Johns Hopkins University
Spring/Summer 2007
Vol. 4, No. 2


Informal Workers, Unite

Multiple Tombs Point to Existence of Royal Cemetery

Movies, Language, and Makeup

> Dark Energy Turns out to be Old News

Promising Student Research Funded Through Olton Award

Fish Shed Light on How We Move


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Dark Energy Turns out to be Old News

Host Galaxies of Distant Supernovae Photo Credit: NASA, ESA, and A. Riess (STScI)


Using NASA’s Hubble Space Telescope, researchers have discovered that dark energy, a mysterious repulsive force that makes the universe expand at an ever-faster rate, is not new but rather has been present in the universe for most of its 13-billion-year history.

A team led by Adam Riess, a professor in the Krieger School’s Henry A. Rowland Department of Physics and Astronomy, found that dark energy was already accelerating the expansion of the universe at least as long as 9 billion years ago. This picture of dark energy would be consistent with Albert Einstein’s prediction, nearly a century ago, that a repulsive form of gravity emanates from empty space. The team’s findings were published in the Feb. 10 issue of Astrophysical Journal.

“Although dark energy accounts for more than 70 percent of the energy of the universe, we know very little about it, so each clue is precious,” says Riess, also a Space Telescope Science Institute researcher. In 1998 he led one of the first studies to reveal the presence of dark energy.

Riess and team’s newest findings are based on an analysis of the 24 most distant supernovae known, most found within the last two years. Only Hubble can measure these supernovae because they are too distant, and therefore too faint, to be studied by the largest ground-based telescopes.

Hubble’s new evidence is important, because it will help astrophysicists start ruling out competing explanations that predict that the strength of dark energy changes over time, Riess says.

In addition, the researchers found that the exploding stars, or supernovae, used as markers to measure the expansion of space today look remarkably similar to those that exploded 9 billion years ago and are just now seen by Hubble. This is an important finding, say researchers, because it gives added credibility to the use of these supernovae as tools for tracking the cosmic expansion over most of the universe’s lifetime.