[Paper Review] Evidence for suppression of structure growth in the concordance cosmological model

Nhat-Minh Nguyen, Dragan Huterer+1 more|PubMed|Feb 2, 2023

Cosmology and Gravitation Theories97 references9 citations

TL;DR

The paper presents evidence that the growth rate of cosmic structures is suppressed relative to the GR-based flat ΛCDM prediction, quantified by a higher growth index γ, using Planck CMB and large-scale structure data.

ABSTRACT

We present evidence for a suppressed growth rate of large-scale structure during the dark-energy-dominated era. Modeling the growth rate of perturbations with the "growth index" γ, we find that current cosmological data strongly prefer a higher growth index than the value γ=0.55 predicted by general relativity in a flat Lambda cold dark matter cosmology. Both the cosmic microwave background data from Planck and the large-scale structure data from weak lensing, galaxy clustering, and cosmic velocities separately favor growth suppression. When combined, they yield γ=0.633_{-0.024}^{+0.025}, excluding γ=0.55 at a statistical significance of 3.7σ. The combination of fσ_{8} and Planck measurements prefers an even higher growth index of γ=0.639_{-0.025}^{+0.024}, corresponding to a 4.2σ tension with the concordance model. In Planck data, the suppressed growth rate offsets the preference for nonzero curvature and fits the data equally well as the latter model. A higher γ leads to a higher matter fluctuation amplitude S_{8} inferred from galaxy clustering and weak lensing measurements, and a lower S_{8} from Planck data, effectively resolving the S_{8} tension.

Motivation & Objective

• Motivate and test whether the growth of structure deviates from GR in a flat ΛCDM universe.
• Introduce and employ a growth-index parameterization f(a) = Ωm(a)^γ to quantify growth.
• Combine Planck CMB data with large-scale structure data to constrain γ and related parameters.
• Assess whether allowing a higher γ can reconcile tensions in S8 and ΩK observed in Planck and LSS data.

Proposed method

• Parameterize growth with f(a) = Ωm(a)^γ and D(γ,a) from Eq. (3).
• Model the linear matter power spectrum as P(γ,k,a) = P_today(k,a=1) D^2(γ,a).
• Modify CAMB to implement γ and reproduce standard outputs for γ=0.55.
• Constrain γ and standard cosmological parameters using MCMC with cobaya and priors γ ~ U(0,2).
• Use data sets: fσ8 measurements, Planck 2018 (PL18), DESY1, and BAO; analyze combinations fσ8+PL18, fσ8+DESY1+BAO, and all together.

Experimental results

Research questions

• RQ1 Does the growth rate of structure deviate from GR predictions in flat ΛCDM when γ is allowed to vary?
• RQ2 Can a higher growth index γ reconcile S8 and ΩK tensions between Planck and low-redshift LSS data?
• RQ3 How do different data combinations (Planck, DES, BAO, fσ8) constrain γ and related parameters?
• RQ4 What is the statistical significance of any detected deviation from γ = 0.55?

Key findings

• Combined data prefer γ > 0.55, indicating growth suppression relative to GR in flat ΛCDM.
• fσ8 + PL18 yields γ = 0.639^{+0.024}_{-0.025} (4.2σ exclusion of 0.55).
• All data (fσ8+PL18+DESY1+BAO) give γ = 0.633^{+0.025}_{-0.024} (3.7σ exclusion of 0.55).
• Including γ>0.55 helps reconcile S8 tensions by shifting Planck and LSS S8 estimates toward each other (DES+BAO favor higher S8 with γ>0.55, Planck favors lower S8).
• Allowing curvature to vary shows a degeneracy with γ; PL18 data prefer either ΩK < 0 or γ > 0.55, with comparable fits for the two scenarios.
• The growth-suppression interpretation can mimic effects of A_lens and negative curvature in CMB analyses.