift in frequency due to the expansion. However, there is an additional effect due to the change of coordinate systems from the natural ones in the region where waves are produced to the ones used to measure the pulsar timing residuals. This change is unavoidable as the strong gravitational field in a black hole merger distorts clocks and rules. Harmonic waves produced in such a merger become anharmonic when detected by a cosmological observer. The effect is small but appears to be observable for the type of gravitational waves to which PTA are sensitive and for the favoured values of the cosmological parameters.

hep-th↗

arXiv2017

Bose-Einstein graviton condensate in a Schwarzschild black hole

We analyze in detail a previous proposal by Dvali and Gómez that black holes could be treated as consisting of a Bose-Einstein condensate of gravitons. In order to do so we extend the Einstein-Hilbert action with a chemical potential-like term, thus placing ourselves in a grand-canonical ensemble. The form and characteristics of this chemical potential-like piece are discussed in some detail. We argue that the resulting equations of motion derived from the action could be interpreted as the Gross-Pitaevskii equation describing a graviton Bose-Einstein condensate trapped by the black hole gravitational field. After this, we proceed to expand the ensuring equations of motion up to second order around the classical Schwarzschild metric so that some non-linear terms in the metric fluctuation are kept. Next we search for solutions and, modulo some very plausible assumptions, we find out that the condensate vanishes outside the horizon but is non-zero in its interior. Inspired by a linearized approximation around the horizon we are able to find an exact solution for the mean-field wave function describing the graviton Bose-Einstein condensate in the black hole interior. After this, we can rederive some of the relations involving the number of gravitons $N$ and the black hole characteristics along the lines suggested by Dvali and Gómez.

gr-qc↗

arXiv2015

Double Higgs mechanisms, supermassive stable particles and the vacuum energy

In the present work it is shown that some specific double Higgs like mechanisms may have interesting cosmological applications. A hidden scenario which cast long lived super heavy particles together with an extremely light particle $a$ with mass $m_a\sim 10^{-32}-10^{-33}$eV is presented. The potential energy of this particle models the vacuum energy density of the universe $ρ_c\simeq 10^{-47}\;\hbox{GeV}^4$. The construction of such scenario is non trivial since the presence of light particles may spoil the stability of the heavy particles. However, double Higgs mechanisms may be helpful for overcoming this problem. The hidden sector we propose include fermions with masses near the neutrino mass $m_ν\sim 10^{-2}$eV which arise in terms of a see saw mechanism. Besides, the super heavy particles acquire a mass due to a double Higgs like mechanism of the order of the GUT scale. The gauge group of the model is $\hbox{SU(2)}_L$ and the scalars of the double Higgs mechanism are not charged under these interactions. The light particle $a$ is the Goldstone boson associated to a Peccei-Quinn like symmetry in the double Higgs model. In addition, the double Higgs mechanism posses another CP odd scalar $A^0$, which acquire a mass of the order of the GUT scale. We show that if there is no direct coupling between $A^0$ and $a$, even in presence of indirect couplings, the $A^0$ particle is long lived an may appear in events above the GKZ bound in present times.

hep-th↗