From Osher Doctorow
Why did I reach back to 2005 in Section 20.1 of this thread? Well,
it's because I was reading a paper of 1 Sep 2006 by Jia-Yu Tang, Jochen
Weller, and Alan Zablocki of University College London, "Probing
modified gravity by combining supernovae and galaxy cluster surveys,"
astro-ph/0609028 v1 1 Sep 2006.
Tang et al point out that there are three posible approaches to
acceleration of the Universe: late-time domination of an addition
component in the energy-momentum tensor, modification of the Einstein
Equation(s) on large scales, and large scale inhomogeneity of the
Universe. They emphasize the second approach and cite Carroll et al
(2005) as one of their main references for this approach, although they
use a particular model that isn't the same as Carroll et al's, namely
the Dvali-Gabadadze-Porrati (DGP) model. The latter model is a
braneworld model with a large flat extra dimension and "leakage" of
gravitational degrees of freedom off the brane causing changes in
cosmological evolution. The simplest of these models' modified
Friedman equation is quadratic in the Hubble parameter H or H_DGP with
the non-Hubble term a linear function of energy density of the
constituents of the Universe which is set equal to the matter density
for late times.
Osher Doctorow
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