The need for some kind of negative-pressure "dark energy" constituent to reconcile various cosmological measurements creates one of the deepest mysteries of fundamental physics. Either a non-zero cosmological constant, or a scalar field with more complex equation of state, can be made consistent with current observations, but there is currently no physical motivation for either phenomenon at energy scales anywhere close to that observed. Furthermore, these solutions to the dark energy problem are not unique---for example a more substantive alteration to General Relativity could be at work---and in the absence of strongly motivated theoretical options, we must rely on experimental constraints to guide us toward the correct solution. It is remarkable that Nature presents us with an entirely new phenomenon, the dark energy, that is detectable only with measurements on scales approaching the size of our cosmological horizon. This intrigue has put the dark energy high on the list of scientific priorities for the Department of Energy and NASA (see the announcement of the Joint Dark Energy Mission (JDEM), as well as on various inter-agency prioritizations for physics and astronomy (e.g. the Quarks to Cosmos report of the NRC Committee on the Physics of the Universe).

The first strong evidence for dark energy arose from the use of distant Type Ia supernovae (SNe) as standard candles to map the luminosity-distance-vs-redshift relation DL(z) (Perlmutter, S. et al. 1999, ApJ, 517, 565--586 and Riess, A. G., et al. 1998, AJ, 116, 1009--1038). This is equivalent to measuring the expansion history a(t) of the Universe, which is significantly affected by the stress-energy of the dark energy, to the point where the expansion is currently accelerating.

In the past two years it has become apparent, with substantial contribution from Penn physicists, that weak gravitational lensing (WL) measurements can constrain dark energy properties as well as or better than the SNe. Weak lensing is the (usually) subtle displacement and distortion of the images of background objects due to the deflection of light by foreground mass structures. The extent of these distortions depends upon two factors: the size of the mass concentrations in the Universe and the distances between them and the observer as the universe evolves. The WMAP measurements of the cosmic background radiation determine the amplitude of mass fluctuations 300,000 years after the Big Bang. Subsequent growth through gravitational instability depends upon the rate of expansion of the Universe; hence the properties of dark energy, or possible alterations of gravity, affect the WL signal because they change the amplitude G(z) of mass fluctuations throughout the history of the Universe. The lensing signal further depends upon the distances between lens, source, and observer (just like glass lenses). Hence weak-lensing observables are sensitive to both DL(z) and to G(z). The measurement of two distinct functions will provide more discrimination on dark energy theories than would SNe alone: in particular, most alterations to the laws of gravitation would affect the growth of structure (and/or the law of gravitational deflection of light) in a manner distinct from that of a scalar field even if the two theories were degenerate in DL(z). It is clear, therefore, that any complete investigation of dark energy will require an amibitious WL survey as well as SNe data.

The Supernova/Acceleration Probe (SNAP) is a proposed orbiting optical/near-IR imaging and spectrographic telescope dedicated to dark-energy investigations. As suggested by the name, the initial approach was to use high-redshift Type Ia supernovae to map the expansion history of the Universe to high precision. The resultant design---a 2-meter diffraction-limited telescope with maximal field of view tiled with visible and near-IR detectors---is in fact nearly optimal for the measurement of weak gravitational lensing as well. With substantial guidance from Penn faculty, the SNAP program has evolved to include a substantial WL survey along with SNe measurements. These two approaches now combine to make SNAP the leading candidate for the NASA-DOE Joint Dark Energy Mission.

The University of Pennsylvania is an institutional member of the SNAP collaboration. Penn faculty who have contributed significantly to the SNAP effort in the past 2 years include: