SUPERNOVA COSMOLOGY PROJECT A WINNER
By observing distant, ancient exploding stars, astronomers at the Institute of Astronomy, Cambridge with colleagues at the Lawrence Berkeley Laboratory, California, and elsewhere, have determined that the universe is expanding at an accelerating rate - an observation that implies the existence of a mysterious, self-repelling property of space first proposed by Albert Einstein, which he called the cosmological constant. This extraordinary finding has been named by the journal 'Science' as Breakthrough of the Year for 1998.
The Supernova Cosmology Project, led at Cambridge by Professor Richard Ellis, Dr Richard McMahon and Dr Mike Irwin, and at Berkeley by Dr Saul Perlmutter, shares the citation with the High-z Supernova Search Team, another international collaboration involving astronomers in Australia, Germany and the USA.
The surprising discovery that the expansion of the universe is accelerating - and hence likely to go on expanding forever - is based on observations of stellar explosions known as type Ia supernovae. These supernovae all have the same intrinsic brightness. This means that their apparent brightness as observed from Earth reveals their distance.
By comparing the distance of these exploding stars with the redshifts of their host galaxies, researchers can calculate how fast the universe was expanding at different times in its history. Good results depend upon observing many type Ia supernovae, both near and far. The Supernova Cosmology Project has fully analysed the first 42 out of more than 80 supernovae it has discovered, and more analysis is in progress.
Type Ia supernovae are rare. In a typical galaxy they may occur only two or three times in a thousand years. And to be useful they must be detected within a week or two of the explosion, while the supernova is still increasing in brightness. Prior to implementation of search techniques developed by the Supernova Cosmology Project during the first five years of its existence, finding supernovae - even those in relatively nearby galaxies - was a haphazard proposition which made it difficult to secure telescope time to observe them.
ORIGIN OF THE SUPERNOVA COSMOLOGY PROJECT
"I first attempted this project in 1986 with a small telescope in Chile" explains Richard Ellis, "After months of effort I found the first distant supernova and this demonstrated the feasibility of the current programme. Today's award from 'Science' comes after over a decade of hard work by a large team of astronomers at Cambridge and Berkeley."
"The most important technical breakthrough was the agreement between Cambridge and Berkeley to employ a new panoramic camera, produced by Perlmutter's group, on UK telescopes in the Canary Islands." adds Cambridge astronomer Richard McMahon. "Our team was then able to develop a new strategy that assured discovery of numerous supernovae 'on demand'."
HOW THE TECHNIQUE WORKS
Project member Mike Irwin explains how the strategy works. "Just after a new moon, when the sky is dark, we make images of 50 to 100 patches of sky. Each contains roughly a thousand distant galaxies. Three weeks later the same patches are imaged again. Supernovae occurring anywhere in these fields show up as bright points of light." In these three weeks, the supernovae typically have not yet reached their brightest moment.
"This guarantees that we will have supernovae to study during the best nights for observation, right before the new moon," says project member Richard McMahon. He adds, "Type Ia supernovae are so similar, whether nearby or far away, that the time at which an explosion started can be determined by simply looking at its spectrum. Type Ia supernovae which exploded when the universe was half its present age behave the same as they do today."
By 1994 the Supernova Cosmology Project had proved repeatedly that, with this search technique, a few nights on the world's best telescopes dependably resulted in many new supernova discoveries. "While some of us are surveying distant galaxies from the Cerro Tololo Interamerican Observatory (CTIO) in the Chilean Andes, or at the Isaac Newton Group telescopes in the Canary Islands, others at home are retrieving the data over the Internet and analysing it to find supernova," says team member Richard Ellis. "Then, with the powerful Keck Telescope in Hawaii we confirm spectra and measure redshifts. We call the Hubble Space Telescope into action to study the most distant supernova, as these require much more accurate measurements than we can get from the ground."
Among the supernovae discovered by the Supernova Cosmology Project are the most distant, and therefore the most ancient, ever seen. In 1997, the Cambridge-Berkeley team announced that a supernova with a redshift of 0.83, equivalent to an age of seven billion years, had been found using the CTIO and Keck telescopes and subsequently observed by the Hubble Space Telescope.
As early as 1994, as their early supernova discoveries began to accumulate, members of the Supernova Cosmology Project developed key analytic techniques that could be used to interpret supernova measurements and thereby determine the cause of the expansion rate of the universe. At the time almost everybody assumed that the universe was slowing down, due to gravity acting on the matter in the universe. The question was how quickly was it slowing? What is the mass density of the universe? And, finally, is there enough mass density to eventually reverse the expansion, leading to a "big crunch" finale for the universe.
EVIDENCE FOR EINSTEIN'S COSMOLGICAL CONSTANT
There was also the possibility, unlikely as it seemed, that some intrinsic property of empty space was in play, something called the cosmological constant - a term originally proposed by Einstein in 1917, in an attempt to balance the equations of General Relativity and preserve a picture of a stable universe that would neither expand nor collapse on itself. A dozen years after Einstein introduced the cosmological constant, astronomer Edwin Hubble found that the universe is indeed expanding; Einstein dismissed his cosmological constant idea as "the biggest blunder of my life."
But observations of distant type Ia supernovae placed them significantly farther away than expected from their redshifts, suggesting that Einstein spoke too soon. Something is pushing everything farther apart faster than it did in the early universe. The cosmological constant is the best candidate.
Thus instead of slowing down, as everybody had expected, the expansion of the universe is in fact speeding up. In early January 1998 the Supernova Cosmology Project presented the first compelling evidence that the expansion of the universe is accelerating and that this acceleration is due to the cosmological constant, known by the Greek letter lambda, which may represent as much as 70 percent of the total mass-energy density of the universe. Subsequently, the High-z Supernova Search Team announced that they had found the same result in their data.
Barring change in the value of lambda - whose exact nature remains a mystery - the universe will expand forever.
"It is important that the High-z team has joined in the quest to learn the nature and fate of the cosmos by studying supernovae," says Richard Ellis. "In major scientific programmes such as this it is important to have two competing teams; it leads to healthy scepticism and a high standard of scientific rigour. The most important conclusion so far is, astonishingly, that both teams agree the Universe is accelerating!"
"We are now searching for more supernovae with high redshifts in order to get more information about the early universe," says team member Richard McMahon. "But, we are also looking for supernovae with low redshifts - nearby supernovae - to make sure that young and old type Ia supernovae are essentially the same, and make for dependable standard candles. We want to be sure we aren't being fooled by interstellar dust dimming the supernovae, or stellar explosions that are somehow weaker in the distant past. So far we haven't found anything to shake our confidence, but this is such an unexpected discovery that we'll keep looking for any loopholes."
Using the world's best telescopes, including the Keck Telescope, the Hubble Space Telescope and the UK's Isaac Newton Group's telescopes on the Canary Islands, the Supernova Cosmology Project continues to pursue studies aimed at confirming these astonishing results.
Contacts for this release:
(All at the Institute of Astronomy, University of Cambridge)