Geometry of the Universe

Most astrophysicists today accept an expanding Friedmann model as the most physically plausible representation of the visible universe. The key issue prior to 1980 was whether the Universe is infinite and "open" with a low density of matter, finite and "closed" with a high density of matter (Omega > 1), or perhaps even "critical" with a flat geometry (Omega = 1). A non-zero cosmological constant was then generally discounted. With the advent of the inflationary paradigm proposed by A. Guth in 1980 and with the increasing evidence for dark matter, the theoretically favored geometry has become flat.

Figure 2: Volume and luminosity versus redshift for various cosmological models, relative to a standard model with Omega = 0 and null cosmological constant.

However, the issue of geometry must ultimately be addressed by observations and not theory. Thus far the majority of observational tests (most of them local) continue to support an open Universe; DEEP is an ideal experiment to tackle this question at high redshift.

In particular, DEEP's measurements of internal velocities should help to break this deadlock. Internal velocities are independent of the geometry of the Universe , yet provide a method to separate galaxies of different masses. A typical L* galaxy today has velocity width around 450 km/s for emission lines and velocity dispersion of around 200 km/s for absorption lines. By design, both values are easily measurable with DEIMOS. With such "mass" measures, we can, for the first time, count the numbers of objects of a given mass per unit volume over time. This is the fundamental datum both for the volume test and for measuring the amount of galaxy merging versus time.

November 1999