Dark-energy dynamics required to solve the cosmic coincidence

Chas A. Egan and Charles H. Lineweaver
Phys. Rev. D 78, 083528 – Published 20 October 2008

Abstract

Dynamic dark-energy (DDE) models are often designed to solve the cosmic coincidence (why, just now, is the dark-energy density ρde the same order of magnitude as the matter density ρm?) by guaranteeing ρdeρm for significant fractions of the age of the Universe. However, such behavior is neither sufficient nor necessary to solve the coincidence problem. Cosmological processes constrain the epochs during which observers can exist. Therefore, what must be shown is that a significant fraction of observers see ρdeρm. Precisely when, and for how long, must a DDE model have ρdeρm in order to solve the coincidence? We explore the coincidence problem in dynamic dark-energy models using the temporal distribution of terrestrial-planet-bound observers. We find that any realistic DDE model which can be parametrized as w=w0+wa(1a) over a few e-folds has ρdeρm for a significant fraction of observers in the Universe. This demotivates DDE models specifically designed to solve the coincidence using long or repeated periods of ρdeρm.

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  • Received 26 December 2007

DOI:https://doi.org/10.1103/PhysRevD.78.083528

©2008 American Physical Society

Authors & Affiliations

Chas A. Egan*

  • Department of Astrophysics, School of Physics, University of New South Wales, Sydney, Australia

Charles H. Lineweaver

  • Research School of Astronomy and Astrophysics, Australian National University, Canberra, Australia

  • *Visiting the Research School of Astronomy and Astrophysics, Australian National University. chas@mso.anu.edu.au
  • Also at Planetary Science Institute, Australian National University; Research School of Earth Sciences, Australian National University.

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Issue

Vol. 78, Iss. 8 — 15 October 2008

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