Astronomers from the United States and Europe have used a gravitational lens – a distant, light-bending clump of dark matter – to make a new estimate of the Hubble constant, which determines the size and age of the universe. The Hubble constant previously has been calculated by using NASA's Hubble Space Telescope to look at distant supernovae, and by measurements of the cosmic microwave background –radiation leftover from the Big Bang, according to Chris Fassnacht, associate professor of physics at the University of California, Davis. The new method provides an independent check on the other two, he said. In this image of a gravitational lens, objects A, B, C and D are all images of the same background object, distorted by the lens. G1 and G2 are two galaxies within the lens itself. (Photo: Hubble Space Telescope) A gravitational lens is a distant object, such as a galaxy surrounded by dark matter, that exerts a gravitational pull on light passing through it. Other galaxies behind the lens, from our point of view, appear distorted. In the case of the object B1608+656, astronomers on Earth see four distorted images of the same background object. Fassnacht began studying B1608+656 as a graduate student a decade ago. Because the mass distribution of the lens is now well understood as a result of recent Hubble Space Telescope observations, it is possible to use it to calculate the Hubble constant, he said. It works something like this: Two photons leave the background galaxy at the same time and travel around the lens, their paths distorted in different ways by the gravitational field so that they arrive on Earth at slightly different times. Based on that time delay, it is possible to calculate the distance of the entire route, and then infer the Hubble constant. The timing is set by waiting for a change in the background object – for example, for it to become more luminous. If the travel times are slightly different, the different images of the background object will seem to brighten at slightly different times. Gravitational lensing has never before been used in such a precise way, said co-author Philip Marshall of the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at the U.S. Department of Energy’s SLAC National Accelerator Laboratory and Stanford University. Several groups are now working on extending the technique with other gravitational lenses. The study, which appears in the March issue of The Astrophysical Journal, was led by Sherry Suyu of University of Bonn, Germany. Besides Marshall and Fassnacht, the coauthors include Stefan Hilbert of the University of Bonn; Matthew Auger and Tommaso Treu of the University of California, Santa Barbara; Roger Blandford of KIPAC and Stanford University; and Leon Koopmanns of Kapteyn Astronomical Institute, The Netherlands. For more information, visit: www.ucdavis.edu