To explain the meaning of dark energy, Dr. Alex Filippenko, internationally acclaimed astronomer and cosmologist, presented a talk to a jam-packed audience at the Institute for Astronomy, IfA, in Pukalani on June 24. Filippenko is one of the world’s most highly cited astronomers and the recipient of numerous awards for his research. He is a member of both teams that discovered the accelerating expansion of the universe—a discovery named the top science breakthrough of 1998. He has appeared in numerous television documentaries on the History Channel and has produced four introductory astronomy video courses with the Teaching Company. Filippenko, currently teaching at University of California at Berkeley, has been elected to the National Academy of Sciences and has co-authored an award-winning textbook.
“It’s called dark energy because we can’t see it and we don’t know what’s behind it,” said Filippenko. “The basic thought is that there must be elementary particles in the universe that form the basis of it, but we don’t know what they are.
“Dark energy is often identified with Einstein’s Cosmological Constant,” said Filippenko.
The connection between Einstein’s work and Filippenko’s work is fascinating. Soon after Einstein published his General Theory of Relativity in 1916, he realized that his complicated equations predicted that the universe could not simply remain static. Mathematically, the universe had to be either expanding or contracting. However, in 1916, astronomers had no observational evidence for either motion.
“By 1917, Einstein concluded that something was missing from his theory,” explained Filippenko.
On sheer intuition, Einstein added another term to his equations that would exactly balance the expansion or contraction, theoretically allowing the universe to remain the same size that it seemed to be. That extra term included the Cosmological Constant.
“If the term were really true, the Cosmological Constant would mean that a previously unknown force existed throughout the universe. The force would be a strange type of anti-gravity that causes galaxies to repel each other at large distances,” Filippenko said.
However, in 1929, Astronomer Edwin Hubble, for whom the Hubble Space Telescope is named, announced new observations indicating that the universe was indeed expanding. When he heard of Hubble’s discovery, Einstein reportedly called the cosmological term “the biggest blunder of my life.” Apparently, Einstein’s intuition had been wrong in this case. But the story does not stop there.
“The expansion discovered by Hubble revolutionized astronomy,” said Filippenko.
Astronomers expected the expansion rate to be changing, simply because galaxies pull on each other gravitationally, thereby resisting the expansion to some degree.
“It seems crazy, because gravity always pulls and slows the expansion—it never pushes,” Filippenko said. “Yet some force appears to be pushing the universe apart.”
If the acceleration continues indefinitely, the ultimate result will be that galaxies outside the local super-cluster will move beyond the cosmic horizon. They will no longer be visible, because their line-of-sight velocity becomes greater than the speed of light.
In the 1990s, the exploration of distant galaxies continued on Mauna Kea on Hawai‘i Island.
“We rely heavily on the Keck Observatory to determine how much the universe has expanded,” Filippenko said.
How can we tell how far away extremely faint galaxies really are?
“One of the answers is Type 1a supernovae,” he said. Because the true brightness of this type of exploding star is fairly consistent, the apparent brightness tells us how far away each one is. “Using the large twin Keck Telescopes, astronomers are able to identify many supernovae per hour, and determine their distances accurately.”
“When the supernovae turned out to be progressively fainter than expected at larger distances, we knew the expansion was accelerating,” Filippenko said.
Some repulsive force, like the one Einstein proposed, seems to be the cause. This is one of the most fundamental problems in physics and cosmology today.
“We now think that the universe extends far, far beyond what we can see,” added Filippenko. “Seventy-three percent of it is dark energy, 23 percent of it is dark matter, and the rest—only about 4 percent—is everything we see. It appears that science is not over. In fact, most of it has not even been discovered,” he said.
Then what is the fate of the universe? Will it collapse into a “Big Crunch,” or will it continue to expand forever into a “Big Rip” or a “Runaway Universe?” “We don’t know,” said Filippenko.
What dark energy really is and whether it will stay constant remains to be seen. No matter which scenario is correct, the universe apparently has at least tens of billions of years left. That should leave us plenty of time to find answers.
For more information, Goo-gle Dark Energy, Alex Filippenko and Keck Observatory.
