“Some say the world will end in fire, some say in ice.”— Robert Frost.
It’s perhaps the scariest question one can contemplate. Will the universe die? Will all the planets, stars and galaxies in the cosmos, and all the myriad forms of life that they sustain, eventually cease to exist? Astronomers long have known that the earth is doomed; in about 5 billion years the sun will run out of hydrogen fuel and swell to gigantic size, vaporizing our planet. But will there always be new stars born from the ashes of their predecessors, and new planets for life to colonize?
Most cosmologists — the scientists who study the shape, structure, and history of the universe — are not very hopeful on this point. The universe has been expanding ever since the Big Bang started nearly 14 billion years ago. In the late 1990s researchers were shocked to discover that the cosmic expansion isn’t slowing down as they had expected. Instead, it’s speeding up. The implications are dire: If the universal acceleration continues, the multitude of galaxies that now pepper the night sky will recede from view. Our local group of galaxies — the Milky Way, Andromeda, and a few dozen lesser lights — will be surrounded by darkness, like a lone campfire in a vast black desert. In about 100 trillion years our aging galaxy no longer will have enough hydrogen to produce new stars, and the embers of our diminished universe will slowly cool and decay. After 10¹°° years — an unimaginably long but not infinite stretch of time — nothing will be left except an extremely diffuse scattering of particles. (Imagine one electron surrounded by billions of light-years of empty space.) This is the icy apocalypse that Robert Frost envisioned in his poem “Fire and Ice.” Or as T.S. Eliot put it in “The Hollow Men”: “This is the way the world ends, not with a bang but a whimper.”
In the past few years, though, some theorists have held out the promise of a fiery rebirth. Princeton University cosmologist Paul Steinhardt is one of the architects of the cyclic model, a hypothesis that challenges the conventional view of our universe’s beginning and makes very different predictions about its end. If the cyclic model is correct, the universe would stop expanding about a trillion years from now, then slowly contract for another 10 billion years. The contraction would be so gradual that the future inhabitants of our galaxy wouldn’t notice anything amiss except for a shift in the values of certain physical constants. “They would see that something strange was happening with the laws of physics,” Steinhardt says. “And then the universe would be blitzed with huge amounts of energy.” The result would be another Big Bang, injecting new matter and radiation into the cosmos and triggering a new round of expansion. All traces of civilization would be annihilated, but perhaps life could arise again in the next cosmic cycle.
Neither the fiery nor icy future seems particularly pleasant, but it would be nice to know which one we’re facing. Luckily, the clues to our ultimate fate may be all around us, hidden in plain sight. At the start of the Big Bang the universe was a hot, seething soup of protons, electrons, and photons (particles of light), but after 380,000 years the soup cooled enough to allow protons and electrons to form hydrogen atoms. The universe suddenly became transparent, and the primordial photons have been streaming across the cosmos ever since, appearing in present-day telescopes as a sea of radiation called the cosmic microwave background (CMB). Cosmologists now are using ever more precise instruments to examine this radiation because it offers a snapshot of the universe in its infancy, with intricate fluctuations that may reveal exactly how it was born and how it might die.
Princeton has a special connection to the CMB. In 1964, a group of researchers in the University’s physics department — including Robert Dicke ’39, Jim Peebles, David Wilkinson, and Peter Roll — was preparing to build an instrument that could detect the primordial radiation. They were gathered for a lunchtime meeting when Dicke got a phone call from Arno Penzias, a Bell Labs scientist who was trying to find the source of some odd static he’d picked up with his laboratory’s radio antenna in nearby Holmdel, N.J. Dicke quickly realized that Penzias had discovered the CMB. After he hung up the phone, he turned to his colleagues and said, “Well, boys, we’ve been scooped.” (Penzias and his Bell Labs co-worker Robert Wilson later received the Nobel Prize.)
In the following decades, however, Princeton researchers made up for the scoop by participating in several studies that investigated the properties of the CMB. The first measurements of the radiation indicated that it was isotropic — that is, all the photons shared a temperature of 2.7 kelvins (2.7 degrees Celsius above absolute zero), no matter which direction they were coming from. This finding suggested that the primordial soup of the infant universe was almost perfectly smooth. But in the early 1990s a NASA satellite called the Cosmic Background Explorer found slight variations in the CMB’s temperature: The photons coming from some spots in the sky were a few ten-thousandths of a degree hotter or colder than the photons coming from other spots. In 2001 NASA launched the Wilkinson Microwave Anisotropy Probe, which made even more detailed observations of the CMB from a parking spot in deep space known as the L2 Sun-Earth Lagrangian point, nearly a million miles from our planet. The probe was named in honor of David Wilkinson, the Princeton cosmologist who died in 2002. The mission’s science team includes Lyman Page and Norman Jarosik of Princeton’s physics department and David Spergel ’82, now the chairman of the astrophysics department.
Although the Wilkinson probe still is collecting data, its results already have revolutionized cosmology. The observed CMB temperature fluctuations closely match the pattern predicted by the theory of inflation, which was developed in the 1980s to address the paradox of the universe’s smoothness. At that time, scientists couldn’t understand why widely separated parts of the universe look so much alike; because there is no mechanism that could mix such a vast cosmic soup, researchers had expected it to be a lot lumpier. Cosmologists tried to solve the problem by proposing that in the first moments of the Big Bang, something called the inflaton field triggered an ultrafast expansion of the universe that swiftly stretched microscopic volumes into smooth, flat expanses of space. The theory predicted, however, that the early universe wouldn’t be perfectly smooth. The brief epoch of inflation would leave a distinctive pattern of high- and low-density patches in the primordial soup — which correspond to the hot and cold spots in the CMB — and the high-density patches later would evolve into the galaxies and galaxy clusters that we see today.
The CMB observations support this hypothesis, but not all cosmologists are convinced that inflation jump-started the universe. The theory has a few holes in it. First, scientists don’t know what the inflaton field is, much less why it appeared when it did or what existed before it came on the scene. Second, one of the consequences of the theory is that inflation must be an ongoing, never-ending process — the inflaton field generates an infinite number of universes, each emerging like a bubble from a glass of champagne. As a result, the theory can’t explain many of the properties of our own universe, except to say that they arose by chance.
In an effort to give inflation a firmer footing, some researchers have tried to derive the hypothesis from string theory, the proposed “theory of everything” that attempts to unify all the forces of nature. Professors and graduate students from Princeton and other universities wrestled with this problem at a conference titled “The Big Bang and Beyond” that was held at Jadwin Hall last October. As the cosmologists scribbled equations on the blackboard of the colloquium room and continued the abstruse discussions in the faculty lounge while slurping tea and gobbling cookies, it became clear that merging inflation with string theory would be a formidable task. Moreover, the inflationary models don’t explain why our universe is now expanding at an accelerating pace. Cosmologists use the term “dark energy” to describe the unknown agent that is causing the acceleration, but the phenomenon remains just as mysterious as the origins of the Big Bang.