POLARSKI David
Fonction : Permanent
Organisme : Université Montpellier II
Professeur (HDR)
Directeur(trice) de thèse de :
CALDERON R.,
David.POLARSKI
umontpellier.fr
0467149329
Bureau: 10, Etg: 1, Bât: 13  Site : Campus Triolet
Domaines de Recherche:  Physique/Relativité Générale et Cosmologie Quantique

Dernieres productions scientifiques :


End of cosmic growth
Auteur(s): Linder Eric V., Polarski D.
(Article) Publié:
Phys.rev.d, vol. 99 p.023503 (2019)
Texte intégral en Openaccess :
Ref HAL: hal01914524_v1
Ref Arxiv: 1810.10547
Ref INSPIRE: 1700428
DOI: 10.1103/PhysRevD.99.023503
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1 citation
Résumé: 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 that the “why now” problem is equally striking when viewed in terms of the shutdown 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.



Consistency of modified gravity with a decreasing $G_{\rm eff}(z)$ in a $\Lambda$CDM background
Auteur(s): Gannouji Radouane, Kazantzidis Lavrentios, Perivolaropoulos Leandros, Polarski D.
(Article) Publié:
Phys.rev.d, vol. 98 p.104044 (2018)
Texte intégral en Openaccess :
Ref HAL: hal01886014_v1
Ref Arxiv: 1809.07034
Ref INSPIRE: 1694683
DOI: 10.1103/PhysRevD.98.104044
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1 citation
Résumé: Recent analyses [S. Nesseris et al., Phys. Rev. D 96, 023543 (2017)PRVDAQ2470001010.1103/PhysRevD.96.023543; L. Kazantzidis and L. Pervolaropoulos, Phys. Rev. D 97, 103503 (2018)PRVDAQ2470001010.1103/PhysRevD.97.103503] have indicated that an effective Newton’s constant Geff(z) decreasing with redshift may relieve the observed tension between the Planck15 best fit ΛCDM cosmological background (i.e., Planck15/ΛCDM) and the corresponding ΛCDM background favored by growth fσ8 and weak lensing data. We investigate the consistency of such a decreasing Geff(z) with some viable scalartensor models and f(R) theories. We stress that f(R) theories generically cannot lead to a decreasing Geff(z) for any cosmological background. For scalartensor models we deduce that in the context of a ΛCDM cosmological background, a decreasing Geff(z) is not consistent with a large BransDicke parameter ωBD,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 Geff(z) and Geff,0=G would have a characteristic signature in its growth index γ with 0.61≲γ0≲0.69 and large slopes γ0′, 0.16≲γ0′≲0.4, which is a characteristic signature of a decreasing (with z) Geff(z)



Consistency of the expansion of the Universe with density perturbations
Auteur(s): Gannouji Radouane, Polarski D.
(Article) Publié:
Phys.rev.d, vol. 98 p.083533 (2018)
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Ref Arxiv: 1805.08230
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DOI: 10.1103/PhysRevD.98.083533
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1 citation
Résumé: Assuming a simple form for the growth index γ(z) depending on two parameters γ0≡γ(z=0) and γ1≡γ′(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 quasistatic growth index and γ1≈0.02 are favored. We find further the lower bounds γ0≳0.53 and γ1≳0.15 for models inside GR. Models outside GR having the same background expansion as ΛCDM and arbitrary γ(z) with γ0=γ0ΛCDM, satisfy Geff,0>G for γ1>γ1ΛCDM, and Geff,0



When is the growth index constant?
Auteur(s): Polarski D., Starobinsky Alexei A., Giacomini Hector
(Article) Publié:
Jcap 1612, vol. no 12 p.037 (2016)
Texte intégral en Openaccess :
Ref HAL: hal02021381_v1
Ref Arxiv: 1610.00363
DOI: 10.1088/14757516/2016/12/037
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6 citations
Résumé: The growth index $\gamma$ 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 $\gamma$ 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 $\gamma$ 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 $\Omega_m$ appears to be too small. An infinite number of solutions, with $\Omega_m$ and $\gamma$ both constant, are found with $w_{DE}=0$ corresponding to Einsteinde 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 dustlike matter deep in the matter era, but with $\Omega_m <1$, requires $G_{\rm eff} > G$ and an example is given using scalartensor gravity for a range of admissible values of $\gamma$. For constant $w_{DE}$ inside GR, departure from a quasiconstant value is limited until today. Even a large variation of $w_{DE}$ may not result in a clear signature in the change of $\gamma$. The change however is substantial in the future and the asymptotic value of $\gamma$ is found while its slope with respect to $\Omega_m$ (and with respect to $z$) diverges and tends to $\infty$.
Commentaires: 16 pages, 5 figure; incorrect reference corrected; v3 matches published version in JCAP; v4: comment added and expanded references. Réf Journal: JCAP 1612, no 12, 037 (2016)



Bouncing Universes in ScalarTensor Gravity Around Conformal Invariance
Auteur(s): Bruno Boisseau, Giacomini Hector, Polarski D.
(Article) Publié:
Journal Of Cosmology And Astroparticle Physics, vol. p.1605, 037 (2016)
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Ref HAL: hal01292660_v1
Ref Arxiv: 1603.06648v1
DOI: 10.1088/14757516/2016/05/048
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5 citations
Résumé: We consider the possibility to produce a bouncing universe in the framework of scalartensor 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.

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