• Accepted Paper

Exploring the growth-index (γ) tension with $\Lambda_{\rm s}$CDM

Luis A. Escamilla, Özgür Akarsu, Eleonora Di Valentino, Emre Özülker, and J. Alberto Vazquez

Phys. Rev. D - Accepted 8 September, 2026

DOI: https://doi.org/10.1103/gs8j-1s7r

Abstract

Recent observational analyzes have revealed a significant tension in the growth index γ, which characterizes the growth rate of cosmic structures. When γ is treated as a free parameter within ΛCDM, the PL18+fσ8 combination yields γ≈0.64, in ∼4σ tension with the theoretically expected value γ≈0.55 (assuming general relativity), suggesting a preference for suppressed late-time growth relative to the standard expectation. In this work, we investigate whether the $\Lambda_{\rm s}$CDM framework —featuring a rapid sign-switching cosmological constant (mirror AdS-to-dS transition) in the late universe at redshift z†∼2— can alleviate the γ tension together with the H0 and S8 tensions. We find that $\gamma\Lambda_{\rm s}$CDM is able to reduce the γ and H0 tensions relative to γΛCDM while maintaining low S8 values. For PL18+fσ8, the combination underlying the γ tension, $\gamma\Lambda_{\rm s}$CDM yields z†>1.58 (95% C.L.) and reduces the γ and H0 discrepancies from 3.7σ and 4.2σ in γΛCDM to 2.1σ and 1.1σ, respectively, while leaving $\chi^2_{\rm min}$ essentially unchanged. The largest improvement among the free-z† analyses is obtained for PL18+fσ8+PP&SH0ES, for which the marginalized posterior constraint is z†=1.82−0.28+0.32 (95% C.L.); the γ and H0 tensions are reduced to 1.8σ and 0.6σ, respectively, and the model also provides an improved fit to the data. The efficacy of the mechanism is, however, dataset dependent: the standard 3D BAO measurements used in the present analysis —from SDSS-IV/eBOSS and the first year of DESI observations— tend to push the transition to higher redshifts, z†≳2.3–2.4, and weaken the simultaneous alleviation, particularly of the H0 tension. We also examine the benchmark case with fixed z†=1.7, previously identified in studies of the standard (γ≃0.55) $\Lambda_{\rm s}$CDM framework as a redshift of interest for alleviating multiple major cosmological tensions —including those in H0, $M_{\rm B}$, and S8. For the restricted combinations in which it is considered, this benchmark yields low residual tensions (e.g., a 1.6σ γ tension and an effectively vanishing H0 tension for PL18+fσ8), albeit with somewhat worse fits than γΛCDM. It is, however, excluded at more than 2σ by the free-z† analyses once either the uncalibrated PantheonPlus SN~Ia sample or the 3D BAO data are included. Our results indicate that $\Lambda_{\rm s}$CDM can contribute to the joint alleviation of multiple cosmological tensions, although its impact is dataset dependent.

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