Bright Lights, Dark Energy, and a Quite Curious Coefficient:  Thermonuclear
Supernovae and the Equation of State for the Universe

Mike Guidry
Department of Physics and Astronomy
University of Tennessee

Physics Division
Oak Ridge National Laboratory

An equation of state is a relationship among thermodynamic variables that
typically goes beyond the information supplied by thermodynamics alone.  What
then is the equation of state for the Universe as a whole?  We believe that this
most profound of questions has a conceptually simple answer:  the pressure of
the Universe is proportional to its energy density. Causality arguments require
that the coefficient of proportionality be less than or equal to 1, but it is
only within the past decade that observations have begun to close in on the
actual value of this elusive coefficient.  These observations indicate that the
Universe is permeated by a mysterious "dark energy" causing the expansion of the
Universe to accelerate.  This requires the coefficient to have a value less than
-1/3 (which is most curious, since this means that the Universe on large scales
has an equation of state fundamentally different from any ever measured in a
local laboratory within that Universe!).  To constrain it further requires
parallel improvements in observational technology and the theoretical
understanding of those observations.  The key tool is comparison of observed
brightness with expected brightness for some of the most luminous objects in the
sky, Type Ia (thermonuclear) supernovae.  I will provide an overview of these
issues that is accessible to non-astronomers, and describe our own efforts to
contribute through a more fundamental understanding of the Type Ia mechanism.