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The reason for the relatively long break in postings on this Blog is that I have been studying some new developments in cosmology. Here is the preliminary 'result' of this study, with updates to come...
The standard cosmological model assumes large-scale homogeneity and then uses a simplified solution to Einstein's field equations to find the equations of motion. This lead to the so-called Friedmann-Lemaître-Robertson-Walker (FLRW) metric and the present 'best-buy' Lambda-Cold-Dark-Matter (LCDM) cosmic model.
The 'Lambda' indicates the cosmological constant or more generally, 'dark energy', a mysterious concept that contributes to the gravity that constrains the expansion of the universe, but has negative pressure that causes the apparent expansion rate to increase. I say 'apparent', because the increase is not directly measured, but inferred from the distance/luminosity ratio of supernovae type SN1a.
Fig. 1:[1] 
From this SN1a data, an expansion curve like the solid line shown if Fig. 1 results. Without dark energy, an expansion curve like the dotted line was expected (that was before 1998).
Nowadays there are more than one rival models trying to get rid of the dark energy. One 'promising' view is that of David Wiltshire of New Zealand[2]. He is one of a few that have tackled the issue that the Universe is not homogeneous, head-on. Wiltshire's approach is deceptively simple: since we are sitting inside a gravitationally bound structure with a definite deviation from the general large-scale homogeneity, we observe the distant universe with a 'bias'.
The bias has to do with the relativistic fact that clocks inside our Galaxy must run slower than hypothetical clocks inside the large voids (empty regions) of the universe. Wiltshire proposes a way to average these clock rates (actually he does a bit more averaging than just clock rates) and showed that this enables a quite different model to fit all the observational data – and it does so without dark energy! To make things even more interesting, he claims to have solved some of the 'paradoxes' that plaque the standard model, e.g., the so-called "Sandage–de Vaucouleurs paradox"[3].
The relevant papers are new, arXiv dated 1 Nov 2007, so the jury is still out on them. If proven even close to valid, this may revolutionize cosmology for decades to come.
Jorrie
[1] Figure from Relativity 4 Engineers, chapter 16, where more details on the curve is located. (Free chapter download available)
[2] http://arxiv.org/abs/0709.0732: "Exact solution to the averaging problem in cosmology" is a preprint of a short Physics Review type letter, which summarizes and references the more complete papers.
[3] Quote from Wiltshire's full paper: "This puzzle, the Sandage-de Vaucouleurs paradox, arises since we expect that the statistical scatter in peculiar velocities of galaxies as a fraction of their recession velocity should be large until the scale of homogeneity is approached. In fact, on the scale of 20 Mpc, of order 10% of the scale of homogeneity, the scatter ought to be so large that no linear Hubble flow should be derivable, statistically speaking. Yet, 20 Mpc is the local scale over which Hubble originally obtained his famous linear law. By conventional understanding this does not make sense."
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Re: Cosmic Clocks
