PASADENA, Calif.—Based on an ongoing study of exploding stars in the distant universe, astrophysicists have concluded that the effect of the "dark energy" that is speeding up the expansion of the universe is within 10 percent of that of Albert Einstein's celebrated cosmological constant. Cosmologists regard this result as a major step forward in understanding the nature of this mysterious property of the universe.
Reporting in an upcoming issue of the journal Astronomy and Astrophysics, an international team using a variety of instruments, including the 10-meter Keck telescopes, show the extent to which supernovae that erupt across the universe compare to those closer to home. Measuring the receding motion of supernovae at great distances has been intensely investigated since 1998, when researchers discovered that supernovae of a given recessional velocity seem to be fainter than they would be if the expansion of the universe were slowing down. This result, which has been observed consistently for the last eight years, strongly implies that the expansion rate of the universe is increasing.
The cause of this acceleration may be some form of exotic energy that causes space to push outwards. Einstein originally proposed a mathematical fudge-factor he called the cosmological constant that would preserve the notion of a universe with no beginning and no end. But when Edwin Hubble demonstrated that the universe was expanding, Einstein abandoned the cosmological constant as his "biggest blunder."
The best way to study the dark energy, whatever it is, continues to be faraway supernovae, says Richard Ellis, the Steele Family Professor of Astronomy at the California Institute of Technology and one of the authors of the paper. "Improved observations of distant supernovae are the most immediate way in which we can learn more about the mysterious dark energy," Ellis says. "The present study is a very big step forward in quantity and quality and amazingly suggests that Einstein was pretty close to the mark."
The research project is known as the Supernova Legacy Survey (SNLS), which aims to discover and examine 700 distant supernovae to map out the history of the expansion of the universe. The survey confirms earlier discoveries that the expansion of the universe proceeded more slowly in the past and is speeding up today. However, the crucial step forward is the discovery that Einstein's 1917 explanation of a constant energy term for empty space fits the new supernova data very well.
The current paper is based on about one-tenth of the imaging data that will be obtained by the end of the survey. Future results are expected to double or even triple the precision of these findings and conclusively solve several remaining mysteries about the nature of dark energy.
"The significance is huge," said Professor Ray Carlberg, of the department of astronomy and astrophysics at the University of Toronto. "Our particular observation is at odds with a number of theoretical ideas about the nature of dark energy. They generally predict that it should change its form as the universe expands, and as far as we can see, it doesn't."
According to Carlberg, the findings suggest that if a human being were to stand on the surface of Earth when the universe is 10-to-20 times its current age and look up at the night sky, most of the galaxies that we take for granted will be so far away that they'll be virtually invisible, with perhaps only one galaxy in our visible universe.
The researchers located distant supernovae using an innovative, 384-million pixel camera called MegaCam, built by the Commissariat à l'Energie Atomique, a unit of the French atomic energy agency. "Because of its wide field of view—you can fit four moons in an image—it allows us to measure simultaneously several supernovae, which are rare events," said lead investigator Pierre Astier, a researcher at the Centre National de la Recherche Scientifique (CNRS).
Ellis contributed a critical piece to this work, using spectrographs mounted on the 10-meter Keck telescopes on Mauna Kea in Hawaii. "Representative supernovae from the program have been examined more closely using the giant aperture of the Keck telescope," he says. "I find these distant supernovae are strikingly similar to those seen locally, validating their use as cosmic yardsticks and hence strongly supporting our scientific conclusions."
The Supernova Legacy Survey is a collaborative international effort that uses images from the Canada-France-Hawaii Telescope, a 3.6-metre telescope atop Mauna Kea, a dormant Hawaiian volcano. Over nearly 500 nights of observing time, the researchers identified a few dozen bright pixels of distant supernovae, then examined their spectra using some of the largest telescopes on earth, including the Keck telescopes and the Frederick C. Gillett Gemini North Telescope on Mauna Kea, the Gemini South Telescope on the Cerro Pachón mountain in the Chilean Andes, and the European Southern Observatory Very Large Telescope (VLT) at the Paranal Observatory in Atacama, Chile.
The research was funded by the Canada-France-Hawaii Telescope, the French agency Commissariat à l'Energie Atomique (CEA), Centre National de la Recherche Scientifique (CNRS), Institut National des Sciences de l'Univers du CNRS, the Natural Sciences and Engineering Research Council of Canada, the National Research Council of Canada's Herzberg Institute of Astrophysics, the Gemini Observatory, the W.M. Keck Observatory, and the Very Large Telescope Project.
Written by Robert Tindol