Why comparable? A multiverse explanation of the dark matter-baryon coincidence

Raphael Bousso and Lawrence Hall
Phys. Rev. D 88, 063503 – Published 3 September 2013

Abstract

The densities of dark and baryonic matter are comparable: ζρD/ρBO(1). This is surprising because they are controlled by different combinations of low-energy physics parameters. Here we consider the probability distribution over ζ in 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+ζ)1, if the causal patch is used to regulate infinities. This suppression can counteract a prior distribution favoring large ζ, selecting ζO(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+ζ)1 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.

  • Figure
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  • Received 15 May 2013

DOI:https://doi.org/10.1103/PhysRevD.88.063503

© 2013 American Physical Society

Authors & Affiliations

Raphael Bousso and Lawrence Hall

  • Department of Physics and Center for Theoretical Physics, University of California, Berkeley, California 94720, USA
  • Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

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Issue

Vol. 88, Iss. 6 — 15 September 2013

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