# $\Lambda_S$CDM model: $\Lambda$CDM model with a sign switching cosmological constant'

Authors
Publication Date
Aug 26, 2021
Source
HAL
Keywords
Language
English
Inspired by the recent conjecture that the universe has transitioned from AdS vacua to dS vacua in the late universe made via graduated dark energy, we extend the $\Lambda$CDM model by a cosmological constant' ($\Lambda_{\rm s}$) that switches sign at certain redshift, $z_\dagger$, and name it as $\Lambda_{\rm s}$CDM. We discuss the construction and theoretical features of this model, and find out that, when the consistency of $\Lambda_{\rm s}$CDM with the CMB data is ensured, (i) $z_\dagger\gtrsim1.1$ is implied by the condition that the universe monotonically expands, (ii) $H_0$ is inversely correlated with $z_\dagger$ and reaches $\approx74.5~{\rm km\, s^{-1}\, Mpc^{-1}}$ for $z_\dagger=1.5$, (iii) $H(z)$ presents an excellent fit to the Ly-$\alpha$ measurements provided that $z_\dagger\lesssim 2.34$. We further investigate the model constraints by using the full Planck CMB data, with and without BAO data. We find that the CMB data alone does not constrain $z_\dagger$ but CMB+BAO dataset favors the sign switch of $\Lambda_{\rm s}$ providing the constraint: $z_\dagger=2.44\pm0.29$ (68% CL). Our analysis reveals that the lower and upper limits of $z_\dagger$ are controlled by the Galaxy and Ly-$\alpha$ BAO measurements, respectively, and the larger $z_{\dagger}$ values imposed by the Galaxy BAO data prevent the model from achieving the highest local $H_0$ measurements. In general, $\Lambda_{\rm s}$CDM (i) relaxes the $H_0$ tension while being fully consistent with the TRGB measurement, (ii) removes the discrepancy with the Ly-$\alpha$ measurements, (iii) relaxes the $S_8$ tension, and (iv) finds a better agreement with the BBN constraints of physical baryon density. We find no strong statistical evidence to discriminate between the $\Lambda_{\rm s}$CDM and $\Lambda$CDM models. However, interesting and promising features of $\Lambda_{\rm s}$CDM provide an upper edge over $\Lambda$CDM.