The universe is simple.
How simple?
As simple as the image below. This is the cosmic background radiation as detected with a Bell Labs radio telescope in 1964. The band across the middle is the center of our galaxy. The rest is the humming echo of the Big Bang, uniform in every direction—just as theorists had been predicting. “Which is an amazing thing,” P. James E. Peebles—one of the very same cosmologists who helped predict it—recalls thinking. “But there it is: The universe is simple.”
As Einstein once famously said, “The most incomprehensible thing about the universe is that it is comprehensible.” But why should it be? Why would something so vast and complex and old be within the comprehension of a species that spent millennia believing it occupied the center of existence? Yet century after century cosmologists have operated under the assumption that the universe is simple, and it appears to have worked—at least so far.
That assumption goes back to Copernicus. The picture of the heavens he inherited from the ancients was crowded with invisible spheres that carried the moon, sun, planets, and stars. The geometry to explain those motions was embroidered with epicycles and deferents—circles, and circles within circles, and circles adjacent to circles, all fabricated by astronomers over the course of a couple of millennia in an attempt to make sense of the motions of the celestial bodies around a stationary Earth. The problem with this picture, Copernicus realized, was that it divided the universe into two realms, the terrestrial and the celestial. What if the universe instead was one big happy realm? Once Copernicus removed the Earth from its place of privilege and set it in orbit around the sun, he arrived at equations that predicted the motions of the heavens with far greater accuracy. A century and a half later, Isaac Newton used the sun-centered model to create his law of universal gravitation —emphasis on “universal.” By uniting the physics of the terrestrial with the physics of the celestial, he showed that Copernicus was right: The universe is simple.
For the next three centuries, the discoveries of moons and planets and comets corroborated Newton’s idea, with one exception: an aberration in the orbit of Mercury. In 1915 Einstein fixed that problem, via the general theory of relativity, by reconceiving gravity not as a force that acts across space but as a property of space itself. Two years later, he published a paper exploring the “cosmological considerations” of this new view of gravity. What might this tweaked law of universal gravitation have to say about the history and structure of the universe? To keep the math simple, Einstein and then other theorists had to assume the universe was simple, too. So they returned to Copernicus’s assumption: The Earth doesn’t have a privileged position in the universe. On the largest scale, the cosmos would look the same in every direction no matter where you are in it.
Which was what the 1964 vision of the cosmic background radiation revealed. This picture of the universe, however, was almost too simple. Where were the subtle fluctuations in temperature that would represent the seeds of the galaxies, clusters of galaxies, and superclusters of galaxies—everything that would grow into the universe as we know it?
To answer that question, NASA set to work designing a satellite to look for those fluctuations. In 1991 and 1992, that satellite, the Cosmic Background Explorer (COBE), found them—differences in the temperature at a level of one part in 100,000:
I met the the co-principal investigator of that project, George Smoot, in his office at the University of California, Berkeley, just days after he won the Nobel Prize in physics. Never a particularly calm presence, he was even more animated on this occasion. Underslept and overadrenalized, he shouted, “Time and time again the universe has turned out to be really simple!”
Sitting across from him, nodding emphatically, was fellow physicist Saul Perlmutter of Lawrence Berkeley National Laboratory. “It’s like, why are we able to understand the universe at our level?” he said, echoing Einstein.
Yet Perlmutter himself is among the scientists whose work has most threatened the notion that the universe will be ultimately comprehensible. In 1998, he was the leader of one of the two teams that found the expansion of the universe is not slowing down, as you might naively expect, but speeding up. (He would share the Nobel for that discovery in 2011.) At first physicists considered the discovery of “dark energy” difficult to accept— a force more powerful than gravity on a cosmic scale? —but in 2003 came the first results from the successor to COBE, the Wilkinson Microwave Anistropy Probe (WMAP):
By reading the patterns in those even finer fluctuations, cosmologists could calculate the portion of the universe that takes the form of dark energy: 72.8 percent. So what is it?
Yet before theorists can begin to answer that question, they need to know how dark energy behaves. Does it vary across space and over time, or is it constant? The successor to WMAP, the Planck satellite, should provide a strong clue when its results are released early next year. So far, though, all the data from less precise experiments are pointing toward dark energy being constant. In that case, theorists agree, the answer to “What is dark energy?” will require them to unite the physics of the very big (relativity) with the physics of the very small (quantum mechanics), just as Newton had united the physics of the terrestrial with the physics of the celestial.
“We shouldn’t be shocked that we’re finding a few surprises,” Perlmutter later told me. “Based on just some fragment of information, and a very interesting theory of Einstein’s, people were able to try out the simplest possible model of the universe. ‘We don’t know anything but let’s imagine that it’s as simple as it could possibly be, because we have no other information to go on.’ And then they said, ‘Let’s take a few more pieces of information,’ and those pieces of information fit, and they fit well into this ridiculously simple, intentionally cartoonish picture.”
But now? We don’t know what the vast majority of the universe is. And, physicists acknowledge, we might never know. The universe just might be incomprehensible after all. But assuming the solution exists, Perlmutter at least has faith as to what it will look like: Copernicus’ solution, and Newton’s, and Einstein’s.
“Something,” he said, “equally elegantly simple.”
