arXiv subjects

Spyros Basilakos

Publications and source records attributed to Spyros Basilakos.

At least 127 records · Page 7Linked to original sources

arXiv2013

Cosmological equivalence between the Finsler-Randers space-time and the DGP gravity model

We perform a detailed comparison between the Finsler-Randers cosmological model and the Dvali, Gabadadze and Porrati braneworld model. If we assume that the spatial curvature is strictly equal to zero then we prove the following interesting proposition: {\it despite the fact that the current cosmological models have a completely different geometrical origin they share exactly the same Hubble expansion}. This implies that the Finsler-Randers model is cosmologically equivalent with that of the DGP model as far as the cosmic expansion is concerned. At the perturbative level we find that the Finsler-Randers growth index of matter perturbations is $γ_{FR} \simeq 9/14$ which is somewhat lower than that of DGP gravity ($γ_{DGP} \simeq 11/16$) implying that the growth factor of the Finsler-Randers model is slightly different ($\sim 0.1-2%$) from the one provided by the DGP gravity model.

gr-qc↗

arXiv2012

Generalizing the running vacuum energy model and comparing with the entropic-force models

We generalize the previously proposed running vacuum energy model by including a term proportional to \dot{H}, in addition to the existing H^2 term. We show that the added degree of freedom is very constrained if both low redshift and high redshift data are taken into account. Best-fit models are undistinguishable from LCDM at the present time, but could be distinguished in the future with very accurate data at both low and high redshifts. We stress the formal analogy at the phenomenological level of the running vacuum models with recently proposed dark energy models based on the holographic or entropic point of view, where a combination of \dot{H} and H^2 term is also present. However those particular entropic formulations which do not have a constant term in the Friedmann equations are not viable. The presence of this term is necessary in order to allow for a transition from a decelerated to an accelerated expansion. In contrast, the running vacuum models, both the original and the generalized one introduced here contain this constant term in a more natural way. Finally, important conceptual issues common to all these models are emphasized.

gr-qc↗

arXiv2012

The $Λ$CDM growth rate of structure revisited

We re-examine the growth index of the concordance $Λ$ cosmology in the light of the latest 6dF and {\em WiggleZ} data. In particular, we investigate five different models for the growth index $γ$, by comparing their cosmological evolution using observational data of the growth rate of structure formation at different redshifts. Performing a joint likelihood analysis of the recent supernovae type Ia data, the Cosmic Microwave Background shift parameter, Baryonic Acoustic Oscillations and the growth rate data, we determine the free parameters of the $γ(z)$ parametrizations and we statistically quantify their ability to represent the observations. We find that the addition of the 6dF and {\em WiggleZ} growth data in the likelihood analysis improves significantly the statistical results. As an example, considering a constant growth index we find $Ω_{m0}=0.273\pm 0.011$ and $γ=0.586^{+0.079}_{-0.074}$.

astro-ph.CO↗

arXiv2012

Determining the Hubble constant using Giant extragalactic HII regions and HII galaxies

We report the first results of a long term program aiming to provide accurate independent estimates of the Hubble constant (H0) using the L-sigma distance estimator for Giant extragalactic HII regions (GEHR) and HII galaxies. We have used VLT and Subaru high dispersion spectroscopic observations of a local sample of HII galaxies, identified in the SDSS DR7 catalogue in order to re-define and improve the L(Hbeta)-sigma distance indicator and to determine the Hubble constant. To this end we utilized as local calibration or `anchor' of this correlation, GEHR in nearby galaxies which have accurate distance measurements determined via primary indicators. Using our best sample of 69 nearby HII galaxies and 23 GEHR in 9 galaxies we obtain H0=74.3 +- 3.1 (statistical) +- 2.9 (systematic) km /s Mpc, in excellent agreement with, and independently confirming, the most recent SNe Ia based results.

astro-ph.CO↗

arXiv2012

Dynamics and constraints of the dissipative Liouville cosmology

In this article we investigate the properties of the FLRW flat cosmological models in which the cosmic expansion of the Universe is affected by a dilaton dark energy (Liouville scenario). In particular, we perform a detailed study of these models in the light of the latest cosmological data, which serves to illustrate the phenomenological viability of the new dark energy paradigm as a serious alternative to the traditional scalar field approaches. By performing a joint likelihood analysis of the recent supernovae type Ia data (SNIa), the differential ages of passively evolving galaxies, and the Baryonic Acoustic Oscillations (BAOs) traced by the Sloan Digital Sky Survey (SDSS), we put tight constraints on the main cosmological parameters. Furthermore, we study the linear matter fluctuation field of the above Liouville cosmological models. In this framework, we compare the observed growth rate of clustering measured with those predicted by the current Liouville models. Performing a chi^2 statistical test we show that the Liouville cosmological model provides growth rates that match sufficiently well with the observed growth rate. To further test the viability of the models under study, we use the Press-Schechter formalism to derive their expected redshift distribution of cluster-size halos that will be provided by future X-ray and Sunyaev-Zeldovich cluster surveys. We find that the Hubble flow differences between the Liouville and the LambdaCDM models provide a significantly different halo redshift distribution, suggesting that the models can be observationally distinguished.

astro-ph.CO↗

arXiv2012

The growth index of matter perturbations and modified gravity

We place tight constraints on the growth index $γ$ by using the recent growth history results of 2dFGRS, SDSS-LRG, VIMOS-VLT deep Survey (VVDS) and {\em WiggleZ} datasets. In particular, we investigate several parametrizations of the growth index $γ(z)$, by comparing their cosmological evolution using observational growth rate data at different redshifts. Utilizing a standard likelihood analysis we find that the use of the combined growth data provided by the 2dFGRS, SDSS-LRG, VVDS and {\em WiggleZ} galaxy surveys, puts the most stringent constraints on the value of the growth index. As an example, assuming a constant growth index we obtain that $γ=0.602\pm 0.055$ for the concordance $Λ$CDM expansion model. Concerning the Dvali-Gabadadze-Porrati gravity model, we find $γ=0.503\pm 0.06$ which is lower, and almost $3σ$ away, from the theoretically predicted value of $γ_{DGP}\simeq 11/16$. Finally, based on a time varying growth index we also confirm that the combined growth data disfavor the DGP gravity.

astro-ph.CO↗

arXiv2012

Confronting the relaxation mechanism for a large cosmological constant with observations

In order to deal with a large cosmological constant a relaxation mechanism based on modified gravity has been proposed recently. By virtue of this mechanism the effect of the vacuum energy density of a given quantum field/string theory (no matter how big is its initial value in the early universe) can be neutralized dynamically, i.e. without fine tuning, and hence a Big Bang-like evolution of the cosmos becomes possible. Remarkably, a large class F^n_m of models of this kind, namely capable of dynamically adjusting the vacuum energy irrespective of its value and size, has been identified. In this paper, we carefully put them to the experimental test. By performing a joint likelihood analysis we confront these models with the most recent observational data on type Ia supernovae (SNIa), the Cosmic Microwave Background (CMB), the Baryonic Acoustic Oscillations (BAO) and the high redshift data on the expansion rate, so as to determine which ones are the most favored by observations. We compare the optimal relaxation models F^n_m found by this method with the standard or concordance LambdaCDM model, and find that some of these models may appear as almost indistinguishable from it. Interestingly enough, this shows that it is possible to construct viable solutions to the tough cosmological fine tuning problem with models that display the same basic phenomenological features as the concordance model.

gr-qc↗

arXiv2011

Hubble expansion and structure formation in the "running FLRW model" of the cosmic evolution

A new class of FLRW cosmological models with time-evolving fundamental parameters should emerge naturally from a description of the expansion of the universe based on the first principles of quantum field theory and string theory. Within this general paradigm, one expects that both the gravitational Newton's coupling, G, and the cosmological term, Lambda, should not be strictly constant but appear rather as smooth functions of the Hubble rate. This scenario ("running FLRW model") predicts, in a natural way, the existence of dynamical dark energy without invoking the participation of extraneous scalar fields. In this paper, we perform a detailed study of these models in the light of the latest cosmological data, which serves to illustrate the phenomenological viability of the new dark energy paradigm as a serious alternative to the traditional scalar field approaches. By performing a joint likelihood analysis of the recent SNIa data, the CMB shift parameter, and the BAOs traced by the Sloan Digital Sky Survey, we put tight constraints on the main cosmological parameters. Furthermore, we derive the theoretically predicted dark-matter halo mass function and the corresponding redshift distribution of cluster-size halos for the "running" models studied. Despite the fact that these models closely reproduce the standard LCDM Hubble expansion, their normalization of the perturbation's power-spectrum varies, imposing, in many cases, a significantly different cluster-size halo redshift distribution. This fact indicates that it should be relatively easy to distinguish between the "running" models and the LCDM cosmology using realistic future X-ray and Sunyaev-Zeldovich cluster surveys.

astro-ph.CO↗

arXiv2010

Quantum Gravity Corrections and Entropy at the Planck time

We investigate the effects of Quantum Gravity on the Planck era of the universe. In particular, using different versions of the Generalized Uncertainty Principle and under specific conditions we find that the main Planck quantities such as the Planck time, length, mass and energy become larger by a factor of order 10-10^{4} compared to those quantities which result from the Heisenberg Uncertainty Principle. However, we prove that the dimensionless entropy enclosed in the cosmological horizon at the Planck time remains unchanged. These results, though preliminary, indicate that we should anticipate modifications in the set-up of cosmology since changes in the Planck era will be inherited even to the late universe through the framework of Quantum Gravity (or Quantum Field Theory) which utilizes the Planck scale as a fundamental one. More importantly, these corrections will not affect the entropic content of the universe at the Planck time which is a crucial element for one of the basic principles of Quantum Gravity named Holographic Principle.

hep-th↗

arXiv2010

The Physical Properties of the Cosmic Acceleration

The observed late-time acceleration of the cosmic expansion constitutes a fundamental problem in modern theoretical physics and cosmology. In an attempt to weight the validity of a large number of dark energy models, I use the recent measurements of the expansion rate of the Universe, the clustering of galaxies the CMB fluctuations as well as the large scale structure formation, to put tight constraints on the different models.

astro-ph.CO↗

arXiv2009

Interacting Dark Matter as an Alternative to Dark Energy

We investigate the global dynamics of the universe within the framework of the Interacting Dark Matter (IDM) scenario. Considering that the dark matter obeys the collisional Boltzmann equation, we can obtain analytical solutions of the global density evolution, which can accommodate an accelerated expansion, equivalent to either the {\em quintessence} or the standard $Λ$ models. This is possible if there is a disequilibrium between the DM particle creation and annihilation processes with the former process dominating, which creates an effective source term with negative pressure. Comparing the predicted Hubble expansion of one of the IDM models (the simplest) with observational data, we find that the effective annihilation term is quite small, as suggested by various experiments.

astro-ph.CO↗

arXiv2009

Solving the main cosmological puzzles with a generalized time varying vacuum energy

We study the dynamics of the FLRW flat cosmological models in which the vacuum energy density varies with time, $Λ(t)$. In particular, we investigate the dynamical properties of a generalized vacuum model and we find that under certain conditions the vacuum term in the radiation era varies as $Λ(z) \propto (1+z)^{4}$, while in the matter era we have $Λ(z)\propto (1+z)^{3}$ up to $z\simeq 3$ and $Λ(z)=Λ$ for $z\le 3$. The confirmation of such a behavior would be of paramount importance because it could provide a solution to the cosmic coincidence problem as well as to the fine tuning problem, without changing the well known (from the concordance $Λ$ cosmology) Hubble expansion.

astro-ph.CO↗

arXiv2009

Precision Cosmology from X-ray AGN clustering

We place tight constraints on the main cosmological parameters of spatially flat cosmological models by using the recent angular clustering results of XMM-{\it Newton} soft (0.5-2 keV) X-ray sources (Ebrero et al. 2009a), which have a redshift distribution with a median of $z\sim 1$. Performing a standard likelihood procedure, assuming a constant in comoving coordinates AGN clustering evolution, the AGN bias evolution model of Basilakos et al. (2008) and the WMAP5 value of $σ_8$, we find stringent simultaneous constraints in the ($Ω_{m}, $w) plane, with $Ω_{m}= 0.26\pm 0.05$, w$=-0.93^{+0.11}_{-0.19}$.

astro-ph.CO↗

arXiv2009

Cosmological implications and structure formation from a time varying vacuum

We study the dynamics of the FLRW flat cosmological models in which the vacuum energy varies with time, $Λ(t)$. In this model we find that the main cosmological functions such as the scale factor of the universe and the Hubble flow are defined in terms of exponential functions. Applying a joint likelihood analysis of the recent supernovae type Ia data, the Cosmic Microwave Background shift parameter and the Baryonic Acoustic Oscillations traced by the Sloan Digital Sky Survey (SDSS) galaxies, we place tight constraints on the main cosmological parameters of the $Λ(t)$ scenario. Also, we compare the $Λ(t)$ model with the traditional $Λ$ cosmology and we find that the former model provides a Hubble expansion which compares well with that of the $Λ$ cosmology. However, the $Λ(t)$ scenario predicts stronger small scale dynamics, which implies a faster growth rate of perturbations with respect to the usual $Λ$-cosmology, despite the fact that they share the same equation of state parameter. In this framework, we find that galaxy clusters in the $Λ(t)$ model appear to form earlier than in the $Λ$ model.

astro-ph.CO↗

arXiv2008

The Aspen--Amsterdam Void Finder Comparison Project

Despite a history that dates back at least a quarter of a century studies of voids in the large--scale structure of the Universe are bedevilled by a major problem: there exist a large number of quite different void--finding algorithms, a fact that has so far got in the way of groups comparing their results without worrying about whether such a comparison in fact makes sense. Because of the recent increased interest in voids, both in very large galaxy surveys and in detailed simulations of cosmic structure formation, this situation is very unfortunate. We here present the first systematic comparison study of thirteen different void finders constructed using particles, haloes, and semi--analytical model galaxies extracted from a subvolume of the Millennium simulation. The study includes many groups that have studied voids over the past decade. We show their results and discuss their differences and agreements. As it turns out, the basic results of the various methods agree very well with each other in that they all locate a major void near the centre of our volume. Voids have very underdense centres, reaching below 10 percent of the mean cosmic density. In addition, those void finders that allow for void galaxies show that those galaxies follow similar trends. For example, the overdensity of void galaxies brighter than $m_B = -20 $ is found to be smaller than about -0.8 by all our void finding algorithms.

astro-ph↗

arXiv2008

Dynamics and chaos in the unified scalar field Cosmology

We study the dynamics of the closed scalar field FRW cosmological models in the framework of the so called Unified Dark Matter (UDM) scenario. Performing a theoretical as well as a numerical analysis we find that there is a strong indication of chaos in agreement with previous studies. We find that a positive value of the spatial curvature is essential for the appearance of chaoticity, though the Lyapunov number seems to be independent of the curvature value. Models that are close to flat exhibit a chaotic behavior after a long time while pure flat models do not exhibit any chaos. Moreover, we find that some of the semiflat models in the UDM scenario exhibit similar dynamical behavior with the Lambda cosmology despite their chaoticity. Finally, we compare the measured evolution of the Hubble parameter derived from the differential ages of passively evolving galaxies with that expected in the semiflat unified scalar field cosmology. Based on a specific set of initial conditions we find that the UDM scalar field model matches well the observational data.

astro-ph↗

arXiv2007

The tension of cosmological magnetic fields as a contribution to dark energy

We propose that cosmological magnetic fields generated in regions of finite spatial dimensions may manifest themselves in the global dynamics of the Universe as `dark energy'. We test our model in the context of spatially flat cosmological models by assuming that the Universe contains non-relativistic matter $ρ_m\propto α^{-3}$, dark energy $ρ_{Q}\propto α^{-3(1+w)}$, and an extra fluid with $ρ_{B} \propto α^{n-3}$ that corresponds to the magnetic field. We place constraints on the main cosmological parameters of our model by combining the recent supernovae type Ia data and the differential ages of passively evolving galaxies. In particular, we find that the model which best reproduces the observational data when $Ω_m=0.26$ is one with $Ω_{B}\simeq 0.03$, $n\simeq 7.68$, $Ω_{Q}\simeq 0.71$ and $w\simeq -0.8$.

astro-ph↗

arXiv2007

Cosmology from XMM High-Z AGN Clustering

We review the high-redshift X-ray selected AGN clustering, based on the XMM/2dF survey, and compare it with other recent XMM-based results. Using the recent Hasinger et al. (2005) and La Franca et al. (2005) luminosity functions we find that the spatial clustering lengths, derived using Limber's inversion equation, are $\sim 17$ and 20 $h^{-1}$ Mpc for the soft and hard band sources while their median redshifts are ${\bar z}\sim 1.2$ and 0.8, respectively. The corresponding bias factors at $z=\bar{z}$ are $\sim 5.3\pm 0.9$ and $\sim 5.1\pm1.1$, respectively. Within the framework of a flat cosmological model we find that our results support a model with $Ω_m \simeq 0.26$, $σ_8\simeq 0.75$, $h\simeq 0.72$, w$\simeq -0.9$ (in excellent agreement with the 3 year WMAP results). We also find the present day bias of X-ray AGNs to be $b_{\circ}\sim 2$.

astro-ph↗