arXiv subjects

L. N. Granda

Publications and source records attributed to L. N. Granda.

At least 19 recordsLinked to original sources

arXiv2022

Modified gravity with disappearing cosmological constant

New corrections to General Relativity are considered in the context of modified $f(R)$ gravity, that satisfy cosmological and local gravity constraints. The proposed models behave asymptotically as $R-2Λ$ at large curvature and show the vanishing of the cosmological constant at the flat spacetime limit. The chameleon mechanism and thin shell restrictions for local systems were analyzed, and bounds on the models were found. The steepness of the deviation parameter $m$ at late times leads to measurable signal of scalar-tensor regime in matter perturbations, that allows to detect departures form the $Λ$CDM model. The theoretical results for the evolution of the weighted growth rate $fσ_8(z)$, from the proposed models, were analyzed.

gr-qc↗

arXiv2022

$Λ$CDM without cosmological constant

A type of exponential correction to General Relativity gives viable modified gravity model of dark energy. The model behaves as $R-2Λ$ at large curvature where an effective cosmological constant appears, but it becomes zero in flat space time. The cosmic evolution of the main density parameters is consistent with current observations. The thin shell conditions for the Solar system were analyzed. Apart from satisfying cosmological and local gravity restrictions, the model may also show measurable differences with $Λ$CDM at recent times. The current value of the deviation parameter $m$ for scales relevant to the matter power spectrum can be larger than $10^{-6}$. The growth index of matter density perturbations is clearly different from that of the $Λ$CDM. The theoretical predictions of the model for the weighted growth rate were analyzed in the light of the $fσ_8$-tension.

gr-qc↗

arXiv2020

Modified gravity with an exponential function of curvature

The role of an exponential function of the scalar curvature in the modified gravity is analyzed. Two models are proposed. A toy model that complies with local and cosmological constraints and gives appropriate qualitative description of the cosmic evolution. This model contains a saddle matter-dominant critical point that can evolve towards a late time de Sitter attractor. Initial conditions have been proposed, showing that this toy model has an acceptable matter era and gives an approximate qualitative behavior of cosmic evolution. A second viable model, behaves very close to $Λ$CDM at early times and can satisfy local and cosmological constraints. It behaves as $R-2Λ$ at $R\rightarrow \infty$ and tends to zero at $R\rightarrow 0$, containing flat spacetime solution. The model gives viable cosmological trajectories that, as the first model, connect the matter dominated point with a late time de Sitter attractor. The cosmic evolution of the main density parameters in this model is consistent with current observations with an equation of state very close to $-1$.

gr-qc↗

arXiv2020

Higgs inflation with non-minimal derivative coupling to gravity

We consider an extension of Higgs inflation in which the Higgs field is non-minimally coupled to gravity through its kinetic term. We analyzed power-law coupling functions with positive or negative integer power and found that the Higgs boson can drive a successful inflation only for the cases $n=2,1,0,-1$. Theoretical predictions for both tensor to scalar ratio $r$ and scalar spectral index $n_s$ are within the 2018 \textit{Planck} $95\%$ CL. The behavior of the self coupling $λ$ with respect to the scalar field at the horizon crossing was obtained, and It was found that it can take values in the interval $λ\sim (10^{-7}, 0.3)$.

gr-qc↗

arXiv2019

Higgs Inflation with linear and quadratic curvature corrections

We consider a single scalar field inflation model with Higgs potential and curvature corrections given by non-minimal derivative coupling to gravity and coupling to the Gauss-Bonnet invariant. Exact analytical expressions, within the slow-roll approximation, are obtained for the main physical quantities. These corrections lead to successful inflation driven by the $ϕ^4$-potential with the main inflationary observables in the regions restricted by the latest Planck data. It is shown that these curvature corrections can make the $ϕ^4$ potential not only compatible with the current CMB observations, but also consistent with the Standard Model Higgs phenomenology, achieving the possibility that the Higgs boson acts as the primordial inflaton.

gr-qc↗

arXiv2019

Modified Gravity in the framework of holographic dark energy

The modified gravity is considered in the framework of the holographic dark energy. An analysis of the autonomous system, the critical points and their stability is presented. Unlike the dark energy models based on $f(R)$, it is found that working in the holographic frame enriches the possibility of accelerated and matter type points for different cosmological scenarios, making viable trajectories of successful $f(R)$ models that are not allowed without the consideration of the holographic framework. The implications for the Hu-Sawicki model are analyzed.

gr-qc↗

arXiv2019

Cosmology in a model with Lagrange multiplier, and Gauss-Bonnet and non-minimal kinetic couplings

A scalar-tensor model with Gauss-Bonnet and non-minimal kinetic couplings is considered, in which ghost modes are eliminated via a Lagrange multiplier constraint. A reconstruction procedure is deviced for the scalar potential and Lagrange multiplier, valid for any given cosmological scenario. In particular, inflationary and dark energy cosmologies of different types (power-law, Little-Rip, de Sitter, quasi de Sitter) are reconstructed in such models. It is shown that, for various choices of the kinetic coupling terms, it is possible to obtain a viable inflationary phenomenology compatible with the most accurate values of the observational indices.

gr-qc↗

arXiv2019

Slow-Roll Inflation in Scalar-Tensor Models

The linear and quadratic perturbations for a scalar-tensor model with non-minimal coupling to curvature, coupling to the Gauss-Bonnet invariant and non-minimal kinetic coupling to the Einstein tensor are developed. The quadratic action for the scalar and tensor perturbations is constructed and the power spectra for the primordial scalar and tensor fluctuations are given. A consistency relation that is useful to discriminate the model from the standard inflation with canonical scalar field was found. For some power-law potentials it is shown that the Introduction of additional interactions, given by non-minimal, kinetic and Gauss-Bonnet couplings, can lower the tensor-to-scalar ratio to values that are consistent with latest observational constraints, and the problem of large fields in chaotic inflation can be avoided.

gr-qc↗

arXiv2019

Slow-Roll Inflation with Exponential Potential in Scalar-Tensor Models

A study of the slow-roll inflation for an exponential potential in the frame of the scalar-tensor theory is performed, where non-minimal kinetic coupling to curvature and non-minimal coupling of the scalar field to the Gauss-Bonnet invariant are considered. Different models were considered with couplings given by exponential functions of the scalar field, that lead to graceful exit from inflation and give values of the scalar spectral index and the tensor-to-scalar ratio in the region bounded by the current observational data. Special cases were found, where the coupling functions are inverse of the potential, that lead to inflation with constant slow-roll parameters, and it was posible to reconstruct the model parameters for given $ns$ and $r$. In first-order approximation the standard consistency relation maintains its validity in the model with non-minimal coupling, but it modifies in presence of Gauss-Bonnet coupling. The obtained Hubble parameter during inflation, $H\sim 10^{-5} M_p$ and the energy scale of inflation $V^{1/4}\sim 10^{-3} M_p$, are consistent with the upper bounds set by latest observations.

hep-th↗

arXiv2019

$Λ$CDM-like models with future singularities

We consider new models of dark energy with finite time future singularities, by introducing the pressure density as a function of the scale factor. This approach gives acceptable phenomenological models of dark energy, practically indistinguishable from the cosmological constant up to the present, which face future singularities at finite time and finite scale factor. Exact scalar field model representation was found for quintessence, Big Rip and type III singularity models. The simple form of the equation of state allows to establish a relationship between its current value, $w_0$, and the time or redshift at which the singularity takes place. The effect on the growth of matter perturbations was calculated.

gr-qc↗

arXiv2019

Finite scale factor and future singularities

The main characteristic of the dark energy is its negative pressure. In a homogeneous and isotropic FRW background, we consider several models for the dark energy fluid, which lead to finite time future singularities of the type I-IV, by introducing the pressure density as a function of the scale factor. This approach gives acceptable behavior of the dark energy equation of state. We give various numerical examples of models with type I-IV singularities, that show very similar late time behavior, making it difficult to determine the type of singularity that would take place in the future.

gr-qc↗

arXiv2018

Reconstructing modified gravity with holographic vacuum energy density

A reconstruction of modified gravity is proposed by establishing a correspondence between the effective density of the modified gravity and the holographic density. The non-homogeneous term in the modified Friedmann equation, generated by the vacuum (holographic) energy density, lead to reconstructed models that contain explicitly, as part of the solution, the Einstein-Hilbert term. It was shown that the $Λ$CDM-type cosmic histories allow the general relativity Lagrangian with cosmological constant as a particular reconstructed solution. The Starobinsky term appears in some reconstructed solutions, and an approximate reconstruction of the Chaplygin gas cosmology was performed in terms of elementary functions of curvature.

gr-qc↗

arXiv2018

The speed of gravitational waves and power-law solutions in a scalar-tensor model

One of the most relevant solutions in any cosmological model concerning the evolution of the universe is the power-law solution. For the scalar-tensor model of dark energy with kinetic and Gauss Bonnet couplings, it is shown that we can conserve the power-law solution and at the same time meet the recent observational bound on the speed of gravitational waves. In the FRW background the anomalous contribution to the speed of gravitational waves, coming from the kinetic and Gauss-Bonnet couplings, cancel each other for power-law solutions. It is shown that by simple restriction on the model parameters we can achieve a non-time-dependent cancellation of the defect in the velocity of the gravitational waves. The model can realize the cosmic expansion with contributions from the kinetic and Gauss-Bonnet couplings of the order of ${\cal O}(1)$ to the dark energy density parameter. The results are valid on the homogeneous FRW background and the limitations of the approach are discussed.

gr-qc↗

arXiv2017

Dynamical analysis for a scalar-tensor model with kinetic and non-minimal couplings

We study the autonomous system for a scalar-tensor model of dark energy with non-minimal coupling to curvature and non-minimal kinetic coupling to the Einstein tensor. The critical points describe important stable asymptotic scenarios including quintessence, phantom and de Sitter attractor solutions. Two functional forms for the coupling functions and the scalar potential were considered: power-law and exponential functions of the scalar field. For power-law couplings, the restrictions on stable quintessence and phantom solutions lead to asymptotic freedom regime for the gravitational interaction. The model with dimensionless kinetic coupling constant gives stable de Sitter solutions. For the exponential functions the stable quintessence, phantom or de Sitter solutions, allow asymptotic behaviors where the effective Newtonian coupling can reach either the asymptotic freedom regime or constant value. The phantom solutions could be realized without appealing to ghost degrees of freedom. Transient inflationary and radiation dominated phases can also be described.

gr-qc↗

arXiv2017

Dynamical analysis for a scalar-tensor model with Gauss-Bonnet and non-minimal couplings

We study the autonomous system for a scalar-tensor model of dark energy with Gauss-Bonnet and non-minimal couplings. The critical points describe important stable asymptotic scenarios including quintessence, phantom and de Sitter attractor solutions. Two functional forms for the coupling functions and the scalar potential were considered: power-law and exponential functions of the scalar field. For the exponential functions the existence of stable quintessence, phantom or de Sitter solutions, allows an asymptotic behavior where the effective Newtonian coupling becomes constant. The phantom solutions could be realized without appealing to ghost degrees of freedom. Transient inflationary and radiation dominated phases can also be described.

gr-qc↗

arXiv2016

Phase space analysis for a scalar-tensor model with kinetic and Gauss-Bonnet couplings

We study the phase space for an scalar-tensor string inspired model of dark energy with non minimal kinetic and Gauss Bonnet couplings. The form of the scalar potential and of the coupling terms is of the exponential type, which give rise to appealing cosmological solutions. The critical points describe a variety of cosmological scenarios that go from matter or radiation dominated universe to dark energy dominated universe. There were found trajectories in the phase space departing from unstable or saddle fixed points and arriving to the stable scalar field dominated point corresponding to late-time accelerated expansion.

gr-qc↗

arXiv2014

Dark Energy from Gauss-Bonnet and non-minimal couplings

We consider a scalar-tensor model of dark energy with Gauss-Bonnet and non-minimal couplings. Exact cosmological solutions were found in absence of potential, that give equations of state of dark energy consistent with current observational constraints, but with different asymptotic behaviors depending on the couplings of the model. A detailed reconstruction procedure is given for the scalar potential and the Gauss-Bonnet coupling for any given cosmological scenario. Particularly, we consider conditions for the existence of a variety of cosmological solutions with accelerated expansion, including quintessence, phantom, de Sitter, Little Rip. For the case of quintessence and phantom we have found a scalar potential of the Albrecht-Skordis type, where the potential is an exponential with a polynomial factor.

gr-qc↗

arXiv2014

A Viable model for modified gravity

We propose a model for modified gravity that meets the conditions of viability. The model has stable constant curvature solution and for an special case contains flat space time solution. The model also leads to matter stability under small perturbations of scalar curvature. We give an example of restrictions on the parameters that give large enough scalaron mass to avoid detectable corrections to the Newton's law. The model describes inflation and late time accelerated expansion with an effective cosmological constant for the inflationary epoch and small effective cosmological constant for the current accelerated phase.

gr-qc↗