[NOTE: This post originally appeared at our new home at Scientific American.]
A few years ago, soon after moving to Los Angeles, an old grad school buddy of the Time Lord came to town, Brian Schmidt, and we took him to a nearby tapas eatery for nibbles and pisco sours. I remember they were shooting a scene from a Will Smith movie that night, so nearby storefronts were riddled with fake bullet holes, and the odd fake gunfire and explosion interrupted our conversation. Unfazed, Brian regaled us with tales of his life in Australia, where he juggles research with running his very own winery -- hence his Twitter handle, @CosmicPinot. After saying farewell, I commented to the Time Lord as we walked back home how much I liked Brian: "You have really nice friends." (It's true; pretty much all of the Time Lord's pals are delightful, but then, I'm partial to physicists.) He agreed, and added, "And you know what else? He will absolutely win the Nobel Prize some day."
That day has arrived, perhaps a bit earlier than even Schmidt and his colleagues expected. This morning it was announced that the 2011 Nobel Prize in Physics had been awarded to Saul Perlmutter of the University of California, Berkeley; Brian Schmidt of the Australian National University in Weston Creek; and Adam Riess, an astronomy professor at Johns Hopkins University and the Space Telescope Science Institute, "for their studies of exploding stars that revealed that the expansion of the universe is accelerating." Schmidt was the leader of the High-Z Supernova Search Team and Reiss was lead author on the resulting paper; Perlmutter was the leader of the Supernova Cosmology Project and also lead author on that resulting paper. They've had a friendly mock rivalry going on ever since. (Photo: Schmidt, left, dukes it out with Perlmutter, right, over who made the best measurement of the rate of expansion. Credit: Texas A&M cosmologist Nicholas Suntzeff.)
It's a fitting coda to what turns out to be just the beginning of an epic saga of the quest to unlock the mysteries of the cosmos. Because the most likely explanation we have (so far) for this observed acceleration is a mysterious thing called dark energy that makes up a whopping 73% of all the "stuff" in the universe.
Einstein's Fudge Factor
Once upon a time, physicists believed the cosmos was static and unchanging, a celestial clockwork mechanism that would run forever. When Albert Einstein was forming his theory of general relativity in 1917, his calculations indicated that the universe should be expanding. But all the observations up to then showed a static universe. So he figured his calculations were incorrect, and introduced a mathematical “fudge factor” into his equations, known as the cosmological constant, or lambda. It implied the existence of a repulsive force pervading space that counteracts the gravitational attraction holding the galaxies together. This balanced out the “push” and “pull” so that the universe would indeed be static.
Einstein should have trusted his instincts. Twelve years later, Edwin Hubble was studying distant galaxies, and noticed an intriguing effect in the light they emitted: it had a pronounced “Doppler shift” toward the red end of the electromagnetic spectrum. Basically, when a light source is moving towards an observer, the wavelength of its emitted light compresses and shifts to the blue end of the spectrum. When moving away from the observer, the wavelength stretches, and the light shifts to the red end of the spectrum. Hubble reasoned that this could only be happening if the light were traveling across space that is expanding.
The conclusion was inescapable. Einstein’s original equations had been correct, and there was no need for a cosmological constant. The cosmos was still expanding. That's why Einstein famously denounced lambda as his “greatest blunder.”
That discovery turned cosmology on its head. If the universe were still expanding, scientists reasoned, eventually the attractive force of gravity would slow down the rate of expansion. They spent the next 70 years trying to measure that rate. If they knew how the rate of expansion was changing over time, they could deduce the shape of the universe. And its shape was believed to determine its fate. Matter curves space and time around it and gives rise to what we recognize as gravity. The more matter there is, the stronger the pull of gravity, and the more space will curve – making it more likely that the current expansion would halt and the universe would collapse back in on itself in a “Big Crunch.” If there’s not enough matter, the pull of gravity would gradually weaken as galaxies and other celestial objects move farther apart, and the universe would expand forever with essentially no end.
A flat universe, with just the right balance of matter, would mean that the expansion will slow down indefinitely, without recollapsing. A flat universe was the favored option; scientists just needed to precisely measure the acceleration rate to confirm the prediction.
Once again, Einstein was a bit too hasty in dismissing his work. In 1998, two separate teams of physicists measured the change in the universe’s expansion rate, using distant supernovae as mileposts: one led by Perlmutter, the other by Schmidt. The Time Lord shared an office with Schmidt back in the early 1990s. As he tells it in his book, From Eternity to Here:
I was the idealistic theorist and he was the no-nonsense observer. In those days, when the technology of large-scale surveys in astronomy was just in its infancy, it was a commonplace belief that measuring the cosmological parameters was a fool's errand, doomed to be plagued by enormous uncertainties that would prevent us from determining the size and shape of the universe with anything like the precision we desired. Brian and I made a bet concerning whether we would be able to accurately measure the total matter density of the universe within 20 years. I said we would; Brian was sure we wouldn't. We were poor graduate students at the time, but purchased a small bottle of vintage port, to be secreted away for two decades before we knew who had won. Happily for both of us, we learned the answer long before then. I won the bet, due in large part to the efforts of Brian himself. We split the bottle of port on the roof of Harvard's Quincy House in 2005.
Why supernovae? They're the best "standard candles" we've got. Because they are among the brightest objects in the universe, these exploding stars can help astronomers determine distances in space. By matching up those distances with how much the light from a supernova has shifted, the two teams could calculate how the expansion rate has changed over time. Light that began its journey across space from a source 10 billion years ago would have a red shift markedly more pronounced than the light that was emitted from a source just 1 billion years ago.
When Hubble made his 1929 measurements, the farthest red-shifted galaxies were roughly 6 million light years away. If expansion was now slowing, supernovae in those distant galaxies should appear brighter and closer than their red shifts would suggest. Instead, just the opposite was true. At high red shifts, the most distant supernovae are dimmer than they would be if the universe were slowing down. The only plausible explanation for this is that instead of gradually slowing down, the expansion of the universe is speeding up.
It was bizarre and completely unexpected. Since 1998, cosmologists have been grappling a whole new set of questions implied by that momentous discovery, the foremost of which is the makeup of the mysterious dark energy that appears to be winning the cosmic tug-of-war. And once again, the discovery turned cosmology on its head.
Now the story goes something like this: very early in the universe’s existence, dark matter dominated. Everything was closer together, so its density was higher than that of the dark energy, and its gravitational pull was stronger. This led to the clumping that formed early galaxies. But as the universe continued to expand, the dark matter density, and hence the gravitational pull, decreased until it was less than that of the dark energy. So instead of the expected slow-down in the expansion rate, the now-dominant dark energy began pushing the universe apart at ever-faster rates.
Where does this dark energy come from? That's the big question. But it’s a testament to Einstein’s genius that even his blunders prove to be significant. Remember his “fudge factor,” Lambda implied the existence of a repulsive form of gravity, and the simplest example of that is the vacuum energy. Quantum physics holds that even the emptiest vacuum is teeming with energy in the form of “virtual” particles that wink in and out of existence, flying apart and coming together in an intricate quantum dance. This roiling sea of virtual particles could give rise to dark energy, giving the universe a little extra push so that it can continue accelerating.
The problem is that the numbers don’t add up. The quantum vacuum contains too much energy: roughly 10120 times too much. So the universe should be accelerating much faster than it is. An alternative theory proposes that the universe may be filled with an even more exotic, fluctuating form of dark energy dubbed “quintessence.” Yet all the observations to date indicate that the dark energy is constant, not fluctuating. So scientists must consider even more possibilities. The dark energy could be the result of the influence of unseen extra dimensions predicted by string theory. Alternatively, the dark energy could be due to neutrinos – the lightest particles of matter – interacting with hypothetical particles called “accelerons.” Some scientists have theorized that dark matter and dark energy emanate from the same source – they just don’t know what that source might be. Yet it’s just as likely that there is no connection, and the two are very different things. Or perhaps there is no such thing as dark energy, and we need to revise Einstein's general theory of relativity, and/or devise a theory of quantum gravity.
Scientists love to explore the unknown, so these are exciting times for cosmologists. Congratulations to Schmidt, Reiss and Perlmutter for a well-deserved honor -- and here's to the future Nobel-worthy discoveries yet to be made!