arXiv subjects

Savvas Nesseris

Publications and source records attributed to Savvas Nesseris.

89 records · Page 5Linked to original sources

arXiv2014

A novel null test for the $Λ$CDM model with growth-rate data

Current and upcoming surveys will measure the cosmological parameters with an extremely high accuracy. The primary goal of these observations is to eliminate some of the currently viable cosmological models created to explain the late time accelerated expansion (either real or only inferred). However, most of the statistical tests used in cosmology have a strong requirement: the use of a model to fit the data. Recently there has been an increased interest on finding tests that are model independent, i.e. to have a function that depends entirely on observed quantities and not on the model, see for instance [1]. In this letter we present an alternative consistency check at the perturbative level for a homogeneous and isotropic Universe filled with a dark energy component. This test makes use of the growth of matter perturbations data and it is able to not only test the homogeneous and isotropic Universe but also, within the framework of a Friedmann-Lemaître-Robertson-Walker Universe, if the dark energy component is able to cluster, if there is a tension in the data or if we are dealing with a modification of gravity.

astro-ph.CO↗

arXiv2014

Cosmological perturbations and observational constraints on nonlocal massive gravity

Nonlocal massive gravity can provide an interesting explanation for the late-time cosmic acceleration, with a dark energy equation of state $w_{\rm DE}$ smaller than $-1$ in the past. We derive the equations of linear cosmological perturbations to confront such models with the observations of large-scale structures. The effective gravitational coupling to nonrelativistic matter associated with galaxy clusterings is close to Newton's gravitational constant $G$ for a mass scale $m$ slightly smaller than today's Hubble parameter $H_0$. Taking into account the background expansion history as well as the evolution of matter perturbations $δ_m$, we test for these models with Type Ia Supernovae (SnIa) from Union 2.1, the cosmic microwave background (CMB) measurements from Planck, a collection of baryon acoustic oscillations (BAO), and the growth rate data of $δ_m$. Using a higher value of $H_0$ derived from its direct measurement ($H_0 \gtrsim 70$ km s$^{-1}$ Mpc$^{-1}$) the data strongly support the nonlocal massive gravity model ($-1.1 \lesssim w_{\rm DE} \lesssim -1.04$ in the past) over the $Λ$CDM model ($w_{\rm DE}=-1$), whereas for a lower prior (67 km s$^{-1}$ Mpc$^{-1}$ $\lesssim$ $H_0 \lesssim 70$ km s$^{-1}$ Mpc$^{-1}$) the two models are statistically comparable.

astro-ph.CO↗

arXiv2013

Can the degeneracies in the gravity sector be broken?

It has been long known that by appropriately modifying gravity one can always reproduce the expansion history of any dark energy model, e.g. the $Λ$CDM. This degeneracy cannot be broken with geometric probes like the Type Ia supernovae, the cosmic microwave background or the baryon acoustic oscillations since they are based on the measurement of distances and scales and therefore require the expansion history $H(z)$, so one may only hope to break this degeneracy by using dynamic probes like the growth rate data that track the matter density perturbations. We demonstrate that breaking this degeneracy is not currently possible by explicitly constructing $f(R)$, $f(G)$ and $f(T)$ theories that mimic exactly the $Λ$CDM model at the background level and confronting them against the latest observational cosmological data. We also determine the necessary improvement in the growth rate data in order to discriminate these theories from $Λ$CDM. We found that at least a threefold improvement is necessary, something which will be possible with a survey like Euclid.

astro-ph.CO↗

arXiv2013

Comparative analysis of model-independent methods for exploring the nature of dark energy

We make a comparative analysis of the various independent methods proposed in the literature for studying the nature of dark energy, using four different mocks of SnIa data. In particular, we explore a generic principal components analysis approach, the genetic algorithms, a series of approximations like Padé power law approximants, and various expansions in orthogonal polynomials, as well as cosmography, and compare them with the usual fit to a model with a constant dark energy equation of state w. We find that, depending on the mock data, some methods are more efficient than others at distinguishing the underlying model, although there is no universally better method.

astro-ph.CO↗

arXiv2013

Is the Jeffreys' scale a reliable tool for Bayesian model comparison in cosmology?

We are entering an era where progress in cosmology is driven by data, and alternative models will have to be compared and ruled out according to some consistent criterium. The most conservative and widely used approach is Bayesian model comparison. In this paper we explicitly calculate the Bayes factors for all models that are linear with respect to their parameters. We do this in order to test the so called Jeffreys' scale and determine analytically how accurate its predictions are in a simple case where we fully understand and can calculate everything analytically. We also discuss the case of nested models, e.g. one with $M_1$ and another with $M_2\supset M_1$ parameters and we derive analytic expressions for both the Bayes factor and the Figure of Merit, defined as the inverse area of the model parameter's confidence contours. With all this machinery and the use of an explicit example we demonstrate that the threshold nature of Jeffreys' scale is not a "one size fits all" reliable tool for model comparison and that it may lead to biased conclusions. Furthermore, we discuss the importance of choosing the right basis in the context of models that are linear with respect to their parameters and how that basis affects the parameter estimation and the derived constraints.

astro-ph.CO↗

arXiv2013

Observational constraints on viable f(R) parametrizations with geometrical and dynamical probes

We demonstrate that a wide range of viable f(R) parameterizations (including the Hu & Sawicki and the Starobinsky models) can be expressed as perturbations deviating from the LCDM Lagrangian. We constrain the deviation parameter b using a combination of geometrical and dynamical observational probes. In particular, we perform a joint likelihood analysis of the recent Supernovae Type Ia data, the Cosmic Microwave Background shift parameters, the Baryonic Acoustic Oscillations and the growth rate data provided by the various galaxy surveys. This analysis provides constraints for the following parameters: the matter density Omega_{m0}, the deviation from LCDM parameter b and the growth index gamma(z). We parametrize the growth index gamma(z) in three manners (constant, Taylor expansion around z=0, and Taylor expansion around the scale factor). We point out the numerical difficulty for solving the generalized f(R) Friedman equation at high redshifts due to stiffness of the resulting ordinary differential equation. We resolve this problem by constructing an efficient analytical perturbative method in the deviation parameter b. We demonstrate that this method is highly accurate, by comparing the resulting analytical expressions for the Hubble parameter, with the numerical solutions at low and intermediate redshifts. Surprisingly, despite of its perturbative nature, the accuracy of the method persists even for values of b that are of O(1).

astro-ph.CO↗

arXiv2012

A new perspective on Dark Energy modeling via Genetic Algorithms

We use Genetic Algorithms to extract information from several cosmological probes, such as the type Ia supernovae (SnIa), the Baryon Acoustic Oscillations (BAO) and the growth rate of matter perturbations. This is done by implementing a model independent and bias-free reconstruction of the various scales and distances that characterize the data, like the luminosity $d_L(z)$ and the angular diameter distance $d_A(z)$ in the SnIa and BAO data, respectively, or the dependence with redshift of the matter density $\om_m(a)$ in the growth rate data, $fσ_8(z)$. These quantities can then be used to reconstruct the expansion history of the Universe, and the resulting Dark Energy (DE) equation of state $w(z)$ in the context of FRW models, or the mass radial function $\om_M(r)$ in LTB models. In this way, the reconstruction is completely independent of our prior bias. Furthermore, we use this method to test the Etherington relation, ie the well-known relation between the luminosity and the angular diameter distance, $η\equiv \frac{d_L(z)}{(1+z)^2 d_A(z)}$, which is equal to 1 in metric theories of gravity. We find that the present data seem to suggest a 3-$σ$ deviation from one at redshifts $z\sim 0.5$. Finally, we present a novel way, within the Genetic Algorithm paradigm, to analytically estimate the errors on the reconstructed quantities by calculating a Path Integral over all possible functions that may contribute to the likelihood. We show that this can be done regardless of the data being correlated or uncorrelated with each other and we also explicitly demonstrate that our approach is in good agreement with other error estimation techniques like the Fisher Matrix approach and the Bootstrap Monte Carlo.

astro-ph.CO↗

arXiv2012

Observational constraints on dark energy with a fast varying equation of state

We place observational constraints on models with the late-time cosmic acceleration based on a number of parametrizations allowing fast transitions for the equation of state of dark energy. In addition to the model of Linder and Huterer where the dark energy equation of state $w$ monotonically grows or decreases in time, we propose two new parametrizations in which $w$ has an extremum. We carry out the likelihood analysis with the three parametrizations by using the observational data of supernovae type Ia, cosmic microwave background, and baryon acoustic oscillations. Although the transient cosmic acceleration models with fast transitions can give rise to the total chi square smaller than that in the $Λ$-Cold-Dark-Matter ($Λ$CDM) model, these models are not favored over $Λ$CDM when one uses the Akaike information criterion which penalizes the extra degrees of freedom present in the parametrizations.

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arXiv2011

The WiggleZ Dark Energy Survey: constraining the evolution of Newton's constant using the growth rate of structure

We constrain the evolution of Newton's constant using the growth rate of large-scale structure measured by the WiggleZ Dark Energy Survey in the redshift range $0.1 < z < 0.9$. We use this data in two ways. Firstly we constrain the matter density of the Universe, $\omms$ (assuming General Relativity), and use this to construct a diagnostic to detect the presence of an evolving Newton's constant. Secondly we directly measure the evolution of Newton's constant, $\Geff$, that appears in Modified Gravity theories, without assuming General Relativity to be true. The novelty of these approaches are that, contrary to other methods, they do not require knowledge of the expansion history of the Universe, $H(z)$, making them model independent tests. Our constraints for the second derivative of Newton's constant at the present day, assuming it is slowly evolving as suggested by Big Bang Nucleosynthesis constraints, using the WiggleZ data is $\ddotGeff(t_0)=-1.19\pm 0.95\cdot 10^{-20}h^2 \textrm{yr}^{-2}$, where $h$ is defined via $H_0=100 h km s^{-1}Mpc^{-1}$, while using both the WiggleZ and the Sloan Digital Sky Survey Luminous Red Galaxy (SDSS LRG) data is $\ddotGeff(t_0)=-3.6\pm 6.8\cdot 10^{-21}h^2 \rm{yr}^{-2}$, both being consistent with General Relativity. Finally, our constraint for the rms mass fluctuation $σ_8$ using the WiggleZ data is $σ_8=0.75 \pm 0.08$, while using both the WiggleZ and the SDSS LRG data $σ_8=0.77 \pm 0.07$, both in good agreement with the latest measurements from the Cosmic Microwave Background radiation.

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arXiv2010

Observational constraints on Galileon cosmology

We study the cosmology of a covariant Galileon field with five covariant Lagrangians and confront this theory with the most recent cosmological probes: the type Ia supernovae data (Constitution and Union2 sets), cosmic microwave background (WMAP7) and the baryon acoustic oscillations (SDSS7). In the Galileon cosmology with a late-time de Sitter attractor, there is a tracker that attracts solutions with different initial conditions to a common trajectory. Including the cosmic curvature K, we place observational constraints on two distinct cases: (i) the tracker, and (ii) the generic solutions to the equations of motion. We find that the tracker solution can be consistent with the individual observational data, but it is disfavored by the combined data analysis. The generic solutions fare quite well when a non-zero curvature parameter is taken into account, but the Akaike and Bayesian information criteria show that they are not particularly favored over the LCDM model.

astro-ph.CO↗

arXiv2010

Genetic algorithms and the analysis of SnIa data

The Genetic Algorithm is a heuristic that can be used to produce model independent solutions to an optimization problem, thus making it ideal for use in cosmology and more specifically in the analysis of type Ia supernovae data. In this work we use the Genetic Algorithms (GA) in order to derive a null test on the spatially flat cosmological constant model $Λ$CDM. This is done in two steps: first, we apply the GA to the Constitution SNIa data in order to acquire a model independent reconstruction of the expansion history of the Universe $H(z)$ and second, we use the reconstructed $H(z)$ in conjunction with the Om statistic, which is constant only for the $Λ$CDM model, to derive our constraints. We find that while $Λ$CDM is consistent with the data at the $2σ$ level, some deviations from $Λ$CDM model at low redshifts can be accommodated.

astro-ph.CO↗

arXiv2010

A model independent null test on the cosmological constant

We use the Om statistic and the Genetic Algorithms (GA) in order to derive a null test on the spatially flat cosmological constant model $Λ$CDM. This is done in two steps: first, we apply the GA to the Constitution SNIa data in order to acquire a model independent reconstruction of the expansion history of the Universe $H(z)$ and second, we use the reconstructed $H(z)$ in conjunction with the Om statistic, which is constant only for the $Λ$CDM model, to derive our constraints. We find that while $Λ$CDM is consistent with the data at the $2σ$ level, some deviations from $Λ$CDM model at low redshifts can be accommodated.

astro-ph.CO↗

arXiv2010

The universe is accelerating. Do we need a new mass scale?

We try to address quantitatively the question whether a new mass is needed to fit current supernovae data. For this purpose, we consider an infra-red modification of gravity that does not contain any new mass scale but systematic subleading corrections proportional to the curvature. The modifications are of the same type as the one recently derived by enforcing the "Ultra Strong Equivalence Principle" (USEP) upon a Friedmann-Lemaitre-Robertson-Walker universe in the presence of a scalar field. The distance between two comoving observers is altered by these corrections and the observations at high redshift affected at any time during the cosmic evolution. While the specific values of the parameters predicted by USEP are ruled out, there are regions of parameter space that fit SnIa data very well. This allows an interesting possibility to explain the apparent cosmic acceleration today without introducing either a dark energy component or a new mass scale.

astro-ph.CO↗

arXiv2010

Massive, massless and ghost modes of gravitational waves from higher-order gravity

We linearize the field equations for higher order theories that contain scalar invariants other than the Ricci scalar. We find that besides a massless spin-2 field (the standard graviton), the theory contains also spin-0 and spin-2 massive modes with the latter being, in general, ghost modes. Then, we investigate the possible detectability of such additional polarization modes of a stochastic gravitational wave by ground-based and space interferometric detectors. Finally, we extend the formalism of the cross-correlation analysis, including the additional polarization modes, and calculate the detectable energy density of the spectrum for a stochastic background of the relic gravity waves that corresponds to our model. For the situation considered here, we find that these massive modes are certainly of interest for direct detection by the LISA experiment.

gr-qc↗

arXiv2009

Genetic Algorithms and Supernovae Type Ia Analysis

We introduce genetic algorithms as a means to analyze supernovae type Ia data and extract model-independent constraints on the evolution of the Dark Energy equation of state. Specifically, we will give a brief introduction to the genetic algorithms along with some simple examples to illustrate their advantages and finally we will apply them to the supernovae type Ia data. We find that genetic algorithms can lead to results in line with already established parametric and non-parametric reconstruction methods and could be used as a complementary way of treating SnIa data. As a non-parametric method, genetic algorithms provide a model-independent way to analyze data and can minimize bias due to premature choice of a dark energy model.

astro-ph.CO↗

arXiv2009

Newton's constant in f(R,R_{μν}R^{μν},\Box R) theories of gravity and constraints from BBN

We consider corrections to the Einstein-Hilbert action which contain both higher order and nonlocal terms. We derive an effective Newtonian gravitational constant applicable at the weak field limit and use the primordial nucleosynthesis (BBN) bound and the local gravity constraints on $G_{eff}$ in order to test the viability of several cases of our general Lagrangian. We will also provide a BBN constrain on the $\Box R$ gravitational correction.

astro-ph.CO↗

arXiv2009

Matter density perturbations in modified gravity models with arbitrary coupling between matter and geometry

We consider theories with an arbitrary coupling between matter and gravity and obtain the perturbation equation of matter on subhorizon scales. Also, we derive the effective gravitational constant $G_{eff}$ and two parameters $Σ$ and $η$, which along with the perturbation equation of the matter density are useful to constrain the theory from growth factor and weak lensing observations. Finally, we use a completely solvable toy model which exhibits nontrivial phenomenology to investigate specific features of the theory. We obtain the analytic solution of the modified Friedmann equation for the scale factor $a$ in terms of time $t$ and use the age of the oldest star clusters and the primordial nucleosynthesis bounds in order to constrain the parameters of our toy model.

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