Phasing out Dark Matter Isocurvature with Thermal Misalignment

Abstract: Thermal misalignment provides an alternative to the standard misalignment mechanism for the cosmological production of scalar dark matter. In this framework, feeble couplings to particles in the thermal bath generate a finite-temperature potential that drives the scalar towards large field values early in the radiation era, dynamically inducing the misalignment before the onset of scalar oscillations. As a result, the relic abundance is controlled primarily by particle masses and couplings rather than the initial field value. As a light spectator field, the scalar acquires inflationary fluctuations that are uncorrelated with the adiabatic curvature mode, generically sourcing isocurvature perturbations. We show that, unlike standard misalignment, where light scalars are strongly constrained by cosmic microwave background bounds on dark matter isocurvature for high-scale inflation, thermal misalignment can naturally suppress the isocurvature signal. This occurs through a novel late-time phase offset between the background zero mode and the superhorizon perturbations, which reduces the final dark matter density contrast. Thermal misalignment therefore provides a new and generic route to isocurvature-safe scalar dark matter.