Saul Perlmutter, Brian Schmidt and Adam Riess have been awarded the 2011 Nobel Prize in Physics “for the discovery of the accelerating expansion of the universe through observations of distant supernovae”. Perlmutter, professor of astrophysics at the Lawrence Berkeley National Laboratory and University of California, Berkeley, receives half of the prize, with the other half being shared between Schmidt, distinguished professor at the Australian National University, and Riess, professor of astronomy and physics at Johns Hopkins University and the Space Telescope Science Institute. Their finding led to a dramatic change in perception of the universe by providing evidence for what has become known as “dark energy”.
In 1997 the Supernova Cosmology Project (SCP), led by Perlmutter and the High-z Supernova Search Team, led by Schmidt, were working independently on observations of distant Type 1a supernovae, using them as “standard candles” to measure cosmological distances as a function of time. (All such supernovae have similar intrinsic brightness, so their apparent brightness gives a measure of distance.) They expected to find evidence for a gradual slowdown in the expansion of the universe, resulting from the influence of gravity on the matter it contains.
Instead, the measurements revealed around 50 distant supernovae that appeared to be dimmer than predicted by calculations based on the gravitational effects of matter. In 1997 Gerson Goldhaber – well known in the particle-physics community – was the first person in the SCP team to notice the unexpected effect while plotting the brightness against redshift for Type Ia supernovae that the project had discovered. The same year, Adam Riess, then a research fellow at UC Berkeley who was leading an analysis of supernovae detected by the High-z project, uncovered a similar effect.
The observations pointed to the surprising conclusion that the expansion of the universe is not slowing under the influence of gravity, but is instead accelerating. This in turn implies the existence of some form of gravitationally repulsive “substance”, uniformly distributed across the universe, which counteracts the gravitational attraction of matter. This unknown substance has become known as “dark energy” (CERN Courier September 2003 p23).
The two teams published their results in 1998–1999 and since then their findings have been confirmed not only by further observations of supernovae but also by detailed measurements of fluctuations in the cosmic microwave background radiation and of baryon acoustic oscillations, i.e. clustering of baryonic matter in the early universe that also serves as a “standard ruler” for cosmological distance scales. All of the evidence suggests that dark energy contributes as much as 73% of the mass-energy content of the universe, with 23% from dark matter and only about 4% from normal baryonic matter – but the nature of both dark matter and dark energy remains unknown.