One of the most important and surprising scientific discoveries of the late 20th Century was that the cosmological expansion of space is not slowing down, as had been expected due to the gravitational pull of all the matter in the Universe, but rather is increasing with time.  We do not have a fundamental understanding of the root cause of this accelerating expansion. We label our ignorance with the term “Dark Energy.”  Although only definitively identified a dozen years ago, this Dark Energy dominates the energy density of the Universe.  The phenomenon of Dark Energy poses major challenges to our basic understanding of fundamental forces in the Universe. Although modern theories of physics allow for a component with the properties of Dark Energy to exist, the value of the energy density that we observe is many orders of magnitude smaller than predicted by those theories. On the other hand, the incorporation of Dark Energy into our prevailing theory of cosmology has been enormously successful.  Numerous puzzles that plagued this field for many years have now been solved.  For example, with prior cosmological models, the Universe appeared to be younger than its oldest stars.  When Dark Energy is included in the model, that problem goes away.

To make further progress in this field, we must subject our theories to an increasingly precise series of experiments that test both the consistency of the overall framework and constrain the values of the fundamental cosmological parameters.  These involve detailed measurements of the expansion history of the Universe – correlating the absolute distance to various astrophysical systems or the absolute time of their formation with the recession velocity that we can infer from the colors of the light they emit. There are a number of distinct methods that are being invoked to determine these distances and velocities, but all are subject to possible sources of systematic error that may limit the precision we can ultimately achieve.  We are using the Universe as our “laboratory”, but it is not an especially well-controlled experiment.  There are many potential astrophysical complexities that can cloud the interpretation of the results we are trying to achieve.  Ironing out these complexities is the key challenge of modern experimental cosmology.

International communities of scientists – including astronomers, astrophysicists, cosmologists, and experimental and theoretical particle physicists – have banded together to attack this problem, to design future observational probes of Dark Energy, and to offer theoretical explanations that could be tested with these probes. Scientists i