University of Pennsylvania Physics/Astrophysics Colloquia - ABSTRACT ADDED

Colloquia are preceded by a department tea at 3:30. All talks are given by eminent scientists, renowned for their speaking ability, at a level that is accessible to all first-year graduate students. This is a department-wide event attended by all students, postdocs, and faculty. ABSTRACT: Distance measurements using explosions of white dwarf stars have played a central role in modern cosmology, leading to the remarkable discovery that the universe is presently going through an accelerated expansion. Despite more than a decade of extensive observational and theoretical efforts, the physical origin of this acceleration remains a complete mystery. Candidates include the existence of an unidentified source of a repulsive force by "dark energy" or the possible breakdown of general relativity on cosmological scales. I will discuss the observational evidence for cosmic acceleration and major observational advances that will be made in the near future.