arXiv subjects

S. Basilakos

Publications and source records attributed to S. Basilakos.

At least 19 recordsLinked to original sources

arXiv2026

Charting the expansion of the Universe from z$\sim$0 to z$\sim$14 with HII galaxies

We present an updated Hubble diagram of giant extragalactic H II regions and H II galaxies spanning the redshift interval $z\sim 0$ to $z\sim 14$, extending the use of the $L(\mathrm{H}\beta)-\sigma$ relation as an independent cosmological probe into the epoch of cosmic dawn. Our sample comprises 243 objects, including local anchor systems with primary distance measurements, previously published low- and intermediate-redshift H II galaxies, recent JWST/NIRSpec observations, and a new subsample of extremely high-redshift H II galaxies observed with JWST and with ALMA+JWST for the highest-redshift cases. We homogenise the measurements across the full sample, applying consistent corrections for instrumental and thermal broadening, extinction, and, where required, the transformation from [O III]-based to Balmer-line velocity dispersions. We model lensing effects of high redshift using a log-normal magnification formalism and infer cosmological parameters via a nested-sampling analysis. The $L-\sigma$ relation remains consistent over the full redshift range, showing no evidence for significant evolution even at the highest redshifts currently accessible. For the joint anchor+H II galaxies sample, under a flat $\Lambda$CDM model, we obtain $h=0.725\pm0.040$ and $\Omega_m=0.308^{+0.043}_{-0.053}$. Allowing a constant dark-energy equation of state yields $w_0=-0.96^{+0.53}_{-0.21}$, while a CPL parametrisation gives $w_0=-0.92^{+0.57}_{-0.34}$ and $w_a=-0.48^{+0.60}_{-1.50}$, all consistent, within the uncertainties, with concordance cosmology. These results demonstrate that H II galaxies offer a viable, fully independent tracer of the expansion history across almost the entire age of the Universe, opening a new avenue for testing $\Lambda$CDM and dark-energy evolution well beyond reionisation.

astro-ph.CO↗

arXiv2023

White Paper and Roadmap for Quantum Gravity Phenomenology in the Multi-Messenger Era

The unification of quantum mechanics and general relativity has long been elusive. Only recently have empirical predictions of various possible theories of quantum gravity been put to test, where a clear signal of quantum properties of gravity is still missing. The dawn of multi-messenger high-energy astrophysics has been tremendously beneficial, as it allows us to study particles with much higher energies and travelling much longer distances than possible in terrestrial experiments, but more progress is needed on several fronts. A thorough appraisal of current strategies and experimental frameworks, regarding quantum gravity phenomenology, is provided here. Our aim is twofold: a description of tentative multimessenger explorations, plus a focus on future detection experiments. As the outlook of the network of researchers that formed through the COST Action CA18108 ``Quantum gravity phenomenology in the multi-messenger approach (QG-MM)'', in this work we give an overview of the desiderata that future theoretical frameworks, observational facilities, and data-sharing policies should satisfy in order to advance the cause of quantum gravity phenomenology.

gr-qc↗

arXiv2022

Quantum gravity phenomenology at the dawn of the multi-messenger era -- A review

The exploration of the universe has recently entered a new era thanks to the multi-messenger paradigm, characterized by a continuous increase in the quantity and quality of experimental data that is obtained by the detection of the various cosmic messengers (photons, neutrinos, cosmic rays and gravitational waves) from numerous origins. They give us information about their sources in the universe and the properties of the intergalactic medium. Moreover, multi-messenger astronomy opens up the possibility to search for phenomenological signatures of quantum gravity. On the one hand, the most energetic events allow us to test our physical theories at energy regimes which are not directly accessible in accelerators; on the other hand, tiny effects in the propagation of very high energy particles could be amplified by cosmological distances. After decades of merely theoretical investigations, the possibility of obtaining phenomenological indications of Planck-scale effects is a revolutionary step in the quest for a quantum theory of gravity, but it requires cooperation between different communities of physicists (both theoretical and experimental). This review is aimed at promoting this cooperation by giving a state-of-the art account of the interdisciplinary expertise that is needed in the effective search of quantum gravity footprints in the production, propagation and detection of cosmic messengers.

hep-ph↗

arXiv2021

Applications of the Schwarzschild-Finsler-Randers model

In this article, we study further applications of the Schwarzschild-Finsler-Randers (SFR) model which was introduced in a previous work. In this model, we investigate curvatures and the generalized Kretschmann invariant which plays a crucial role for singularities. In addition, the derived path equations are used for the gravitational redshift of the SFR-model and these are compared with the GR model. Finally, we get some results for different values of parameters of the generalized photonsphere of the SFR-model and we find small deviations from the classical results of general relativity (GR) which may be ought to the possible Lorentz violation effects.

gr-qc↗

arXiv2020

Schwarzschild-like solutions in Finsler-Randers gravity

In this work, we extend for the first time the spherically symmetric Schwarzschild and Schwarzschild-De Sitter solutions with a Finsler-Randers-type perturbation which is generated by a covector $A_γ$. This gives a locally anisotropic character to the metric and induces a deviation from the Riemannian models of gravity. A natural framework for this study is the Lorentz tangent bundle of a spacetime manifold. We apply the generalized field equations to the perturbed metric and derive the dynamics for the covector $A_γ$. Finally, we find the timelike, spacelike and null paths on the Schwarzschild-Randers spacetime, we solve the timelike ones numerically and we compare them with the classic geodesics of general relativity. The obtained solutions are new and they enrich the corresponding literature.

gr-qc↗

arXiv2020

Dynamics in Varying vacuum Finsler-Randers Cosmology

In the context of Finsler-Randers theory we consider, for a first time, the cosmological scenario of the varying vacuum. In particular, we assume the existence of a cosmological fluid source described by an ideal fluid and the varying vacuum terms. We determine the cosmological history of this model by performing a detailed study on the dynamics of the field equations. We determine the limit of General Relativity, while we find new eras in the cosmological history provided by the geometrodynamical terms provided by the Finsler-Randers theory.

gr-qc↗

arXiv2018

Dark energy reconstruction based on the PADE approximation; an expansion around the $Λ$CDM

We study the dynamical properties of dark energy based on a large family of PADE parameterizations for which the dark energy density evolves as a ratio between two polynomials in the scale factor of the universe. Using the latest cosmological data we perform a standard likelihood analysis in order to place constraints on the main cosmological parameters of different PADE models. We find that the basic cosmological parameters, namely $(Ω_{m0},h,σ_{8})$ are practically the same for all PADE parametrizations explored here. Concerning the free parameters which are related to dark energy we show that the best fit values indicate that the equation of state parameter at the present time is in the phantom regime ($w<-1$), however we can not exclude the possibility of $w>-1$ at $1σ$ level. Finally, for the current family of PADE parametrizations we test their ability, via AIC and Jeffreys' scale, to deviate from $Λ$CDM cosmology. Among the current PADE parametrizations, the model which contains two dark energy parameters is the one for which a small but non-zero deviation from $Λ$CDM cosmology is slightly allowed by AIC test. Moreover, based on Jeffreys' scale we show that a deviation from $Λ$CDM cosmology is also allowed and thus the possibility of having a dynamical dark energy in the form of PADE parametrization cannot be excluded.

astro-ph.CO↗

arXiv2018

Cosmological singularities and analytical solutions in varying vacuum cosmologies

We investigate the dynamical features of a large family of running vacuum cosmologies for which $Λ$ evolves as a polynomial in the Hubble parameter. Specifically, using the critical point analysis we study the existence and the stability of singular solutions which describe de-Sitter, radiation and matter dominated eras. We find several classes of $Λ(H)$ cosmologies for which new analytical solutions are given in terms of Laurent expansions. Finally, we show that the Milne universe and the $R_{h}=ct$ model can be seen as perturbations around a specific $Λ(H)$ model, but this model is unstable.

gr-qc↗

arXiv2018

Updated constraints on $f(T)$ models using direct and indirect measurements of the Hubble parameter

We extract observational constraints on $f(T)$ gravity, using the recently proposed statistical method which is not affected by the value of $H_0$ and thus it bypasses the problem of the disagreement in its exact numerical value between Planck and direct measurements. We use direct measurements of the Hubble parameter with the corresponding covariance matrix, and for completeness we perform a joint analysis using the latest data from Supernovae type Ia based on JLA sample, quasi-stellar objects, and Cosmic Microwave Background shift parameter from Planck. We analyze a large family of $f(T)$ models, and we compare the fitting results with $Λ$CDM cosmology using the AIC statistical test. Utilizing only the Hubble parameter data we find that in the case of the power-law $f(T)$ model a small but non-zero deviation from $Λ$CDM cosmology is slightly favored at 1-$σ$, nevertheless the corresponding AIC value shows a statistical equivalence with it. Finally, the join analysis reveals that all $f(T)$ models are very efficient and in very good agreement with observations.

astro-ph.CO↗

arXiv2018

Cartan symmetries and global dynamical systems analysis in a higher-order modified teleparallel theory

In a higher-order modified teleparallel theory cosmological we present analytical cosmological solutions. In particular we determine forms of the unknown potential which drives the scalar field such that the field equations form a Liouville integrable system. For the determination of the conservation laws we apply the Cartan symmetries. Furthermore, inspired from our solutions, a toy model is studied and it is shown that it can describe the Supernova data, while at the same time introduces dark matter components in the Hubble function. When the extra matter source is a stiff fluid then we show how analytical solutions for Bianchi I universes can be constructed from our analysis. Finally, we perform a global dynamical analysis of the field equations by using variables different from that of the Hubble-normalization.

gr-qc↗

arXiv2018

Model selection and constraints from Holographic dark energy scenarios

In this study we combine the expansion and the growth data in order to investigate the ability of the three most popular holographic dark energy models, namely event future horizon, Ricci scale and Granda-Oliveros IR cutoffs, to fit the data. Using a standard $χ^2$ minimization method we place tight constraints on the free parameters of the models. Based on the values of the Akaike and Bayesian information criteria we find that two out of three holographic dark energy models are disfavored by the data, because they predict a non-negligible amount of dark energy density at early enough times. Although the growth rate data are relatively consistent with the holographic dark energy models which are based on Ricci scale and Granda-Oliveros IR cutoffs, the combined analysis provides strong indications against these models. Finally, we find that the model for which the holographic dark energy is related with the future horizon is consistent with the combined observational data.

gr-qc↗

arXiv2017

Finsler-Randers Cosmology: dynamical analysis and growth of matter perturbations

We study for the first time the dynamical properties and the growth index of linear matter perturbations of the Finsler-Randers (FR) cosmological model, for which we consider that the cosmic fluid contains matter, radiation and a scalar field. Initially, for various FR scenarios we implement a critical point analysis and we find solutions which provide cosmic acceleration and under certain circumstances we can have de-Sitter points as stable late-time attractors. Then we derive the growth index of matter fluctuations in various Finsler-Randers cosmologies. Considering cold dark matter and neglecting the scalar field component from the perturbation analysis we find that the asymptotic value of the growth index is $γ_{\infty}^{(FR)}\approx\frac {9}{16}$, which is close to that of the concordance $Λ$ cosmology, $γ^{(Λ)} \approx\frac{6}{11}$. In this context, we show that the current FR model provides the same Hubble expansion with that of Dvali, Gabadadze and Porrati (DGP) gravity model. However, the two models can be distinguished at the perturbation level since the growth index of FR model is $\sim18.2\%$ lower than that of the DPG gravity $γ^{(DGP)} \approx \frac{11}{16}$. If we allow pressure in the matter fluid then we obtain $γ_{\infty}^{(FR)}\approx\frac{9(1+w_{m})(1+2w_{m})}{2[8+3w_{m}% (5+3w_{m})]}$, where $w_{m}$ is the matter equation of state parameter. Finally, we extend the growth index analysis by using the scalar field and we find that the evolution of the growth index in FR cosmologies is affected by the presence of scalar field.

gr-qc↗

arXiv2016

Dynamical symmetries in Brans-Dicke cosmology

In the context of generalised Brans-Dicke cosmology we use the Killing tensors of the minisuperspace in order to determine the unspecified potential of a scalar-tensor gravity theory. Specifically, based on the existence of contact symmetries of the field equations, we find four types of potentials which provide exactly integrable dynamical systems. We investigate the dynamical properties of these potentials by using a critical point analysis and we find solutions which lead to cosmic acceleration and under specific conditions we can have de-Sitter points as stable late-time attractors.

gr-qc↗

arXiv2016

Agegraphic dark energy: growth index and cosmological implications

We study the main cosmological properties of the agegraphic dark energy model at the expansion and perturbation levels. Initially, using the latest cosmological data we implement a joint likelihood analysis in order to constrain the cosmological parameters. Then we test the performance of the agegraphic dark energy model at the perturbation level and we define its difference from the usual $Λ$CDM model. Within this context, we verify that the growth index of matter fluctuations depends on the choice of the considered agegraphic dark energy (homogeneous or clustered). In particular, assuming a homogeneous agegraphic dark energy we find, for the first time, that the asymptotic value of the growth index is $γ\approx 5/9$, which is close to that of the usual $Λ$ cosmology, $γ^{(Λ)} \approx 6/11$. Finally, if the distribution of dark energy is clustered then we obtain $γ\approx 1/2$ which is $\sim 8\%$ smaller than that of the $Λ$CDM model.

astro-ph.CO↗

arXiv2016

Comparison of the spatial and the angular clustering of X-ray AGN

The angular correlation function is a powerful tool for deriving the clustering properties of AGN and hence the mass of the corresponding dark matter halos in which they reside. However, studies based on the application of the angular correlation function on X-ray samples, yield results apparently inconsistent with those based on the direct estimation of the spatial correlation function. The goal of the present paper is to attempt to investigate this issue by analysing a well defined sample. To this end we use the hard-band (2-10 keV) X-ray selected sources of the Chandra AEGIS fields, chosen because of the availability of accurately derived flux sensitivity maps. In particular we use the 186 hard-band sources with spectroscopic redshifts in the range z=0.3-1.3, a range selected in order to contain the bulk of the AGN while minimizing the contribution of unknown clustering and luminosity evolution from very high redshifts. Using the projected spatial auto-correlation function, we derive a clustering comoving length of 5.4+-1.0 Mpc (for gamma=1.8), consistent with results in the literature. We further derive the angular correlation function and the corresponding spatial clustering length using the Limber's inversion equation and a novel parametrization of the clustering evolution model that also takes into account the bias evolution of the host dark matter halo. The Limber's inverted spatial comoving clustering length of 5.5+-1.2 Mpc at a median redshift of z~0.75, matches the directly measured one, from the spatial correlation function analysis, but for a significant non-linear contribution to the growing mode of perturbations, estimated independently from literature results of x_0 at different redshifts. Therefore, using this sample of X-ray AGN and our clustering evolution parametrization we have found an excellent consistency between the angular and spatial clustering analysis.

astro-ph.CO↗

arXiv2015

How clustering dark energy affects matter perturbations

The rate of structure formation in the Universe is different in homogeneous and clustered dark energy models. The degree of dark energy clustering depends on the magnitude of its effective sound speed $c^{2}_{\rm eff}$ and for $c_{\rm eff}=0$ dark energy clusters in a similar fashion to dark matter while for $c_{\rm eff}=1$ it stays (approximately) homogeneous. In this paper we consider two distinct equations of state for the dark energy component, $w_{\rm d}=const$ and $w_{\rm d}=w_0+w_1\left(\frac{z}{1+z}\right)$ with $c_{\rm eff}$ as a free parameter and we try to constrain the dark energy effective sound speed using current available data including SnIa, Baryon Acoustic Oscillation, CMB shift parameter ({\em Planck} and {\em WMAP}), Hubble parameter, Big Bang Nucleosynthesis and the growth rate of structures $fσ_{8}(z)$. At first we derive the most general form of the equations governing dark matter and dark energy clustering under the assumption that $c_{\rm eff}=const$. Finally, performing an overall likelihood analysis we find that the likelihood function peaks at $c_{\rm eff}=0$, however the dark energy sound speed is degenerate with respect to the cosmological parameters, namely $Ω_{\rm m}$ and $w_{\rm d}$.

astro-ph.CO↗

arXiv2015

Nonsingular Decaying Vacuum Cosmology and Entropy Production

The thermodynamic behavior of a decaying vacuum cosmology describing the entire cosmological history evolving between two extreme (early and late time) de Sitter eras is investigated. The thermal evolution from the early de Sitter to the radiation phase is discussed in detail. The temperature evolution law and the increasing entropy function are analytically determined. The entropy of the effectively massless particles is initially zero but evolves continuously to the present day maximum value within the current Hubble radius, $S_0 \sim 10^{88}$ in natural units. By using the Gibbons-Hawking temperature relation for the de Sitter spacetime, it is found that the ratio between the primeval and the late time vacuum energy densities is $ρ_{vI}/ρ_{v0} \sim 10^{123}$, as required by some naive estimates from quantum field theory.

gr-qc↗

arXiv2014

Cosmic acceleration without dark energy: Background tests and thermodynamic analysis

A new cosmic scenario with gravitationally induced particle creation is proposed. In this model the Universe evolves from an early to a late time de Sitter era, with the recent accelerating phase driven only by the negative creation pressure associated with the cold dark matter component. The model can be interpreted as an attempt to reduce the so-called cosmic sector (dark matter plus dark energy) and relate the two cosmic accelerating phases (early and late time de Sitter expansions). A detailed thermodynamic analysis including possible quantum corrections is also carried out. For a very wide range of the free parameters, it is found that the model presents the expected behavior of an ordinary macroscopic system in the sense that it approaches thermodynamic equilibrium in the long run (i.e., as it nears the second de Sitter phase). Moreover, an upper bound is found for the Gibbons-Hawking temperature of the primordial de Sitter phase. Finally, when confronted with the recent observational data, the current `quasi'-de Sitter era, as predicted by the model, is seen to pass very comfortably the cosmic background tests.

gr-qc↗