arXiv subjects

Ioav Waga

Publications and source records attributed to Ioav Waga.

At least 19 recordsLinked to original sources

arXiv2022

Gravitational wave stochastic background in reduced Horndeski theories

We generalize to reduced Horndeski theories of gravity, where gravitational waves (GWs) travel at the speed of light, the expression of a statistically homogeneous and unpolarized stochastic gravitational wave background (SGWB) signal measured as the correlation between the individual signals detected by two interferometers in arbitrary configurations. We also discuss some results found in the literature regarding cosmological distances in modified theories, namely, the simultaneous validity of a duality distance relation for GW signals and of the coincidence between the gravitational wave luminosity distance, based on the energy flux, and the distance inferred from the wave amplitude. This discussion allows us to conclude that the spectral energy density per unit solid angle of an astrophysical SGWB signal has the same functional dependency with the luminosity of each emitting source as in General Relativity (GR). Using the generalized expression of the GW energy-momentum tensor and the modified propagation law for the tensor modes, we conclude that the energy density of a SGWB maintains the same functional relation with the scale factor as in GR, provided that the modified theory coincides with GR in a given hypersurface of constant time. However, the relation between the detected signal and the spectral energy density is changed by the global factor $G_4(φ(t_0))$, thus potentially serving as a probe for modified gravity theories.

gr-qc↗

arXiv2021

New dynamical system approach to Palatini $f(R)$ theories and its application to exponential gravity

The approach of dynamical systems is a useful tool to investigate the cosmological history that follows from modified theories of gravity. It provides qualitative information on the typical background solutions in a parametrized family of models, through the computation of the fixed points and their characters (attractor, repeller or saddle), allowing, for instance, the knowledge of which regions on the parameter space of the models generate the desired radiation, matter and dark energy dominated eras. However, the traditional proposal for building dynamical systems for an $f(R)$ theory in the Palatini formalism assumes the invertibility of a function that depends on the specific Lagrangian functional form, which is not true, for example, for the particular theory of exponential gravity ($f(R)=R-αR_*(1-e^{-R/R_*})$). In this work, we propose an alternative choice of variables to treat $f(R)$ models in their Palatini formulation, which do include exponential gravity. We derive some general results that can be applied to a given model of interest and present a complete description of the phase space for exponential gravity. We show that Palatini exponential gravity theories have a final attractor critical point with an effective equation of state parameter $w_{\text{eff}} = -1$ (for $α>1$), $w_{\text{eff}} = -2/3$ (for $α=1$) and $w_{\text{eff}} = 0$ (for $α<1$). Finally, our analytical results are compared with numerical solutions of the field equations.

gr-qc↗

arXiv2021

Gravitational wave propagation in $f(R)$ models: New parametrizations and observational constraints

Modified gravity (MG) theories predict, in general, that the ratio of gravitational wave (GW) to electromagnetic (EM) luminosity distances, $Ξ$, differs from its general relativity (GR) value of unity at cosmological scales, thus providing another perturbative probe to MG. In this paper, we introduce new phenomenological parametrizations for both the Friedmann-Lemaître-Robertson-Walker (FLRW) background evolution of $f(R)$ models, via the dark energy equation of state parameter, $w_{\text{DE}}$, and for $Ξ$ in this class of theories. We simulate a mock dataset for the Einstein Telescope (ET) of 1000 GW signals from binary neutron star (BNS) mergers and redshift information from their EM counterpart, exploring the consequent constraints on the relevant gravitational, cosmological and phenomenological parameters. As a model of particular interest, we take $γ$-gravity theory and investigate whether it could be distinguished from GR's $Λ$CDM model. We then combine our results with actual data from type Ia supernovae (SNIa) and combined baryon acoustic oscillations (BAO) and cosmic microwave background (CMB) observations. Additionally, we also investigate the potential bounds to $f_{R0}$ for any viable $f(R)$ whose background evolution is indistinguishable from the standard model of cosmology above a certain redshift, showing that, for a $Λ$CDM fiducial model, ET data would provide $|f_{R0}|<10^{-2}$ at a 95\% level. We conclude altogether that probing the redshift evolution of the GW luminosity distance from detections of the ET in its first running decade will not substantially help constraining $f(R)$ theories of gravity.

gr-qc↗

arXiv2016

N-body Simulations of $γ$ Gravity

We have investigated structure formation in the $γ$ gravity $f(R)$ model with {\it N}-body simulations. The $γ$ gravity model is a proposal which, unlike other viable $f(R)$ models, not only changes the gravitational dynamics, but can in principle also have signatures at the background level that are different from those obtained in $Λ$CDM (Cosmological constant, Cold Dark Matter). The aim of this paper is to study the nonlinear regime of the model in the case where, at late times, the background differs from $Λ$CDM. We quantify the signatures produced on the power spectrum, the halo mass function, and the density and velocity profiles. To appreciate the features of the model, we have compared it to $Λ$CDM and the Hu-Sawicki $f(R)$ models. For the considered set of parameters we find that the screening mechanism is ineffective, which gives rise to deviations in the halo mass function that disagree with observations. This does not rule out the model per se, but requires choices of parameters such that $|f_{R0}|$ is much smaller, which would imply that its cosmic expansion history cannot be distinguished from $Λ$CDM at the background level.

astro-ph.CO↗

arXiv2016

Constraining cosmic deceleration-acceleration transition with type Ia supernova, BAO/CMB and H(z) data

We revisit the kink-like parametrization of the deceleration parameter q(z), which considers a transition, at $z_t$, from cosmic deceleration to acceleration. In this parametrization the initial, at $z \gg z_t$, value of the q-parameter is $q_i$, its final, z=-1, value is $q_f$ and the duration of the transition is parametrized by $τ$. By assuming a flat space geometry we obtain constraints on the free parameters of the model using recent data from type Ia supernovae, BAO, CMB and the Hubble parameter H(z). The use of H(z) data introduces an explicit dependence of the combined likelihood on the present value of the Hubble parameter $H_0$, allowing us to explore the influence of different priors when marginalizing over this parameter. We also study the importance of the CMB information in the results by considering data from WMAP7, WMAP9 and Planck. We show that the contours and best fit do not depend much on the different CMB data used and that the considered new BAO data is responsible for most of the improvement in the results. Assuming a flat space geometry, $q_i=1/2$ and expressing the present value of the deceleration parameter $q_0$ as a function of the other three free parameters, we obtain $z_t=0.67^{+0.10}_{-0.08}$, $τ=0.26^{+0.14}_{-0.10}$ and $q_0=-0.48^{+0.11}_{-0.13}$, at 68\% of confidence level, with an uniform prior over $H_0$. If in addition we fix $q_f=-1$, as in flat $Λ$CDM, DGP and Chaplygin quartessence that are special models described by our parametrization, we get $z_t=0.66^{+0.03}_{-0.04}$, $τ=0.33^{+0.04}_{-0.04}$ and $q_0=-0.54^{+0.05}_{-0.07}$, in excellent agreement with flat $Λ$CDM for which $τ=1/3$. We also obtain for flat wCDM, another dark energy model described by our parametrization, the constraint on the equation of state parameter -1.22 < w < -0.78 at more than 99% confidence level.

astro-ph.CO↗

arXiv2014

Degeneracy between CCDM and $Λ$CDM cosmologies

The creation of cold dark matter cosmology model is studied beyond the linear perturbation level. The skewness is explicitly computed and the results are compared to those from the $Λ$CDM model. It is explicitly shown that both models have the same signature for the skewness and cannot be distinguished by using this observable.

astro-ph.CO↗

arXiv2014

Matter creation and cosmic acceleration

We investigate the creation of cold dark matter (CCDM) cosmology as an alternative to explain the cosmic acceleration. Particular attention is given to the evolution of density perturbations and constraints coming from recent observations. By assuming negligible effective sound speed we compare CCDM predictions with redshift-space-distortion based f(z) sigma_8(z) measurements. We identify a subtle issue associated with which contribution in the density contrast should be used in this test and then show that the CCDM results are the same as those obtained with LambdaCDM. These results are then contrasted with the ones obtained at the background level. For the background tests we have used type Ia supernovae data (Union 2.1 compilation) in combination with baryonic acoustic oscillations and cosmic microwave background observations and also measurements of the Hubble parameter at different redshifts. As a consequence of the studies we have performed at both the background and perturbation levels, we explicitly show that CCDM is observationally degenerate with respect to LambdaCDM (dark degeneracy). The need to overcome the lack of a fundamental microscopic basis for the CCDM is the major challenge for this kind of model.

astro-ph.CO↗

arXiv2009

Viable Singularity-Free f(R) Gravity Without a Cosmological Constant

Several authors have argued that self-consistent $f(R)$ gravity models distinct from $Λ$CDM are almost ruled out. Confronting such claims, we present a particular two-parameter $f(R)$ model that: (a) is cosmologically viable and distinguishable from $Λ$CDM; (b) is compatible with the existence of relativistic stars; (c) is free of singularities of the Ricci scalar during the cosmological evolution and (d) allows the addition of high curvature corrections that could be relevant for inflation.

astro-ph.CO↗

arXiv2008

Dark Interactions and Cosmological Fine-Tuning

Cosmological models involving an interaction between dark matter and dark energy have been proposed in order to solve the so-called coincidence problem. Different forms of coupling have been studied, but there have been claims that observational data seem to narrow (some of) them down to something annoyingly close to the $Λ$CDM model, thus greatly reducing their ability to deal with the problem in the first place. The smallness problem of the initial energy density of dark energy has also been a target of cosmological models in recent years. Making use of a moderately general coupling scheme, this paper aims to unite these different approaches and shed some light as to whether this class of models has any true perspective in suppressing the aforementioned issues that plague our current understanding of the universe, in a quantitative and unambiguous way.

astro-ph↗

arXiv2007

When did cosmic acceleration start? How fast was the transition?

Cosmic acceleration is investigated through a kink-like expression for the deceleration parameter (q). The new parametrization depends on the initial (q_i) and final (q_f) values of q, on the redshift of the transition from deceleration to acceleration (z_{t}) and the width of such transition (τ). We show that although supernovae (SN) observations (Gold182 and SNLS data samples) indicate, at high confidence, that a transition occurred in the past (z_{t}>0) they do not, by themselves, impose strong constraints on the maximum value of z_{t}. However, when we combine SN with the measurements of the ratio between the comoving distance to the last scattering surface and the SDSS+2dfGRS BAO distance scale (S_{k}/D_{v}) we obtain, at 95.4% confidence level, z_{t}=0.84+{0.17}-{0.13} and τ=0.51-{0.17}+{0.23} for (S_{k}/D_{v}+Gold182), and z_{t}=0.88-{0.10}+{0.12} and τ=0.35-{0.10}+{0.12} for (S_{k}/D_{v} + SNLS), assuming q_i=0.5 and q_f=-1. We also analyze the general case, q_f\in(-\infty,0) finding the constraints that the combined tests (S_{k}/D_{v} + SNLS) impose on the present value of the deceleration parameter (q_0).

astro-ph↗

arXiv2006

Challenges for scaling cosmologies

A cosmological model that aims at solving the coincidence problem should show that dark energy and dark matter follow the same scaling solution from some time onward. At the same time, the model should contain a sufficiently long matter-dominated epoch that takes place before acceleration in order to guarantee a decelerated epoch and structure formation. So a successful cosmological model requires the occurrence of a sequence of epochs, namely a radiation era, a matter-dominated era and a final accelerated scaling attractor with $Ω_ϕ \simeq 0.7$. In this paper we derive the generic form of a scalar-field Lagrangian that possesses scaling solutions in the case where the coupling $Q$ between dark energy and dark matter is a free function of the field $ϕ$. We then show, rather surprisingly, that the aforementioned sequence of epochs cannot occur for a vast class of generalized coupled scalar field Lagrangians that includes, to our knowledge, all scaling models in the current literature.

astro-ph↗

arXiv2005

Observational Constraints on Silent Quartessence

We derive new constraints set by SNIa experiments (`gold' data sample of Riess et al.), X-ray galaxy cluster data (Allen et al. Chandra measurements of the X-ray gas mass fraction in 26 clusters), large scale structure (Sloan Digital Sky Survey spectrum) and cosmic microwave background (WMAP) on the quartessence Chaplygin model. We consider both adiabatic perturbations and intrinsic non-adiabatic perturbations such that the effective sound speed vanishes (Silent Chaplygin). We show that for the adiabatic case, only models with equation of state parameter $ |α|\lesssim 10^{-2}$ are allowed: this means that the allowed models are very close to \LambdaCDM. In the Silent case, however, the results are consistent with observations in a much broader range, -0.3<α<0.7.

astro-ph↗

arXiv2004

Observational Constraints on Chaplygin Quartessence: Background Results

We derive the constraints set by several experiments on the quartessence Chaplygin model (QCM). In this scenario, a single fluid component drives the Universe from a nonrelativistic matter-dominated phase to an accelerated expansion phase behaving, first, like dark matter and in a more recent epoch like dark energy. We consider current data from SNIa experiments, statistics of gravitational lensing, FR IIb radio galaxies, and x-ray gas mass fraction in galaxy clusters. We investigate the constraints from this data set on flat Chaplygin quartessence cosmologies. The observables considered here are dependent essentially on the background geometry, and not on the specific form of the QCM fluctuations. We obtain the confidence region on the two parameters of the model from a combined analysis of all the above tests. We find that the best-fit occurs close to the $Λ$CDM limit ($α=0$). The standard Chaplygin quartessence ($α=1$) is also allowed by the data, but only at the $\sim2σ$ level.

astro-ph↗

arXiv2002

The end of unified dark matter?

Despite the interest in dark matter and dark energy, it has never been shown that they are in fact two separate substances. We provide the first strong evidence that they are separate by ruling out a broad class of so-called unified dark matter models that have attracted much recent interest. We find that they produce oscillations or exponential blowup of the matter power spectrum inconsistent with observation. For the particular case of generalized Chaplygin gas models, 99.999% of the previously allowed parameter space is excluded, leaving essentially only the standard Lambda-CDM limit allowed.

astro-ph↗

arXiv2002

Constraints on the generalized Chaplygin gas from supernovae observations

We explore the implications of type Ia supernovae (SNIa) observations on flat cosmological models whose matter content is an exotic fluid with equation of state, p=-M^(4(a+1))/rho^a. In this scenario, a single fluid component may drive the Universe from a nonrelativistic matter dominated phase to an accelerated expansion phase behaving, first, like dark matter and in a more recent epoch as dark energy. We show that these models are consistent with current SNIa data for a rather broad range of parameters. However, future SNIa experiments will place stringent constraints on these models, and could safely rule out the special case of a Chaplygin gas (a=1) if the Universe is dominated by a true cosmological constant.

astro-ph↗

arXiv2000

New Constraints from High Redshift Supernovae and Lensing Statistics upon Scalar Field Cosmologies

We explore the implications of gravitationally lensed QSOs and high-redshift SNe Ia observations for spatially flat cosmological models in which a classically evolving scalar field currently dominates the energy density of the Universe. We consider two representative scalar field potentials that give rise to effective decaying $Λ$ (``quintessence'') models: pseudo-Nambu-Goldstone bosons ($V(ϕ)=M^4(1+\cos (ϕ/f)) $) and an inverse power-law potential ($V(ϕ)=M^{4+α}ϕ^{-α}$). We show that a large region of parameter space is consistent with current data if $Ω_{m0} > 0.15$. On the other hand, a higher lower bound for the matter density parameter suggested by large-scale galaxy flows, $Ω_{m0} > 0.3$, considerably reduces the allowed parameter space, forcing the scalar field behavior to approach that of a cosmological constant.

astro-ph↗

arXiv1998

Cosmological constraints from lensing statistics and supernovae on the cosmic equation of state

We investigate observational constraints from lensing statistics and high-z type Ia supernovae on flat cosmological models with nonrelativistic matter and an exotic fluid with equation of state, $p_x=(m/3 -1)ρ_x$. We show that agreement with both tests at the 68% confidence level is possible if the parameter $m$ is low ($m \lesssim 0.85$) and $0.24 \lesssim Ω_{m0} \lesssim 0.38$ with lower values of $Ω_{m0}$ corresponding to higher $m$. We find that a conventional cosmological constant model with $Ω_{m0}\simeq 0.33$ is the best fit model of the combined likelihood.

astro-ph↗

arXiv1998

Weak Gravitational Lensing by Voids

We consider the prospects for detecting weak gravitational lensing by underdensities (voids) in the large-scale matter distribution. We derive the basic expressions for magnification and distortion by spherical voids. Clustering of the background sources and cosmic variance are the main factors which limit in principle the detection of lensing by voids. We conclude that only voids with radii larger than $\sim 100$ \hm have lensing signal to noise larger than unity.

astro-ph↗