arXiv subjects

David Polarski

Publications and source records attributed to David Polarski.

At least 19 recordsLinked to original sources

arXiv2023

Joint reconstructions of growth and expansion histories from stage-IV surveys with minimal assumptions. II. Modified gravity and massive neutrinos

Based on a formalism introduced in our previous work, we reconstruct the phenomenological function $G_{\rm eff}(z)$ describing deviations from General Relativity (GR) in a model-independent manner. In this alternative approach, we model $μ\equiv G_\mathrm{eff}/G$ as a Gaussian process and use forecasted growth-rate measurements from a stage-IV survey to reconstruct its shape for two different toy models. We follow a two-step procedure: (i) we first reconstruct the background expansion history from Supernovae (SNe) and Baryon Acoustic Oscillation (BAO) measurements; (ii) we then use it to obtain the growth history $fσ_8$, that we fit to redshift-space distortions (RSD) measurements to reconstruct $G_\mathrm{eff}$. We find that upcoming surveys such as the Dark Energy Spectroscopic Instrument (DESI) might be capable of detecting deviations from GR, provided the dark energy behavior is accurately determined. We might even be able to constrain the transition redshift from $G\to G_\mathrm{eff}$ for some particular models. We further assess the impact of massive neutrinos on the reconstructions of $G_\mathrm{eff}$ (or $μ$) assuming the expansion history is given, and only the neutrino mass is free to vary. Given the tight constraints on the neutrino mass, and for the profiles we considered in this work, we recover numerically that the effect of such massive neutrinos does not alter our conclusions. Finally, we stress that incorrectly assuming a $Λ$CDM expansion history leads to a degraded reconstruction of $μ$, and/or a non-negligible bias in the ($Ω_\mathrm{m,0}$,$σ_{8,0}$)-plane.

astro-ph.CO↗

arXiv2021

Negative cosmological constant in the dark sector?

We consider the possibility that the dark sector of our Universe contains a negative cosmological constant dubbed $λ$. For such models to be viable, the dark sector should contain an additional component responsible for the late-time accelerated expansion rate ($X$). We explore the departure of the expansion history of these models from the concordance $Λ$ Cold Dark Matter model. For a large class of our models the accelerated expansion is transient with a nontrivial dependence on the model parameters. All models with $w_X>-1$ will eventually contract and we derive an analytical expression for the scale factor $a(t)$ in the neighborhood of its maximal value. We find also the scale factor for models ending in a Big Rip in the regime where dustlike matter density is negligible compared to $λ$. We address further the viability of such models, in particular when a high $H_0$ is taken into account. While we find no decisive evidence for a nonzero $λ$, the best models are obtained with a phantom behavior on redshifts $z\gtrsim 1$ with a higher evidence for nonzero $λ$. An observed value for $h$ substantially higher than $0.70$ would be a decisive test of their viability.

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arXiv2020

Weak gravity on a $Λ$CDM background

We consider Horndeski modified gravity models obeying stability, velocity of gravitational waves $c_T$ equals $c$ and quasistatic approximation (QSA) on subhorizon scales. We assume further a $Λ$CDM background expansion and a monotonic evolution on the cosmic background of the $α$ functions as $α_i= α_{i0}~a^s$ where $i=M,B$, $a$ is the scale factor and $α_{i0}$ ($α_{M0}, α_{B0}$), $s$ are arbitrary parameters. We show that the growth and lensing reduced (dimensionless) gravitational couplings $μ\equiv G_{\rm growth}/G$, $Σ\equiv G_{\rm lensing}/G$ exhibit the following generic properties today: $Σ_0 < 1$ for all viable parameters, $μ_0<1$ (weak gravity today) is favored for small $s$ while $μ_0>1$ is favored for large $s$. We establish also the relation $μ\geq Σ$ at all times. Taking into account the $fσ_8$ and $E_G$ data constrains the parameter $s$ to satisfy $s\lesssim 2$. Hence these data select essentially the weak gravity regime today ($μ_0<1$) when $s<2$, while $μ_0>1$ subsists only marginally for $s\approx 2$. At least the interval $0.5\lesssim s \lesssim 2$ would be ruled out in the absence of screening. We consider further the growth index $γ(z)$ and identify the $(α_{M0},α_{B0},s)$ parameter region that corresponds to specific signs of the differences $γ_0-γ_0^{ΛCDM}$, and $γ_1-γ_1^{ΛCDM}$, where $γ_0\equiv γ\bigl|_{z=0}$ and $γ_1\equiv \frac{{\rm d}γ}{\rm d z}\bigl|_{z=0}$. In this way important information is gained on the past evolution of $μ$. We obtain in particular the signature $γ_0>γ_0^{ΛCDM}$ for $s<2$ in the selected weak gravity region.

gr-qc↗

arXiv2020

Defying the laws of Gravity I: model-independent reconstruction of the Universe expansion from growth data

Using redshift space distortion data, we perform model-independent reconstructions of the growth history of matter inhomogeneity in the expanding Universe using two methods: crossing statistics and Gaussian processes. We then reconstruct the corresponding history of the Universe background expansion and fit it to type Ia supernovae data, putting constraints on $(Ω_\mathrm{m,0},σ_{8,0})$. The results obtained are consistent with the concordance flat-$Λ$CDM model and General Relativity as the gravity theory given the current quality of the inhomogeneity growth data.

astro-ph.CO↗

arXiv2018

The End of Cosmic Growth

The growth of large scale structure is a battle between gravitational attraction and cosmic acceleration. We investigate the future behavior of cosmic growth under both general relativity (GR) and modified gravity during prolonged acceleration, deriving analytic asymptotic behaviors and showing that gravity generally loses and growth ends. We also note the `why now' problem is equally striking when viewed in terms of the shut down of growth. For many models inside GR the gravitational growth index $γ$ also shows today as a unique time between constant behavior in the past and a higher asymptotic value in the future. Interestingly, while $f(R)$ models depart in this respect dramatically from GR today and in the recent past, their growth indices are identical in the asymptotic future and past.

astro-ph.CO↗

arXiv2018

On the consistency of the expansion with the perturbations

Assuming a simple form for the growth index gamma(z) depending on two parameters gamma_0 = gamma(z=0) and gamma_1 = gamma'(z=0), we show that these parameters can be constrained using background expansion data. We explore systematically the preferred region in this parameter space. Inside General Relativity we obtain that models with a quasi-static growth index and gamma_1 = -0.02 are favoured. We find further the lower bounds gamma_0 > 0.53 and gamma_1 > -0.15 for models inside GR. Models outside GR having the same background expansion as LCDM and arbitrary gamma(z) with gamma_0 = gamma_0^{LCDM}, satisfy G_{eff,0}>G for gamma_1 > gamma_1^{LCDM}, and G_{eff,0}<G for gamma_1 < gamma_1^{LCDM}. The first models will cross downwards the value G_{eff}=G on very low redshifts z<0.3, while the second models will cross upwards G_{eff}=G in the same redshift range. This makes the realization of such modified gravity models even more problematic.

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arXiv2018

Consistency of Modified Gravity with a decreasing $G_{\rm eff}(z)$ in a $Λ$CDM background

Recent analyses \cite{Nesseris:2017vor,Kazantzidis:2018rnb} have indicated that an effective Newton's constant $G_{\rm eff}(z)$ decreasing with redshift may relieve the observed tension between the Planck15 best fit $Λ$CDM cosmological background ({\it i.e.} Planck15/$Λ$CDM) and the corresponding $Λ$CDM background favored by growth $fσ_8$ and weak lensing data. We investigate the consistency of such a decreasing $G_{\rm eff}(z)$ with some viable scalar-tensor models and $f(R)$ theories. We stress that $f(R)$ theories generically can not lead to a decreasing $G_{\rm eff}(z)$ for any cosmological background. For scalar-tensor models we deduce that in the context of a $Λ$CDM cosmological background, a decreasing $G_{\rm eff}(z)$ is not consistent with a large Brans-Dicke parameter $ω_{BD,0}$ today. This inconsistency remains and amplifies in the presence of a phantom dark energy equation of state parameter ($w < -1$). However it can be avoided for $w >-1$. We also find that any modified gravity model with the required decreasing $G_{\rm eff}(z)$ and $G_{{\rm eff},0}=G$, would have a characteristic signature in its growth index $γ$ with $0.61\lesssim γ_0\lesssim 0.69$ and large slopes $γ_0'$, $0.16\lesssim γ_0'\lesssim 0.4$, which is a characteristic signature of a decreasing (with $z$) $G_{\rm eff}(z)<G$ on small redshifts. This is a substantial departure today from the quasi-static behaviour in $Λ$CDM with $(γ_0,γ_0')\approx (0.55,-0.02)$.

gr-qc↗

arXiv2017

When is the growth index constant?

The growth index $γ$ is an interesting tool to assess the phenomenology of dark energy (DE) models, in particular of those beyond general relativity (GR). We investigate the possibility for DE models to allow for a constant $γ$ during the entire matter and DE dominated stages. It is shown that if DE is described by quintessence (a scalar field minimally coupled to gravity), this behaviour of $γ$ is excluded either because it would require a transition to a phantom behaviour at some finite moment of time, or, in the case of tracking DE at the matter dominated stage, because the relative matter density $Ω_m$ appears to be too small. An infinite number of solutions, with $Ω_m$ and $γ$ both constant, are found with $w_{DE}=0$ corresponding to Einstein-de Sitter universes. For all modified gravity DE models satisfying $G_{\rm eff}\ge G$, among them the $f(R)$ DE models suggested in the literature, the condition to have a constant $w_{DE}$ is strongly violated at the present epoch. In contrast, DE tracking dust-like matter deep in the matter era, but with $Ω_m <1$, requires $G_{\rm eff} > G$ and an example is given using scalar-tensor gravity for a range of admissible values of $γ$. For constant $w_{DE}$ inside GR, departure from a quasi-constant value is limited until today. Even a large variation of $w_{DE}$ may not result in a clear signature in the change of $γ$. The change however is substantial in the future and the asymptotic value of $γ$ is found while its slope with respect to $Ω_m$ (and with respect to $z$) diverges and tends to $-\infty$.

astro-ph.CO↗

arXiv2015

A critical assessment of some inhomogeneous pressure Stephani models

We consider spherically symmetric inhomogeneous pressure Stephani universes, the center of symmetry being our location. The main feature of these models is that comoving observers do not follow geodesics. In particular, comoving perfect fluids have necessarily a radially dependent pressure. We consider a subclass of these models characterized by some inhomogeneity parameter $β$. We show that also the velocity of sound, like the (effective) equation of state parameter, of comoving perfect fluids acquire away from the origin a time and radial dependent change proportional to $β$. In order to produce a realistic universe accelerating at late times without dark energy component one must take $β< 0$. The redshift gets a modified dependence on the scale factor $a(t)$ with a relative modification of $-9\%$ peaking at $z\sim 4$ and vanishing at the big-bang and today on our past lightcone. The equation of state parameter and the speed of sound of dustlike matter (corresponding to a vanishing pressure at the center of symmetry $r=0$) behave in a similar way and away from the center of symmetry they become negative -- a property usually encountered for the dark energy component only. In order to mimic the observed late-time accelerated expansion, the matter component must significantly depart from standard dust, presumably ruling this subclass of Stephani models out as a realistic cosmology. The only way to accept these models is to keep all standard matter components of the universe including dark energy and take an inhomogeneity parameter $β$ small enough.

gr-qc↗

arXiv2013

Soft singularity crossing and transformation of matter properties

We investigate particular cosmological models, based either on tachyon fields or on perfect fluids, for which soft future singularities arise in a natural way. Our main result is the description of a smooth crossing of the soft singularity in models with an anti-Chaplygin gas or with a particular tachyon field in the presence of dust. Such a crossing is made possible by certain transformations of matter properties. Some of these cosmological evolutions involving tachyons are compatible with SNIa data. We compute numerically their dynamics involving a first soft singularity crossing, a turning point and a second soft singulatity crossing during recollapse, ending in a Big Crunch singularity.

gr-qc↗

arXiv2013

Past and future of some universes

We consider a class of toy models where a spatially flat universe is filled with a perfect fluid. The dynamics is found exactly for all these models. In one family, the perfect fluid is of the phantom type and we find that the universe is first contracting and then expanding while the dynamics is always accelerated. In a second family, the universe is first in an accelerated expansion stage, then in a decelerated expansion stage until it reaches a turning point after which it contracts in a decelerated way (increasing contraction rate) followed by another accelerated stage (decreasing contraction rate). We also consider the possibility to embed this perfect fluid in a realistic cosmology. The first family cannot be viable in a conventional big bang universe and requires a rebound in the very early universe. The second family is viable in the range $0<1+w_{DE,0}\lesssim 0.09$ for a spatially closed universe with a curvature satisfying current bounds. Though many of the models in this family cannot be distinguished today from a universe dominated by a cosmological constant, the present accelerated expansion is transient and these universes will reach a turning point in the future before entering a contraction phase.

astro-ph.CO↗

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↗

arXiv2011

The complementarity of the redshift drift

We derive some basic equations related to the redshift drift and we show how some dark energy (DE) properties can be retrieved from it. We consider in particular three kinds of DE models which exhibit a characteristic signature in their redshift drift while no such signature would be present in their luminosity-distances: a sudden change of the equation of state parameter w_{DE} at low redshifts, oscillating DE and finally an equation of state with spikes at low redshifts. Accurate redshift drift measurements would provide interesting complementary probes for some of these models and for models with varying gravitational coupling. While the redshift drift would efficiently constrain models with a spike at z~1, the signature of the redshift drift for models with large variations at very low redshifts z<0.1 would be unobservable, allowing a large arbitrariness in the present expansion of the universe.

astro-ph.CO↗

arXiv2010

Will the tachyonic Universe survive the Big Brake?

We investigate a Friedmann universe filled with a tachyon scalar field, which behaves as dustlike matter in the past, while it is able to accelerate the expansion rate of the universe at late times. The comparison with type Ia supernovae (SNIa) data allows for evolutions driving the universe into a Big Brake. Some of the evolutions leading to a Big Brake exhibit a large variation of the equation of state parameter at low redshifts which is potentially observable with future data though hardly detectable with present SNIa data. The soft Big Brake singularity occurs at finite values of the scale factor, vanishing energy density and Hubble parameter, but diverging deceleration and infinite pressure. We show that the geodesics can be continued through the Big Brake and that our model universe will recollapse eventually in a Big Crunch. Although the time to the Big Brake strongly depends on the present values of the tachyonic field and of its time derivative, the time from the Big Brake to the Big Crunch represents a kind of invariant timescale for all field parameters allowed by SNIa.

gr-qc↗

arXiv2010

Chameleon dark energy models with characteristic signatures

In chameleon dark energy models, local gravity constraints tend to rule out parameters in which observable cosmological signatures can be found. We study viable chameleon potentials consistent with a number of recent observational and experimental bounds. A novel chameleon field potential, motivated by f(R) gravity, is constructed where observable cosmological signatures are present both at the background evolution and in the growth-rate of the perturbations. We study the evolution of matter density perturbations on low redshifts for this potential and show that the growth index today gamma_0 can have significant dispersion on scales relevant for large scale structures. The values of gamma_0 can be even smaller than 0.2 with large variations of gamma on very low redshifts for the model parameters constrained by local gravity tests. This gives a possibility to clearly distinguish these chameleon models from the Lambda-Cold-Dark-Matter model in future high-precision observations.

astro-ph.CO↗

arXiv2009

The dispersion of growth of matter perturbations in f(R) gravity

We study the growth of matter density perturbations delta_m for a number of viable f(R) gravity models that satisfy both cosmological and local gravity constraints, where the Lagrangian density f is a function of the Ricci scalar R. If the parameter m=Rf_{,RR}/f_{,R} today is larger than the order of 10^{-6}, linear perturbations relevant to the matter power spectrum evolve with a growth rate s=d (ln delta_m)/d (ln a) (a is the scale factor) that is larger than in the LCDM model. We find the window in the free parameter space of our models for which spatial dispersion of the growth index gamma_0= gamma(z=0) (z is the redshift) appears in the range of values 0.40< gamma_0<0.55, as well as the region in parameter space for which there is essentially no dispersion and gamma_0 converges to values around 0.40<gamma_0<0.43. These latter values are much lower than in the LCDM model. We show that these unusual dispersed or converged spectra are present in most of the viable f(R) models with m(z=0) larger than the order of 10^{-6}. These properties will be essential in the quest for f(R) modified gravity models using future high-precision observations and they confirm the possibility to distinguish clearly most of these models from the LCDM model.

astro-ph.CO↗

arXiv2009

Dispersion in the growth of matter perturbations

We consider the linear growth of matter perturbations on low redshifts in modified gravity Dark Energy (DE) models where G_eff(z,k) is explicitly scale-dependent. Dispersion in the growth today will only appear for scales of the order the critical scale ~ λ_{c,0}, the range of the fifth-force today. We generalize the constraint equation satisfied by the parameters γ_0(k) and γ'_0(k) \equiv \frac{dγ(z,k)}{dz}(z=0) to models with G_{eff,0}(k) \ne G. Measurement of γ_0(k) and γ'_0(k) on several scales can provide information about λ_{c,0}. In the absence of dispersion when λ_{c,0} is large compared to the probed scales, measurement of γ_0 and γ'_0 provides a consistency check independent of λ_{c,0}. This applies in particular to results obtained earlier for a viable f(R) model.

astro-ph.CO↗

arXiv2009

Why do cosmological perturbations look classical to us?

According to the inflationary scenario of cosmology, all structure in the Universe can be traced back to primordial fluctuations during an accelerated (inflationary) phase of the very early Universe. A conceptual problem arises due to the fact that the primordial fluctuations are quantum, while the standard scenario of structure formation deals with classical fluctuations. In this essay we present a concise summary of the physics describing the quantum-to-classical transition. We first discuss the observational indistinguishability between classical and quantum correlation functions in the closed system approach (pragmatic view). We then present the open system approach with environment-induced decoherence. We finally discuss the question of the fluctuations' entropy for which, in principle, the concrete mechanism leading to decoherence possesses observational relevance.

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