Bright Lights, Dark Energy, and an 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.  The ideal gas law, which follows from making particular assumptions about the microscopic nature and interactions of the particles in a gas, is a familiar example. If we view the entire Universe as a thermodynamic system, what is its equation of state?  There are fundamental reasons to 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 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 that the coefficient be less than -1/3 (most curious, since it implies that the Universe as a whole has an equation of state fundamentally different from any ever measured in a laboratory), but to constrain its value further requires parallel improvements in observational technology and in our theoretical understanding of those observations.  The key tool is comparison of  observed brightness with expected brightness for some of the brightest lights in the sky, Type Ia (thermonuclear) supernovae.  I will provide an overview of these issues accessible to non-astronomers, and describe our own efforts to contribute through an improved theoretical understanding of the Type Ia mechanism.