Linear matter density perturbations in the $Λ_{\rm s}$CDM model: Examining growth dynamics and addressing the $S_8$ tension
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| Format: | Preprint |
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2025
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| author | Akarsu, Özgür Çam, Arman Paraskevas, Evangelos A. Perivolaropoulos, Leandros |
| author_facet | Akarsu, Özgür Çam, Arman Paraskevas, Evangelos A. Perivolaropoulos, Leandros |
| contents | We investigate linear matter density perturbations in the $Λ_{\rm s}$CDM, in which the $Λ$ is replaced by late-time ($z\sim2$) mirror AdS-dS transition, resulting in distinct growth dynamics. We use two complementary approaches: (i) determining the initial density contrast and its evolution rate for a given collapse scale factor, (ii) computing the collapse scale factor for a specified initial density contrast and evolution rate. We derive analytical solutions for the growth rate $f=Ω_{\rm m}^γ$ and growth index $γ$ in both models. Prior to the transition, the AdS-like $Λ$ reduces cosmic friction, causing linear matter density perturbations to grow more rapidly than in $Λ$CDM; this effect is most pronounced just before the transition, with a growth rate around $15\%$ higher than $Λ$CDM around $z\sim2$. After the transition, $Λ_{\rm s}$CDM behaves similarly to $Λ$CDM but features a larger cosmological constant, leading to higher $H(z)$ and greater cosmic friction that more effectively suppresses growth. Before the transition, $γ$ remains below both the $Λ$CDM and EdS values ($γ\approx6/11$); during the transition, it increases rapidly and then grows gradually, paralleling $Λ$CDM while remaining slightly higher in the post-transition era-though overall, it stays near $γ\sim0.55$. Using the Planck best-fit values, $Ω_{\rm m0}=0.28$ for $Λ_{\mathrm{s}}$CDM and $Ω_{\rm m0}=0.32$ for $Λ$CDM, we find $f=0.49$ and $f=0.53$, respectively. $Λ_{\rm s}$CDM predicts a value of $f=0.48$, recently obtained from LSS data when $γ$ is treated as a free parameter in $Λ$CDM. This suggests that $Λ_{\rm s}$CDM may resolve the structure growth anomaly, without deviating from $γ\sim 0.55$. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2502_20384 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Linear matter density perturbations in the $Λ_{\rm s}$CDM model: Examining growth dynamics and addressing the $S_8$ tension Akarsu, Özgür Çam, Arman Paraskevas, Evangelos A. Perivolaropoulos, Leandros Cosmology and Nongalactic Astrophysics General Relativity and Quantum Cosmology We investigate linear matter density perturbations in the $Λ_{\rm s}$CDM, in which the $Λ$ is replaced by late-time ($z\sim2$) mirror AdS-dS transition, resulting in distinct growth dynamics. We use two complementary approaches: (i) determining the initial density contrast and its evolution rate for a given collapse scale factor, (ii) computing the collapse scale factor for a specified initial density contrast and evolution rate. We derive analytical solutions for the growth rate $f=Ω_{\rm m}^γ$ and growth index $γ$ in both models. Prior to the transition, the AdS-like $Λ$ reduces cosmic friction, causing linear matter density perturbations to grow more rapidly than in $Λ$CDM; this effect is most pronounced just before the transition, with a growth rate around $15\%$ higher than $Λ$CDM around $z\sim2$. After the transition, $Λ_{\rm s}$CDM behaves similarly to $Λ$CDM but features a larger cosmological constant, leading to higher $H(z)$ and greater cosmic friction that more effectively suppresses growth. Before the transition, $γ$ remains below both the $Λ$CDM and EdS values ($γ\approx6/11$); during the transition, it increases rapidly and then grows gradually, paralleling $Λ$CDM while remaining slightly higher in the post-transition era-though overall, it stays near $γ\sim0.55$. Using the Planck best-fit values, $Ω_{\rm m0}=0.28$ for $Λ_{\mathrm{s}}$CDM and $Ω_{\rm m0}=0.32$ for $Λ$CDM, we find $f=0.49$ and $f=0.53$, respectively. $Λ_{\rm s}$CDM predicts a value of $f=0.48$, recently obtained from LSS data when $γ$ is treated as a free parameter in $Λ$CDM. This suggests that $Λ_{\rm s}$CDM may resolve the structure growth anomaly, without deviating from $γ\sim 0.55$. |
| title | Linear matter density perturbations in the $Λ_{\rm s}$CDM model: Examining growth dynamics and addressing the $S_8$ tension |
| topic | Cosmology and Nongalactic Astrophysics General Relativity and Quantum Cosmology |
| url | https://arxiv.org/abs/2502.20384 |