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S. Zerbini

Publications and source records attributed to S. Zerbini.

At least 37 records · Page 2Linked to original sources
arXiv2009

Initial and final de Sitter universes from modified $f(R)$ gravity

Viable models of modified gravity which satisfy both local as well as cosmological tests are investigated. It is demonstrated that some versions of such highly non-linear models exhibit multiply de Sitter universe solutions, which often appear in pairs, being one of them stable and the other unstable. It is explicitly shown that, for some values of the parameters, it is possible to find several de Sitter spaces (as a rule, numerically); one of them may serve for the inflationary stage, while the other can be used for the description of the dark energy epoch. The numerical evolution of the effective equation of state parameter is also presented, showing that these models can be considered as natural candidates for the unification of early-time inflation with late-time acceleration through dS critical points. Moreover, based on the de Sitter solutions, multiply SdS universes are constructed which might also appear at the (pre-)inflationary stage. Their thermodynamics are studied and free energies are compared.

gr-qc↗
arXiv2008

Local temperature for dynamical black holes

A local Hawking temperature was recently derived for any future outer trapping horizon in spherical symmetry, using a Hamilton-Jacobi tunneling method, and is given by a dynamical surface gravity as defined geometrically. Descriptions are given of the operational meaning of the temperature, in terms of what observers measure, and its relation to the usual Hawking temperature for static black holes. Implications for the final fate of an evaporating black hole are discussed.

gr-qc↗
arXiv2008

Phantom and non-phantom dark energy: The cosmological relevance of non-locally corrected gravity

In this paper we have investigated the cosmological dynamics of non-locally corrected gravity involving a function of the inverse d'Alembertian of the Ricci scalar, $f(\Box^{-1} R))$. Casting the dynamical equations into local form, we derive the fixed points of the dynamics and demonstrate the existence and stability of a one parameter family of dark energy solutions for a simple choice, $f(\Box^{-1} R)\sim \exp(α\Box^{-1} R)$. The effective EoS parameter is given by, $w_{\rm eff}=({α-1})/({3α-1})$ and the stability of the solutions is guaranteed provided that $1/3<α<2/3$. For $1/3<α<1/2$ and $1/2<α<2/3$, the underlying system exhibits phantom and non-phantom behavior respectively; the de Sitter solution corresponds to $α=1/2$. For a wide range of initial conditions, the system mimics dust like behavior before reaching the stable fixed point. The late time phantom phase is achieved without involving negative kinetic energy fields. A brief discussion on the entropy of de Sitter space in non-local model is included.

hep-th↗
arXiv2008

Class of viable modified $f(R)$ gravities describing inflation and the onset of accelerated expansion

A general approach to viable modified $f(R)$ gravity is developed in both the Jordan and the Einstein frames. A class of exponential, realistic modified gravities is introduced and investigated with care. Special focus is made on step-class models, most promising from the phenomenological viewpoint and which provide a natural way to classify all viable modified gravities. One- and two-steps models are explicitly considered, but the analysis is extensible to $N$-step models. Both inflation in the early universe and the onset of recent accelerated expansion arise in these models in a natural, unified way. Moreover, it is demonstrated that models in this category easily pass all local tests, including stability of spherical body solution, non-violation of Newton's law, and generation of a very heavy positive mass for the additional scalar degree of freedom.

hep-th↗
arXiv2007

On the Hawking radiation as tunneling for a class of dynamical black holes

The instability against emission of massless particles by the trapping horizon of an evolving black hole is analyzed with the use of the Hamilton-Jacobi method. The method automatically selects one special expression for the surface gravity of a changing horizon. Indeed, the strength of the horizon singularity turns out to be governed by the surface gravity as was defined a decade ago by Hayward using Kodama's theory of spherically symmetric gravitational fields. The theory also applies to point masses embedded in an expanding universe, were the surface gravity is still related to Kodama-Hayward theory. As a bonus of the tunneling method, we gain the insight that the surface gravity still defines a temperature parameter as long as the evolution is sufficiently slow that the black hole pass through a sequence of quasi-equilibrium states.

hep-th↗
arXiv2006

Spectral analysis and zeta determinant on the deformed spheres

We consider a class of singular Riemannian manifolds, the deformed spheres $S^N_k$, defined as the classical spheres with a one parameter family $g[k]$ of singular Riemannian structures, that reduces for $k=1$ to the classical metric. After giving explicit formulas for the eigenvalues and eigenfunctions of the metric Laplacian $Δ_{S^N_k}$, we study the associated zeta functions $ζ(s,Δ_{S^N_k})$. We introduce a general method to deal with some classes of simple and double abstract zeta functions, generalizing the ones appearing in $ζ(s,Δ_{S^N_k})$. An application of this method allows to obtain the main zeta invariants for these zeta functions in all dimensions, and in particular $ζ(0,Δ_{S^N_k})$ and $ζ'(0,Δ_{S^N_k})$. We give explicit formulas for the zeta regularized determinant in the low dimensional cases, $N=2,3$, thus generalizing a result of Dowker \cite{Dow1}, and we compute the first coefficients in the expansion of these determinants in powers of the deformation parameter $k$.

math-ph↗
arXiv2004

Asymptotics of Quasi-normal Modes for Multi-horizon Black Holes

The issue concerning rigorous methods recently developed in deriving the asymptotics of quasi-normal modes is revisited and applied to a generic non rotating multi-horizon black holes solution. Some examples are illustrated and the single horizon cases are also considered. As a result, the asymptotics for large angular momentum parameter is shown to depend on the difference between the maximal or Nariai black hole mass and the ordinary black hole mass. The extremal limit is also discussed and the exact evaluation of the quasi-normal frequencies related to the Nariai space-time is presented, as a consistent check of the general asymptotic formula.

hep-th↗
arXiv2001

On recent strategies proposed for proving the Riemann hypothesis

We comment on some apparently weak points in the novel strategies recently developed by various authors aiming at a proof of the Riemann hypothesis. After noting the existence of relevant previous papers where similar tools have been used, we refine some of these strategies. It is not clear at the moment if the problems we point out here can be resolved rigorously, and thus a proof of the RH be obtained, along the lines proposed. However, a specific suggestion of a procedure to overcome the encountered difficulties is made, what constitutes a step towards this goal.

math-ph↗
arXiv2001

Effective Finite Temperature Partition Function for Fields on Non-Commutative Flat Manifolds

The first quantum correction to the finite temperature partition function for a self-interacting massless scalar field on a $D-$dimensional flat manifold with $p$ non-commutative extra dimensions is evaluated by means of dimensional regularization, suplemented with zeta-function techniques. It is found that the zeta function associated with the effective one-loop operator may be nonregular at the origin. The important issue of the determination of the regularized vacuum energy, namely the first quantum correction to the energy in such case is discussed.

hep-th↗
arXiv2000

One-loop effective potential for scalar and vector fields on higher dimensional noncommutative flat manifolds

The effective potentials for massless scalar and vector quantum field theories on D dimensional manifolds with p compact noncommutative extra dimensions are evaluated by means of dimensional regularization implemented by zeta function techniques. It is found that the zeta function associated with the one-loop operator may not be regular at the origin. Thus, the related heat-kernel trace has a logarithmic term in the short t asymptotics expansion. Consequences of this fact are briefly discussed.

hep-th↗
arXiv2000

The extremal limit of D-dimensional black holes

The extreme limit of a class of D-dimensional black holes is revisited. In the static limit, it is shown that well defined extremal limiting procedure exists and it leads to new solutions of the type AdS2 times constant curvature symmetric spaces.

hep-th↗
arXiv2000

On the dimensional reduced theories

The procedure of the dimensional reduction related to the partition function of a quantum field living in curved space-time which is the warp product of a symmetric space is investigated.

hep-th↗
arXiv1999

Semiclassical Approximation for Chern-Simons Theory and 3-Hyperbolic Invariants

The invariant integration method for Chern-Simons theory defined on the compact hyperbolic manifold Γ\H^3 is verified in the semiclassical approximation. The semiclassical limit for the partition function is presented. We discuss briefly L^2 - analytic torsion and the eta invariant of Atiyah-Patodi-Singer for compact hyperbolic 3-manifolds.

hep-th↗
arXiv1999

Quantum Scalar Field on the Massless (2+1)-Dimensional Black Hole Background

The behavior of a quantum scalar field is studied in the metric ground state of the (2+1)-dimensional black hole of Bañados, Teitelboim and Zanelli which contains a naked singularity. The one-loop BTZ partition function and the associate black hole effective entropy, the expectation value of the quantum fluctuation as well as the renormalized expectation value of the stress tensor are explicitly computed in the framework of the $ζ$-function procedure. This is done for all values of the coupling with the curvature, the mass of the field and the temperature of the quantum state. In the massless conformally coupled case, the found stress tensor is used for determining the quantum back reaction on the metric due to the scalar field in the quantum vacuum state, by solving the semiclassical Einstein equations. It is finally argued that, within the framework of the 1/N expansion, the Cosmic Censorship Hypothesis is implemented since the naked singularity of the ground state metric is shielded by an event horizon created by the back reaction.

gr-qc↗
arXiv1997

Zeta-function regularization, the multiplicative anomaly and the Wodzicki residue

The multiplicative anomaly associated with the zeta-function regularized determinant is computed for the Laplace-type operators $L_1=-\lap+V_1$ and $L_2=-\lap+V_2$, with $V_1$, $V_2$ constant, in a D-dimensional compact smooth manifold $ M_D$, making use of several results due to Wodzicki and by direct calculations in some explicit examples. It is found that the multiplicative anomaly is vanishing for $D$ odd and for D=2. An application to the one-loop effective potential of the O(2) self-interacting scalar model is outlined.

hep-th↗
arXiv1997

Quantum Correction to the Entropy of the (2+1)-Dimensional Black Hole

The thermodynamic properties of the (2+1)-dimensional non-rotating black hole of Bañados, Teitelboim and Zanelli are discussed. The first quantum correction to the Bekenstein-Hawking entropy is evaluated within the on-shell Euclidean formalism, making use of the related Chern-Simons representation of the 3-dimensional gravity. Horizon and ultraviolet divergences in the quantum correction are dealt with a renormalization of the Newton constant. It is argued that the quantum correction due to the gravitational field shrinks the effective radius of a hole and becomes more and more important as soon as the evaporation process goes on, while the area law is not violated.

gr-qc↗
arXiv1995

Quantum Fields and Extended Objects in Space-Times with Constant Curvature Spatial Section

The heat-kernel expansion and $ζ$-regularization techniques for quantum field theory and extended objects on curved space-times are reviewed. In particular, ultrastatic space-times with spatial section consisting in manifold with constant curvature are discussed in detail. Several mathematical results, relevant to physical applications are presented, including exact solutions of the heat-kernel equation, a simple exposition of hyperbolic geometry and an elementary derivation of the Selberg trace formula. With regards to the physical applications, the vacuum energy for scalar fields, the one-loop renormalization of a self-interacting scalar field theory on a hyperbolic space-time, with a discussion on the topological symmetry breaking, the finite temperature effects and the Bose-Einstein condensation, are considered. Some attempts to generalize the results to extended objects are also presented, including some remarks on path integral quantization, asymptotic properties of extended objects and a novel representation for the one-loop (super)string free energy.

hep-th↗