Recently, Fred Cooperstock and Steven Tieu of the University of Victoria proposed that general relativity might be able to explain the observed galactic rotation curves without the need for any "exotic dark matter" (see CERN Courier October 2005 p9). Since then, Mikolaj Korzynski of Warsaw University has argued that the gravitational field in the Cooperstock-Tieu solution is generated "not only by the galaxy matter, but by a thin, singular disc as well" and that their model should be considered unphysical. In simple terms, the model implicitly has some extra matter in it.
D Vogt and P S Letelier of the Universidade Estadual de Campinas in Brazil have now analysed the presence of this singular disc, calculating the physical variables of the disc's energy-momentum tensor. They are led to the conclusion that "the disc is made of exotic matter, either cosmic strings or struts with negative energy density". They also point out that even if the Cooperstock-Tieu model has problems, a proper general relativistic solution should be further investigated.
Further reading
Mikolaj Korzynski 2005 www.arxiv.org/abs/ astro-ph/0508377.
D Vogt and P S Letelier 2005 www.arxiv.org/ abs/astro-ph/0510750.