de Sitter Swampland, $H_0$ tension & observation

Abstract

Realising de Sitter vacua in string theory is challenging. For this reason it has been conjectured that de Sitter vacua inhabit the Swampland of inconsistent low-energy effective theories coupled to gravity. Since de Sitter is an attractor for $Λ$CDM, the conjecture calls $Λ$CDM into question. Reality appears sympathetic to this idea as local measurements of the Hubble constant $H_0$ are also at odds with $Λ$CDM analysis of Planck data. This tension suggests that the de Sitter state is unstable, thereby implying a turning point in the Hubble parameter. We present a model relieving this tension, which predicts a turning at small positive redshift $z_*$ that is dictated by present-day matter density $ω_m$. This feature is easily identified by homogeneous surveys covering redshifts $z \leq 0.1$. We comment on the implications for the Swampland program.

Figures (2)

• Figure 1: Normalised cosmological evolution according to (\ref{['ODE']}) and $\Lambda$CDM (\ref{['EQN_LCDM']}). While evolution are relatively similar in the past ($z>0$), they are vastly distinct in the future ($z<0$).
• Figure 2: Cosmological evolution outside the Swampland is expected to violate NEC ($w<-1)$, inevitably giving rise to a turn $z_*$ point in $H(z)$. Holography predicts $0<z_*<0.1$ as a new observable accessible by homogeneous supernova surveys covering extremely low redshifts of the Local Universe. Shown is the predicted location (\ref{['EQN_zs']}) for various canonical values of $\omega_m$, where $z_*$ for $\omega_m=0.27$ is illustrative for the anticipated location suggested by current cosmological data $H(z)$.