Anti-de Sitter (AdS) vacua, being theoretically important, might have an unexpected impact on the observable universe. We find that in early dark energy (EDE) scenarios the existence of AdS vacua around recombination can effectively lift the CMB-inferred H0 value. As an example, we study a phenomenological EDE model with an AdS phase starting at the redshift z ∼ 2000 and ending shortly after recombination (hereafter the universe will settle down in a Λ > 0 phase until now), and obtain a best-fit H0 = 72.74 km/s/Mpc without degrading the CMB fit compared with the standard ΛCDM model. * yegen14@mails.ucas.ac.cn † yspiao@ucas.ac.cn ΛCDM PhaseAdS Phase

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Recent progress seems to suggest that one must modify General Relativity (GR) to stably violate the null energy condition and avoid the cosmological singularity. However, with higher-order derivative operators of the scalar field (a subclass of the degenerate higher-order scalar-tensor theory), we show that at energies well below the Planck scale, fully stable nonsingular cosmologies can actually be implemented within GR.

The detection of GW170817 and its electromagnetic counterpart has revealed the speed of gravitational waves coincides with the speed of light, c T = 1. Inspired by the possibility that the physics implied by GW170817 might be related with that for the primordial universe, we construct the spatially flat stable (throughout the whole evolution) nonsingular bounce models in the beyond Horndeski theory with c T = 1 and in the degenerate higher-order scalar-tensor (DHOST) theory with c T = 1, respectively. Though it constricts the space of viable models, the constraint of c T = 1 makes the procedure of building models simpler.