With the current convergence of
determinations of the Hubble Constant (e.g.
The Extragalactic Distance
Scale, 1997, Livio, Donahue and
Panagia, eds.) to values within ±25% rather than a
factor of two, and the clear possibility of
determining q0 using high
redshift supernovae (Garnavich et al. 1998), the
major remaining problem in observational cosmology
is the determination of Ω — what is the dark
matter, how much is there, and how is it
distributed?
The most direct approach to the last two
parts of the question has been to study galaxy
dynamics, first through the motions of galaxies in
binaries, groups and clusters, and in the last
decade and a half, driven by the observation of
our motion w.r.t. the Cosmic Microwave Background
(CMB) and thenotion that DM must be clumped on
larger scales than galaxy clusters if
(Ω is to be unity,
through the study of large scale galaxy
flows.
The ratio of the mass density to the
closure mass density, Ω, is thought by most
observers to be ~0.1-0.3, primarily based on the
results of dynamical measurements of galaxy
clusters and, more recently, gravitational lensing
studies of clusters. In contrast, most theoretical
cosmologists opt for a high density universe,
Ω = 1.0, based on the
precepts of the inflation scenario, the difficulty
of forming galaxies in low density models given
the observed smoothness of the microwave
background radiation, and the observational
evidence from the matching of the available large
scale flow measurements (and the absolute
microwave background dipole velocity) to the local
density field. However this last result is
extremely controversial—matching the velocity
field to the density field derived from IRAS (60μ)
selected galaxy samples yields high Ω values
(e.g., Dekel et al. 1993) but matching to
optically selected samples yields low values
(Hudson 1994; Lahav et al. 1994; Santiago et al.
1995). On small scales, the high Ω camp argues
that the true matter distribution is much more
extended than the distribution of galaxies, so the
dynamical mass estimates are biased low.
A fast radio burst in the direction of the Virgo Cluster
