- Accepted Paper
Exploring the growth-index () tension with $\Lambda_{\rm s}$CDM
Phys. Rev. D - Accepted 8 September, 2026
DOI: https://doi.org/10.1103/gs8j-1s7r
Phys. Rev. D - Accepted 8 September, 2026
DOI: https://doi.org/10.1103/gs8j-1s7r
Recent observational analyzes have revealed a significant tension in the growth index , which characterizes the growth rate of cosmic structures. When is treated as a free parameter within CDM, the PL18+ combination yields , in tension with the theoretically expected value (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 — can alleviate the tension together with the and tensions. We find that $\gamma\Lambda_{\rm s}$CDM is able to reduce the and tensions relative to CDM while maintaining low values. For PL18+, the combination underlying the tension, $\gamma\Lambda_{\rm s}$CDM yields (95% C.L.) and reduces the and discrepancies from and in CDM to and , respectively, while leaving $\chi^2_{\rm min}$ essentially unchanged. The largest improvement among the free- analyses is obtained for PL18++PP&SH0ES, for which the marginalized posterior constraint is (95% C.L.); the and tensions are reduced to and , 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, –, and weaken the simultaneous alleviation, particularly of the tension. We also examine the benchmark case with fixed , previously identified in studies of the standard () $\Lambda_{\rm s}$CDM framework as a redshift of interest for alleviating multiple major cosmological tensions —including those in , $M_{\rm B}$, and . For the restricted combinations in which it is considered, this benchmark yields low residual tensions (e.g., a tension and an effectively vanishing tension for PL18+), albeit with somewhat worse fits than CDM. It is, however, excluded at more than by the free- 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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