other NRC committees or advisory groups will address them. The committee calls for three new initiatives—an experiment to map the polarization of the cosmic microwave background, a wide-field space telescope, and a deep underground laboratory. It adds its support to several other initiatives that were previously put forth or set in place and addresses structural issues.

THE ELEVEN QUESTIONS

What Is Dark Matter?

Dark matter dominates the matter in the universe, but questions remain: How much dark matter is there? Where is it? What is it? Of these, the last is the most fundamental. The questions concerning dark matter can be answered by a combination of astronomical and physical experiments. On small astronomical scales, the quantity and location of dark matter can be studied by utilizing its strong gravitational lensing effects on light from distant bright objects and from the distribution and motions of galaxies and hot gas under its gravitational influence. These can be studied using ground-based and space-based optical and infrared telescopes and space-based x-ray telescopes. On larger scales, optical and infrared wide-field survey telescopes can trace the matter distribution via weak gravitational lensing. (Strong gravitational lensing produces multiple images of the lensed objects, while weak gravitational lensing simply distorts the image of the lensed object; see Figure 5.6.) The distribution of dark matter on large scales can be measured by studying motions of galaxies relative to the cosmic expansion. While these observations will measure the quantity and location of dark matter, the ultimate determination of its nature will almost certainly depend on the direct detection of dark matter particles. Ongoing experiments to detect dark matter particles in our Milky Way such as Cold Dark Matter Search II and the US Axion Experiment, future dark-matter experiments in underground laboratories, and accelerator searches for supersymmetric particles at the Fermilab Tevatron or the CERN LHC are all critical. Elements of the program live in the purview of each of three funding agencies, DOE, NASA, and NSF; coordination will be needed to ensure the most effective overall program.

What Is the Nature of the Dark Energy?

There is strong evidence from the study of high-redshift supernovae that the expansion of the universe is accelerating. Fluctuations in the



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