Ref HAL: hal-03946561_v1
Ref Arxiv: 2301.00640
Ref INSPIRE: 2620237
DOI: 10.1103/PhysRevD.108.023504
Ref. & Cit.: NASA ADS
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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 model-independent manner. In this alternative approach, we model $\mu\equiv G_\mathrm{eff}/G$ 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 $f\sigma_8$, 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 $\mu$) 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 non-negligible bias in the ($\Omega_\mathrm{m0}$,$\sigma_{8,0}$)-plane.

Ref HAL: hal-03722260_v1
Ref Arxiv: 2206.13820
Ref INSPIRE: 2102704
DOI: 10.1103/PhysRevD.106.083513
WoS: 000875169200002
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2 Citations
Résumé:

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 $f_{\mathrm DE}(z) = \rho_\mathrm{DE}(z)/\rho_\mathrm{DE,0}$ 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 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 $\Lambda$CDM at $\gtrsim2\sigma$.

Ref HAL: tel-03612038_v1
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Les observations des dernières décennies ont fait émerger un modèle standard en cosmologie, appelé modèle de concordance. Le paradigme qui s'est imposé grâce aux observations aux échelles cosmologiques est celui d'un univers contenant deux phases d'expansion accélérée. La première permet d'expliquer l'origine des galaxies et amas de galaxies que l'on observe de nos jours, tandis que la deuxième rend compte de l'expansion accélérée actuelle, confirmée par de nombreuses observations. Le mécanisme physique à l'origine de cette expansion accélérée reste une des plus grandes questions ouvertes en physique. Néanmoins, des observations provenant du rayonnement fossile cosmologique (Cosmic microwave background - CMB), des oscillations acoustiques baryoniques (Baryon Acoustic Oscillations -BAO), des supernovae du type Ia (SNeIa) et de nombreuses autres observations, semblent en accord avec l'introduction d'une constante cosmologique Lambda dans les équations du champ de la gravitation (les équations d'Einstein). Par ailleurs, une composante de matière ``sombre'' froide (non-relativiste) -appelée Cold Dark Matter ou CDM- est un ingrédient nécessaire pour expliquer la croissance et la formation des grandes structures dans l'Univers. Ce modèle repose aussi sur des hypothèses concernant les fluctuations cosmologiques primordiales (supposées d'origine inflationnaire). En fait, le modèle LCDM de base est entièrement caractérisé par 6 paramètres libres.

Ref HAL: hal-03184607_v1
DOI: 10.1103/PhysRevD.103.023526
WoS: WOS:000609014700004
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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 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.

Ref HAL: hal-02423733_v1
Ref Arxiv: 1912.06958
Ref INSPIRE: 1770954
DOI: 10.1103/PhysRevD.101.103501
WoS: 000529824500006
Ref. & Cit.: NASA ADS
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Résumé:

We analyze the global behavior of the growth index of cosmic inhomogeneities in an isotropic homogeneous universe filled by cold nonrelativistic 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 (Ωm=0,γ∞) in the future and (Ωm=1,γ-∞) 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) ϵΩmtot remains unclustered, we find that the limit of the growth index in the past γ-∞ϵ does not depend on the equation of state of DE, in sharp contrast with the case ϵ=0 (for which γ-∞ is obtained). We show indeed that there is a mathematical discontinuity: one cannot obtain γ-∞ by taking limϵ→0γ-∞ϵ (i.e., the limits ϵ→0 and Ωmtot→1 do not commute). We recover in our analysis that the value γ-∞ϵ corresponds to tracking DE in the asymptotic past with constant γ=γ-∞ϵ found earlier.

Ref HAL: hal-02266321_v1
Ref Arxiv: 1908.00117
Ref INSPIRE: 1747566
DOI: 10.1103/PhysRevD.100.083503
WoS: 000488510300002
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1 Citation
Résumé:

We perform here a global analysis of the growth index γ behavior from deep in the matter era till the far future. For a given cosmological model in general relativity (GR) or in modified gravity, the value of γ(Ωm) is unique when the decaying mode of scalar perturbations is negligible. However, γ∞, the value of γ in the asymptotic future, is unique even in the presence of a non-negligible 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(Ωm) to be monotonically decreasing (or increasing). This condition can be violated inside GR for varying wDE though generically γ(Ωm) will be monotonically decreasing (like ΛCDM), except in the far future and past. A bump or a dip in Geff can also lead to a significant and rapid change in the slope dγdΩm. On a ΛCDM background, a γ substantially lower (higher) than 0.55 with a negative (positive) slope reflects the opposite evolution of Geff. In Dvali-Gabadadze-Porrati (DGP) models, γ(Ωm) is monotonically increasing except in the far future. While DGP gravity becomes weaker than GR in the future and wDGP→-1, we still get γ∞DGP=γ∞ΛCDM=23. In contrast, despite GeffDGP→G in the past, γ does not tend to its value in GR because dGeffDGPdΩm|-∞≠0.