Shedding Light on the Nature of Dark Energy June 7, 2001 A team of scientists has suggested tests that should be able to shed light on the mysterious dark energy that is thought to be causing the expansion of the universe to accelerate. The tests should determine whether the present epoch is a special time in the history of the universe, or whether extra dimensions of space or modifications of the theory of gravity are required. As presented today by Kevork Abazajian of the University of California, San Diego (UCSD) at the summer meeting of the American Astronomical Society in Pasadena, CA, observations of distant supernovas, quasars and clusters of galaxies can be used to determine whether the simplest model for dark energy is correct, or radical departures are needed. Some of these observations are possible with today's technology, whereas others may require specially designed space observatories. Observations of distant supernovas, the cosmic microwave background, and the distribution of galaxies all point to the existence of significant amounts of dark energy in our universe. This dark energy contributes a majority of the mass-energy, and causes an acceleration of the expansion of the universe in recent times. The mystery, in addition to the unknown nature of this dark energy, is why the dark energy is just starting to be effective in the present epoch. If the theoretical amount of dark energy is tuned to fit the observations, then dark energy was negligible for essentially all of the previous history of the universe, but for the indefinite future all other forms of energy will become negligible and the universe will undergo an ever more rapid expansion. "The good news is that simple models with dark energy can provide an excellent fit to many observations", said Abazajian. "The bad news is that these models give no explanation of the implied cosmic coincidence that we are living in a very special time in the history of the universe." "It is possible to get around the coincidence problem by modifying the theory of gravity, or invoking extra dimensions", explained Neal Dalal, a coauthor who is also from UCSD. "The tests we have proposed can give us insight into whether these far-out ideas are worth pursuing." In particular, Abazajian, Dalal and their colleagues show that effects of dark energy on the evolution of the universe can be assessed through observations of the dimming of supernovas with distance, the apparent angular separation of intergalactic clouds in front of quasars, (extremely bright cores of galaxies that can be seen at great distances), and the rate of formation of giant clusters of galaxies with cosmic time. Present supernova surveys, together with data being accumulated by the Sloan Digital Sky Survey, and deep X-ray surveys by NASA's Chandra X-ray Observatory and the European XMM-Newton X-ray observatory may provide enough information to confirm or deny the simplest dark energy model. The proposed Supernova Acceleration Probe, (SNAP), an optical infrared space telescope, would make an even more definitive test. Other members of the UCSD team are Elizabeth Jenkins and Aneesh Manohar, also of UCSD. This research was supported by the U.S. Department of Energy, NASA, and the ARCS foundation. Science Contacts: Kevork Abazajian (858) 534-5687 kev@physics.ucsd.edu Neal Dalal (858) 534-5687 endall@physics.ucsd.edu To read more about cosmic coincidences and dark energy see the Physical Review Focus article: "A Cosmic Yardstick" CASS/UCSD