$H_0$ Tension, Swampland Conjectures and the Epoch of Fading Dark Matter

TL;DR

This work tests a string-theory–motivated cosmology in which a rolling scalar field drives dark energy and, via the distance conjecture, couples to a dark-matter tower resulting in fading dark matter. The authors implement a two-exponential quintessence potential $V(\phi)= B\,e^{-b\phi}+ C\,e^{-c\phi}$ and a dark-matter mass $m(\phi)= m_0\,\exp(-\tilde{c}\phi)$ that becomes light after a transition at $\phi_0$, turning on late-time coupling around $z\sim 15$. Using Planck, BAO, Pantheon, and SH0ES data with CLASS and MontePython, they find a nonzero $\tilde{c}$ near $0.3$ (≈$2.8\sigma$) which improves the fit by about $2\sigma$ relative to $\Lambda$CDM, raising $H_0$ and partially alleviating the $S_8$ tension. The model also predicts a nontrivial evolution of $w_{DE}(z)$ and a long-range fifth force in the dark sector, with distinctive future cosmological and astrophysical signatures testable by upcoming experiments such as CMB polarization missions.

Abstract

Motivated by the swampland dS conjecture, we consider a rolling scalar field as the source of dark energy. Furthermore, the swampland distance conjecture suggests that the rolling field will lead at late times to an exponentially light tower of states. Identifying this tower as residing in the dark sector suggests a natural coupling of the scalar field to the dark matter, leading to a continually reducing dark matter mass as the scalar field rolls in the recent cosmological epoch. The exponent in the distance conjecture, $\tilde{c}$, is expected to be an $\mathcal{O}(1)$ number. Interestingly, when we include the local measurement of $H_0$, our model prefers a non-zero value of the coupling $\tilde{c}$ with a significance of $2.8σ$ and a best-fit at $\tilde{c} \sim 0.3$. Modifying the recent evolution of the universe in this way improves the fit to data at the $2σ$ level compared to $Λ$CDM. This string-inspired model automatically reduces cosmological tensions in the $H_0$ measurement as well as $σ_8$.

Figures (9)

• Figure 1: Schematic dependence of the mass gap $m(\phi)$ on the value of $\phi$. At large positive and negative field values the mass gap decreases exponentially, and a tower of states become light.
• Figure 2: Evolution of the Hubble parameter in our best-fit models with $c=0.1$ relative to that of our best-fit $\Lambda$CDM. Quintessence-only models lead to a smaller value of $H_0$, whereas models with coupled dark matter -- dark energy lead to a larger $H_0$. The coupling becomes relevant at $z\approx 15$.
• Figure 3: We show the evolution of $w(z)$ in our models. In the presence of coupling of dark matter with the scalar field $(1+w_{\rm DE})$ can be negative as discussed in section \ref{['sec:IIIC']}.
• Figure 4: Contours showing 1 and 2$\sigma$ bounds for $c,\tilde{c}$ including HST (shaded blue) and without HST (green). We see that including the HST dataset disfavors the $\tilde{c}=0$ point.
• Figure 5: Posteriors for $H_0$ in $\Lambda$CDM, scalar field quintessence and fading dark matter models described in table \ref{['tab:bestfit']}. The green shaded regions show the 1, 2, 3 and 4$\sigma$ ranges for the SH0ES measurement Riess:2019cxk.