arXiv subjects

Emmanuel N. Saridakis

Publications and source records attributed to Emmanuel N. Saridakis.

At least 271 records · Page 15Linked to original sources

arXiv2013

Cosmological behavior in extended nonlinear massive gravity

We perform a detailed dynamical analysis of various cosmological scenarios in extended (varying-mass) nonlinear massive gravity. Due to the enhanced freedom in choosing the involved free functions, this cosmological paradigm allows for a huge variety of solutions that can attract the universe at late times, comparing to scalar-field cosmology or usual nonlinear massive gravity. Amongst others, it accepts quintessence, phantom, or cosmological-constant-like late-time solutions, which moreover can alleviate the coincidence problem. These features seem to be general and non-sensitive to the imposed ansantzes and model parameters, and thus extended nonlinear massive gravity can be a good candidate for the description of nature.

astro-ph.CO↗

arXiv2013

Inflation in (Super-)renormalizable Gravity

We investigate a (super-)renormalizable and ghost-free theory of gravity, showing that under a natural (exponential) ansatz of the form factor and a suitable truncation it can give rise to the Starobinsky inflationary theory in cosmological frameworks, and thus offering a theoretical justification of its origin. We study the corresponding inflationary evolution and we examine the generation of curvature perturbations, adapting the $f(R)$-like equations in a symmetry-reduced FLRW metric. Furthermore, we analyze how the ultraviolet regime of a simply renormalizable and unitary theory of gravity is also compatible with the Starobinsky action, and hence we show that such a theory could account for an inflationary phase of the Universe in the ultraviolet regime.

hep-th↗

arXiv2013

Dynamical analysis of generalized Galileon cosmology

We perform a detailed dynamical analysis of generalized Galileon cosmology, incorporating also the requirements of ghost and instabilities absence. We find that there are not any new stable late-time solutions apart from those of standard quintessence. Furthermore, depending on the model parameters the Galileons may survive at late times or they may completely disappear by the dynamics, however the corresponding observables are always independent of the Galileon terms, determined only by the usual action terms. Thus, although the Galileons can play an important role at inflationary or at recent times, in the future, when the universe will asymptotically reach its stable state, they will not have any effect on its evolution.

astro-ph.CO↗

arXiv2013

Phantom crossing and quintessence limit in extended nonlinear massive gravity

We investigate the cosmological evolution in a universe governed by the extended, varying-mass, nonlinear massive gravity, in which the graviton mass is promoted to a scalar-field. We find that the dynamics, both in flat and open universe, can lead the varying graviton mass to zero at late times, offering a natural explanation for its hugely-constrained observed value. Despite the limit of the scenario towards standard quintessence, at early and intermediate times it gives rise to an effective dark energy sector of a dynamical nature, which can also lie in the phantom regime, from which it always exits naturally, escaping a Big-Rip. Interestingly enough, although the motivation of massive gravity is to obtain an IR modification, its varying-mass extension in cosmological frameworks leads to early and intermediate times modification instead.

gr-qc↗

arXiv2012

Constraining Horava-Lifshitz gravity from neutrino speed experiments

We constrain Horava-Lifshitz gravity using the results of OPERA and ICARUS neutrino speed experiments, which show that neutrinos are luminal particles, examining the fermion propagation in the earth's gravitational field. In particular, investigating the Dirac equation in the spherical solutions of the theory, we find that the neutrinos feel an effective metric with respect to which they might propagate superluminally. Therefore, demanding not to have superluminal or subluminal motion we constrain the parameters of the theory. Although the excluded parameter regions are very narrow, we find that the detailed balance case lies in the excluded region.

gr-qc↗

arXiv2012

Charged black holes in nonlinear massive gravity

We find static charged black hole solutions in nonlinear massive gravity. In the parameter space of two gravitational potential parameters $(α, β)$ we show that below the Compton wavelength the black hole solutions reduce to that of Reissner-Nordström via the Vainshtein mechanism in the weak field limit. In the simplest case with $α=β=0$ the solution exhibits the vDVZ discontinuity but ordinary General Relativity is recovered deep inside the horizon due to the existence of electric charge. For $α\neq0$ and $β=0$, the post-Newtonian parameter of the charged black hole evolves to that of General Relativity via the Vainshtein mechanism within a macroscopic distance; however, a logarithmic correction to the metric factor of the time coordinate is obtained. When $α$ and $β$ are both nonzero, there exist two branches of solutions depending on the positivity of $β$. When $β<0$, the strong coupling of the scalar graviton weakens gravity at distances smaller than the Vainshtein radius. However, when $β>0$ the metric factors exhibit only small corrections compared to the solutions obtained in General Relativity, and under a particular choice of $β=α^2/6$ the standard Reissner-Nordström-de Sitter solution is recovered.

hep-th↗

arXiv2012

Bounce and cyclic cosmology in extended nonlinear massive gravity

We investigate non-singular bounce and cyclic cosmological evolutions in a universe governed by the extended nonlinear massive gravity, in which the graviton mass is promoted to a scalar-field potential. The extra freedom of the theory can lead to certain energy conditions violations and drive cyclicity with two different mechanisms: either with a suitably chosen scalar-field potential under a given Stuckelberg-scalar function, or with a suitably chosen Stuckelberg-scalar function under a given scalar-field potential. Our analysis shows that extended nonlinear massive gravity can alter significantly the evolution of the universe at both early and late times.

astro-ph.CO↗

arXiv2012

Exact charged black-hole solutions in D-dimensional f(T) gravity: torsion vs curvature analysis

We extract exact charged black-hole solutions with flat transverse sections in the framework of D-dimensional Maxwell-f(T) gravity, and we analyze the singularities and horizons based on both torsion and curvature invariants. Interestingly enough, we find that in some particular solution subclasses there appear more singularities in the curvature scalars than in the torsion ones. This difference disappears in the uncharged case, or in the case where f(T) gravity becomes the usual linear-in-T teleparallel gravity, that is General Relativity. Curvature and torsion invariants behave very differently when matter fields are present, and thus f(R) gravity and f(T) gravity exhibit different features and cannot be directly re-casted each other.

hep-th↗

arXiv2012

Solar system constraints on f(T) gravity

We use recent observations from solar system orbital motions in order to constrain f(T) gravity. In particular, imposing a quadratic f(T) correction to the linear-in-T form, which is a good approximation for every realistic case, we extract the spherical solutions of the theory. Using these spherical solutions to describe the Sun's gravitational field, we use recently determined supplementary advances of planetary perihelia, to infer upper bounds on the allowed f(T) corrections. We find that the maximal allowed divergence of the gravitational potential in f(T) gravity from that in the teleparallel equivalent of General Relativity is of the order of 6.2 \times 10^{-10}, in the applicability region of our analysis. This is much smaller than the corresponding (significantly small too) divergence that is predicted from cosmological observations, as expected. Such a tiny allowed divergence from the linear form should be taken into account in f(T) model building.

gr-qc↗

arXiv2012

Circularly symmetric solutions in three-dimensional Teleparallel, f(T) and Maxwell-f(T) gravity

We present teleparallel 3D gravity and we extract circularly symmetric solutions, showing that they coincide with the BTZ and Deser-de-Sitter solutions of standard 3D gravity. However, extending into f(T) 3D gravity, that is considering arbitrary functions of the torsion scalar in the action, we obtain BTZ-like and Deser-de-Sitter-like solutions, corresponding to an effective cosmological constant, without any requirement of the sign of the initial cosmological constant. Finally, extending our analysis incorporating the electromagnetic sector, we show that Maxwell-f(T) gravity accepts deformed charged BTZ-like solutions. Interestingly enough, the deformation in this case brings qualitatively novel terms, contrary to the pure gravitational solutions where the deformation is expressed only through changes in the coefficients. We investigate the singularities and the horizons of the new solutions, and amongst others we show that the cosmic censorship can be violated. Such novel behaviors reveal the new features that the f(T) structure brings in 3D gravity.

gr-qc↗

arXiv2012

Phase-Space analysis of Teleparallel Dark Energy

We perform a detailed dynamical analysis of the teleparallel dark energy scenario, which is based on the teleparallel equivalent of General Relativity, in which one adds a canonical scalar field, allowing also for a nonminimal coupling with gravity. We find that the universe can result in the quintessence-like, dark-energy-dominated solution, or to the stiff dark-energy late-time attractor, similarly to standard quintessence. However, teleparallel dark energy possesses an additional late-time solution, in which dark energy behaves like a cosmological constant, independently of the specific values of the model parameters. Finally, during the evolution the dark energy equation-of-state parameter can be either above or below -1, offering a good description for its observed dynamical behavior and its stabilization close to the cosmological-constant value.

gr-qc↗

arXiv2012

A New Class of Four-Dimensional N=1 Supergravity with Non-minimal Derivative Couplings

In the N=1 four-dimensional new-minimal supergravity framework, we supersymmetrise the coupling of the scalar kinetic term to the Einstein tensor. This coupling, although introduces a non-minimal derivative interaction of curvature to matter, it does not introduce harmful higher-derivatives. For this construction, we employ off-shell chiral and real linear multiplets. Physical scalars are accommodated in the chiral multiplet whereas curvature resides in a linear one.

hep-th↗

arXiv2012

Entropic Force Scenarios and Eternal Inflation

We examine various entropic inflation scenarios, under the light of eternality. After describing the inflation realization and the normal condition for inflation to last at the background level, we investigate the conditions for eternal inflation with the effect of thermal fluctuations produced from standard radiation and from the holographic screen. Furthermore, we incorporate stochastic quantum fluctuations through a phenomenological, Langevin analysis, studying whether they can affect the inflation eternality. In single-holographic-screen scenarios eternality can be easily obtained, while in double-screen considerations inflation is eternal only in the high-energy regime. Thus, from the cosmological point of view, one should take these into account before he can consider entropic gravity as a candidate for the description of nature. However, form the string theory point of view, inflation eternality may form the background for the "Landscape" of string/M theory vacua, leading to new perspectives in entropy gravity.

hep-th↗

arXiv2011

Observational Constraints on Teleparallel Dark Energy

We use data from Type Ia Supernovae (SNIa), Baryon Acoustic Oscillations (BAO), and Cosmic Microwave Background (CMB) observations to constrain the recently proposed teleparallel dark energy scenario based on the teleparallel equivalent of General Relativity, in which one adds a canonical scalar field, allowing also for a nonminimal coupling with gravity. Using the power-law, the exponential and the inverse hyperbolic cosine potential ansatzes, we show that the scenario is compatible with observations. In particular, the data favor a nonminimal coupling, and although the scalar field is canonical the model can describe both the quintessence and phantom regimes.

astro-ph.CO↗

arXiv2011

Horava-Lifshitz cosmology with generalized Chaplygin gas

We investigate cosmological scenarios of generalized Chaplygin gas in a universe governed by Horava-Lifshitz gravity. We consider both the detailed and non-detailed balance versions of the gravitational background, and we include the baryonic matter and radiation sectors. We use observational data from Type Ia Supernovae (SNIa), Baryon Acoustic Oscillations (BAO), and Cosmic Microwave Background (CMB), along with requirements of Big Bang Nucleosynthesis (BBN), to constrain the parameters of the model, and we provide the corresponding likelihood contours. We deduce that the present scenario is compatible with observations. Additionally, examining the evolution of the total equation-of-state parameter, we find in a unified way the succession of the radiation, matter, and dark energy epochs, consistently with the thermal history of the universe.

astro-ph.CO↗

arXiv2011

Observational constraints on phantom power-law cosmology

We investigate phantom cosmology in which the scale factor is a power law, and we use cosmological observations from Cosmic Microwave Background (CMB), Baryon Acoustic Oscillations (BAO) and observational Hubble data, in order to impose complete constraints on the model parameters. We find that the power-law exponent is $β\approx-6.51^{+0.24}_{-0.25}$, while the Big Rip is realized at $t_s\approx104.5^{+1.9}_{-2.0}$ Gyr, in 1$σ$ confidence level. Providing late-time asymptotic expressions, we find that the dark-energy equation-of-state parameter at the Big Rip remains finite and equal to $w_{DE}\approx -1.153$, with the dark-energy density and pressure diverging. Finally, we reconstruct the phantom potential.

astro-ph.CO↗

arXiv2011

Vanishing Cosmological Constant by Gravitino-Dressed Compactification of 11D Supergravity

We consider compactifications induced by the gravitino field of eleven dimensional supergravity. Such compactifications are not trivial in the sense that the gravitino profiles are not related to pure bosonic ones by means of a supersymmetry transformation. The basic property of such backgrounds is that they admit $ψ$-torsion although they have vanishing Riemann tensor. Thus, these backgrounds may be considered also as solutions of the teleparallel formulation of supergravity. We construct two classes of solutions, one with both antisymmetric three-form field, gravity and gravitino and one with only gravity and gravitino. In these classes of solutions, the internal space is a parallelized compact manifold, so that it does not inherit any cosmological constant to the external spacetime. The latter turns out to be flat Minkowski in the maximally symmetric case. The elimination of the cosmological constant in the spontaneously compactified supergravity seems to be a generic property based on the trading of the cosmological constant for parallelizing torsion.

hep-th↗

arXiv2011

"Teleparallel" Dark Energy

Using the "teleparallel" equivalent of General Relativity as the gravitational sector, which is based on torsion instead of curvature, we add a canonical scalar field, allowing for a nonminimal coupling with gravity. Although the minimal case is completely equivalent to standard quintessence, the nonminimal scenario has a richer structure, exhibiting quintessence-like or phantom-like behavior, or experiencing the phantom-divide crossing. The richer structure is manifested in the absence of a conformal transformation to an equivalent minimally-coupled model.

hep-th↗