Abstract

Most dark matter (DM) models set the DM relic density by some interaction with Standard Model particles. Such models generally assume the existence of Standard Model particles early on, with the DM relic density a later consequence of those interactions. Perhaps a more compelling assumption is that DM is not part of the Standard Model sector and a population of DM too is generated at the end of inflation. This democratic assumption does not necessarily provide a natural value for the DM relic density, and superficially leads to too much entropy in the dark sector. We address the latter issue by the late decay of heavy particles produced at early times, associating the DM relic density with the lifetime of a long-lived state. We ask what it would take for this scenario to be compatible with observations in what we call Flooded Dark Matter (FDM) and discuss several interesting consequences. One is that DM can be very light and furthermore, light DM is in some sense the most natural scenario in FDM as it is compatible with larger couplings of the decaying particle. Moreover, the decay of the field with the smallest coupling and hence the longest lifetime dominates the entropy and possibly the matter content of the Universe, a principle we refer to as 'Maximum Baroqueness'. We also show that the dark sector should be colder than the ordinary sector, relaxing the free-streaming constraints on light DM. We will discuss the implications for the core-cusp problem in a follow-up paper. FDM also has interesting baryogenesis implications. One possibility is that both DM and baryon asymmetries are simultaneously diluted by a late entropy dump. Alternatively, FDM is compatible with an elegant non-thermal leptogenesis implementation in which decays of a heavy RH neutrino lead to late time reheating of the Standard Model and provide suitable conditions for creation of a lepton asymmetry.

Highlights

  • We have examined the cosmological implications of assuming dark matter arises in an egalitarian fashion along with ordinary matter following inflation, and that there is not necessarily any significant interaction between the different sectors

  • This problem can be addressed if a heavy state decays into the Standard Model, but leaves the dark matter entropy intact

  • We do not guarantee that the dark matter energy density is naturally explained, but we do introduce what might be a very generic cosmological scenario with interesting consequences that is worth pursuing

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Summary

Flooded Dark Matter

We give a description of the evolution of the Universe within the FDM framework and derive the relevant constraints on the parameter space.

Standard Model reheating from late decays
Dark matter from late decay
Additional constraints
Allowed parameter space
Baryogenesis
See-saw neutrino model
Leptogenesis with nonthermal right-handed neutrino production
Concluding remarks
A Efficiency of Standard Model reheating
B Nonrelativistic dark matter prior to reheating
C Multiple non-degenerate heavy states
Full Text
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