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C. Bryja (CCSF)
We present a cosmological model in which a time-dependent "cosmological constant", \Lambda, arises from virtual gravitons which possess such low energies that their Heisenberg lifetimes exceed the age of the universe. Assuming the wavelengths of gravitons stretch in proportion to the cosmic expansion scale length, there exists a time-dependent energy threshold below which a virtual graviton lifetime would be extended indefinitely in an accelerating universe. Unlike an exponentially inflationary vacuum, the effective equation of state of such infinitely long-lived virtual gravitons (ILVGs) would maintain consistency with the proposed holographic bound on the total entropy contained within the cosmological apparent horizon. Interestingly, the functional time-dependence of the modeled ILVG energy density matches the time-dependence of \Omega - 1 throughout the eras of radiation, matter, and ultimate ILVG dominance. If a graviton is a quantum of spacetime curvature, then we may observe a non-zero \Lambda because cosmic curvature, traced by \Omega - 1, is not zero. In this model, the effective value of \Lambda \propto (\Omega - 1) would have increased until the recent onset of cosmic acceleration and decreased thereafter. Saturation of the holographic entropy bound is shown to be consistent with a universe dominated by a nearly critical density of ILVGs. A postulated equivalence with the saturated holographic bound for black holes then suggests a present-era value of \Lambda(13.7 Gyr) = 1.61 \times 10-35 s-2 if \OmegaILVG = 1. Since the dark energy presently accounts for ~3/4 of the critical density, we should expect to find \Lambda ~1.2 \times 10-35 s-2, in excellent agreement with the observed value. The predicted past variation of \Lambda(t) also lies comfortably within present observational constraints.
A substantial portion of this work was first presented at the 13th General Conference of the European Physical Society in Bern, Switzerland, on July 12, 2005.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.