F. Kuik:
Hi,

http://www.nature.com/nsu/030630/030630-7.html

What do you guys think of this theory of this Turner guy?

If you read the article referenced in the url, you'll find the claims
are quite modest and this doesn't represent a "theory", per se. The
authors make a modification to general relativity which essentially gives
predictions which are consistent with observation and other theories,
namely accelerated expansion at early (i.e., inflation) and late times. In
the opinion of the authors, their proposal is a preferable alternative to
dark matter for reasons they specify in their article. High energy
theorists doing supersymmetry would probably disagree, so it's rather
early to draw any conclusions one way or the other. The validty of of
their proposal will depend upon what else comes of it. In particular, the
authors note in their article that many other details remain to be
checked.

Dark matter and this proposal both have theoretical advantages and
disadvantages. Dark matter offers the possibility of a connection between
particle physics and gravity through supersymmetric type theories, but
requires some rather dubious "fine tuning". The theory mentioned in the
url above provides a gravitational reason for the expansion which doesn't
require the same _sort_ of fine tuning. It also doesn't really help much
in consolidating particle physics with gravity and doesn't offer any
compelling reason for the additional term, unless by some chance, the
additional term could be used to establish some deeper connection to
particle physics. All anyone can say, is wait and see.