These snapshots, taken by NASA's Hubble Space Telescope, reveal five supernovae, or exploding stars, and their host galaxies. The arrows in the top row of images point to the supernovae. The bottom row shows the host galaxies before or after the stars exploded. The supernovae exploded between 3.5 and 10 billion years ago. (Credit: NASA; ESA; and A. Riess, STScI)
Observations made by NASA's Hubble Space Telescope of a special type of supernovae contributed to research on the expansion of the universe that today was honored with the 2011 Nobel Prize in Physics.
Adam Riess, an astronomer at the Space Telescope Science Institute and Krieger-Eisenhower professor in physics and astronomy at The Johns Hopkins University in Baltimore, was a member of a team awarded the Nobel Prize in Physics by the Royal Swedish Academy of Sciences. The academy recognized him for leadership in the High-z Team's 1998 discovery that the expansion rate of the universe is accelerating, a phenomenon widely attributed to a mysterious, unexplained "dark energy" filling the universe. Critical parts of the work were done with NASA's Hubble Space Telescope.
Riess shares the prize with Saul Perlmutter, an astrophysicist at the University of California, Berkeley, and the Lawrence Berkeley National Laboratory, whose Supernova Cosmology Project team published similar results shortly after those published by Riess and High-z teammate Brian Schmidt of the Australian National University. Both teams shared the Peter Gruber Foundation's 2007 Cosmology Prize -- a gold medal and $500,000 -- for the discovery of dark energy, which Science Magazine called "The Breakthrough Discovery of the Year" in 1998.
"The work of Riess and others has completely transformed our understanding of the universe," said Waleed Abdalati, NASA chief scientist. "This award also recognizes the tremendous contributions of the technological community that engineered, deployed, and serviced the Hubble Space Telescope, which continues to open new doors to discovery after more than 20 years of peering deep into the universe. With the future launch of the even more powerful James Webb Space Telescope, NASA is ensuring more revolutionary science discoveries like these."
Space Telescope Science Institute director Matt Mountain added, "The power of this discovery is that NASA has kept Hubble going for 20 years. This meant that Adam was able to track the history of the universe using science instruments that were upgraded from one servicing mission to the next. That is why this work has been recognized with the Nobel Prize."
Riess led the study for the High-z Supernova Search Team of highly difficult and precise measurements of objects that spanned 7 billion light years that resulted in the 1998 discovery that many believe has changed astrophysics forever: an accelerated expansion of the universe propelled by dark energy.
"We originally set out to use a special kind of exploding star called 'supernovae' to measure how fast the universe was expanding in the past and to compare it to how fast it is expanding now," Riess recalled. "We anticipated finding that gravity had slowed the rate of expansion over time. But that's not what we found." Instead, Riess' team was startled to discern that the rate of expansion was actually speeding up.
Richard Griffiths, Hubble program scientist in the Astrophysics Division at NASA Headquarters, Washington, said, "The role of the Hubble Space Telescope in this work was to measure how the brightness of some of the most distant supernovae changed over time. This established the acceleration of the universe and by inference that the agent of acceleration is 'dark energy.'"
The importance of Hubble was to obtain images of the high-redshift supernovae of type Ia, exploding white dwarfs that have accreted gas from their companion stars in a binary system and reached a mass limit beyond which they can no longer support themselves against gravity. Since the brightness of these supernovae change with time in a way that correlates with their intrinsic peak brightness, observations of their light can point to how bright, and therefore how distant, their host galaxies are.
The precision of Hubble measurements of the high redshift supernovae, which had been discovered from the ground, was crucial in the demonstration that distant supernovae were fainter than expected, and that the initial deceleration of the universe has astoundingly transformed into an accelerating expansion due to the effects of dark energy.
Although Hubble played a critical role in the discovery of dark energy, nearly every major observatory on Earth also contributed to the study of this mysterious energy. Ground-based telescopes run by the National Optical Astronomy Observatories, especially the 4-meter Blanco telescope at the Cerro Tololo International Observatory in Chile, and at the Kitt Peak National Observatory in Arizona, as well as European telescopes on the Canary Islands, are credited with discovering of the majority of the supernovae ultimately used to track the expansion rate of the universe. The astronomers also used the W. M. Keck Observatory in Hawaii, the MMT Observatory in Arizona, and European Southern Observatory's 3.6-meter telescope in Chile to measure the spectra of the discovered supernovae and the distances of their host galaxies.
"One of the most exciting things about dark energy is that it seems to live at the very nexus of two of our most successful theories of physics: quantum mechanics, which explains the physics of the small, and Einstein's Theory of General Relativity, which explains the physics of the large, including gravity," Riess said.
"Currently, physicists have to choose between those two theories when they calculate something. Dark energy is giving us a peek into how to make those two theories operate together. Nature somehow must know how to bring these both together, and it is giving us some important clues. It's up to us to figure out what [those clues] are saying."
Riess is continuing his Hubble Space Telescope observations of distant supernovae to characterize dark energy. He also is involved in searching for the exploding stars with the Panoramic Survey Telescope and Rapid Response System, a series of ground-based telescopes at the University of Hawaii's Institute for Astronomy. The sky survey is expected to find thousands of new supernovae.
The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center manages the telescope. The Space Telescope Science Institute conducts Hubble science operations and is operated for NASA by the Association of Universities for Research in Astronomy, Inc.