arXiv subjects

Emmanuel N. Saridakis

Publications and source records attributed to Emmanuel N. Saridakis.

At least 199 records · Page 11Linked to original sources

arXiv2018

Modified cosmology through nonextensive horizon thermodynamics

We construct modified cosmological scenarios through the application of the first law of thermodynamics in the universe horizon, but using the generalized, nonextensive Tsallis entropy instead of the usual Bekenstein-Hawking one. We result to modified cosmological equations that possess the usual ones as a particular limit, but which in the general case contain extra terms that appear for the first time, that constitute an effective dark energy sector quantified by the nonextensive parameter $δ$. When the matter sector is dust, we extract analytical expressions for the dark energy density and equation-of-state parameters, and we extend these solutions to the case where radiation is present too. We show that the universe exhibits the usual thermal history, with the sequence of matter and dark-energy eras, and according to the value of $δ$ the dark-energy equation-of-state parameter can be quintessence-like, phantom-like, or experience the phantom-divide crossing during the evolution. Even in the case where the explicit cosmological constant is absent, the scenario at hand can very efficiently mimic $Λ$CDM cosmology, and is in excellent agreement with Supernovae type Ia observational data.

gr-qc↗

arXiv2018

New observational constraints on $f(T)$ gravity through gravitational-wave astronomy

We investigate the new observational constraints on $f(T)$ gravity that arise from the effects of primordial gravitational waves (GWs) on the cosmic microwave background (CMB) anisotropies and the BB spectrum. We first show that on the GWs propagation in $f(T)$ gravity we obtain only an amplitude modification and not a phase one, comparing to the case of general relativity in the background of $Λ$CDM cosmology. Concerning primordial GWs we find that the more the model departs from general relativity the larger is the GW amplitude decay, and thus a possible future detection would bring the viable $f(T)$ gravity models five orders of magnitude closer to $Λ$CDM cosmology comparing to standard cosmological constraints. Additionally, we use the CLASS code and both data from the Planck probe, as well as forecasts from the near-future CORE collaboration, and we show that possible non-trivial constraints on the tensor-to-scalar ratio would offer a clear signature of $f(T)$ gravity. Finally, we discuss on the possibility to use the properties of the GWs that arise from neutron stars mergers in order to extract additional constrains on the theory.

gr-qc↗

arXiv2018

Observational constraints on dynamical dark energy with pivoting redshift

We investigate the generalized Chevallier-Polarski-Linder (CPL) parametrization, which contains the pivoting redshift $z_p$ as an extra free parameter. We use various data combinations from cosmic microwave background (CMB), baryon acoustic oscillations (BAO), redshift space distortion (RSD), weak lensing (WL), joint light curve analysis (JLA), cosmic chronometers (CC), and we include a Gaussian prior on the Hubble constant value, in order to extract the observational constraints on various quantities. For the case of free $z_p$ we find that for all data combinations it always remains unconstrained, and there is a degeneracy with the current value of the dark energy equation of state $w_0$. For the case where $z_p$ is fixed to specific values, and for the full data combination, we find that with increasing $z_p$ the mean value of $w_0$ slowly moves into the phantom regime, however the cosmological constant is always allowed within 1$σ$ confidence-level. However, the significant effect is that with increasing $z_p$ the correlations between $w_0$ and $w_a$ change from negative to positive, with the case $z_p =0.35$ corresponding to no correlation. This feature indeed justifies why a non-zero pivoting redshift should be taken into account.

astro-ph.CO↗

arXiv2018

The effective field theory approach of teleparallel gravity, $f(T)$ gravity and beyond

We develop the effective field theory approach to torsional modified gravities, a formalism that allows for the systematic investigation of the background and perturbation levels separately. Starting from the usual effective field theory approach to curvature-based gravity, we suitably generalize it at the background level by including terms of the contracted torsion tensor, and at the perturbation level by including pure torsion perturbative terms and mixed perturbative terms of torsion and curvature. Having constructed the effective field theory action of general torsional modified gravity, amongst others we focus on $f(T)$ gravity and we perform a cosmological application. We investigate the scalar perturbations up to second order, and we derive the expressions of the Newtonian constant and the post Newtonian parameter $γ$. Finally, we apply this procedure to two specific and viable $f(T)$ models, namely the power-law and the exponential ones, introducing a new parameter that quantifies the deviation from general relativity and depends on the model parameters. Since this parameter can be expressed in terms of the scalar perturbation mode, a precise measurement of its evolution could be used as an alternative way to impose constraints on $f(T)$ gravity and break possible degeneracies between different $f(T)$ models.

gr-qc↗

arXiv2018

Cosmological Fluids with Logarithmic Equation of State

We investigate the cosmological applications of fluids having an equation of state which is the analog to the one related to the isotropic deformation of crystalline solids, that is containing logarithmic terms of the energy density, allowing additionally for a bulk viscosity. We consider two classes of scenarios and we show that they are both capable of triggering the transition from deceleration to acceleration at late times. Furthermore, we confront the scenarios with data from Supernovae type Ia (SN Ia) and Hubble function observations, showing that the agreement is excellent. Moreover, we perform a dynamical system analysis and we show that there exist asymptotic accelerating attractors, arisen from the logarithmic terms as well as from the viscosity, which in most cases correspond to a phantom late-time evolution. Finally, for some parameter regions we obtain a nearly de Sitter late-time attractor, which is a significant capability of the scenario since the dark energy, although dynamical, stabilizes at the cosmological constant value.

gr-qc↗

arXiv2018

Observational Constraints on Oscillating Dark-Energy Parametrizations

We perform a detailed confrontation of various oscillating dark-energy parame-trizations with the latest sets of observational data. In particular, we use data from Joint Light Curve analysis (JLA) sample from Supernoave Type Ia, Baryon Acoustic Oscillations (BAO) distance measurements, Cosmic Microwave Background (CMB) observations, redshift space distortion, weak gravitational lensing, Hubble parameter measurements from cosmic chronometers, and we impose constraints on four oscillating models. From the analyses we find that the best-fit characters of almost all models are bent towards the phantom region, nevertheless in all of them the quintessential regime is also allowed within 1$σ$ confidence-level. Furthermore, the deviations from $Λ$CDM cosmology are not significant, however for two of the models they could be visible at large scales, through the impact on the temperature anisotropy of the CMB spectra and on the matter power spectra. Finally, we peform the Bayesian analysis, which shows that the current observational data support the $Λ$CDM paradigm over this set of oscillating dark-energy parametrizations.

astro-ph.CO↗

arXiv2018

Bounce and cyclic cosmology in new gravitational scalar-tensor theories

We study the bounce and cyclicity realization in the framework of new gravitational scalar-tensor theories. In these theories the Lagrangian contains the Ricci scalar and its first and second derivatives, in a specific combination that makes them free of ghosts, and transformed into the Einstein frame they are proved to be a subclass of bi-scalar extensions of general relativity. We present analytical expressions for the bounce requirements, and we examine the necessary qualitative behavior of the involved functions that can give rise to a given scale factor. Having in mind these qualitative forms, we reverse the procedure and we construct suitable simple Lagrangian functions that can give rise to a bounce or cyclic scale factor.

gr-qc↗

arXiv2018

Gravitational Waves in Modified Teleparallel Theories

We investigate the gravitational waves and their properties in various modified teleparallel theories, such as $f(T), f(T,B)$ and $f(T,T_G)$ gravities. We perform the perturbation analysis both around a Minkowski background, as well as in the case where a cosmological constant is present, and for clarity we use both the metric and tetrad language. For $f(T)$ gravity we verify the result that no further polarization modes comparing to general relativity are present at first-order perturbation level, and we show that in order to see extra modes one should look at third-order perturbations. For non-trivial $f(T,B)$ gravity, by examining the geodesic deviation equations, we show that extra polarization models, namely the longitudinal and breathing modes, do appear at first-order perturbation level, and the reason for this behavior is the fact that although the first-order perturbation does not have any effect on $T$, it does affect the boundary term $B$. Finally, for $f(T,T_G)$ gravity we show that at first-order perturbations the gravitational waves exhibit the same behavior to those of $f(T)$ gravity. Since different modified teleparallel theories exhibit different gravitational wave properties, the dvancing gravitational-wave astronomy would help to alleviate the degeneracy not only between curvature and torsional modified gravity but also between different subclasses of modified teleparallel gravities.

gr-qc↗

arXiv2018

Ricci-Gauss-Bonnet holographic dark energy

We present a model of holographic dark energy in which the Infrared cutoff is determined by both the Ricci and the Gauss-Bonnet invariants. Such a construction has the significant advantage that the Infrared cutoff, and consequently the holographic dark energy density, does not depend on the future or the past evolution of the universe, but only on its current features, and moreover it is determined by invariants, whose role is fundamental in gravitational theories. We extract analytical solutions for the behavior of the dark energy density and equation-of-state parameters as functions of the redshift. These reveal the usual thermal history of the universe, with the sequence of radiation, matter and dark energy epochs, resulting in the future to a complete dark energy domination. The corresponding dark energy equation-of-state parameter can lie in the quintessence or phantom regime, or experience the phantom-divide crossing during the cosmological evolution, and its asymptotic value can be quintessence-like, phantom-like, or be exactly equal to the cosmological-constant value. Finally, we extract the constraints on the model parameters that arise from Big Bang Nucleosynthesis.

gr-qc↗

arXiv2018

Phase Portraits of general f(T) Cosmology

We use dynamical system methods to explore the general behaviour of $f(T)$ cosmology. In contrast to the standard applications of dynamical analysis, we present a way to transform the equations into a one-dimensional autonomous system, taking advantage of the crucial property that the torsion scalar in flat FRW geometry is just a function of the Hubble function, thus the field equations include only up to first derivatives of it, and therefore in a general $f(T)$ cosmological scenario every quantity is expressed only in terms of the Hubble function. The great advantage is that for one-dimensional systems it is easy to construct the phase space portraits, and thus extract information and explore in detail the features and possible behaviours of $f(T)$ cosmology. We utilize the phase space portraits and we show that $f(T)$ cosmology can describe the universe evolution in agreement with observations, namely starting from a Big Bang singularity, evolving into the subsequent thermal history and the matter domination, entering into a late-time accelerated expansion, and resulting to the de Sitter phase in the far future. Nevertheless, $f(T)$ cosmology can present a rich class of more exotic behaviours, such as the cosmological bounce and turnaround, the phantom-divide crossing, the Big Brake and the Big Crunch, and it may exhibit various singularities, including the non-harmful ones of type II and type IV. We study the phase space of three specific viable $f(T)$ models offering a complete picture. Moreover, we present a new model of $f(T)$ gravity that can lead to a universe in agreement with observations, free of perturbative instabilities, and applying the Om(z) diagnostic test we confirm that it is in agreement with the combination of SNIa, BAO and CMB data at 1$σ$ confidence level.

gr-qc↗

arXiv2018

Cosmological dynamics of mimetic gravity

We present a detailed investigation of the dynamical behavior of mimetic gravity with a general potential for the mimetic scalar field. Performing a phase-space and stability analysis, we show that the scenario at hand can successfully describe the thermal history of the universe, namely the successive sequence of radiation, matter, and dark-energy eras. Additionally, at late times the universe can either approach a de Sitter solution, or a scaling accelerated attractor where the dark-matter and dark-energy density parameters are of the same order, thus offering an alleviation of the cosmic coincidence problem. Applying our general analysis to various specific potential choices, including the power-law and the exponential ones, we show that mimetic gravity can be brought into good agreement with the observed behavior of the universe. Moreover, with an inverse square potential we find that mimetic gravity offers an appealing unified cosmological scenario where both dark energy and dark matter are characterized by a single scalar field, and where the cosmic coincidence problem is alleviated.

gr-qc↗

arXiv2018

Unification of Dark Matter - Dark Energy in Generalized Galileon Theories

We present a unified description of the dark matter and the dark energy sectors, in the framework of shift-symmetric generalized Galileon theories. Considering a particular combination of terms in the Horndeski Lagrangian in which we have not introduced a cosmological constant or a matter sector, we obtain an effective unified cosmic fluid whose equation of state $w_U$ is zero during the whole matter era, namely from redshifts $z\sim3000$ up to $z\sim2-3$. Then at smaller redshifts it starts decreasing, passing the bound $w_U=-1/3$, which marks the onset of acceleration, at around $z\sim0.5$. At present times it acquires the value $w_U=-0.7$. Finally, it tends toward a de-Sitter phase in the far future. This behaviour is in excellent agreement with observations. Additionally, confrontation with Supernovae type Ia data leads to a very efficient fit. Examining the model at the perturbative level, we show that it is free from pathologies such as ghosts and Laplacian instabilities, at both scalar and tensor sectors, at all times.

gr-qc↗

arXiv2018

$f(T)$ gravity after GW170817 and GRB170817A

The combined observation of GW170817 and its electromagnetic counterpart GRB170817A reveals that gravitational waves propagate at the speed of light in high precision. We apply the effective field theory approach to investigate the experimental consequences for the theory of $f(T)$ gravity. We find that the speed of gravitational waves within $f(T)$ gravity is exactly equal to the light speed, and hence the constraints from GW170817 and GRB170817A are trivially satisfied. The results are verified through the standard analysis of cosmological perturbations. Nevertheless, by examining the dispersion relation and the frequency of cosmological gravitational waves, we observe a deviation from the results of General Relativity, quantified by a new parameter. Although its value is relatively small in viable $f(T)$ models, its possible future measurement in advancing gravitational-wave astronomy would be the smoking gun of testing this type of modified gravity.

gr-qc↗

arXiv2017

The Chern-Simons current in systems of DNA-RNA transcriptions

A Chern-Simons current, coming from ghost and anti-ghost fields of supersymmetry theory, can be used to define a spectrum of gene expression in new time series data where a spinor field, as alternative representation of a gene, is adopted instead of using the standard alphabet sequence of bases $A, T, C, G, U$. After a general discussion on the use of supersymmetry in biological systems, we give examples of the use of supersymmetry for living organism, discuss the codon and anti-codon ghost fields and develop an algebraic construction for the trash DNA, the DNA area which does not seem active in biological systems. As a general result, all hidden states of codon can be computed by Chern-Simons 3 forms. Finally, we plot a time series of genetic variations of viral glycoprotein gene and host T-cell receptor gene by using a gene tensor correlation network related to the Chern-Simons current. An empirical analysis of genetic shift, in host cell receptor genes with separated cluster of gene and genetic drift in viral gene, is obtained by using a tensor correlation plot over time series data derived as the empirical mode decomposition of Chern-Simons current.

physics.gen-ph↗

arXiv2017

Emergent universe in theories with natural UV cutoffs

We investigate the realization of the emergent universe scenario in theories with natural UV cutoffs, namely a minimum length and a maximum momentum, quantified by a new deformation parameter in the generalized uncertainty principle. We extract the Einstein static universe solutions and we examine their stability through a phase-space analysis. As we show, the role of the new deformation parameter is crucial in a twofold way: Firstly, it leads to the appearance of new Einstein static universe critical points, that are absent in standard cosmology. Secondly, it provides a way for a graceful exit from the Einstein static universe into the expanding thermal history, that is needed for a complete and successful realization of the emergent universe scenario.

gr-qc↗

arXiv2017

Viscous Cosmology for Early- and Late-Time Universe

From a hydrodynamicist's point of view the inclusion of viscosity concepts in the macroscopic theory of the cosmic fluid would appear most natural, as an ideal fluid is after all an abstraction (excluding special cases such as superconductivity). Making use of modern observational results for the Hubble parameter plus standard Friedmann formalism, we may extrapolate the description of the universe back in time up to the inflationary era, or we may go to the opposite extreme and analyze the probable ultimate fate of the universe. In this review we discuss a variety of topics in cosmology when it is enlarged in order to contain a bulk viscosity. Various forms of this viscosity, when expressed in terms of the fluid density or the Hubble parameter, are discussed. Furthermore, we consider homogeneous as well as inhomogeneous equations of state. We investigate viscous cosmology in the early universe, examining the viscosity effects on the various inflationary observables. Additionally, we study viscous cosmology in the late universe, containing current acceleration and the possible future singularities, and we investigate how one may even unify inflationary and late-time acceleration. Finally, we analyze the viscosity-induced crossing through the quintessence-phantom divide, we examine the realization of viscosity-driven cosmological bounces, and we briefly discuss how the Cardy-Verlinde formula is affected by viscosity.

gr-qc↗

arXiv2017

Observational constraints on successful model of quintessential Inflation

We study quintessential inflation using a generalized exponential potential $V(ϕ)\propto exp(-λϕ^n/Mpl^n), n>1$, the model admits slow-roll inflation at early times and leads to close-to-scaling behaviour in the post inflationary era with an exit to dark energy at late times. We present detailed investigations of the inflationary stage in the light of the Planck 2015 results, study post-inflationary dynamics and analytically confirm the existence of an approximately scaling solution. Additionally, assuming that standard massive neutrinos are non-minimally coupled, makes the field $ϕ$ dominant once again at late times giving rise to present accelerated expansion of the Universe. We derive observational constraints on the field and time-dependent neutrino masses. In particular, for $n=6 (8)$, the parameter $λ$ is constrained to be,$\log λ> -7.29 (-11.7)$; the model produces the spectral index of the power spectrum of primordial scalar (matter density) perturbations as $ n_s = 0.959 \pm 0.001 (0.961 \pm 0.001)$ and tiny tensor-to-scalar ratio, $r<1.72 \times 10^{-2} (2.32 \times 10^{-2})$ respectively. Consequently, the upper bound on possible values of the sum of neutrino masses $Σm_ν \lesssim 2.5$ eV significantly enhances compared to that in the standard $Λ$CDM model.

gr-qc↗

arXiv2017

Inflationary cosmology in unimodular $F(T)$ gravity

We investigate the inflationary realization in the context of unimodular $F(T)$ gravity, which is based on the $F(T)$ modification of teleparallel gravity, in which one imposes the unimodular condition through the use of Lagrange multipliers. We develop the general reconstruction procedure of the $F(T)$ form that can give rise to a given scale-factor evolution, and then we apply it in the inflationary regime. We extract the Hubble slow-roll parameters that allow us to calculate various inflation-related observables, such as the scalar spectral index and its running, the tensor-to-scalar ratio, and the tensor spectral index. Then, we examine the particular cases of de Sitter and power-law inflation, of Starobinsky inflation, as well as inflation in a specific model of unimodular $F(T)$ gravity. As we show, in all cases the predictions of our scenarios are in a very good agreement with Planck observational data. Finally, inflation in unimodular $F(T)$ gravity has the additional advantage that it always allows for a graceful exit for specific regions of the model parameters.

gr-qc↗