Impact of freeze-in on dark matter isocurvature

Dark matter freeze-in is a compelling cosmological production mechanism in which all orsome of the observed abundance of dark matter is generated through feeble interactions it has withthe Standard Model. In this work we present the first analysis of freeze-in dark matterfluctuations and consider two benchmark models: freeze-in through the direct decay of a heavyvector boson and freeze-in through pair annihilation of Standard Model particles in the thermalbath. We provide a theoretical framework for determining the impact of freeze-in on curvature anddark matter isocurvature perturbations. We determine freeze-in dark matter fluid properties fromfirst principles, tracking its evolution from its relativistic production to its final cold state,and calculate the evolution of the dark matter isocurvature perturbation. We find that in theabsence of initial isocurvature, the freeze-in production of dark matter does not sourceisocurvature. However, for an initial isocurvature perturbation seeded by inflation, thenonthermal freeze-in process may allow for a fraction of the isocurvature to persist, in contrastto the exponential suppression it receives in the case of thermal dark matter. In either case,the evolution of the curvature mode is unaffected by the freeze-in process. We show sensitivityprojections of future cosmic microwave background experiments to the amplitude of uncorrelated,totally anticorrelated, and totally correlated dark matter isocurvature perturbations. From theseprojections, we infer the sensitivity to the abundance of freeze-in dark matter that sustains somefraction of the primordial isocurvature.

Note:

31 pages, 7 figures. V2 provides details of our analysis and fixes a numerical error, which changes our results presented in v1

cosmological perturbation theory
dark matter theory
physics of the early universe
electron positron: annihilation
lead: production
cosmic background radiation: anisotropy
dark matter: mass
isocurvature: perturbation
inflation
sensitivity

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Figures(7)

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