In the Dark
A mysterious force is tearing the universe apart!
- By Ed Regis
- Air & Space magazine, March 2005
(Page 4 of 6)
When many astronomers heard about invisible forces catapulting the universe hither and yon, the very first thing that popped into their heads was Albert Einstein’s so-called cosmological constant. In 1917, when Einstein published a paper on cosmology, the astronomical evidence of the era still suggested a static universe—Hubble’s landmark observations of receding galaxies were still a dozen years away. However, Einstein realized that according to both the Newtonian law of universal gravitation and his own theory of general relativity, the universe couldn’t be eternally static. Plainly, something had to be done to make his own theory agree with the empirical evidence of the day. And so to make his general relativity equations describe the universe as fixed and unmoving, Einstein introduced a “cosmological term,” a sort of mathematical fine-tuning of his theory that, lo and behold, yielded a static universe after all.
Today we would regard Einstein’s cosmological constant as a fudge factor, a tweak, or, less charitably, a kluge. It was the very image of a gimmick cooked up for no other reason than to “solve” an otherwise intractable problem—and indeed when Einstein learned of Edwin Hubble’s discovery that the universe was expanding, he repudiated the cosmological constant, calling it the “biggest blunder” of his life and saying that it was “theoretically unsatisfactory anyway.”
But Einstein had regarded the cosmological term as representing an inherent property of empty space—an unknown something that pushed against the pull of gravity—and it was exactly this sort of anti-gravitational force that later astronomers needed in order to explain the newly found acceleration of the universe. And so when Adam Riess and his 21 co-authors published their 1998 findings in the Astronomical Journal, the paper’s title was “Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant.”
The modern version of the physical force represented by Einstein’s cosmological constant is a phenomenon called “vacuum energy.” Supposedly, according to the esoteric rules of quantum mechanics, a vacuum is not merely empty nothingness; rather it’s just barely something—or at least it can be something, some of the time. Riess, now at the Space Telescope Science Institute in Baltimore, explains: “The uncertainty principle says that the vacuum can borrow energy from nothing, if it has it for a very short amount of time.” This energy—dark energy—exists in the form of virtual particles, which live on borrowed time and borrowed energy. Where do these virtual particles come from and how do they create anti-gravity? “That’s the $64,000 question,” Riess says. “I mean, really, that is the thing we don’t understand. We think it’s a property of the vacuum that has to do with quantum mechanics, that even a vacuum still has energy in it.
“But it’s not the property of being empty that makes the vacuum have anti-gravity,” he adds. “It’s actually the existence of virtual particles that we weren’t really aware of that’s causing the anti-gravity. Apparently, if this is all correct, the vacuum still does have energy in it, in the form of these virtual particles.”
Whatever. Vacuum energy is such a strange and unintuitive phenomenon that it has an equally strange and unintuitive consequence: The bigger the vacuum, the more dark energy there is. And so it stands to reason that since the universe is expanding, there should have been a time in the distant past when the vacuum—and hence the amount of dark energy—were smaller than they are now. And in that case, the relative strength of dark matter would have been proportionately greater than it is now, which means that at some point in the past the universe should have been slowing down after all.
In 2001, Riess found that the Hubble Space Telescope had made repeated images of an extremely distant Type 1a supernova, SN 1997ff, an object that was more than 10 billion years old. It turned out that the object appeared brighter than it would have been if the universe had been expanding at the same rate throughout its history. In other words, the universe had been slowing down way back then, 10 billion years ago. And then it had speeded up.
Later images made by Riess and his colleagues enabled them to determine that the transition occurred some five or six billion years ago. This was the Big Jerk. (Sorry—that’s what they call it.) That was when the universe had expanded to a point at which its dark matter had become dilute enough, and its attractive force had therefore become weak enough, for the anti-gravitational push of dark energy to rise up and overpower it. Says Riess: “As the universe moved through time, it slowly removed its foot from the brake pedal until the point when the accelerator became stronger than the brake and started jerking the car forward.”