Einstein was right! (for the wrong reason)
Astronomers struggle with basic theories about the universe, following the discovery of
Michael Levi is finding a golden opportunity in Albert Einstein's "greatest blunder."
In 1917, Einstein pulled a fudge factor out of thin air to coax his equations on general relativity into describing a universe astronomers actually saw.
Three years ago, two teams of astronomers startled the scientific world with evidence that this figment of Einstein's chalk board - which the legendary physicist later repudiated - has a measurable impact on the universe.
Now, what Einstein called a cosmological constant appears to be "the largest form of energy in the universe," Dr. Levi, a physicist, says. And, he laments, "we know nothing about it."
That is about to change.
On June 30, the National Aeronautics and Space Administration is slated to launch a spacecraft that will map the afterglow of the Big Bang in unprecedented detail. Data from the craft are expected to carry further clues about the "dark energy" Einstein's fudge factor describes.
Meanwhile, at the Lawrence Berkeley National Laboratory in Berkeley, Calif., Levi and
colleague Saul Perlmutter are designing a space-based telescope that will use light from exploding stars in distant galaxies to trace the history of this energy's influence on the cosmos.
These projects are two among several aimed at unraveling the mysteries of dark energy, which has earned that moniker less for its lack of luminosity than for the ignorance surrounding it, some researchers say.
Fundamentally bizarre phenomenon
Indeed, the very idea of dark energy in the cosmos "is so bizarre from a fundamental-physics point of view that in their heart of hearts, people are still extremely skeptical," says Scott Dodelson, an astrophysicist and theoretician at the Fermi National Accelerator Laboratory in Batavia, Ill. "They say: 'You've got to prove it to me again and again - and you've got to prove it in different ways - or I won't believe it.' "
For his part, Einstein invoked dark energy out of expedience.
As he watched his equations on general relativity unfold, they led to the conclusion that the fabric of space-time is not holding steady, but could either expand or contract, depending on the universe's shape and on how densely it is packed with matter. Above a certain threshold, a densely packed universe would collapse under the pull of its combined gravity. Below that threshold, gravity grip loosens and the universe expands.
At the time, however, astronomers saw no large-scale motion. So Einstein added the cosmological constant, which applied brakes to the universe his equations described. When Edwin Hubble published his evidence in 1925 that the universe was expanding, and distant objects were receding from us at faster rates than close ones, Einstein tossed the cosmological constant out the window.
"Einstein was pulling something out of a hat," says Sean Carroll, an assistant professor of physics at the University of Chicago. "He thought of it as an extra term in his equations changing the response of spacetime to ordinary matter."
But, he adds, "We know now that the term he added is precisely equivalent in all ways" to a form of energy that has come to be known as vacuum energy.
Take a volume of space, he continues, strip it of every form of matter, and general relativity still allows the vacuum to contain energy. Its density would be constant: The amount of vacuum energy in a cubic inch of space in our solar system would match that of a cubic inch of space billions of light-years away. Depending on the value assigned to it, this energy could either retard or accelerate the expansion of the cosmos.
Many cosmologists hold that the universe got its kick-start with the sudden release of vast amounts of pent-up vacuum energy. During the universe's first billion trillion trillionth of a second, it burst from subatomic size to nearly its current volume. The result was the Big Bang, the primordial explosion that scientists say gave birth to the universe.
Yet only in 1998 were astronomers able to spot what looked to be the action of vacuum energy on the universe.
Two teams - one led by Dr. Perlmutter, the other by Brian Schmidt, with the Mt. Stromlo Siding Springs Observatories in Australia - reported that light from distant supernovae was dimmer than inflation theories said it should be. Currently, inflation holds that all the "stuff" in the universe is just dense enough to prevent collapse, but that expansion will slow without ever reaching a stop.
Yet light measurements taken from the supernovae, roughly 7 billion light-years away, indicated that the galaxies hosting the supernovae were farther away then they should have been if the universe was decelerating.
Last month, Lawrence Berkeley's Peter Nugent and Adam Riess of the Space Telescope Science Institute bolstered the 1998 results with data from a supernova more distant yet, which does fit with the expected rate of deceleration.
One explanation, researchers say, is that scientists have now bracketed the period in the universe's history when matter thinned sufficiently for gravity to give way to the universe's residual vacuum energy, which counteracts gravity and pushes clusters and superclusters of galaxies away from each other.
The most recent observation virtually eliminates objections that the 1998 data might merely be showing the effect of dust obscuring the supernovae, or other measurement errors, says Dr. Nugent, who also is a member of Perlmutter's team.
The Hubble Space Telescope result Dr. Riess and Nugent reported also "gives us a look at the epoch when gravity was dominant," Nugent says.
This distance scale is likely to be one of the most fruitful for followup studies of vacuum energy - if that's what it is - LBL's Levi says, because farther out, when the universe was younger and smaller, matter's higher density would give gravity the advantage over the much weaker vacuum energy. Any closer than about 7 billion light-years, and the vacuum energy's effect would be swamped by "local" regions of space where matter is relatively dense.
This rough distance is the target region for SNAP, a space telescope Perlmutter and Levi have proposed to tease more information out of supernovae. Known as type-1A supernovae, these stellar explosions yield consistent levels and changes of brightness as they evolve.
But type-1A explosions are rare, so researchers say they must gather repeat images of thousands of galaxies in less than two weeks to ensure they can spot an explosion soon enough to track it through its entire cycle.
With initial funding from the Department of Energy, the team is designing a 1.8-meter orbiting telescope with a million-pixel camera to fill that role. If all goes well, Levi estimates that the telescope could be ready for launch in 2008.
Next month, NASA is scheduled to launch the Microwave Anisotropy Probe, a spacecraft that will map the microwave background radiation from the Big Bang with extreme accuracy.
Tiny changes in the density of the radiation are thought to be the seeds from which galaxies and larger cosmic structures evolved. Buried in those fingerprints of the early universe are signatures that will help cosmologists refine their estimates of the universe's density and the share of that density that different forms of "stuff" account for, says MAP lead scientist Charles Bennett, with the Goddard Space Flight Center in Greenbelt, Md.
Density defines future of the cosmos
Each cosmological theory predicts a certain pattern in the radiation, he says, turning density measurements into "a powerful tool" for pointing toward the correct theory.
Recent microwave background measurements from balloon-borne instruments in Antarctica have provided stunning confirmations of the inflation theories, researchers say. They yield a "flat" universe in which 5 percent of its density consists of matter and forms of energy humans can detect, 25 percent dark matter, which is inferred from the movement of galaxies, clusters, and super clusters. The remaining 65 percent consists of vacuum energy or its equivalent.
For Dodelson, it's an amazing time for astrophysics. "The time scale for change in cosmology is typically 500 years," he says.
"Five years ago, if someone told you there's a cosmological constant, you'd say he's crazy. Today its just the reverse. It might take 100 years to figure out what this stuff is. But it's remarkable we're living at this time."
(c) Copyright 2001. The Christian Science Monitor