Volume 1, Issue 5
Supernovas Illuminate Dark Energy
Einstein called it his biggest blunder. His early equations for general relativity accounted for a strange cosmic antigravity precisely balancing the attractive force of gravity. But after Edwin Hubble discovered that the universe is expanding, Einstein yanked the mysterious force from his equations. Several years ago though, UC Berkeley astronomer Alex Filippenko helped prove that Einstein was right after all. In fact, the magnitude of the antigravity effect is greater than Einstein had even thought. The universe is not only expanding, but it's speeding up every day. Why? And how fast? Filippenko is now asking those questions, using exploding stars as his cosmic mile markers.
A recipient of UC Berkeley's two most coveted teaching awards, Alex Filippenko is also the co-author of the introductory astronomy textbook, "The Cosmos: Astronomy in the New Millennium." (photo by Jock McDonald)
Filippenko is a world-renowned hunter of supernovas, massive stars that die a violent death. Because of their power, supernovas are much more visible to observers compared to other objects in distant space. In September, Filippenko used the NASA Hubble Space Telescope to image the brightest and closest supernova of the decade, just 11 million light-years from Earth and glowing with the intensity of 200 million suns.
Classified by the process by which they die, Type Ia supernova are the most commonly observed, primarily because they are the most powerful of the dying stars. Filippenko and his colleagues have found several hundred of the objects. By measuring a supernova's brightness and determining its true power through various observational techniques, scientists can determine its distance from Earth.
"It's like looking at the label of a lightbulb to see how powerful it is," Filippenko says.
A star explodes (arrow) in a galaxy 11 million light years away. The heart of the galaxy, NGC 2403, is the glowing region at lower left. Sprinkled across the region are pink areas of star birth. The myriad of faint stars visible in the Hubble image belong to the galaxy, but the handful of very bright stars in the image belong to our own Milky Way and are only a few hundred to a few thousand light-years away. (Photo courtesy NASA, ESA, A.V. Filippenko [UC Berkeley], P. Challis [Harvard-Smithsonian Center for Astrophysics], et al.)
Once a supernova's distance is determined, it can also provide clues to the rate of cosmic expansion. As objects recede from the Earth, the wavelengths of the light they emit lengthen, shifting into the red part of the spectrum. Measuring a supernova's "redshift" indicates its speed.
Until 1998, astronomers believed that the universe's expansion as a result of the Big Bang was slowing down due to gravity and might eventually reverse itself. Imagine throwing a ball into the air. At some point, it will slow down and fall back to the ground. But what if you tossed it upwards and it kept going, gaining speed as it went? Observations of dozens of supernovas by Filippenko, physics professor and Lawrence Berkeley National Laboratory (LBNL) scientist Saul Perlmutter, and their colleagues showed that this is exactly what's happening with objects in the universe. The supernovas turned out to be farther away from Earth than the scientists predicted based on the redshifts.
"It appears there is some funny energy in the universe that makes it expand forever, but at an ever increasing rate," Filippenko said at the time.
The invisible force is now known as dark energy. While it may account for as much as two-thirds of the mass in the universe, scientists really have no idea what it is. One theory is that it's vacuum energy of empty space that exerts a negative pressure, as postulated by quantum physics. Others suggest that it's a low-energy field called quintessence, whose detailed properties are still unknown.
According to Filippenko, supernovas may contain clues to solve that puzzle, considered by many to be the hottest mystery in modern physics. Uncovering the secret of dark energy, he says, may dramatically increase our knowledge about what really happened about 14 billion years ago during the Big Bang, and inform scientific predictions about the ultimate fate of our universe.
"Unless the dark energy changes into an attractive force in the future, we think that the universe will expand forever, eventually becoming cold and dark," Filippenko says.
A before and after shot of a galaxy where the UC Berkeley team discovered a supernova (noted by the arrow) using their Katzman Automatic Imaging Telescope. (courtesy the researchers)
To gain insight into dark energy's traits--its pressure versus density, for example--Filippenko employs several state-of-the-art telescopes. Based on his track record, he's granted time on the world's most advanced systems, from the Keck Observatory in Hawaii to the Hubble Space Telescope orbiting the Earth. In January though, NASA announced that due to safety concerns it would no longer service the telescope. Already, components vital to Filippenko's research have failed.
"The announcement was a huge blow to all of us," he says.
Fortunately in October, NASA agreed to consider robotic servicing machines to keep the telescope operational for another five to seven years. Meanwhile, just east of San Jose, a robotic telescope built by Filippenko's group keeps a constant vigil for supernovas every clear night. Filippenko calls the 76-centimeter diameter Katzman Automatic Imaging Telescope (KAIT) a "supernova search engine." This year alone, the group has discovered more than sixty supernovae using KAIT's unblinking eye.
"Cosmology is spurring a revolution in physics and I'm excited to contribute to it in any way I can," Filippenko says.