Researchers use supernovae to measure expansion of the universe

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Researchers at the Nearby Supernova Factory, based at the Lawrence Berkeley National Laboratory, have found a more accurate method to measure cosmological distances by using supernovae.

This discovery is another step in understanding the universe’s expansion, which was discovered less than 20 years ago by researchers studying Type Ia supernovae.

The Nearby Supernova Factory, or SNfactory, is an international scientific collaboration developing Type Ia supernovae as tools to study the expansion of the universe and explore dark energy. A supernova is the explosion of a star that releases tremendous amounts of radiation, resulting in a brightness that could rival entire galaxies in the weeks after the explosion.

The particular explosion that births a Type Ia supernova can result either from a single white dwarf — a dying star that is approximately Earth-sized yet dense enough to contain a mass similar to that of our sun — after it borrows mass from a companion star, or from the merging of two orbiting white dwarfs.

This difference in a Type Ia’s birth represents just one of several variations that can result in different brightnesses between these supernovae. The study tested the hypothesis that if the supernovae have highly similar spectra — a display of electromagnetic radiation from an object — then their brightness should also be highly similar.

“We skip over the question of: ‘Why are the brightnesses slightly different and go straight to which have same brightness?’ ” said Greg Aldering, a researcher on the study and the principal investigator at the SNfactory.

Using the “twin method,” researchers reduced this dispersion from 15 percent to 8 percent by identifying supernova “twins” with closely matched spectra.

Astronomers use brightness to determine distances of celestial objects. Kyle Boone, a campus graduate student who worked on the research, compared the supernovae to light bulbs that dim as the distance between them and the observer increases. The measurement of the dimming determines how far away the supernovae are, allowing researchers to map the expansion of the universe.

The method allows the distances to these supernovae to be measured with about twice the accuracy of previous techniques, according to Aldering.

“The significance of reducing the intrinsic brightness dispersion among Type Ia supernovae is that we will be able to more accurately trace the expansion history of the Universe,” said Alex Filippenko, a campus astronomy professor who was a member of both teams that discovered the expansion, in an email.

The Type Ia supernovae may hold the key to understanding dark energy, the unknown force that counteracts gravity to propel the accelerated expansion of the universe.

“We’re simply trying to understand how dark energy behaves, and that will then be a constraint for people understanding what it could be,” Aldering said.

Contact Amelia Mineiro at [email protected].

A previous version of this article incorrectly stated that the universe’s expansion was discovered less than 20 years ago. In fact, the acceleration of the universe’s expansion was discovered less than 20 years ago, but the expansion of the universe was discovered almost 100 years ago.

A previous version of this article also incorrectly stated that campus astronomy professor Alex Filippenko was a member of both teams that discovered the expansion. In fact, he was a member of both teams that discovered that the expansion was accelerating.