**Title:**
The Missing Energy Problem

**Speaker:**
Paul Steinhardt

**Abstract:**
Growing evidence suggests that the matter density of the universe
is significantly less than the critical density and that
the expansion of the universe is accelerating.
If true, this may emerge as one of the most profound and
significant discoveries of cosmology, for it would mean
that some form of exotic missing energy with negative pressure
(i.e., not dark matter) fills the universe.
This talk will discuss
the leading candidates for the missing energy: the
venerable cosmological constant and a recent proposal, quintessence.
The discussion will focus on the theoretical and observational
differences.

**References for students: **

- "Cosmic Concordance and Quintessence," (with L. Wang, R. Caldwell, and J.P. Ostriker), astro-ph/9901388
- "Quintessence, Cosmic Coincidence, and the Cosmological Constant," (with I. Zlatev and L. Wang), Phys. Rev. Lett. 82, 896-899 (1999).
- "The Imprint of Gravitational Waves in Models Dominated by a Dynamical Cosmic Scalar Field," (with R.R. Caldwell) Phys.Rev. D57, 6057-6064 (1998).