Dick Bond, Canadian Institute for Advanced Theoretical Astrophysics

Research scientist Michele Limon and Princeton professor Suzanne Staggs *93 take a break in front of the ACT telescope in 2007. Staggs is working on a microwave telescope called QUIET that will begin operating nearby.

Steinhardt, who helped develop the theory of inflation during the 1980s, is now skeptical of that hypothesis. Although he doesn’t fit the conventional image of a scientific rebel — he’s an easygoing professor with a tidy office in Jadwin Hall — he has a feisty, independent spirit. In 2001 he and Neil Turok of Cambridge University published their first paper on an alternative to inflation called the ekpyrotic model. (Ekpyrosis is the Greek word for conflagration.) The model was inspired by string theory, which posits that the fundamental particles and forces actually are minuscule strings vibrating in extra dimensions of space that we can’t perceive. In one variant of the theory, most of the strings in our universe are attached to a three-dimensional boundary called a brane. Although these strings are stuck to the brane like magnets on a refrigerator, the brane itself can travel through the extra dimensions. But gravity also can extend into the extra dimensions, which raises the possibility that one brane could attract another. When Steinhardt and Turok considered what would happen if another brane collided with ours, they found that the impact would infuse our universe with energy and force it to expand — an event that looks very much like the Big Bang. And because the branes would ripple slightly as they approached each other, the collision would produce the small fluctuations in density observed in the CMB studies.

Better yet, Steinhardt and Turok later realized that their alternative hypothesis also can explain cosmic acceleration. After the collision the branes would bounce apart, but they would still be gravitationally attracted to each other, and this attraction would speed up the expansion of our universe. Eventually the branes would stop moving apart and hurtle toward each other again, causing a slow contraction of our universe and finally another Big Bang. Then the cycle would repeat indefinitely, with the branes clashing like a pair of cosmic cymbals. This updated version of the hypothesis, which was renamed the cyclic model, is quite different from the earlier idea of the Big Crunch, in which the universe would collapse under the force of its own gravity. In the cyclic model, the matter in our universe doesn’t re-concentrate; rather, the gravitational field between the branes periodically pumps new energy into our cosmos.

Of course, there are reasons to be skeptical of the cyclic model, too. Researchers can’t say exactly what happens at the moment when the branes collide. And, as with eternal inflation, theorists aren’t sure whether the cycling had a beginning, and if so, what could have started it. Still, the cyclic model appears to be a viable competitor to inflation. Best of all, the rival theories make different predictions about the detailed characteristics of the CMB, and upcoming observations of the radiation may soon determine which hypothesis is closer to the truth. Princeton cosmologists are taking a leading role in these studies, and even some undergraduates have joined the effort by focusing their junior papers and senior theses on the distinctions between inflation and the cyclic model.

One of the most important distinctions involves something called non-Gaussianity. Most inflationary models predict that the temperature fluctuations in the CMB should be very close to Gaussian — that is, the probability distribution of the fluctuations should follow the classic bell-shaped curve, with virtually equal numbers of hot and cold spots (no matter how big or small the spots are). The cyclic model, however, would produce a slightly lopsided, non-Gaussian distribution, with more cold spots than hot. The results from the Wilkinson probe have not yet shown convincing evidence for non-Gaussianity in the CMB, but this year the European Space Agency plans to launch a spacecraft called Planck that will deliver a more fine-grained map of the temperature fluctuations. If their distribution turns out to be more non-Gaussian than inflation predicts, the case for the cyclic model will be strengthened.

Another way to test the hypotheses is to look for signs of gravitational waves in the CMB. According to Einstein’s theory of relativity, when a mass accelerates in a certain way, it creates ripples in the surrounding space, just as a stone thrown into a pond creates ripples in the water. If inflation triggered the Big Bang, it would have released a flood of gravitational waves that would have polarized the CMB. (Polarized light is oriented in a particular direction; polarized sunglasses reduce glare by filtering out horizontally oriented light.) Because the cyclic model would not produce such strong gravitational waves, the detection of this kind of polarization in the CMB would be compelling evidence for inflation. Unfortunately, the Planck spacecraft cannot identify this elusive polarization signal, but cosmologists may be able to search for it using new telescopes controlled from the ground and carried by balloons.

Fresh from their success with the Wilkinson probe, Spergel, Page, and several other Princeton researchers are now analyzing data from the Atacama Cosmology Telescope, a microwave observatory on a mountain called Cerro Toco in the Atacama Desert of northern Chile. It’s the highest fixed telescope in the world, situated at an altitude of 17,000 feet to minimize the interference from microwaves emitted in the atmosphere. The site is so high that oxygen deprivation is a job hazard for the professors and grad students who run the telescope; they usually stay on the mountain for only six hours at a time. “If I stay up there any longer, my brain slows down a bit,” says Joseph Fowler, a cosmologist in Princeton’s physics department. “I start to lose my energy and alertness.” The hard drives that store the telescope’s observations — about 1,000 gigabytes are collected every four days — must be kept in pressurized boxes because the drives would crash in the thin air. The flood of data is too torrential to be transmitted over the Internet, so the researchers retrieve it using a method that Fowler calls HDOA: Hard Drives On Airplane.

Unlike the Wilkinson and Planck spacecraft, the Atacama telescope doesn’t provide full-sky maps of the CMB, but it offers better resolution for the sections it scans, measuring temperature fluctuations as small as one arc-minute across (one-sixtieth of an angular degree). At that scale, researchers expect to see subtle changes in the CMB caused by the passage of the primordial photons through and around the galaxy clusters that have formed since the radiation was emitted. The results may help cosmologists chart the development of the clusters and determine when dark energy began to accelerate the expansion of the universe. Meanwhile, another team from Princeton, led by Suzanne Staggs *93, is contributing to the design and construction of a microwave telescope called QUIET that soon will begin operating at a nearby site in the Atacama Desert. QUIET will measure the polarization of the CMB, concentrating in particular on a signal that would be generated by inflationary gravitational waves.  

Other Princeton researchers are pinning their hopes on a balloon-borne observatory called Spider that will observe the CMB while floating through the upper atmosphere. Bill Jones ’98, an assistant professor in Princeton’s physics department who worked on the design of the Wilkinson probe when he was a Princeton undergraduate, says Spider is scheduled to go on a qualifying flight above Australia in the spring of 2010. If that flight goes well, Spider will embark on a 45-day mission in 2011. Its instruments are designed specifically to look for the polarization from gravitational waves. And because Spider will cruise at an altitude of 85,000 to 110,000 feet — well above the thickest part of the atmosphere — the observatory will get a clear view of the CMB across 60 percent of the sky.

The fact that so many scientists are studying the CMB is a reflection of how much cosmology has changed in the past 20 years. A field that once was dominated by blue-sky thinkers — the kind of theorists who delight in constructing elegant mathematical frameworks for the universe — is coming down to earth. The precise observations from new telescopes are putting more and more constraints on cosmological theories such as inflation and the cyclic model. Theorists can try to tweak these hypotheses so that they fit the new data, but eventually the weight of evidence will rule out one of them -- or perhaps both. “That’s the way things are supposed to work in science,” says Steinhardt. “Nature will tell you that it’s time to move on.”

Spergel, who is familiar with both the theoretical and observational sides of cosmology, believes that some questions about the universe may be easier to answer than others. Over the past decade scientists have firmly established our universe’s size (infinite), shape (flat) and age (13.7 billion years), and in the next 10 years researchers finally may determine the nature of dark matter. This mysterious material, not to be confused with dark energy, never has been directly observed because it doesn’t emit or absorb light, but most astronomers believe that it pervades and surrounds all galaxies, which would fly apart at the seams if not for dark matter’s gravity. Theorists have proposed several exotic particles as candidates for dark matter, and researchers are hunting for their fleeting signatures with the help of particle accelerators, underground detectors, and spacecraft observatories. But dark energy, Spergel believes, won’t be as easy to pin down. “We need a lucky discovery to figure out dark energy,” he says. “We need another hint.”

Learning how the universe began and how it may end would be an even bigger prize. Although inflation and the cyclic model can explain an impressive number of observations, Steinhardt acknowledges that both theories could be wrong. But the fact that researchers can use astronomical observations to test these ideas is encouraging. “I don’t see how you can’t be fascinated by the fact that we can do measurements to decide the issue,” he says. “We don’t have to leave it to philosophical discussion. And that’s amazing.”    

Mark Alpert ’82 is a contributing editor at Scientific American and the author of Final Theory (Touchstone, 2008), a thriller about Albert Einstein and the search for the “theory of everything.”

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