Abstract
We present a novel source of dark energy, which is motivated by the prevalence of hidden sectors in string theory models and is consistent with all of the proposed swampland conjectures. Thermal effects hold a light hidden sector scalar at a point in field space that is not a minimum of its zero temperature potential. This leads to an effective "cosmological constant", with an equation of state $w=-1$, despite the scalar's zero temperature potential having only a 4D Minkowski or AdS vacuum. For scalar masses $\lesssim \mu$eV, which could be technically natural via sequestering, there are large regions of phenomenologically viable parameter space such that the induced vacuum energy matches the measured dark energy density. Additionally, in many models a standard cosmological history automatically leads to the scalar having the required initial conditions. We study the possible observational signals of such a model, including at fifth force experiments and through $\Delta N_{\rm eff}$ measurements. Similar dynamics that are active at earlier times could resolve the tension between different measurements of $H_0$ and can lead to a detectable stochastic gravitational wave background.
Highlights
The microscopic nature of dark energy, which constitutes around 70% of the energy density in the Universe today and drives its current accelerated expansion, is one of the biggest questions in fundamental physics
Considering observational and model building constraints, we show that the measured dark energy density can be explained for scalar masses ≲10−6 eV, which is still much heavier than the masses required for quintessence
The idea that finite temperature effects may result in a temperature-dependent vacuum energy that leads to cosmic acceleration in the early universe has previously been proposed under the name of thermal inflation [34,35]
Summary
The microscopic nature of dark energy, which constitutes around 70% of the energy density in the Universe today and drives its current accelerated expansion, is one of the biggest questions in fundamental physics. Motivated by the richness of typical string compactifications, we consider the possibility that there could be several hidden sectors, at diverse mass scales, each of which sources a component of dark energy These could lead to detectable gravitational wave signatures and can provide a scenario for early dark energy [33], which has been argued to resolve the tension between astrophysical and cosmological measurements of the Hubble parameter. The idea that finite temperature effects may result in a temperature-dependent vacuum energy that leads to cosmic acceleration in the early universe has previously been proposed under the name of thermal inflation [34,35] These papers, and the subsequent literature, focused on dynamics arising from a “flaton” scalar field in the visible sector, at temperatures around the tera-electron-volt (TeV) scale.
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