Supernovae survey provides new clues to nature of mysterious dark energy

Measurements of 11 exploding stars spread throughout the visible universe made by the Hubble Space Telescope confirm an earlier, ground-based study which produced the first evidence that the universe is not only expanding, but expanding at an increasing rate.

The new study, which has been posted online [http://www.arxiv.org/abs/astro-ph/0309368] and will soon appear in the Astrophysical Journal, also provides some tantalizing new insights into the nature of the mysterious repulsive force, dubbed dark energy, that appears to be propelling this run-away expansion.

“As far as the ultimate fate of the universe goes, the most straightforward conclusion is that over the next few billion years it is going to become an increasingly thin, cold and boring place,” says Robert Knop, the assistant professor of physics and astronomy at Vanderbilt University who led the analysis of the supernova data for the Supernova Cosmology Project (SCP), an international collaboration of 48 scientists directed from Lawrence Berkeley National Laboratory in California.

Using the Hubble Space Telescope, Knop and his colleagues measured the light curves and spectra of a special kind of exploding star, called a Type 1A supernova, that occurs in binary star systems made up of a normal star and a collapsed star called a white dwarf. Basically, the white dwarf pulls material from its companion until it reaches a critical size, at which point it is consumed in a giant thermonuclear explosion. Astronomers consider Type 1A supernovae to be so similar that their brightness provides a dependable gauge of their distance and so bright they are visible billions of light years away.

Knowing this, astronomers can get a good estimate of the distance of a Type 1A supernova by comparing its brightness curve with those of comparable stellar outbursts that have taken place nearby: The dimmer the image the greater the distance. Because it takes light time to travel these great, intergalactic distances, as astronomers look farther out into the universe they are also looking back in time. So the estimates of the distances of the supernovae also provides their approximate ages as well. By measuring the extent to which the light spectrum of each of these images has been shifted to longer, redder wavelengths – a phenomenon called redshift – the astronomers can determine how much the universe has expanded since the time when the star exploded. As the universe expands, the wavelength of light is stretched right along with the fabric of the space through which it is traveling. (For relatively nearby “local galaxies,” this redshift looks just like the Doppler shift produced by the velocity at which these galaxies are moving away from our galaxy.)

By comparing the redshifts and look-back times of the supernovae, the astronomers can measure the rate at which the universe is expanding. The fact that the exploding stars are dimmer and older than expected based on their redshift indicates that the universe is expanding at an increasing rate, something like raisins in a loaf of raisin bread that is rising faster and faster. The new study reinforces the initial discovery made five years ago that the expansion rate of the universe appears to be speeding up, rather than slowing down as most scientists had expected. The discovery was made independently by the Supernova Cosmology Project and a competing group, the High-Z Supernova Search Team.

One of the most serious criticisms of the initial studies was the possibility that dust from the distant galaxies may have dimmed the images of the supernovae enough to skew their results. This is called the “host-galaxy extinction hypothesis.”

The initial studies were done using data from supernovae obtained primarily with ground-based telescopes. Because the supernovae images obtained by the Hubble Space Telescope (HST) are unaffected by the Earth’s atmosphere, they are not only sharper and stronger than those taken from the ground, but also their colors are more accurate. The improved color measurements provided the scientists with a more stringent test of the host-galaxy extinction problem. In addition to absorbing and scattering the supernovae’s light, the galactic dust should also make a supernova’s light redder, much as the sun looks redder at sunset because of dust in the atmosphere. Because the Hubble data show no anomalous reddening with distance, Knop says, the supernovae “pass the test with flying colors.”

“Limiting such uncertainties is crucial for using supernovae – or any other astronomical observations – to explore the nature of the universe,” says Ariel Goobar, a member of SCP and a professor of particle astrophysics at Stockholm University in Sweden. The extinction test, says Goobar, “eliminates any concern that ordinary host-galaxy dust could be a source of bias for these cosmological results at high-redshifts.”

The new analysis also provides tighter estimates of the relative density of matter and dark energy in the universe. Using straightforward assumptions, the initial studies estimated that the composition of the cosmos is 63 to 80 percent dark energy and 20 to 37 percent matter of all types. The new study narrows this range to 68 to 81 percent dark energy and 19 to 32 percent miscellaneous matter. In addition, the new data provides a more accurate measure of just how effective dark energy is at pushing the universe apart.

Among the numerous attempts to explain the nature of dark energy, some are allowed by these new measurements — including the cosmological constant originally proposed by Albert Einstein — but others are ruled out, including some of the simplest models of the theories known as quintessence.

The current study points the way to the next generation of supernova research: In the future, the SuperNova/Acceleration Probe, or SNAP satellite, is being designed to identify thousands of Type 1A supernovae and measure their spectra and their light curves from the earliest moments, through maximum brightness, until their light has died away. Saul Perlmutter, the astrophysicist at the Berkeley Lab who heads up the Supernova Cosmology Project, is leading an international group of collaborators who are developing SNAP with the support of the U.S. Department of Energy’s Office of Science.