“The most plausible explanation,” Riess said later, “is that the light from the supernovas, which exploded billions of years ago, traveled a greater distance than theorists had predicted. And this explanation, in turn, led to the conclusion that the expansion of the universe is actually speeding up, not slowing down.” So surprising was this result that, initially, many of the scientists involved were too embarrassed to publish it. What if they were wrong? After repeatedly analyzing their data, however, and seeing no flaw in it, they did, with Riess as the lead author.
Odd as the supernova findings were, they were buttressed within a few years by two other types of observations. The first was a survey of the cosmic microwave background, the distant remnants of the Big Bang. In 2001, data from NASA’s Wilkinson Microwave Anisotropy Probe, an observatory orbiting the sun 900 million miles beyond Earth orbit, discovered variations in the temperature of the background radiation; the variations, through a tortuous chain of astrophysical reasoning, also pointed to cosmic acceleration. And then last year, a third study, involving measurements that NASA’s Earth-orbiting Chandra X-ray Observatory made of hot gas inside galaxy clusters, confirmed the results of the other two.
The Chandra observations also enabled the scientists to estimate what proportion of the universe consists of what type of substance. The conclusion was that 75 percent of the universe is made up of dark energy, 21 percent is dark matter, and only a scant four percent is ordinary matter—the stuff you can see and touch.
Finally there was no longer any doubt: Cosmic expansion was speeding up, and the visible universe of ordinary, everyday, boring reality was being eclipsed by a haze of mystic dark stuff. The question was: What did it all mean?
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.