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 $H_0$ value. As an example, we study a phenomenological EDE model with an AdS phase starting at the redshift $z\sim 2000$ and ending shortly after recombination (hereafter the universe will settle down in a $\mathrm{\Lambda }>0$ phase until now), and obtain a best-fit $H_0=72.74\mathrm{km}/\mathrm{s}/\mathrm{Mpc}$ without degrading the CMB fit compared with the standard $\mathrm{\Lambda }\mathrm{CDM}$ model.

The nonsingular bounce models usually suffer from the ghost or gradient instabilities, as has been proved recently. In this paper, we propose a covariant effective theory for stable nonsingular bounce, which has the quadratic order of the second order derivative of the field ϕ but the background set only by P (ϕ, X). With it, we explicitly construct a fully stable nonsingular bounce model for the ekpyrotic scenario.

Recent Planck data show the anomalies of cosmic microwave background (CMB) fluctuations on large angular scales, which confirms the early observations by WMAP. We continue studying an inflationary model, in which before the slow-roll inflation the universe is in a contracting phase, and fit the model with the Planck data. We show that this model may generate not only the power deficit at low $l$, but also a large hemispherical power asymmetry in the CMB. We also discuss the implication of the result to the eternal inflation scenario.

Based on the effective field theory (EFT) of cosmological perturbations, we explicitly clarify the pathology in nonsingular cubic Galileon models and show

In prerecombination resolutions of the Hubble tension, such as early dark energy, new physics before recombination shifts the values of relevant cosmological parameters so that the models can fit with cosmic microwave background and baryon acoustic oscillations observations as well as LambdaCDM does. In this paper, we clarify how the parameter shifts are related with $\delta H_0$, particularly we find the shift of primordial scalar spectral index scales as $\delta n_s\simeq 0.4\frac{\delta H_0}{H_0}$ by performing the Monte Carlo Markov chain analysis with the $\mathrm{Planck}2018+\mathrm{BAO}+\mathrm{Pantheon}+\mathrm{R}19+\mathrm{Keck}$ Array/BICEP dataset. A novel point of our result is that if the current $H_0$ measured locally is correct, complete resolution of the Hubble tension seems to be pointing to a scale invariant Harrison-Zeldovich spectrum, i.e., $n_s=1$ for $H_0\sim 73\mathrm{km}/\mathrm{s}/\mathrm{Mpc}$.

We investigate the constraints on early dark energy (EDE) by combining the most recent CMB observations available, ACT DR4, SPT-3G, and Planck2018 (${\ell }_{\mathrm{TT},\max }=1000$) data. This combined CMB dataset favors nonzero EDE fractions and large Hubble constants. The inclusion of $\mathrm{BAO}+\mathrm{Pantheon}$ data has little effect on the results, leads to $H_0=71.6(72.9{)}_{-1.5}^{+2.0}$ and $73.17(72.74{)}_{-0.77}^{+0.55}\mathrm{km}/\mathrm{s}/\mathrm{Mpc}$ for axionlike EDE and AdS-EDE, respectively. The axionlike EDE can fit the data significantly better ($\mathrm{\Delta }{\chi }^2\lesssim -10$) than $\mathrm{\Lambda }\mathrm{CDM}$, which is mainly driven by the ACT data. It is found again that if the current $H_0$ measured locally is correct, complete resolution of the Hubble tension seems to be pointing toward a scale invariant Harrison-Zeldovich spectrum of primordial scalar perturbation, i.e., $n_s=1$ for $H_0\sim 73\mathrm{km}/\mathrm{s}/\mathrm{Mpc}$.