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Research Keynote Address:
Professor of Astronomy
University of California, Berkeley
The measured distances of type Ia (white dwarf) supernovae as a function of redshift (z) have shown that the expansion of the Universe is currently accelerating, probably due to the presence of dark energy (X) having a negative pressure, such as Einstein's cosmological constant (Lambda). Combining all of the data with existing results from large-scale structure surveys, we find a best fit for Omega_M and Omega_X of 0.28 and 0.72 (respectively), in excellent agreement with the values derived independently from WMAP measurements of the cosmic microwave background radiation. A number of possible systematic effects (dust, supernova evolution) do not seem to eliminate the need for Omega_X > 0. Moreover, analyses of very distant supernovae (z = 1.0-1.7) reveal an early epoch of deceleration, followed by acceleration. Several groups have recently measured hundreds of supernovae with z = 0.2-0.8, to determine the equation-of-state parameter of the dark energy, w_X = P/(rho c^2). The best-fit value is w_X = -1, and its first derivative (dw/dz) is consistent with zero, suggesting that the dark energy may indeed be the cosmological constant or something nearly indistinguishable from it. Any viable theory of quantum gravity will need to be consistent with this result.