Paper: astro-ph/0605198
Date: Mon, 8 May 2006 14:38:52 GMT (141kb)

Title: Cosmological Acceleration: Dark Energy or Modified Gravity?

Authors: Sidney Bludman

Report-no: DESY 06-063
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We review the evidence for recently accelerating cosmological expansion or
"dark energy", either a negative pressure constituent in General Relativity
(Dark Energy) or modified gravity (Dark Gravity), without any Dark Energy
constituent. If constituent Dark Energy does not exist, so that our universe is
now dominated by pressure-free matter, Einstein gravity must be modified at low
curvature. The vacuum symmetry of any Robertson-Walker universe then
characterizes Dark Gravity as low- or high-curvature modifications of Einstein
gravity. The dynamics of either kind of "dark energy" cannot be derived from
the homogeneous expansion alone, but requires also observing the growth of
inhomogeneities. Present and projected observations are all consistent with a
small fine tuned cosmological constant, with nearly static Dark Energy, or with
gravity modified at cosmological scales. The growth of cosmological
fluctuations will potentially distinguish static "dark energy" from dynamic
"dark energy" with equation of state $w(z)$ either changing rapidly or tracking
the background matter. But to cosmologically distinguish $\Lambda$CDM from
modified gravity will require a weak lensing shear survey more ambitious than
any now projected. Dvali-Gabadadze-Porrati modifications of Einstein gravity
may also be detected in refined bservations in the solar system or at the
intermediate Vainstein scale.

Dark Energy's epicyclic character, failure to explain the original Cosmic
Coincidence ("Why now?") without fine tuning, inaccessibility to laboratory or
solar system tests, along with braneworld theories, now motivate future
precision solar system, Vainstein-scale and cosmological-scale studies of Dark
Gravity.

\\ ( http://arXiv.org/abs/astro-ph/0605198 , 141kb)