arXiv subjects

D. Polarski

Publications and source records attributed to D. Polarski.

At least 19 recordsLinked to original sources

arXiv2022

Joint reconstructions of growth and expansion histories from stage-IV surveys with minimal assumptions I: Dark Energy beyond $Λ$

Combining Supernovae, Baryon Acoustic Oscillations and Redshift-Space Distortions data from the next generation of (Stage-IV) cosmological surveys, we aim to reconstruct the expansion history up to large redshifts using forward-modeling of $f_{\mathrm DE}(z) = ρ_\mathrm{DE}(z)/ρ_\mathrm{DE,0}$ with Gaussian processes (GP). In order to reconstruct cosmological quantities at high redshifts where few or no data are available, we adopt a new approach to GP which enforces the following minimal assumptions: a) Our cosmology corresponds to a flat Friedman-Lemaître-Robertson-Walker (FLRW) universe; b) An Einstein de Sitter (EdS) universe is obtained on large redshifts. This allows us to reconstruct the perturbations growth history from the reconstructed background expansion history. Assuming various DE models, we show the ability of our reconstruction method to differentiate them from $Λ$CDM at $\gtrsim2σ$.

astro-ph.CO↗

arXiv2022

Bouncing Cosmological Isotropic Solutions in Scalar-Tensor Gravity

Bouncing non-singular isotropic cosmological solutions are investigated in a simple model of scalar-tensor gravity. New families of such solutions are found and their properties are presented and analyzed using an effective potential as the main tool. Bouncing solutions are shown to exist for a Higgs-like self-interaction potential which is bounded from below, in contrast to previous solutions that appeared in the literature based on potentials which were unbounded from below. In the simplest version of a scalar field with the quartic potential and conformal coupling to gravity, bouncing spatially flat solutions either have the Hubble function diverging in the past before the bounce, but with a well-behaved future, or are globally regular but unstable with respect to anisotropic or inhomogeneous perturbations at some finite values of the scalar field and curvature. Regular solutions can only exist in the part of the parameter space where the maximum of the effective potential is larger than the first zero of the potential, and gravity becomes repulsive at the bounce.

gr-qc↗

arXiv2020

Global properties of the growth index: mathematical aspects and physical relevance

We analyze the global behaviour of the growth index of cosmic inhomogeneities in an isotropic homogeneous universe filled by cold non-relativistic matter and dark energy (DE) with an arbitrary equation of state. Using a dynamical system approach, we find the critical points of the system. That unique trajectory for which the growth index $γ$ is finite from the asymptotic past to the asymptotic future is identified as the so-called heteroclinic orbit connecting the critical points $(Ω_m=0,~γ_{\infty})$ in the future and $(Ω_m=1,~γ_{-\infty})$ in the past. The first is an attractor while the second is a saddle point, confirming our earlier results. Further, in the case when a fraction of matter (or DE tracking matter) $\varepsilon Ω^{\rm tot}_m$ remains unclustered, we find that the limit of the growth index in the past $γ_{-\infty}^{\varepsilon}$ does not depend on the equation of state of DE, in sharp contrast with the case $\varepsilon=0$ (for which $γ_{-\infty}$ is obtained). We show indeed that there is a mathematical discontinuity: one cannot obtain $γ_{-\infty}$ by taking $\lim_{\varepsilon \to 0} γ^{\varepsilon}_{-\infty}$ (i.e. the limits $\varepsilon\to 0$ and $Ω^{\rm tot}_m\to 1$ do not commute). We recover in our analysis that the value $γ_{-\infty}^{\varepsilon}$ corresponds to tracking DE in the asymptotic past with constant $γ=γ_{-\infty}^{\varepsilon}$ found earlier.

astro-ph.CO↗

arXiv2019

Global properties of the growth index of matter inhomogeneities in the universe

We perform here a global analysis of the growth index $γ$ behaviour from deep in the matter era till the far future. For a given cosmological model in GR or in modified gravity, the value of $γ(Ω_{m})$ is unique when the decaying mode of scalar perturbations is negligible. However, $γ_{\infty}$, the value of $γ$ in the asymptotic future, is unique even in the presence of a nonnegligible decaying mode today. Moreover $γ$ becomes arbitrarily large deep in the matter era. Only in the limit of a vanishing decaying mode do we get a finite $γ$, from the past to the future in this case. We find further a condition for $γ(Ω_{m})$ to be monotonically decreasing (or increasing). This condition can be violated inside general relativity (GR) for varying $w_{DE}$ though generically $γ(Ω_{m})$ will be monotonically decreasing (like $Λ$CDM), except in the far future and past. A bump or a dip in $G_{\rm eff}$ can also lead to a significant and rapid change in the slope $\frac{dγ}{dΩ_{m}}$. On a $Λ$CDM background, a $γ$ substantially lower (higher) than $0.55$ with a negative (positive) slope reflects the opposite evolution of $G_{\rm eff}$. In DGP models, $γ(Ω_{m})$ is monotonically increasing except in the far future. While DGP gravity becomes weaker than GR in the future and $w^{DGP}\to -1$, we still get $γ_{\infty}^{DGP}= γ_{\infty}^{ΛCDM}=\frac{2}{3}$. In contrast, despite $G^{DGP}_{\rm eff}\to G$ in the past, $γ$ does not tend to its value in GR because $\frac{dG^{DGP}_{\rm eff}}{dΩ_{m}}\Big|_{-\infty}\ne 0$.

astro-ph.CO↗

arXiv2016

Bouncing Universes in Scalar-Tensor Gravity Around Conformal Invariance

We consider the possibility to produce a bouncing universe in the framework of scalar-tensor gravity when the scalar field has a nonconformal coupling to the Ricci scalar. We prove that bouncing universes regular in the future with essentially the same dynamics as for the conformal coupling case do exist when the coupling deviates slightly from it. This is found numerically for more substantial deviations as well. In some cases however new features are found like the ability of the system to leave the effective phantom regime.

gr-qc↗

arXiv2015

Scalar Field Cosmologies With Inverted Potentials

Regular bouncing solutions in the framework of a scalar-tensor gravity model were found in a recent work. We reconsider the problem in the Einstein frame (EF) in the present work. Singularities arising at the limit of physical viability of the model in the Jordan frame (JF) are either of the Big Bang or of the Big Crunch type in the EF. As a result we obtain integrable scalar field cosmological models in general relativity (GR) with inverted double-well potentials unbounded from below which possess solutions regular in the future, tending to a de Sitter space, and starting with a Big Bang. The existence of the two fixed points for the field dynamics at late times found earlier in the JF becomes transparent in the EF.

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arXiv2015

Bouncing Universes in Scalar-Tensor Gravity Models admitting Negative Potentials

We consider the possibility to produce a bouncing universe in the framework of scalar-tensor gravity models in which the scalar field potential may be negative, and even unbounded from below. We find a set of viable solutions with nonzero measure in the space of initial conditions passing a bounce, even in the presence of a radiation component, and approaching a constant gravitational coupling afterwards. Hence we have a model with a minimal modification of gravity in order to produce a bounce in the early universe with gravity tending dynamically to general relativity (GR) after the bounce.

gr-qc↗

arXiv2009

The growth of matter perturbations in f(R) models

We consider the linear growth of matter perturbations on low redshifts in some $f(R)$ dark energy (DE) models. We discuss the definition of dark energy (DE) in these models and show the differences with scalar-tensor DE models. For the $f(R)$ model recently proposed by Starobinsky we show that the growth parameter $γ_0\equiv γ(z=0)$ takes the value $γ_0\simeq 0.4$ for $Ω_{m,0}=0.32$ and $γ_0\simeq 0.43$ for $Ω_{m,0}=0.23$, allowing for a clear distinction from $Λ$CDM. Though a scale-dependence appears in the growth of perturbations on higher redshifts, we find no dispersion for $γ(z)$ on low redshifts up to $z\sim 0.3$, $γ(z)$ is also quasi-linear in this interval. At redshift $z=0.5$, the dispersion is still small with $Δγ\simeq 0.01$. As for some scalar-tensor models, we find here too a large value for $γ'_0\equiv \frac{dγ}{dz}(z=0)$, $γ'_0\simeq -0.25$ for $Ω_{m,0}=0.32$ and $γ'_0\simeq -0.18$ for $Ω_{m,0}=0.23$. These values are largely outside the range found for DE models in General Relativity (GR). This clear signature provides a powerful constraint on these models.

astro-ph↗

arXiv2007

Pointer states for primordial fluctuations in inflationary cosmology

Primordial fluctuations in inflationary cosmology acquire classical properties through decoherence when their wavelengths become larger than the Hubble scale. Although decoherence is effective, it is not complete, so a significant part of primordial correlations remains up to the present moment. We address the issue of the pointer states which provide a classical basis for the fluctuations with respect to the influence by an environment (other fields). Applying methods from the quantum theory of open systems (the Lindblad equation), we show that this basis is given by narrow Gaussians that approximate eigenstates of field amplitudes. We calculate both the von Neumann and linear entropy of the fluctuations. Their ratio to the maximal entropy per field mode defines a degree of partial decoherence in the entropy sense. We also determine the time of partial decoherence making the Wigner function positive everywhere which, for super-Hubble modes during inflation, is virtually independent of coupling to the environment and is only slightly larger than the Hubble time. On the other hand, assuming a representative environment (a photon bath), the decoherence time for sub-Hubble modes is finite only if some real dissipation exists.

astro-ph↗

arXiv2007

Scalar-Tensor Dark Energy Models

We present here some recent results concerning scalar-tensor Dark Energy models. These models are very interesting in many respects: they allow for a consistent phantom phase, the growth of matter perturbations is modified. Using a systematic expansion of the theory at low redshifts, we relate the possibility to have phantom like DE to solar system constraints.

astro-ph↗

arXiv2005

On the equation of state of Dark Energy

The formalism in order to obtain the Dark Energy equation of state is extended to non-flat universes and we consider the inequalities that must be satisfied by Phantom Dark Energy in this case. We show that due to a non-vanishing spatial curvature satisfying the observational bounds, the uncertainty on the determination of the Dark Energy equation of state parameter $w$, when it is taken constant, can be significant and that it is minimal for some redshift $z_{cr}\sim 3$. We consider the potential of future measurements of the gravitational waves emitted by binaries at high redshifts $z>z_{cr}$ to reduce this uncertainty. Results obtained here should also be relevant for a weakly varying equation of state with $w\approx -1$.

astro-ph↗

arXiv2004

Transient Accelerated Expansion and Double Quintessence

We consider Double Quintessence models for which the Dark Energy sector consists of two coupled scalar fields. We study in particular the possibility to have a transient acceleration in these models. In both Double Quintessence models studied here, it is shown that if acceleration occurs, it is necessarily transient. We consider also the possibility to have transient acceleration in two one-field models, the Albrecht-Skordis model and the pure exponential. Using separate conservative constraints (marginalizing over the other parameters) on the effective equation of state $w_{eff}$, the relative density of the Dark Energy $Ω_{Q,0}$ and the present age of the universe, we construct scenarios with a transient acceleration that has already ended at the present time, and even with no acceleration at all, but a less conservative analysis using the CMB data rules out the last possibility. The scenario with a transient acceleration ended by today, can be implemented for the range of cosmological parameters $Ω_{m,0}\gtrsim 0.35$ and $h\lesssim 0.68$.

astro-ph↗

arXiv2004

Peculiar Relics from Primordial Black Holes in the Inflationary Paradigm

Depending on various assumptions on the energy scale of inflation and assuming a primordial power spectrum of a Broken Scale Invariance (BSI) type, we explore the possibility for Primordial Black Holes (PBH) and Planck relics to contribute substantially to cold dark matter in the Universe. A recently proposed possibility to produce planck relics in 4-dimensional string gravity is considered. Possible experimental detection through gravitational waves is further explored. We stress that inflation with a low energy scale, and also possibly when Planck relics are produced, leads unavoidably to relics originating from PBHs that are not effectively classical during their formation, rendering the usual formalism inadequate for them.

astro-ph↗

arXiv2002

Galactic Cosmic Rays from PBHs and Primordial Spectra with a Scale

We consider the observational constraints from the detection of antiprotons in the Galaxy on the amount of Primordial Black Holes (PBH) produced from primordial power spectra with a bumpy mass variance. Though essentially equivalent at the present time to the constraints from the diffuse $γ$-ray background, they allow a widely independent approach and they should improve sensibly in the nearby future. We discuss the resulting constraints on inflationary parameters using a Broken Scale Invariance (BSI) model as a concrete example.

astro-ph↗

arXiv2000

Accelerating Universes with Scaling Dark Matter

Friedmann-Robertson-Walker universes with a presently large fraction of the energy density stored in an $X$-component with $w_X<-1/3$, are considered. We find all the critical points of the system for constant equations of state in that range. We consider further several background quantities that can distinguish the models with different $w_X$ values. Using a simple toy model with a varying equation of state, we show that even a large variation of $w_X$ at small redshifts is very difficult to observe with $d_L(z)$ measurements up to $z\sim 1$. Therefore, it will require accurate measurements in the range $1<z<2$ and independent accurate knowledge of $Ω_{m,0}$ (and/or $Ω_{X,0}$) in order to resolve a variable $w_X$ from a constant $w_X$.

gr-qc↗

arXiv2000

Scalar-tensor gravity in an accelerating universe

We consider scalar-tensor theories of gravity in an accelerating universe. The equations for the background evolution and the perturbations are given in full generality for any parametrization of the Lagrangian, and we stress that apparent singularities are sometimes artifacts of a pathological choice of variables. Adopting a phenomenological viewpoint, i.e., from the observations back to the theory, we show that the knowledge of the luminosity distance as a function of redshift up to z ~ (1-2), which is expected in the near future, severely constrains the viable subclasses of scalar-tensor theories. This is due to the requirement of positive energy for both the graviton and the scalar partner. Assuming a particular form for the Hubble diagram, consistent with present experimental data, we reconstruct the microscopic Lagrangian for various scalar-tensor models, and find that the most natural ones are obtained if the universe is (marginally) closed.

gr-qc↗

arXiv2000

Detectability of the primordial origin of the gravitational wave background in the Universe

The appearance of peaks in various primordial fluctuation Fourier power spectra is a generic prediction of the inflationary scenario. We investigate whether future experiments, in particular the satellite experiment PLANCK, will be able to detect the possible appearance of these peaks in the B-mode polarization multipole power spectrum. This would yield a conclusive proof of the presence of a primordial background of gravitational waves.

gr-qc↗

arXiv2000

Entropy of gravitons produced in the early Universe

Gravitons produced from quantum vacuum fluctuations during an inflationary stage in the early Universe have zero entropy as far as they reflect the time evolution (squeezing) of a pure state, their large occupation number notwithstanding. A non-zero entropy of the gravitons (classical gravitational waves (GW) after decoherence) can be obtained through coarse graining. The latter has to be physically justified {\it and} should not contradict observational constraints. We propose two ways of coarse graining for which the fixed temporal phase of each Fourier mode of the GW background still remains observable: one based on quantum entanglement, and another one following from the presence of a secondary GW background. The proposals are shown to be mutually consistent. They lead to the result that the entropy of the primordial GW background is significantly smaller than it was thought earlier. The difference can be ascribed to the information about the regular (inflationary) initial state of the Universe which is stored in this background and which reveals itself, in particular, in the appearance of primordial peaks (acoustic peaks in the case of scalar perturbations) in the multipole spectra of the CMB temperature anisotropy and polarization.

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