Hubble sinks in the low-redshift swampland

Local determinations of the Hubble constant $H_0$ favor a higher value than Planck based on cosmic microwave background and $\mathrm{\Lambda }$ cold dark matter ($\mathrm{\Lambda }\mathrm{CDM}$). Through a model-independent expansion, we show that low redshift ($z\lesssim 0.7$) data comprising baryon acoustic oscillations, cosmic chronometers, and Type Ia supernovae have a preference for quintessence models that lower $H_0$ relative to $\mathrm{\Lambda }\mathrm{CDM}$. In addition, we confirm that an exponential coupling to dark matter cannot alter this conclusion in the same redshift range. Our results leave open the possibility that a coupling in the matter-dominated epoch, potentially even in the dark ages, may yet save $H_0$ from sinking in the string theory swampland.

Figure 1

Bounds on the Chevallier-Polarski-Linderr (CPL) parametrization [36, 37], $w(z)=w_0+w_{a}z/(1+z)$, which are constructed directly from the chains of an MCMC exploration of cosmic chronometer, BAOs, and supernovae data restricted to $z\le 0.7$.

Figure 2

Distribution of our 87,675 models, which survive our cuts, as a function of ${\chi }^2$ versus $H_0$. The dashed green line highlights the ${\chi }^2$ for $\mathrm{\Lambda }\mathrm{CDM}$ and all models that raise $H_0$ also increase the ${\chi }^2$ value. In contrast 53,744 models lower both $H_0$ and ${\chi }^2$.

Figure 3

Distribution of 80,732 surviving models as a function of ${\chi }^2$ versus $H_0$ for the coupling $C=0.1$. Here all models that raise $H_0$ also raise the ${\chi }^2$, whereas 46,665 models lower $H_0$ and ${\chi }^2$.

Figure 4

The 1 and $2\sigma$ intervals for the normalized potential $V/V_0$ corresponding to CPL models that are consistent with CMB, BAO and Pantheon. For illustrative processes we plot the simplest quintessence model, $V/V_0=e^{-\lambda \varphi }$ for different values of $\lambda$.