POLARSKI David
Fonction : Permanent
Emerite Université
(HDR)
David.POLARSKI
umontpellier.fr
Bureau: 10, Etg: 1, Bât: 13  Site : Campus Triolet
Domaines de Recherche:  Physique/Relativité Générale et Cosmologie Quantique

Dernieres productions scientifiques :


Joint reconstructions of growth and expansion histories from stageIV surveys with minimal assumptions II: Modified gravity and massive neutrinos
Auteur(s): Calderon R., L'Huillier Benjamin, Polarski D., Shafieloo Arman, Starobinsky Alexei A.
(Document sans référence bibliographique) Texte intégral en Openaccess :
Ref HAL: hal03946561_v1
Ref Arxiv: 2301.00640
Ref INSPIRE: 2620237
Ref. & Cit.: NASA ADS
Exporter : BibTex  endNote
Résumé: 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 modelindependent manner. In this alternative approach, we model $\mu\equiv G_\mathrm{eff}/G$ as a Gaussian process and use forecasted growthrate measurements from a stageIV survey to reconstruct its shape for two different toymodels. We follow a twostep 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 $f\sigma_8$, that we fit to redshiftspace 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 $\mu$) 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 do not alter our conclusions. Finally, we stress that incorrectly assuming a $\Lambda$CDM expansion history leads to a degraded reconstruction of $\mu$, and/or a nonnegligible bias in the ($\Omega_\mathrm{m0}$,$\sigma_{8,0}$)plane.



Joint reconstructions of growth and expansion histories from stageIV surveys with minimal assumptions I: Dark Energy beyond $\Lambda$
Auteur(s): Calderon R., L'Huillier B., Polarski D., Shafieloo A., Starobinsky A.A.
(Document sans référence bibliographique) Texte intégral en Openaccess :
Ref HAL: hal03722260_v1
Ref Arxiv: 2206.13820
Ref INSPIRE: 2102704
Ref. & Cit.: NASA ADS
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Résumé: Combining Supernovae, Baryon Acoustic Oscillations and RedshiftSpace Distortions data from the next generation of (StageIV) cosmological surveys, we aim to reconstruct the expansion history up to large redshifts using forwardmodeling of $f_{\mathrm DE}(z) = \rho_\mathrm{DE}(z)/\rho_\mathrm{DE,0}$ 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 FriedmanLemaîtreRobertsonWalker (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 $\Lambda$CDM at $\gtrsim2\sigma$.



Bouncing cosmological isotropic solutions in scalartensor gravity
Auteur(s): Polarski D., Starobinsky A.A., Verbin Y.
(Article) Publié:
Jcap, vol. 01 p.052 (2022)
Texte intégral en Openaccess :
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Ref Arxiv: 2111.07319
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DOI: 10.1088/14757516/2022/01/052
Ref. & Cit.: NASA ADS
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Résumé: Bouncing nonsingular isotropic cosmological solutions are investigated in a simple model of scalartensor 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 Higgslike selfinteraction 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 wellbehaved 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.



Negative cosmological constant in the dark sector?
Auteur(s): Calderon R., Gannouji Radouane, L'Huillier Benjamin, Polarski D.
(Article) Publié:
Physical Review D, vol. 103 p.023526 (2021)
Texte intégral en Openaccess :
Ref HAL: hal03184607_v1
DOI: 10.1103/PhysRevD.103.023526
WoS: WOS:000609014700004
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Résumé: 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 latetime accelerated expansion rate (X). We explore the departure of the expansion history of these models from the concordanceΛ cold dark matter (ΛCDM) model. For a large class of our models, the accelerated expansion is transient with a nontrivial dependence on the model parameters. All models with wX>−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 H0 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≳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.



Weak gravity on a $\Lambda$CDM background
Auteur(s): Gannouji Radouane, Perivolaropoulos Leandros, Polarski D., Skara Foteini
(Article) Publié:
Physical Review D, vol. 103 p.063509 (2021)
Texte intégral en Openaccess :
Ref HAL: hal03022688_v1
Ref Arxiv: 2011.01517
Ref INSPIRE: 1827879
DOI: 10.1103/PhysRevD.103.063509
Ref. & Cit.: NASA ADS
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Résumé: We consider Horndeski modified gravity models obeying stability, velocity of gravitational waves cT equals c and quasistatic approximation on subhorizon scales. We assume further a Lambda cold dark matter background expansion and a monotonic evolution on the cosmic background of the α functions as αi=αi0as where i=M, B, a is the scale factor and αi0 (αM0,αB0), s are arbitrary parameters. We show that the growth and lensing reduced (dimensionless) gravitational couplings μ≡Ggrowth/G, Σ≡Glensing/G exhibit the following generic properties today: Σ0<1 for all viable parameters, μ0<1 (weak gravity today) is favored for small s while μ0>1 is favored for large s. We establish also the relation μ≥Σ at all times. Taking into account the fσ8 and EG data we constrain the parameter s to satisfy s≲2. Hence these data select essentially the weak gravity regime today (μ0<1) when s<2, while μ0>1 subsists only marginally for s≈2. At least the interval 0.5≲s≲2 would be ruled out in the absence of screening. We consider further the growth index γ(z) and identify the (αM0,αB0,s) parameter region that corresponds to specific signs of the differences γ0γ0ΛCDM, and γ1γ1ΛCDM, where γ0≡γz=0 and γ1≡dγdzz=0. In this way important information is gained on the past evolution of μ. We obtain in particular the signature γ0>γ0ΛCDM for s<2 in the selected weak gravity region.

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