Cosmic Expansion: Supernovas shed light on dark energy

Eight years ago, astronomers made an astonishing discovery: The rate at which the universe is expanding, assumed to have been steadily slowing since the Big Bang, is in fact speeding up. The entity revving up cosmic expansion remains elusive (SN: 5/22/04, p. 330: Dark Doings), but scientists have dubbed it dark energy. This week, a team reported gains in its efforts to understand and describe it.

SUPERDATA. Astronomers studied 71 type 1a supernovas to shed new light on dark energy. Here, the supernova is the right-hand white spot inside the box to the left of the central bright star. CFHTLS, SNLS, Terapix

The work provides new hints that dark energy might be distributed uniformly throughout space and time (SN: 2/28/04, p. 132: Available to subscribers at Wrenching Findings: Homing in on dark energy). It further suggests that dark energy closely resembles the cosmological constant that Albert Einstein introduced into his theory of gravity in 1917 but quickly abandoned. In Einstein’s theory, the constant can either add to or oppose gravity’s tug.

Like the initial discovery of cosmic acceleration, the new study relies on light detected from type 1a supernovas: elderly stars that die explosive deaths. Isobel Hook of the University of Oxford in England and her colleagues identified 71 of these supernovas in images taken by the Canada-France-Hawaii Telescope atop Hawaii’s Mauna Kea. Using several of the world’s biggest telescopes, the researchers obtained spectra of the supernovas, confirming their identity and indicating how long ago they exploded.

Hook reviewed the team’s findings on Dec. 11 at a cosmology meeting in Chicago honoring the late astrophysicist David Schramm. Details of the observations, from the first year of a 5-year study called the Supernova Legacy Survey, appear in an upcoming Astronomy & Astrophysics.

Type 1 supernovas serve as markers for cosmic expansion because each of them has roughly the same intrinsic brightness, as lightbulbs of the same wattage do. That enables astronomers to calculate the body’s distance from Earth. Furthermore, a spectrum of the light emitted by a supernova reveals how fast its host galaxy was receding at the time the star exploded. With this information, astronomers can reconstruct how fast the universe was expanding at different times during its history.

Analyzing their 71 supernovas, Hook and her colleagues found that dark energy affects cosmic expansion as a cosmological constant would. However, that conclusion depends on some simplifying assumptions. For instance, the researchers adopted a model in which the composition of dark energy doesn’t vary over time. However, several models predict some variation.

The new data are important, “but the types of analyses that we’re doing of dark energy are very simplistic,” comments theorist Josh A. Frieman of the Fermi National Accelerator Laboratory in Batavia, Ill., and the University of Chicago. At the meeting, Frieman announced that his own team had just identified 139 type 1a supernovas using data from the Sloan Digital Sky Survey.

By comparing hundreds of supernovas at earlier and later times, researchers should soon be able to test more-realistic models of dark energy than they’ve had before. That improvement will shed new light on dark energy’s true nature, Frieman says.

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