The Expansion Rate and the Size of the Universe; Magnificent Cosmos; Scientific American Presents; by Freedman; 6 Page(s)

Our Milky Way and all other galaxies are moving away from one another as a result of the big bang, the fiery birth of the universe. As we near the end of the millennium, it is interesting to reflect that during the 20th century, cosmologists discovered this expansion, detected the microwave background radiation from the original explosion, deduced the origin of chemical elements in the universe and mapped the largescale structure and motion of galaxies. Despite these advances, elementary questions remain. When did the colossal expansion begin? Will the universe expand forever, or will gravity eventually halt its expansion and cause it to collapse back on itself?

For decades, cosmologists have attempted to answer such questions by measuring the universe's size-scale and expansionrate. To accomplish this task, astronomers must determine both how fast galaxies are moving and how far away they are. Techniques for measuring the velocities of galaxies are well established, but estimating the distances to galaxies has proved far more difficult. During the past decade, several independent groups of astronomers have developed better methods for measuring the distances to galaxies, leading to completely new estimates of the expansion rate. Recently the superb resolution of the Hubble Space Telescope has extended and strengthened the calibration of the extragalactic distance scale, leading to new estimates of the expansion rate.