< The 570-Megapixel camera forming the heart of the Dark Energy Survey experiment
The observation that the expansion of the universe is speeding up ("accelerated expansion") creates the need for some kind of negative-pressure constituent to reconcile various cosmological measurements. The generic name for such stuff is "dark energy" and its existence (which is still not firmly established) constitutes one of the deepest mysteries of fundamental physics. Either a non-zero cosmological constant, or a scalar field with a 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 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 dark energy high on the list of scientific priorities for the Department of Energy, NASA and the National Science Foundation. This has been recognized for some time and is exemplified by 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) and most recently by the Decadal Survey of Astronomy and Astrophysics. The executive summary of this report lists the Large Synoptic Survey Telescope (LSST) as the top priority for new ground-based observatories over the next decade and a space-based widefield telescope (WFIRST) as the top priority for space missions costing more than $1 billion. Penn scientists expect to play leading roles in LSST and possibly in WFIRST as well. These are so-called Stage 4 experiments, meant to elucidate the properties of dark energy to the systematic error limits we can currently foresee as allowable by the universe. A near-term or Stage 3 experiment, is the Dark Energy Survey or DES. Penn is an institutional member of DES and contributing heavily to the software pipelines for weak lensing and supernovae identification.

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.

Over the past 5 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. Both the DES and LSST will perform weak lensing measurements in the quest to understand the dark energy equation of state and its evolution.

Penn faculty who are part of the dark energy experimental efforts are