arXiv subjects
Publications and source records attributed to Spyros Basilakos.
In the context of $f\left( R\right) $-gravity with a spatially flat FLRW metric containing an ideal fluid, we use the method of invariant transformations to specify families of models which are integrable. We find three families of $f(R)$ theories for which new analytical solutions are given and closed-form solutions are provided.
The relationship between the integrated H$β$ line luminosity and the velocity dispersion of the ionized gas of HII galaxies and giant HII regions represents an exciting standard candle that presently can be used up to redshifts z ~ 4. Locally it is used to obtain precise measurements of the Hubble constant by combining the slope of the relation obtained from nearby ($z \leq $ 0.2) HII galaxies with the zero point determined from giant HII regions belonging to an `anchor sample' of galaxies for which accurate redshift-independent distance moduli are available. We present new data for 36 giant HII regions in 13 galaxies of the anchor sample that includes the megamaser galaxy NGC 4258. Our data is the result of the first four years of observation of our primary sample of 130 giant HII regions in 73 galaxies with Cepheid determined distances. Our best estimate of the Hubble parameter is $71.0\pm2.8(random)\pm2.1(systematic)$ km /s Mpc This result is the product of an independent approach and, although at present less precise than the latest SNIa results, it is amenable to substantial improvement.
In this paper we present conjoined constraints on several cosmological models from the expansion history $H(z)$ and cosmic growth $fσ_8(z)$. The models we study include the CPL $w_0w_a$ parametrization, the Holographic Dark Energy (HDE) model, the Time varying vacuum ($Λ_t$CDM) model, the Dvali, Gabadadze and Porrati (DGP) and Finsler-Randers (FRDE) models, a power law $f(T)$ model and finally the Hu-Sawicki $f(R)$ model. In all cases we perform a simultaneous fit to the SnIa, CMB, BAO, $H(z)$ and growth data, while also following the conjoined visualization of $H(z)$ and $fσ_8(z)$ as in Linder (2017). Furthermore, we introduce the Figure of Merit (FoM) in the $H(z)-fσ_8(z)$ parameter space as a way to constrain models that jointly fit both probes well. We use both the latest $H(z)$ and $fσ_8(z)$ data, but also LSST-like mocks with $1\%$ measurements and we find that the conjoined method of constraining the expansion history and cosmic growth simultaneously is able not only to place stringent constraints on these parameters but also to provide an easy visual way to discriminate cosmological models. Finally, we confirm the existence of a tension between the growth rate and Planck CMB data and we find that the FoM in the conjoined parameter space of $H(z)-fσ_8(z)$ can be used to discriminate between the $Λ$CDM model and certain classes of modified gravity models, namely the DGP and $f(T)$.
We put constraints on dark energy properties using the PADE parameterisation, and compare it to the same constraints using Chevalier-Polarski-Linder (CPL) and $Λ$CDM, at both the background and the perturbation levels. The dark energy equation of state parameter of the models is derived following the mathematical treatment of PADE expansion. Unlike CPL parameterisation, the PADE approximation provides different forms of the equation of state parameter which avoid the divergence in the far future. Initially, we perform a likelihood analysis in order to put constraints on the model parameters using solely background expansion data and we find that all parameterisations are consistent with each other. Then, combining the expansion and the growth rate data we test the viability of PADE parameterisations and compare them with CPL and $Λ$CDM models respectively. Specifically, we find that the growth rate of the current PADE parameterisations is lower than $Λ$CDM model at low redshifts, while the differences among the models are negligible at high redshifts. In this context, we provide for the first time growth index of linear matter perturbations in PADE cosmologies. Considering that dark energy is homogeneous we recover the well known asymptotic value of the growth index, namely $γ_{\infty}=\frac{3(w_{\infty}-1)}{6w_{\infty}-5}$, while in the case of clustered dark energy we obtain $γ_{\infty}\simeq \frac{3w_{\infty}(3w_{\infty}-5)}{(6w_{\infty}-5)(3w_{\infty}-1)}$. Finally, we generalize the growth index analysis in the case where $γ$ is allowed to vary with redshift and we find that the form of $γ(z)$ in PADE parameterisation extends that of the CPL and $Λ$CDM cosmologies respectively.
In this Essay we investigate the observational signatures of Loop Quantum Cosmology (LQC) in the CMB data. First, we concentrate on the dynamics of LQC and we provide the basic cosmological functions. We then obtain the power spectrum of scalar and tensor perturbations in order to study the performance of LQC against the latest CMB data. We find that LQC provides a robust prediction for the main slow-roll parameters, like the scalar spectral index and the tensor-to-scalar fluctuation ratio, which are in excellent agreement within $1σ$ with the values recently measured by the Planck collaboration. This result indicates that LQC can be seen as an alternative scenario with respect to that of standard inflation.
We study the spherical collapse model (SCM) in the framework of spatially flat power law $f(T) \propto (-T)^{b}$ gravity model. We find that the linear and non-linear growth of spherical overdensities of this particular $f(T)$ model are affected by the power-law parameter $b$. Finally, we compute the predicted number counts of virialized haloes in order to distinguish the current $f(T)$ model from the expectations of the concordance $Λ$ cosmology. Specifically, the present analysis suggests that the $f(T)$ gravity model with positive (negative) $b$ predicts more (less) virialized objects with respect to those of $Λ$CDM.
The growth index of matter fluctuations is computed for ten distinct accelerating cosmological models and confronted to the latest growth rate data via a two-step process. First, we implement a joint statistical analysis in order to place constraints on the free parameters of all models using solely background data. Second, using the observed growth rate of clustering from various galaxy surveys we test the performance of the current cosmological models at the perturbation level while either marginalizing over $σ_8$ or having it as a free parameter. As a result, we find that at a statistical level, i.e. after considering the best-fit $χ^2$ or the value of the Akaike information criterion, most models are in very good agreement with the growth rate data and are practically indistinguishable from $Λ$CDM. However, when we also consider the internal consistency of the models by comparing the theoretically predicted values of $(γ_0, γ_1)$, i.e. the value of the growth index $γ(z)$ and its derivative today, with the best-fit ones, we find that the predictions of three out of ten dark energy models are in mild tension with the best-fit ones when $σ_8$ is marginalized over. When $σ_8$ is free we find that most models are not only in mild tension, but also predict low values for $σ_8$. This could be attributed to either a systematic problem with the growth-rate data or the emergence of new physics at low redshifts, with the latter possibly being related to the well-known issue of the lack of power at small scales. Finally, by utilizing mock data based on an LSST-like survey we show that with future surveys and by using the growth index parameterization, it will be possible to resolve the issue of the low $σ_8$ but also the tension between the fitted and theoretically predicted values of $(γ_0, γ_1)$.
The correlation between emission-line luminosity (L) and profile width (sigma) for HII Galaxies provides a powerful method to measure the distances to galaxies over a wide range of redshifts. In this paper we use SDSS spectrophotometry to explore the systematics of the correlation using the [OIII]5007 lines instead of Halpha or Hbeta to measure luminosities and line widths. We also examine possible systematic effects involved in measuring the profile-widths and the luminosities through different apertures. We find that the green L-sigma relation defined using [OIII]5007 luminosities is significantly more sensitive than Hbeta to the effects of age and the physical conditions of the nebulae, which more than offsets the advantage of the higher strength of the [OIII]5007 lines. We then explore the possibility of mixing [OIII]5007 profile-widths with SDSS Hbeta luminosities using the Hubble constant H0 to quantify the possible systematic effects. We find the mixed L(Hbeta) sigma[OIII] relation to be at least as powerful as the canonical L-sigma relation as a distance estimator, and we show that the evolutionary corrections do not change the slope and the scatter of the correlation, and therefore, do not bias the L-sigma distance indicator at high redshifts. Locally, however, the luminosities of the Giant HII Regions that provide the zero-point calibrators are sensitive to evolutionary corrections and may bias the Hubble constant if their mean ages, as measured by the equivalent widths of Hbeta, are significantly different from the mean age of the HII Galaxies. Using a small sample of 16 ad-hoc zero point calibrators we obtain a value of H0 = 66.4\pm4.5 km s^-1 Mpc^-1 for the Hubble constant, which is fully consistent with the best modern determinations, and that is not biased by evolutionary corrections.
We study the main properties of the warm inflationary model based on Barrow's solution for the scale factor of the universe. Within this framework we calculate analytically the basic slow roll parameters for different versions of warm inflation. We test the performance of this inflationary scenario against the latest observational data and we verify that the predicted spectral index and the tensor-to-scalar fluctuation ratio are in excellent agreement with those of {\it Planck 2015}. Finally, we find that the current predictions are consistent with those of viable inflationary models.
We describe the primeval inflationary phase of the early Universe within a quantum field theoretical (QFT) framework that can be viewed as the effective action of vacuum decay in the early times. Interestingly enough, the model accounts for the "graceful exit" of the inflationary phase into the standard radiation regime. The underlying QFT framework considered here is Supergravity (SUGRA), more specifically an existing formulation in which the Starobinsky-type inflation (de-Sitter background) emerges from the quantum corrections to the effective action after integrating out the gravitino fields in their (dynamically induced) massive phase. We also demonstrate that the structure of the effective action in this model is consistent with the generic idea of renormalization group (RG) running of the cosmological parameters, specifically it follows from the corresponding RG equation for the vacuum energy density as a function of the Hubble rate, $ρ_Λ(H)$. Overall our combined approach amounts to a concrete-model realization of inflation triggered by vacuum decay in a fundamental physics context which, as it turns out, can also be extended for the remaining epochs of the cosmological evolution until the current dark energy era.
We use the HII galaxies $L - σ$ relation and the resulting Hubble expansion cosmological probe of a sample of just 25 high-$z$ (up to $z \sim 2.33$) HII galaxies, in a joint likelihood analysis with other well tested cosmological probes (CMB, BAOs) in an attempt to constrain the dark energy equation of state (EoS). The constraints, although still weak, are in excellent agreement with those of a similar joint analysis using the well established SNIa Hubble expansion probe. Interestingly, even with the current small number of available high redshift HII galaxies, the HII/BAO/CMB joint analysis gives a 13% improvement of the quintessence dark energy cosmological constraints compared to the BAO/CMB joint analysis. We have further performed extensive Monte Carlo simulations, with a realistic redshift sampling, to explore the extent to which the use of the $L - σ$ relation, observed in HII galaxies, can constrain effectively the parameter space of the dark energy EoS. The simulations predict substantial improvement in the constraints when increasing the sample of high-$z$ HII galaxies to 500, a goal that can be achieved in reasonable observing times with existing large telescopes and state-of-the-art instrumentation.
We use a kinematic parametrisation of the luminosity distance to measure the angular distribution on the sky of time derivatives of the scale factor, in particular the Hubble parameter H_0, the deceleration parameter q_0, and the jerk parameter j_0. We apply a recently published method to complement probing the inhomogeneity of the large-scale structure by means of the inhomogeneity in the cosmic expansion. This parametrisation is independent of the cosmological equation of state, which renders it adequate to test interpretations of the cosmic acceleration alternative to the cosmological constant. For the same analytical toy model of an inhomogeneous ensemble of homogenous pixels, we derive the backreaction term in j_0 due to the fluctuations of {H_0,q_0} and measure it to be of order 10^{-2} times the corresponding average over the pixels in the absence of backreaction. In agreement with that computed using a Lambda CDM parametrisation of the luminosity distance, the backreaction effect on q_0 remains below the detection threshold. Although the backreaction effect on j_0 is about ten times that on q_0, it is also below the detection threshold. Hence backreaction remains unobservable both in q_0 and in j_0.
ID11 is an actively star forming extremely compact galaxy and Ly alpha emitter at z=3.117 that is gravitationally magnified by a factor of ~17 by the cluster of galaxies Hubble Frontier Fields AS1063. Its observed properties resemble those of low luminosity HII galaxies or Giant HII regions like 30-Doradus in the LMC. Using the tight correlation correlation between the Balmer-line luminosities and the width of the emission lines (typically L(Hbeta)-sigma(Hbeta)) valid for HII galaxies and Giant HII regions to estimate its total luminosity, we are able to measure the lensing amplification of ID11. We obtain an amplification of 23 +- 11 similar within errors to the value of ~17 estimated or predicted by the best lensing models of the massive cluster Abell S1063. We also compiled from the literature luminosities and velocity dispersions for a set of lensed compact starforming regions. There is more scatter in the L-sigma correlation for these lensed systems but on the whole the results tend to support the lensing models estimates of the magnification. Our result indicates that the amplification can be independently measured using the L-sigma relation in lensed Giant HII regions or HII galaxies. It also supports the suggestion, even if lensing model dependent, that the L-sigma relation is valid for low luminosity high-z objects. Ad-hoc observations of lensed starforming systems are required to accurately determine the lensing amplification.
We show that the recent results of \ [Int. J. Mod. Phys. D 25 (2016) 1650051] on the application of Lie/Noether symmetries in scalar field cosmology are well-known in the literature while the problem could have been solved easily under a coordinate transformation. That follows from the property, that the admitted group of invariant transformations of dynamical system is independent on the coordinate system.
The thermal history of a large class of running vacuum models in which the effective cosmological term is described by a truncated power series of the Hubble rate, whose dominant term is $Λ(H) \propto H^{n+2}$, is discussed in detail. Specifically, by assuming that the ultra-relativistic particles produced by the vacuum decay emerge into space-time in such a way that its energy density $ρ_r \propto T^{4}$, the temperature evolution law and the increasing entropy function are analytically calculated. For the whole class of vacuum models explored here we findthat the primeval value of the comoving radiation entropy density (associated to effectively massless particles) starts from zero and evolves extremely fast until reaching a maximum near the end of the vacuum decay phase, where it saturates. The late time conservation of the radiation entropy during the adiabatic FRW phase also guarantees that the whole class of running vacuum models predicts thesame correct value of the present day entropy, $S_{0} \sim 10^{87-88}$ (in natural units), independently of the initial conditions. In addition, by assuming Gibbons-Hawking temperature as an initial condition, we find that the ratio between the late time and primordial vacuum energy densities is in agreement with naive estimates from quantum field theory, namely, $ρ_{Λ0}/ρ_{ΛI} \sim10^{-123}$. Such results are independent on the power $n$ and suggests that the observed Universe may evolve smoothly between two extreme, unstable, nonsingular de Sitter phases.
We provide for the first time the growth index of linear matter fluctuations of the power law $f(T) \propto (-T)^{b}$ gravity model. We find that the asymptotic form of this particular $f(T)$ model is $γ\approx \frac{6}{11-6b}$ which obviously extends that of the $Λ$CDM model, $γ_Λ\approx 6/11$. Finally, we generalize the growth index analysis of $f(T)$ gravity in the case where $γ$ is allowed to vary with redshift.
We consider the application of group invariant transformations in order to constrain a flat isotropic and homogeneous cosmological model, containing of a Brans-Dicke scalar field and a perfect fluid with a constant equation of state parameter $w$, where the latter is not interacting with the scalar field in the gravitational action integral. The requirement that the Wheeler-DeWitt equation be invariant under one-parameter point transformations provides us with two families of power-law potentials for the Brans-Dicke field, in which the powers are functions of the Brans-Dicke parameter $ω_{BD}$ and the parameter $w$. The existence of the Lie symmetry in the Wheeler-DeWitt equation is equivalent to the existence of a conserved quantity in field equations and with oscillatory terms in the wavefunction of the universe. This enables us to solve the field equations. For a specific value of the conserved quantity, we find a closed-form solution for the Hubble factor, which is equivalent to a cosmological model in general relativity containing two perfect fluids. This provides us with different models for specific values of the parameters $ω_{BD},$ and $w$. Finally, the results hold for the specific case where the Brans-Dicke parameter $ω_{BD}$ is zero, that is, for the O'Hanlon massive dilaton theory, and consequently for $f\left( R\right) $ gravity in the metric formalism.
We derive for the first time the growth index of matter perturbations of the FLRW flat cosmological models in which the vacuum energy depends on redshift. A particularly well motivated model of this type is the so-called quantum field vacuum, in which apart from a leading constant term $Λ_0$ there is also a $H^{2}$-dependence in the functional form of vacuum, namely $Λ(H)=Λ_{0}+3ν(H^{2}-H^{2}_{0})$. Since $|ν|\ll1$ this form endows the vacuum energy of a mild dynamics which affects the evolution of the main cosmological observables at the background and perturbation levels. Specifically, at the perturbation level we find that the growth index of the running vacuum cosmological model is $γ_{Λ_{H}} \approx \frac{6+3ν}{11-12ν}$ and thus it nicely extends analytically the result of the $Λ$CDM model, $γ_Λ\approx 6/11$.