- Cosmology and Gravitation Theories
- Black Holes and Theoretical Physics
- Geophysics and Gravity Measurements
- Galaxies: Formation, Evolution, Phenomena
- Relativity and Gravitational Theory
- Advanced Differential Geometry Research
- Dark Matter and Cosmic Phenomena
- Pulsars and Gravitational Waves Research
- Astrophysical Phenomena and Observations
- Particle physics theoretical and experimental studies
- Solar and Space Plasma Dynamics
- Earth Systems and Cosmic Evolution
- Advanced Mathematical Physics Problems
- Neutrino Physics Research
- Stochastic processes and statistical mechanics
- Space Science and Extraterrestrial Life
- advanced mathematical theories
- Mathematical Dynamics and Fractals
- Astronomy and Astrophysical Research
- Physics of Superconductivity and Magnetism
- Advanced Differential Equations and Dynamical Systems
- Optimization and Variational Analysis
- Distributed and Parallel Computing Systems
- Bayesian Methods and Mixture Models
- Superconducting Materials and Applications
Pontificia Universidad Católica de Valparaíso
2015-2024
University of KwaZulu-Natal
2013-2015
Tokyo University of Science
2010-2013
Laboratoire de Physique Théorique
2006-2012
Université de Montpellier
2006-2012
Centre National de la Recherche Scientifique
2007-2012
University of Oslo
2012
Inter-University Centre for Astronomy and Astrophysics
2009-2010
We derive the conditions under which dark energy models whose Lagrangian densities $f$ are written in terms of Ricci scalar $R$ cosmologically viable. show that cosmological behavior $f(R)$ can be understood by a geometrical approach consisting studying $m(r)$ curve on $(r,m)$ plane, where $m\ensuremath{\equiv}R{f}_{,RR}/{f}_{,R}$ and $r\ensuremath{\equiv}\ensuremath{-}R{f}_{,R}/f$ with ${f}_{,R}\ensuremath{\equiv}\mathrm{d}f/\mathrm{d}R$. This allows us to classify into four general...
We consider the viability of dark energy (DE) models in framework scalar–tensor theory gravity, including possibility having a phantom DE at small redshifts z as admitted by supernova luminosity–distance data. For z, generic solution for these is constructed form power series without any approximation. Necessary constraints to be today and cross divide line p = −ρ are presented. Considering solar system constraints, we find post-Newtonian parameters that γPN < 1 γPN,0 ≈ model viable, βPN,0 >...
We consider the covariant galileon gravity taking into account third order and fourth scalar field Lagrangians L_3(\pi) L_4(\pi) consisting of three four $\pi$'s with five derivatives acting on them respectively. The background dynamical equations are set up for system under consideration stability self accelerating solution is demonstrated in general setting. extended this study to case fifth theory. For spherically symmetric static background, we spell out conditions suppression force...
We consider the linear growth of matter perturbations in various dark energy (DE) models. show existence a constraint valid at $z=0$ between background and parameters parameters. For $Λ$CDM $γ'_0\equiv \frac{dγ}{dz}_0$ lies very narrow interval $-0.0195 \le γ'_0 -0.0157$ for $0.2 Ω_{m,0}\le 0.35$. Models with constant equation state inside General Relativity (GR) are characterized by quasi-constant $γ'_0$, $Ω_{m,0}=0.3$ example we have $γ'_0\approx -0.02$ while $γ_0$ can nonnegligible...
We consider an Einstein-scalar-Gauss-Bonnet gravitational theory, and argue that at early times the Ricci scalar can be safely ignored. then demonstrate pure scalar-Gauss-Bonnet with a quadratic coupling function, naturally supports inflationary---de Sitter---solutions. During inflation, field decays exponentially its effective potential remains always bounded. The theory also contains solutions where these de Sitter phases possess natural exit mechanism are replaced by linearly...
We study the structure of neutron stars in $f(R)=R+\alpha R^{2}$ theory gravity (Starobinsky model), an exact and non-perturbative approach. In this model, apart from standard General Relativistic junction conditions, two extra namely continuity curvature scalar its first derivative needs to be satisfied. For exterior Schwarzschild solution, has zero at stellar surface. show that for some equation state (EoS) matter, matching all conditions surface star is impossible. Hence model brings...
We consider the possibility that dark sector of our Universe contains a negative cosmological constant dubbed $\ensuremath{\lambda}$. For such models to be viable, should contain an additional component responsible for late-time accelerated expansion rate ($X$). explore departure history these from concordance$\mathrm{\ensuremath{\Lambda}}$ cold matter ($\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$) model. large class models, is transient with nontrivial dependence on model parameters. All...
We consider the linear growth of matter perturbations on low redshifts in some f(R) dark energy (DE) models. discuss definition these models and show differences with scalar-tensor DE For model recently proposed by Starobinsky we that parameter γ0≡γ(z = 0) takes value γ0 ≃ 0.4 for Ωm,0 0.32 0.43 0.23, allowing a clear distinction from ΛCDM. Though scale-dependence appears higher redshifts, find no dispersion γ(z) up to z ∼ 0.3, is also quasi-linear this interval. At redshift 0.5, still small...
In this paper we examine the cosmological consequences of fourth order Galileon gravity. We carry out detailed investigations underlying dynamics and demonstrate stability one de Sitter phase. The stable phase contains a field $π$ which is an increasing function time (\dotπ>0). Using required suppression fifth force, supernovae, BAO CMB data, constrain parameters model. find that matter coupling parameter $β$ constrained to small numerical values such $β$<0.02. also show third in...
We study the growth of matter density perturbations ${\ensuremath{\delta}}_{m}$ for a number viable $f(R)$ gravity models that satisfy both cosmological and local constraints, where Lagrangian $f$ is function Ricci scalar $R$. If parameter $m\ensuremath{\equiv}R{f}_{,RR}/{f}_{,R}$ today larger than order ${10}^{\ensuremath{-}6}$, linear relevant to power spectrum evolve with rate $s\ensuremath{\equiv}d\mathrm{ln}{\ensuremath{\delta}}_{m}/d\mathrm{ln}a$ ($a$ scale factor) in...
We consider asymptotically stable scalar–tensor dark energy (DE) models for which the equation of state parameter wDE tends to zero in past. The viable are phantom type today: however, this phantomness is milder than general relativity if we take into account varying gravitational constant when dealing with SNIa data. study further growth matter perturbations and find a scaling behaviour on large redshifts could provide an important constraint. In particular, our close standard , while it...
In chameleon dark energy models, local gravity constraints tend to rule out parameters in which observable cosmological signatures can be found. We study viable potentials consistent with a number of recent observational and experimental bounds. A novel field potential, motivated by $f(R)$ gravity, is constructed where are present both at the background evolution growth rate perturbations. matter density perturbations on low redshifts for this potential show that index today...
The $f(R)$ gravity models proposed by Hu-Sawicki and Starobinsky are generic for local constraints to be evaded. large deviations from these result in either violation of or the modifications indistinguishable cosmological constant. curvature singularity is but can avoided, provided that proper fine-tuning imposed on evolution scalaron high regime. In principle, problem circumvented incorporating quadratic correction Lagrangian, though it might quite challenging probe relevant region numerically.
In this work, we consider a generalized gravitational theory that contains the Einstein term, scalar field, and quadratic Gauss-Bonnet (GB) term. We focus on early-universe dynamics, demonstrate simple choice of coupling function between field term simplifying assumption regarding role Ricci can lead to new, analytical, elegant solutions with interesting characteristics. first argue, in context two different models, presence at early times (when curvature is strong) does not affect actual...
We investigate cosmological behavior in the quasidilaton nonlinear massive gravity. perform a detailed dynamical analysis and examine stable late-time solutions relevant to cosmic acceleration. demonstrate that dark-energy-dominated, cosmological-constant-like solution is late attractor of dynamics. also analyze evolution universe at intermediate times, showing observed epoch sequence can be easily obtained model under consideration. Furthermore, we study nonsingular bounce turnaround which...
Charged black holes are known to suffer from an interior instability associated with the presence of Cauchy horizon. Recently, a hairy charged hole was proposed that avoids formation It is natural question whether this might manifest in exterior solution. In paper, we have analyzed stability hole. Our results show vector perturbations stable, along scalar sector for $l=0$. We also computed corresponding quasinormal modes (QNMs) and quasibound states (QBSs). Among six degrees freedom...
Recent analyses [S. Nesseris et al., Phys. Rev. D 96, 023543 (2017); L. Kazantzidis and Pervolaropoulos, 97, 103503 (2018)] have indicated that an effective Newton's constant ${G}_{\mathrm{eff}}(z)$ decreasing with redshift may relieve the observed tension between Planck15 best fit $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ cosmological background (i.e., $\mathrm{Planck}15/\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$) corresponding favored by growth $f{\ensuremath{\sigma}}_{8}$ weak lensing...
The large scale structure bispectrum in the squeezed limit couples with small scales. Since relativity is important at scales and non-linear loop corrections are scales, proper calculation of observed this requires a relativistic calculation. We compute matter general weak field approximation. as involved existing second-order results. find several differences Newtonian such non-cancellation IR divergences, need to renormalize background, fact that initial conditions must be set second order...
We study the stability under linear odd-parity perturbations of static spherically symmetric black holes in Horndeski gravity.We derive master equation for these and obtain conditions no-ghost Laplacian instability.In order hole solutions to be stable, we their generalized "Regge-Wheeler potential".It turns out that problem is reduced an algebraic where three functions characterizing should positive outside horizon prove stability.We found are similar instability conditions.We apply our...
In this paper, we carry out a study of viable cosmological models in $f(R)$ gravity at the background level. We use observable parameters like $\ensuremath{\Omega}$ and $\ensuremath{\gamma}$ to form an autonomous system equations show that under consideration exhibit two different regimes their time evolution, namely, phantom phase followed by quintessencelike behavior. employ state finder emphasize characteristic discriminative signature these models.
We study quantum stability bound on the mass of scalaron in generic theories $f(R)$ gravity. show that these scenarios, increases faster with local density environment than one loop correction to it thereby leading violation chameleon mass. The introduction quadratic curvature corrections action are shown stabilize model.
In this paper, we study existence and stability of static black holes in Lovelock theories with a particular focus on pure holes. We derive the equation from action without using S-deformation approach. It turns out that though pure-Lovelock hole even dimension is always unstable, however introduction $\Lambda$ stablizes it by prescribing lower threshold mass while there also exists an upper bound which given horizon. dimensionally continued as well analogue BTZ all odd dimensions.
In this paper we show that pure Lovelock static Schwarzschild's analogue black hole in dimensions $d>3N+1$, where $N$ is the degree of polynomial action, stable even though Gauss-Bonnet $N=2$ unstable dimension $d<7$. We also discuss and compare quasinormal modes for corresponding Einstein same dimension. find perturbations decay with characteristic time which weakly dimensional dependent as it depends only on gravitational potential background solution, while frequency oscillations...
We study the stability of static black holes in generalized Einstein-Maxwell-scalar theories. derive master equations for odd and even parity perturbations. The sufficient necessary conditions under odd-parity perturbations are derived. show that these usually not similar to energy simplest case a minimally coupled scalar field. obtain even-parity also derived speed propagation five degrees freedom obtained class theories which all propagate at light. Finally, we have applied our results...