Why Comparable? A Multiverse Explanation of the Dark Matter-Baryon Coincidence

Abstract

The densities of dark and baryonic matter are comparable: ζ= ρ_D / ρ_B ~ O(1). This is surprising because they are controlled by different combinations of low-energy physics parameters. Here we consider the probability distribution over ζin the landscape. We argue that the Why Comparable problem can be solved without detailed anthropic assumptions, and independently of the nature of dark matter. Overproduction of dark matter suppresses the probability like 1/(1+ζ), if the causal patch is used to regulate infinities. This suppression can counteract a prior distribution favoring large ζ, selecting ζ~ O(1). This effect not only explains the Why Comparable coincidence but also renders otherwise implausible models of dark matter viable. For the special case of axion dark matter, Wilczek and independently Freivogel have already noted that a 1/(1+ζ) suppression prevents overproduction of a GUT-scale QCD axion. If the dark matter is the LSP, the effect can explain the moderate fine-tuning of the weak scale in simple supersymmetric models.

20 pages JHEP format, 2 figures; v2: added references and a comment on multi-component dark matter; v3: final version for PRD, minor clarifications

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Cited by in corpus (18)

• Axion Cosmology
• A Model For Halo Formation With Axion Mixed Dark Matter
• Axion cold dark matter in view of BICEP2 results
• Inflation after False Vacuum Decay: Observational Prospects after Planck
• The Degree of Fine-Tuning in our Universe -- and Others
• The Weak Scale from BBN
• Grand Unification, Axion, and Inflation in Intermediate Scale Supersymmetry
• The Multiverse and Particle Physics
• WIMPs and Un-Naturalness
• The cosmological moduli problem and naturalness
• Minimal Asymmetric Dark Matter
• Radiative PQ Breaking and the Higgs Boson Mass
• Is the magnitude of the Peccei-Quinn scale set by the landscape?
• The impact of the cosmological constant on past and future star formation
• Cavitation model of the initial stage of Big Bang
• Anthropic Origin of the Neutrino Mass from Cooling Failure
• Localization on the Landscape and Eternal Inflation
• A brief history of the multiverse