arXiv subjects
Publications and source records attributed to Lorenzo Sebastiani.
In this paper, we have investigated the possibility to have inflation from inhomogeneous viscous fluids by taking into account the brane correction coming from string-inspired five dimensional Einsten's gravity. We have realized several kinds of viable solutions for early-time acceleration. At the end of inflation, the classical Einstein's gravity is recovered and fluids produce decelerated expansion.
In this paper, we analyze a two coupled fluids model by investigating several solutions for accelerated universe in flat FRW space-time. One of the fluids can be identified with the matter and the model possesses the standard matter solution also. Beyond the removal of the coincidence problem, we will see how the coupling may change the description of the energy contents of the universe and which features can be aquired with respect to the standard decoupled cases.
We investigate the bounce cosmology induced by inhomogeneous viscous fluids in FRW space-time (non necessarly flat), taking into account the early-time acceleration after the bounce. Different forms for the scale factor and several examples of fluids will be considered. We also analyze the relation between bounce and finite-time singularities and between the corresponding fluids realizing this scenarios. In the last part of the work, the study is extended to the framework of $f(R)$-modified gravity, where the modification of gravity may also be considered as an effective (viscous) fluid producing the bounce.
We consider inhomogeneous viscous fluids in flat Friedmann-Robertson-Walker universe. We analyze different kinds of such fluids and investigate the possibility to reproduce the current cosmic acceleration providing a different future evolution with respect to the Cosmological Constant case. In particular, we study the presence of finite-future time singularities. We also discuss a general class of "integrable" viscous fluid models whose bulk viscosities obey to a common differential equation.
In the present work, we review some general aspects of modified gravity theories, investigating mathematical and physical properties and, more specifically, the feature of viable and realistic models able to reproduce the dark energy epoch and the early-time inflation. We will discuss the black hole solutions in generalized theories of gravity: it is of fundamental interest to understand how properties and laws of black holes in General Relativity can be addressed in the framework of modified theories. In particular, we will discuss the energy issue and the possibility to derive the First Law of thermodynamics from the field equations. Then, in the analysis of cosmological solutions, we will pay a particular attention to the occurrence of finite-time future singularities and to the possibility to avoid them in $\mathcal F(R,G)$-gravity. Furthermore, realistic models of $F(R)$-gravity will be analyzed in the detail. A general feature occurring in matter era will be shown, namely the high derivatives of Hubble parameter may be influenced by the high frequency oscillation of the dark energy and some correction term is required in order to stabilize the theory at high redshift. The inflationary scenario is also carefully analyzed and an unified description of the universe is risen. In the final part of the work, we will have a look at the last developments in modified gravity, namely we will investigate cosmological and black hole solutions in a covariant field theory of gravity and we will introduce the extended "teleparallel" $F(T)$-gravity theories. A nice application to the dark matter problem will be presented.
A toy model of Einstein gravity with a Gauss-Bonnet classically "entropic" term mimicking a quantum correction is considered. The static black hole solution due to Tomozawa is found and generalized with the inclusion of non trivial horizon topology, and its entropy evaluated deriving the first law by equations of motion. As a result the Bekenstein-Hawking area law turns to be corrected by a logarithmic area term. A Misner-Sharp expression for the mass of black hole is found. Within a Friedmann-Lemaître-Robertson-Walker (FLRW) cosmological setting, the model is used in order to derive modified Friedmann equations. Such new equations are shown to reproduce the first law with the same formal entropy and quasi local energy of the static case, but here within a FLRW space-time interpreted as a dynamical cosmological black hole. A detailed analysis of cosmological solutions is presented, and it is shown that the presence of the correction term provides regular solutions and interesting phases of acceleration and decelerations, as well as, with negligible matter, exact de Sitter solutions.
Thermodynamics of extended gravity static spherically symmetric black hole solutions is investigated. The energy issue is discussed making use of the derivation of Clausius relation from equations of motion, evaluating the black hole entropy by the Wald method and computing the related Hawking temperature.
The Lagrangian derivation of the Equations of Motion for topological static spherically symmetric metrics in $\mathcal F (R,G)$-modified gravity is presented and the related solutions are discussed. In particular, a new topological solution for the model $\mathcal F (R,G)=R+\sqrt{G}$ is found. The black hole solutions and the First Law of thermodynamic are analyzed. Furthermore, the coupling with electromagnetic field is also considered and a Maxwell solution is derived.
Spherically symmetric static topological black hole solutions associated with some extended higher order gravitational models in the presence of a Maxwell-field are derived by means of simple Lagrangian method, based on spherically symmetric reduction. Some new topological black hole solutions are presented and the validity of the First Law is investigated, and in these cases an expression for the energy is provided.
In the case of a large class of static spherically symmetric black hole solutions in higher order modified gravity models, an expression for the associated energy is proposed and identified as a quantity proportional to the constant of integration, which appears in the explicit solution. The identification is achieved making use of derivation of the First Law of black hole physics from the equations of motion, evaluating independently the Entropy via Wald method and the Hawking temperature via the tunneling method. Several non trivial examples are discussed, including a new topological higher derivative black hole solution.
A Lagrangian derivation of the Equation of Motion (EOM) for static spherically symmetric metrics in F(R) modified gravity is presented. For a large class of metrics, our approach permits to reduce the EOM to a single equation and we show how it is possible to construct exact solutions in $F(R)$-gravity. All known exact solutions are recovered. We also exibit a new non trivial solution with non constant Ricci scalar.
We study effects of cosmic fluids on finite-time future singularities in modified $f(R,G)$-gravity, where $R$ and $G$ are the Ricci scalar and the Gauss-Bonnet invariant, respectively. We consider the fluid equation of state in the general form, $ω=ω(ρ)$, and we suppose the existence of a bulk viscosity. We investigate quintessence region ($ω>-1$) and phantom region ($ω<-1$) and the possibility to change or avoid the singularities in $f(R,G)$-gravity. Finally, we study the inclusion of quantum effects in large curvatures regime.
The dark energy issue is focusing the attention of an incresing number of physicists all over the world. Among the possible alternatives in order to explain what as been named the "Mystery of the Millennium" are the so-called Modified Theories of Gravity. A crucial test for such models is represented by the existence and (if this is the case) the properties of their black hole solutions. Nowadays, to our knowledge, only two non-trivial, spherically symmetric, solutions with vanishing cosmological constant are known by Barrow & Clifton (2005) and Deser, Sarioglu & Tekin (2008). Aim of the paper is to discuss some features of such solutions, with emphasis on their thermodynamic properties such as entropy and temperature, little progress being possible along the way which leads to a consistent definition of mass.
We study finite-time future singularities in $\mathcal{F}(R,G)$-gravity, where $R$ and $G$ are the Ricci scalar and the Gauss-Bonnet invariant, respectively. In particular, we reconstruct the $F(G)$-gravity and $\mathcal{F}(R,G)$-gravity models realizing the finite-time future singularities. We discuss a possible way to cure the finite-time future singularities in $\mathcal{F}(R,G)$-gravity by taking into account higher-order curvature corrections or effects of viscous fluids.
We study effects of viscous fluid coupled with dark matter in our universe. We consider bulk viscosity in the cosmic fluid and we suppose the existence of a coupling between fluid and dark matter, in order to reproduce a stable de Sitter universe protected against future-time singularities. More general inhomogeneous fluids are studied related to future singularities.
The stability issue of generalized modified gravitational models is discussed with particular emphasis to de Sitter solutions. Two approaches are briefly presented.
We study all four types of finite-time future singularities emerging in late-time accelerating (effective quintessence/phantom) era from $\mathcal{F}(R,G)$-gravity, where $R$ and $G$ are the Ricci scalar and the Gauss-Bonnet invariant, respectively. As an explicit example of $\mathcal{F}(R,G)$-gravity, we also investigate modified Gauss-Bonnet gravity, so-called $F(G)$-gravity. In particular, we reconstruct the $F(G)$-gravity and $\mathcal{F}(R,G)$-gravity models where accelerating cosmologies realizing the finite-time future singularities emerge. Furthermore, we discuss a possible way to cure the finite-time future singularities in $F(G)$-gravity and $\mathcal{F}(R,G)$-gravity by taking into account higher-order curvature corrections. The example of non-singular realistic modified Gauss-Bonnet gravity is presented. It turns out that adding such non-singular modified gravity to singular Dark Energy makes the combined theory to be non-singular one as well.