Refined Hubble Constant Narrows Possible Explanations For Dark Energy

Office of News and Information - Johns Hopkins University - 901 South Bond Street, Suite 540 - Baltimore, Maryland 21231 - Phone: 443-287-9960 - Fax: 443-287-9920 May 7, 2009 - FOR IMMEDIATE RELEASE - JH Media Contact: Lisa De Nike - 443-287-9960, Lde [a] jhu (p) edu - Space Telescope Science Institute Contact: Ray Villard - 410-338-4514, villard [a] stsci (p) edu Whatever dark energy is, explanations for it have less wiggle room following a Hubble Space Telescope observation that has refined the measurement of the universe's present expansion rate to a precision where the error is smaller than five percent. The new value for the expansion rate, known as the Hubble constant, or Ho (after Edwin Hubble who first measured the expansion of the universe nearly a century ago), is 74.2 kilometers per second per megaparsec (error margin of ± 3. The results agree closely with an earlier measurement gleaned from Hubble of 72 ± 8 km/sec/megaparsec, but are now more than twice as precise. The Hubble measurement, conducted by the SHOES (Supernova Ho for the Equation of State) Team and led by Adam Riess, a professor in the Henry A. Rowland Department of Physics and Astronomy at Johns Hopkins University’s Krieger School of Arts and Sciences and at the Space Telescope Science Institute, uses a number of refinements to streamline and strengthen the construction of a cosmic "distance ladder," a billion light-years in length, that astronomers use to determine the universe's expansion rate.