Lecture aims to shed light on dark energy

Alex Filippenko

Alex Filippenko, astronomy professor at UC-Berkeley, will give the annual Bunyan Lecture May 4 at 7:30 p.m.

When Berkeley astronomy Professor Alex Filippenko and his team scoured the universe with their telescopes in the 1990s, they were on an astronomical dig for artifacts. Instead of looking for ancient cities, the team searched for ancient rays of light from exploding stars. Distant light is old light, and it can illuminate the history of our universe.

Filippenko found the exploding stars, or supernovae, but they were farther away than they should have been, based on prior data. Not only was the universe expanding—it was expanding at an accelerating rate. An unknown force was making the galaxies repel each other with an extra push, the team concluded. They dubbed this force dark energy, and Science magazine considered it the biggest breakthrough of 1998.

On Monday, May 4, at 7:30 p.m., Filippenko will deliver his presentation, "Dark Energy and the Runaway Universe," at Stanford's Braun Auditorium in the Mudd Chemistry Building, as the Astronomy Program's 27th annual Bunyan Lecturer. The presentation, punctuated with the color and comedy that earned Filippenko the student honor as "Best Professor" at the University of California-Berkeley six times, is free and open to the public.

Astronomers ask some of the grandest questions, Filippenko has said: "When did the universe begin? How will it end? What will its fate be far into the future?" The light from supernovae billions of light-years away (and thus, billions of years ago) provides astronomers cosmic clues.

To help explain what the team found, Filippenko likens the universe to an apple. If one were to toss an apple into the air, there are two theoretical outcomes. It could rise, stop and fall due to Earth's gravity. This would be like a universe that expands outward, stops, then collapses inward. On the other hand, the apple could somehow overcome Earth's gravity and keep rising. This would resemble a universe that keeps expanding.

Either way, the rising apple, or expanding universe, should slow down.

"Because all stars, galaxies, etc. are made of matter, and all of the matter (and energy) in the universe pulls on all other matter (and energy); we thus expected the expansion to slow down," Filippenko said in an e-mail. But which apple represents our universe?

Filippenko and his team concluded the universe is like an apple that gets tossed, rises and then accelerates, rising faster and faster. After ruling out possible errors in their data analysis, the team realized some antigravitational force was jolting the universe's expansion. The team's discovery of this force, later called dark energy, was voted as Science magazine's "Science Breakthrough of 1998," according to Filippenko's UC-Berkeley website.

Dark energy is important to science, Filippenko said, because it can unite two conflicting schools of thought.

"It is a different face of gravity," Filippenko said, "and may help us constrain theories that try to unite the two great pillars of modern physics: quantum mechanics and the general theory of relativity, which are completely inconsistent with each other when one considers large amounts of energy in a small volume."

Albert Einstein first conceived of dark energy as part of his cosmological constant, a mathematical attempt to explain why he thought the universe was static rather than collapsing inward due to gravity. Upon astronomer Edwin Hubble's discovery that the universe was actually expanding and not static, Einstein renounced the cosmological constant as his "biggest blunder," Filippenko said.

Essentially, the modern-day discovery of dark energy resurrects Einstein's concept of an antigravitational force. Einstein's mistake was not in conceptualizing the antigravitational force, but in giving it an equal value in his mathematical equations, Filippenko said. Even though gravity may reign supreme in our galaxy's neighborhood, dark energy dominates the farthest stretches of our universe.

Since Filippenko's team first made their discovery, additional observations have supported their findings. These observations include work from the Katzman Automatic Imaging Telescope (KAIT), a robotic telescope at San Jose's Lick Observatory that is programmed to observe up to 8,000 galaxies per week. This tool led to the discovery of more than 300 supernovae over the last five years.

Even after the discovery of dark energy's existence, the exact nature of this force, which makes up nearly 75 percent of the universe's total energy content, remains shrouded in mystery.

"There are hundreds of hypotheses, but we don't know which one is right," Filippenko said.

The annual Bunyan Lecture series was spawned from an endowment left by James T. Bunyan, a scholar at the Hoover Institution. The lectures "inquire into man's changing vision of the cosmos and of human destiny as revealed in the latest discoveries in the fields of astronomy and space exploration," according to the series website.

Professor Vahe Petrosian, head of the Stanford Astronomy Program, said Filippenko, like all Bunyan Lecturers, is a great public speaker who can make cosmology accessible to the layperson. Petrosian hopes Filippenko's lecture will fire up a curiosity in attendees about the perplexing nature of dark energy.

"Nobody knows what it is," Petrosian said. "That's a question we still don't know the answer to. People will learn we still have a lot of work to do."

Emmanuel Romero is a science-writing intern at the Stanford News Service.