Lab Scientist: More Evidence for Expanding Universe

January 15, 1999

By Lynn Yarris, lcyarris@lbl.gov

Spinoffs from the Supernova Cosmology Project, named last month as a co-winner of Science magazine's 1998 Breakthrough of the Year, continued to make news last week at the annual meeting of the American Astronomical Society, held in Austin, Texas. Key collaboration member Gerson Goldhaber, a Berkeley Lab physicist, made the case that observations of a stretching of the light curves of distant exploding stars (the more distant the star the greater the stretching) provide direct evidence for an expanding universe.

Gerson Goldhaber of the Laboratory's Supernova Cosmology Project.

Goldhaber also demonstrated how a "time-dilation" factor related to the expansion of the universe serves to refute alternative proposals as an explanation of the high redshifts seen in distant type Ia supernovae.

The Supernova Cosmology Project is an international collaboration headed by Berkeley Lab astrophysicist Saul Perlmutter. This collaboration shared the Science award with another international collaboration, the High-z Supernova Search Team led by Brian Schmidt of Australia's Mount Stromlo and Siding Spring Observatories, for the discovery that the universe is expanding at an accelerating rate. The discovery was made through the observation of type Ia supernovae.

Because all type Ia supernovae have the same intrinsic light curves (a measurement of the rise and fall of their light output), they can act as clocks over cosmological distances. Having traveled a longer time to get here, light from more distant supernovae shows a higher redshift (a shift in spectral lines to longer wavelengths) than light from relatively nearby supernovae.

Redshifts in the spectral lines of distant stars were first observed in 1926 by Edwin Hubble, who attributed the phenomenon to an expansion of the universe. Since then, rival theories have been submitted to explain redshifts without the need for an expanding universe.

In 1995 Goldhaber showed that for a few type Ia supernovae the observation of a time-dilation factor could serve as a direct test of cosmological expansion. Since then, he and his colleagues have measured the light curves of 35 high-redshift supernovae from the more than 80 discovered by the Supernova Cosmology Project collaboration. The data for each of these 35 supernovae were shown to share more than 1400 experimental points along a common light curve when a cosmological expansion factor was taken into account.

"In an expanding universe, the light curve spread should behave the same as the redshift of spectral features," says Goldhaber. "We have concluded that our data are in agreement with this behavior."

Without factoring in cosmological expansion, the light curves of the individual supernovae could not be combined. Goldhaber calls this a "strong argument" against the "tired light" theory, in which the observed redshift in Type Ia supernovae is attributed to the loss of energy by photons as they travel across the universe.

"A mechanism that degrades photon energy, such as tired light, would not affect the spacing between light emitted at different epochs of the light curve, and hence would not affect its shape," Goldhaber says. "Our findings can only be explained by the expansion of the universe."