Showing papers by "Salvatore Capozziello published in 2015"

f(T) teleparallel gravity and cosmology

Abstract: Over the past decades, the role of torsion in gravity has been extensively investigated along the main direction of bringing gravity closer to its gauge formulation and incorporating spin in a geometric description. Here we review various torsional constructions, from teleparallel, to Einstein-Cartan, and metric-affine gauge theories, resulting in extending torsional gravity in the paradigm of f(T) gravity, where f(T) is an arbitrary function of the torsion scalar. Based on this theory, we further review the corresponding cosmological and astrophysical applications. In particular, we study cosmological solutions arising from f(T) gravity, both at the background and perturbation levels, in different eras along the cosmic expansion. The f(T) gravity construction can provide a theoretical interpretation of the late-time universe acceleration, and it can easily accommodate with the regular thermal expanding history including the radiation and cold dark matter dominated phases. Furthermore, if one traces back to very early times, a sufficiently long period of inflation can be achieved and hence can be investigated by cosmic microwave background observations, or alternatively, the Big Bang singularity can be avoided due to the appearance of non-singular bounces. Various observational constraints, especially the bounds coming from the large-scale structure data in the case of f(T) cosmology, as well as the behavior of gravitational waves, are described in detail. Moreover, the spherically symmetric and black hole solutions of the theory are reviewed. Additionally, we discuss various extensions of the f(T) paradigm. Finally, we consider the relation with other modified gravitational theories, such as those based on curvature, like f(R) gravity, trying to enlighten the subject of which formulation might be more suitable for quantization ventures and cosmological applications.

Cosmological inflation in F(R,G) gravity

Abstract: Cosmological inflation is discussed in the framework of $F(R,\mathcal{G})$ gravity where $F$ is a generic function of the curvature scalar $R$ and the Gauss--Bonnet topological invariant $\mathcal{G}$. The main feature that emerges in this analysis is the fact that this kind of theory can exhaust all the curvature budget related to curvature invariants without considering derivatives of $R$, ${R}_{\ensuremath{\mu}\ensuremath{ u}}$, ${R}_{\ensuremath{\sigma}\ensuremath{\mu}\ensuremath{ u}}^{\ensuremath{\lambda}}$, etc., in the action. Cosmological dynamics results driven by two effective masses (lengths) are related to the $R$ scalaron and the $\mathcal{G}$ scalaron working respectively at early and very early epochs of cosmic evolution. In this sense, a double inflationary scenario naturally emerges.

Generalized energy conditions in Extended Theories of Gravity

Abstract: In this work, we consider the further degrees of freedom related to curvature invariants and scalar fields in extended theories of gravity (ETG) These new degrees of freedom can be recast as ``effective fluids'' that differ in nature with respect to the standard matter fluids generally adopted as sources of the field equations It is, thus, somewhat misleading to apply the standard general relativistic energy conditions to this effective energy-momentum tensor, as the latter contains the matter content and a geometrical quantity, which arises from the specific ETG considered Here we explore this subtlety, extending our previous work, in particular, to cases with the contracted Bianchi identities with diffeomorphism invariance and to cases with generalized explicit curvature-matter couplings, which imply the nonconservation of the energy-momentum tensor Furthermore, we apply the analysis to specific ETGs, such as scalar-tensor gravity and $f(R)$ gravity Thus, in the context of ETGs, interesting results appear such as matter that may exhibit unusual thermodynamical features, for instance, gravity that retains its attractive character in the presence of large negative pressures; or alternatively, we verify that repulsive gravity may occur for standard matter

Extreme neutron stars from Extended Theories of Gravity

Abstract: We discuss neutron stars with strong magnetic mean fields in the framework of Extended Theories of Gravity. In particular, we take into account models derived from f(R) and f() extensions of General Relativity where functions of the Ricci curvature invariant R and the Gauss-Bonnet invariant are respectively considered. Dense matter in magnetic mean field, generated by magnetic properties of particles, is described by assuming a model with three meson fields and baryons octet. As result, the considerable increasing of maximal mass of neutron stars can be achieved by cubic corrections in f(R) gravity. In principle, massive stars with M > 4M☉ can be obtained. On the other hand, stable stars with high strangeness fraction (with central densities ρc ~ 1.5–2.0 GeV/fm3) are possible considering quadratic corrections of f() gravity. The magnetic field strength in the star center is of order 6–8 × 1018 G. In general, we can say that other branches of massive neutron stars are possible considering the extra pressure contributions coming from gravity extensions. Such a feature can constitute both a probe for alternative theories and a way out to address anomalous self-gravitating compact systems.

Transition redshift in f (T ) cosmology and observational constraints

Abstract: We extract constraints on the transition redshift ${z}_{\text{tr}}$, determining the onset of cosmic acceleration, predicted by an effective cosmographic construction, in the framework of $f(T)$ gravity. In particular, employing cosmography we obtain bounds on the viable $f(T)$ forms and their derivatives. Since this procedure is model independent, as long as the scalar curvature is fixed, we are able to determine intervals for ${z}_{\text{tr}}$. In this way we guarantee that the Solar-System constraints are preserved and, moreover, we extract bounds on the transition time and the free parameters of the scenario. We find that the transition redshifts predicted by $f(T)$ cosmology, although compatible with the standard $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ predictions, are slightly smaller. Finally, in order to obtain observational constraints on $f(T)$ cosmology, we perform a Monte Carlo fitting using supernova data, involving the most recent Union 2.1 data set.

Hybrid Metric-Palatini Gravity

Abstract: Recently, the phenomenology of f(R) gravity has been scrutinized. This scrutiny has been motivated by the possibility to account for the self-accelerated cosmic expansion without invoking dark energy sources. Besides, this kind of modified gravity is capable of addressing the dynamics of several self-gravitating systems alternatively to the presence of dark matter. It has been established that both metric and Palatini versions of these theories have interesting features but also manifest severe and different downsides. A hybrid combination of theories, containing elements from both these two formalisms, turns out to be also very successful accounting for the observed phenomenology and is able to avoid some drawbacks of the original approaches. This article reviews the formulation of this hybrid metric-Palatini approach and its main achievements in passing the local tests and in applications to astrophysical and cosmological scenarios, where it provides a unified approach to the problems of dark energy and dark matter.

Hybrid metric-Palatini gravity

Abstract: Recently, the phenomenology of f(R) gravity has been scrutinized motivated by the possibility to account for the self-accelerated cosmic expansion without invoking dark energy sources. Besides, this kind of modified gravity is capable of addressing the dynamics of several self-gravitating systems alternatively to the presence of dark matter. It has been established that both metric and Palatini versions of these theories have interesting features but also manifest severe and different downsides. A hybrid combination of theories, containing elements from both these two formalisms, turns out to be also very successful accounting for the observed phenomenology and is able to avoid some drawbacks of the original approaches. This article reviews the formulation of this hybrid metric-Palatini approach and its main achievements in passing the local tests and in applications to astrophysical and cosmological scenarios, where it provides a unified approach to the problems of dark energy and dark matter.

Magnetic neutron stars in f(R) gravity

Abstract: Neutron stars with strong magnetic fields are considered in the framework of f(R) gravity. In order to describe dense matter in magnetic field, the model with baryon octet interacting through σρω-fields is used. The hyperonization process results in softening the equation of state (EoS) and in decreasing the maximal mass. We investigate the effect of strong magnetic field in models involving quadratic and cubic corrections in the Ricci scalar R to the Hilbert–Einstein action. For large fields, the Mass–Radius relation differs considerably from that of General Relativity only for stars with masses close to the maximal one. Another interesting feature is the possible existence of more compact stable stars with extremely large magnetic fields (∼6×1018 G instead of ∼4×1018 G as in GR) in the central regions of the stars. Due to cubic terms, a significant increasing of the maximal mass is possible.

Constraining models of extended gravity using Gravity Probe B and LARES experiments

Abstract: We consider models of extended gravity and in particular, generic models containing scalar-tensor and higher-order curvature terms, as well as a model derived from noncommutative spectral geometry. Studying, in the weak-field approximation (the Newtonian and post-Newtonian limit of the theory), the geodesic and Lense-Thirring processions, we impose constraints on the free parameters of such models by using the recent experimental results of the Gravity Probe B (GPB) and Laser Relativity Satellite (LARES) satellites. The imposed constraint by GPB and LARES is independent of the torsion-balance experiment, though it is much weaker.

Constraining $f(R)$ Gravity by the Large-Scale Structure

Abstract: Over the past few decades, general relativity and the concordance ΛCDM model have been successfully tested using several different astrophysical and cosmological probes based on large datasets (precision cosmology). Despite their successes, some shortcomings emerge due to the fact that general relativity should be revised at infrared and ultraviolet limits and to the fact that the fundamental nature of dark matter and dark energy is still a puzzle to be solved. In this perspective, ƒ(R) gravity has been extensively investigated, being the most straightforward way to modify general relativity and to overcame some of the above shortcomings. In this paper, we review various aspects of ƒ(R) gravity at extragalactic and cosmological levels. In particular, we consider a cluster of galaxies, cosmological perturbations and N-body simulations, focusing on those models that satisfy both cosmological and local gravity constraints. The perspective is that some classes of ƒ(R) models can be consistently constrained by the large-scale structure.

Selection effects in gamma-ray burst correlations: consequences on the ratio between gamma-ray burst and star formation rates

Abstract: Gamma-ray bursts (GRBs) visible up to very high redshift have become attractive targets as potential new distance indicators. It is still not clear whether the relations proposed so far originate from an unknown GRB physics or result from selection effects. We investigate this issue in the case of the L{sub X} -T{sub a}{sup ∗} (hereafter LT) correlation between the X-ray luminosity L{sub X} (T{sub a} ) at the end of the plateau phase, T{sub a} , and the rest-frame time T{sub a}{sup ∗}. We devise a general method to build mock data sets starting from a GRB world model and taking into account selection effects on both time and luminosity. This method shows how not knowing the efficiency function could influence the evaluation of the intrinsic slope of any correlation and the GRB density rate. We investigate biases (small offsets in slope or normalization) that would occur in the LT relation as a result of truncations, possibly present in the intrinsic distributions of L{sub X} and T{sub a}{sup ∗}. We compare these results with the ones in Dainotti et al. showing that in both cases the intrinsic slope of the LT correlation is ≈ – 1.0. This method is general andmore » therefore relevant for investigating whether or not any other GRB correlation is generated by the biases themselves. Moreover, because the farthest GRBs and star-forming galaxies probe the reionization epoch, we evaluate the redshift-dependent ratio Ψ(z) = (1 + z){sup α} of the GRB rate to the star formation rate. We found a modest evolution –0.2 ≤ α ≤ 0.5 consistent with a Swift GRB afterglow plateau in the redshift range 0.99 < z < 9.4.« less

Connecting early and late universe by f(R) gravity

Abstract: Inflation and dark energy are two of the most relevant aspects of modern cosmology. These different epochs provide the universe is passing through accelerated phases soon after the Big–Bang and at present stage of its evolution. In this review paper, we discuss that both eras can be, in principle, described by a geometric picture, under the standard of f(R) gravity. We give the fundamental physics motivations and outline the main ingredients of f(R) inflation, quintessence and cosmography. This wants to be a quick summary of f(R) paradigm without claiming of completeness.

Invariant solutions and Noether symmetries in hybrid gravity

Abstract: Symmetries play a crucial role in physics and, in particular, the Noether symmetries are a useful tool both to select models motivated at a fundamental level, and to find exact solutions for specific Lagrangians. In this work, we apply Noether point symmetries to metric-Palatini hybrid gravity in order to select the $f(\mathcal{R})$ functional form and to find analytical solutions for the field equations and for the related Wheeler-DeWitt (WDW) equation. It is important to stress that hybrid gravity implies two definitions of curvature scalar: $R$ for standard metric gravity and $\mathcal{R}$ for further degrees of freedom related to the Palatini formalism. We use conformal transformations in order to find out integrable $f(\mathcal{R})$ models. In this context, we explore two conformal transformations of the forms $d\ensuremath{\tau}=N(a)dt$ and $d\ensuremath{\tau}=N(\ensuremath{\phi})dt$. For the former, we found two cases of $f(\mathcal{R})$ functions where the field equations admit Noether symmetries. In the second case, the Lagrangian reduces to a Brans-Dicke-like theory with a general coupling function. For each case, it is possible to transform the field equations by using normal coordinates to simplify the dynamical system and to obtain exact solutions. Furthermore, we perform quantization and derive the WDW equation for the minisuperspace model. The Lie point symmetries for the WDW equation are determined and used to find invariant solutions. In particular, hybrid gravity introduces a further term in cosmic dynamics whose interpretation is related to the signature of an auxiliary scalar field. Solutions are compared with $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$.

Noether Symmetry Approach for teleparallel-curvature cosmology

Abstract: We consider curvature-teleparallel F(R,T) gravity, where the gravitational Lagrangian density is given by an arbitrary function of the Ricci scalar R and the torsion scalar T. Using the Noether Symmetry Approach, we show that the functional form of the F(R,T) function can be determined by the presence of symmetries. Furthermore, we obtain exact solutions through the presence of conserved quantities and the reduction of cosmological dynamical system. Example of particular cosmological models are considered.

Gravitational waves in fourth order gravity

Abstract: In the post-Minkowskian limit approximation, we study gravitational wave solutions for general fourth-order theories of gravity. Specifically, we consider a Lagrangian with a generic function of curvature invariants $f(R, R_{\alpha\beta}R^{\alpha\beta}, R_{\alpha\beta\gamma\delta }R^{\alpha\beta\gamma\delta})$ . It is well known that when dealing with General Relativity such an approach provides massless spin-two waves as propagating degree of freedom of the gravitational field while this theory implies other additional propagating modes in the gravity spectra. We show that, in general, fourth order gravity, besides the standard massless graviton is characterized by two further massive modes with a finite-distance interaction. We find out the most general gravitational wave solutions in terms of Green functions in vacuum and in presence of matter sources. If an electromagnetic source is chosen, only the modes induced by $R_{\alpha\beta}R^{\alpha\beta}$ are present, otherwise, for any $f(R)$ gravity model, we have the complete analogy with tensor modes of General Relativity. Polarizations and helicity states are classified in the hypothesis of plane wave.

External Stability for Spherically Symmetric Solutions in Lorentz Breaking Massive Gravity

Abstract: We discuss spherically symmetric solutions for point-like sources in Lorentz-breaking massive gravity theories. This analysis is valid for Stuckelberg’s effective field theory formulation, for Lorentz Breaking Massive Bigravity and general extensions of gravity leading to an extra term −S r γ added to the Newtonian potential. The approach consists in analyzing the stability of the geodesic equations, at the first order (deviation equation). The main result is a strong constrain in the space of parameters of the theories. This motivates higher order analysis of geodesic perturbations in order to understand if a class of spherically symmetric Lorentz-breaking massive gravity solutions, for self-gravitating systems, exists. Stable and phenomenologically acceptable solutions are discussed in the no-trivial case S ≠ 0.

Can f(R) gravity contribute to (dark) radiation

Abstract: We discuss the possibility that suitable modifications of gravity could account for some amount of the radiation we observe today, in addition to the possibility of explaining the present speed up of the universe. We start introducing and reviewing cosmological reconstruction methods for metric f(R) theories of gravity that can be considered as one of the straightforward modifications of Einstein's gravity as soon as f(R)≠ R. We then take into account two possible f(R) models which could give rise to (dark) radiation. Constraints on the models are found by using the Planck Collaboration 2015 data within a cosmographic approach and by obtaining the matter power spectrum of those models. The conclusion is that f(R) gravity can only contribute minimally to the (dark) radiation to avoid departures from the observed matter power spectrum at the smallest scales (of the order of 0.01Mpc−1), i.e., precisely those scales that exited the horizon at the radiation dominated epoch. This result could strongly contribute to select reliable f(R) models.

Unifying inflation with late-time acceleration in BIonic system

Abstract: In this research, we propose a new model that allows to unify inflation, deceleration and acceleration phases of expansion history in BIonic system. In this model, in the beginning, there have been $k$ black fundamental strings that transited to the BIon configuration at a corresponding point. At this point, two universe brane and universe antibrane have been created, interacted with each other via one wormhole and inflated. With decreasing temperature, the energy of this wormhole flowed into universe branes and lead to inflation. After a short time, wormhole died, inflation ended and deceleration epoch started. With approaching two universe brane and antibrane together, tachyon was born, grew and caused creation of one new wormhole. At this time, two universe brane and antibrane connected again and late-time acceleration era of the universe began. We compare our model with previous unified phantom model and observational data and obtain some cosmological parameters like temperature in terms of time. We also find that deceleration parameter is negative during inflation and late-time acceleration epochs and positive during deceleration era. This means that our model is consistent with previous prediction and cosmological experiments.

Noether Symmetry Approach for Dirac-Born-Infeld Cosmology

Abstract: We consider the Noether Symmetry Approach for a cosmological model derived from a tachyon scalar field T with a Dirac–Born–Infeld Lagrangian and a potential V(T). Furthermore, we assume a coupled canonical scalar field ϕ with an arbitrary interaction potential B(T, ϕ). Exact solutions are derived consistent with the accelerated behavior of cosmic fluid.

String duality transformations in $f(R)$ gravity from Noether symmetry approach

Abstract: We select $f(R)$ gravity models that undergo scale factor duality transformations. As a starting point, we consider the tree-level effective gravitational action of bosonic String Theory coupled with the dilaton field. This theory inherits the Busher's duality of its parent String Theory. Using conformal transformations of the metric tensor, it is possible to map the tree-level dilaton-graviton string effective action into $f(R)$ gravity, relating the dilaton field to the Ricci scalar curvature. Furthermore, the duality can be framed under the standard of Noether symmetries and exact cosmological solutions are derived. Using suitable changes of variables, the string-based $f(R)$ Lagrangians are shown in cases where the duality transformation becomes a parity inversion.

Emergence and expansion of cosmic space as due to M0-branes

Abstract: Recently, Padmanabhan (arXiv:1206.4916 [hep-th]) discussed that the difference between the number of degrees of freedom on the boundary surface and the number of degrees of freedom in a bulk region causes the accelerated expansion of the universe. The main question arising is: what is the origin of this inequality between the surface degrees of freedom and the bulk degrees of freedom? We answer this question in M-theory. In our model, first M0-branes are compactified on one circle and ND0-branes are created. Then ND0-branes join each other, grow, and form one D5-branes. Next, the D5-brane is compactified on two circles and our universe’s D3-brane, two D1-branes and some extra energies are produced. After that, one of the D1-branes, which is closer to the universe’s brane, gives its energy into it, and this leads to an increase in the difference between the numbers of degrees of freedom and the occurring inflation era. With the disappearance of this D1-brane, the number of degrees of freedom of boundary surface and bulk region become equal and inflation ends. At this stage, extra energies that are produced due to the compactification cause an expansion of the universe and deceleration epoch. Finally, another D1-brane dissolves in our universe’s brane, leads to an inequality between degrees of freedom, and there occurs a new phase of acceleration.

Cosmological evolution of thermal relic particles in f(R) gravity

Abstract: By considering $f(R)$ gravity models, the cosmic evolution is modified with respect to the standard $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ scenario. In particular, the thermal history of particles results is modified. In this paper, we derive the evolution of relics particles (weakly interacting massive particles) assuming a reliable $f(R)$ cosmological solution and taking into account observational constraints. The connection to the PAMELA experiment is also discussed. Results are consistent with constraints coming from BICEP2 and PLANCK experiments.

Extended Theories of Gravity with Generalized Energy Conditions

Abstract: We address the problem of the energy conditions in modified gravity taking into account the additional degrees of freedom related to scalar fields and curvature invariants. The latter are usually interpreted as generalized geometrical fluids that differ in meaning with respect to the matter fluids generally considered as sources of the field equations. In extended gravity theories the curvature terms are encapsulated in a tensor Hab and a coupling g(Ψi) that can be recast as effective Einstein field equations, with corrections to the energy-momentum tensor of matter. The formal validity of standard energy inequalities does not assure basic requirements such as the attractive nature of gravity, so we argue that the energy conditions have to be considered in a wider sense.