On the consistency of the expansion with the perturbations

Abstract

Assuming a simple form for the growth index gamma(z) depending on two parameters gamma_0 = gamma(z=0) and gamma_1 = gamma'(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 quasi-static growth index and gamma_1 = -0.02 are favoured. We find further the lower bounds gamma_0 > 0.53 and gamma_1 > -0.15 for models inside GR. Models outside GR having the same background expansion as LCDM and arbitrary gamma(z) with gamma_0 = gamma_0^{LCDM}, satisfy G_{eff,0}>G for gamma_1 > gamma_1^{LCDM}, and G_{eff,0}<G for gamma_1 < gamma_1^{LCDM}. The first models will cross downwards the value G_{eff}=G on very low redshifts z<0.3, while the second models will cross upwards G_{eff}=G in the same redshift range. This makes the realization of such modified gravity models even more problematic.

8 pages, 11 figures, v2 accepted for publication in PRD, updated analysis, conclusions unchanged

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Cited by in corpus (6)

• Consistency of Modified Gravity with a decreasing $G_{\mathrm{eff}}(z)$ in a $\Lambda$CDM background
• Global properties of the growth index of matter inhomogeneities in the universe
• Is gravity getting weaker at low z? Observational evidence and theoretical implications
• Weak gravity on a $\Lambda$CDM background
• Global properties of the growth index: mathematical aspects and physical relevance
• $\Lambda$CDM-like models with future singularities