60 research outputs found

    Observed Smooth Energy is Anthropically Even More Likely As Quintessence Than As Cosmological Constant

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    For a universe presently dominated by static or dynamic vacuum energy, cosmological constant (LCDM) or quintessence (QCDM), we calculate the asymptotic collapsed mass fraction as function of the present ratio of vacuum energy to clustered mass, \Omega_{Q0}/\Omega_{m0}. Identifying these collapsed fractions as anthropic probabilities, we find the value \Omega_{Q0}/\Omega_{m0} \sim 2 to be reasonably likely in LCDM, and very likely in QCDM.Comment: 4 pages, 2 tables, 1 figur

    Tracking Quintessence Would Require Two Cosmic Coincidences

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    Good tracking requires that the quintessence energy fraction slowly increase while the roll λdlnV/ϰdϕ\lambda\equiv -d\ln V/\varkappa d\phi slowly decreases, but is not yet truly slow-rolling. The supernova bound on the present quintessence equation of state requires either (1) a cosmological constant or other fine-tuned "crawling quintessence" or (2) "roll-over quintessence" that tracked until recently, but now became slow rolling, because of a sharp increase in potential curvature. Thus, fine-tuning is required by constant equation of state and inverse power potentials, but can be avoided by the SUGRA and Skordis-Albrecht potentials and other good trackers, provided quintessence energy domination and slow roll {\em both} began only recently. This makes the time in which we live special in {\em two} respects.Comment: 10 pages, including 2 figures, 3 table

    Vacuum Energy: If Not Now, Then When?

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    We review the cosmological evidence for a low matter density universe and a cosmological constant or dynamical vacuum energy and address the cosmolog$ coincidence problem: why is the matter density about one-half the vacuum energy {\em now}. This is reasonble, following the anthropic argument of Efstathiou and of Martel, Schapiro & Weinberg.Comment: 4 pages (latex

    Gravitational vacuum energy in our recently accelerating universe

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    We review current observations of the homogeneous cosmological expansion which, because they measure only kinematic variables, cannot determine the dynamics driving the recent accelerated expansion. The minimal fit to the data, the flat ΛCDM\Lambda CDM model, consisting of cold dark matter and a cosmological constant, interprets 4Λ4\Lambda geometrically as a classical spacetime curvature constant of nature, avoiding any reference to quantum vacuum energy. (The observed Uehling and Casimir effects measure forces due to QED vacuum polarization, but not any quantum material vacuum energies.) An Extended Anthropic Principle, that Dark Energy and Dark Gravity be indistinguishable, selects out flat ΛCDM\Lambda CDM. Prospective cosmic shear and galaxy clustering observations of the growth of fluctuations are intended to test whether the 'dark energy' driving the recent cosmological acceleration is static or moderately dynamic. Even if dynamic, observational differences between an additional negative-pressure material component within general relativity (Dark Energy) and low-curvature modifications of general relativity (Dark Gravity) will be extremely small.Comment: 3 pages, from Proceedings of the Casimir Workshop, to be published by IOP in Journal of Physics Conference Serie