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sizingup Inflation By Steve Nadis in january 1980, a young Stanford physicist named Alan Guth unveiled a brilliant idea that had just one drawback: it didn’t work. At the time, Guth (now a professor at MIT) was fully aware of this shortcoming, yet he was convinced of the idea’s impor- tance nevertheless. History shows his faith to have been well placed. Unlike most 25-year-old ideas that don’t quite work, this one, which Guth called “inflation,” was not dis- carded long ago. Instead, the notion of a fleeting yet explosive growth spurt in the universe’s earliest mo- ments has become a cornerstone of modern cosmology. University of Chicago astrophysicist Michael Turner goes further, calling inflation “the most important idea in cosmology since the Big Bang.” 32 November 2005 Sky & Telescope What put the bang in the Big Bang? A physical force whose nature remains cloaked in mystery. S&T illustration by Casey B. Reed Sky & Telescope November 2005 33 psizing up inflation When Guth first conceived of inflation, he doubted that were delayed by “supercooling,” so that it occurred at a lower the idea would be rigorously tested within his lifetime. But temperature than otherwise would have been the case (just inflation has already passed numerous observational hur- as supercooled water turns to ice well below its normal dles with flying colors. Now, with the age of “precision cos- freezing point). Late one night in December 1979, Guth mology” upon them, astronomers hope to see whether this discovered another consequence of supercooling: it would powerful idea holds up to even closer scrutiny. propel the universe into a state of exponential growth. Inflation stands at a critical threshold, claims MIT cos- Inflation was thus born. mologist Max Tegmark. “For the first time, inflation theory The accelerated growth Guth proposed didn’t just dilute is bumping against data. We’re finally getting to the point magnetic monopoles to unobservably low densities. It also where we can kill off a lot of models.” But the cup is only solved numerous cosmological puzzles, explaining why the half full, as the saying goes. Even if the idea withstands the universe is flat, as observed; why it’s so smooth; and even challenges posed by ever more stringent measurements, why it produced the small deviations from complete bland- theorists still have to explain exactly how inflation works. ness that eventually generated galaxies and galaxy clusters. How does inflation accomplish these feats? Before an- Birth of an Idea swering that question, let’s first review some of the theory’s Guth, of course, had no idea what he was getting into when, basics. Before the universe was a tiny fraction of a second in the late 1970s, he embarked on the path that led to infla- old, the theory holds, it already had completed a rapid tion. In fact, he knew little about cosmology at the time. burst of exponential expansion lasting perhaps only 10–35 The initial problem he took on, with help from Cornell second, during which time its volume increased by a factor University physicist Henry Tye, related to magnetic monopoles of 1090 or more. Fueling this outlandish growth was an — hypothetical particles that carry lone north or south exotic energy field — the inflaton (not inflation) field — that poles. Guth and Tye’s calculations suggested that fantasti- turned gravity on its head. During the brief inflationary cally large numbers of these particles should have been epoch, the cosmos was filled with this invisible fog, which produced in the Big Bang. Yet none has ever been detected. pushed space apart and stretched it out. Guth and Tye showed that monopole production would This inflation-driving substance had another unusual be suppressed if a phase transition in the early universe property: it was hard to dilute, maintaining a constant or nearly constant density even as the volume of space it inhabited expanded like mad. Fortunately for life as we 1040 Inflation Present know it, inflation’s gravity-defying energy field was unstable, period day and it eventually decayed into matter and the radiation now seen as the cosmic microwave background (CMB). It 1020 was this transition that allowed the universe to follow a far 1 more leisurely expansion over the last 13 /2 billion years. Inflation made the observable universe geometrically Inflationary theory 1 Standard theory “flat” in the same way that inflating a balloon flattens a small vable today (meters) patch on the balloon’s surface. It also explains why today’s Standard universe is so remarkably smooth, yet not too smooth to cosmology 1 mm OF THE UNIVERSE A SHORT HISTORY 10–20 form stars, galaxies, and galaxy clusters. The uniformity –3 1026m (10 m) results from blowing up a tiny region — one small enough to have achieved thermodynamic equilibrium — into a vast Inflationary –40 region encompassing the visible realm. (This addresses the 10 cosmology so-called horizon problem that arises in an inflation-free cos- 3 x 10–27m 26 mos, where energy would have had to travel 100 times Radius of universe obser 10 m faster than the speed of light in order to bring disparate 10–60 SOURCE: ALAN GUTH; INSET: JOSEPH SILK, JOSEPH SOURCE: ALAN GUTH; INSET: regions into thermal equilibrium.) –40 –30 –20 –10 10 10 10 10 10 1 10 Conversely, the seeds of today’s cosmic structures origi- Time (seconds) nated when quantum fluctuations created lumps in the Before inflation entered the picture, most cosmologists believed that to- otherwise uniform tapestry of space-time and inflation day’s observable universe — the region within which light has had time then blew them up to macroscopic proportions. Since these to reach us — was about 1 millimeter across when it was 10–35 second old. random, short-lived enhancements of mass and energy were Although small, this mm-wide region was far vaster than the distance that continuously produced while space stretched outward, light or heat could have traveled since the Big Bang. By contrast, inflation inflation generated fluctuations of roughly the same strength posits that space expanded exponentially during the universe’s first across a broad range of spatial scales, leading to a so-called 10–35 second (or thereabouts), allowing regions that once were in ther- “scale-invariant” spectrum — precisely what cosmologists mal contact to temporarily be taken out of each other’s view. This graph observe today. shows how the region of space that we can see today has grown in both conventional and inflationary cosmologies. Note that the graph is loga- An Evolving Theory rithmic: moving horizontally by 11 mm corresponds to multiplying the Although Guth’s original inflation explained many mysteri- 10 1 unit of time by a factor of 10 (10 billion), while 6 /2 mm on the vertical ous aspects of our universe, the idea was terminally flawed axis corresponds to a 10-billionfold increase in size. — as he noted himself in 1981, when he wrote his first paper 34 November 2005 Sky & Telescope Edg e of visi ble un COSMOLOGICAL PUZZLE NO. 1 iv A er se on the subject. How so? Bubbles generated randomly dur- The Horizon Problem ing the transition to a post-inflationary state would have (A) Shown here in false color, this map of the destroyed the uniformity that inflation had established, cosmic microwave background (CMB) from the producing a universe far more inhomogeneous than the WMAP satellite dramatizes what actually are one we see today. tiny (parts per hundred thousand) deviations “New inflation” — conceived in 1982 by Andrei Linde from the microwave sky’s overall temperature (now at Stanford University) and independently by Paul of 2.7° Kelvin. If the Earth were as smooth as Steinhardt and Andreas Albrecht (now at Princeton Univer- the microwave sky, its highest mountains sity and the University of California, Davis, respectively) — would be no taller than New York City’s sky- solved that problem by modifying the primordial phase scrapers. (B) The radiation emanated about 1 transition. Bubbles still formed, but they grew to such 13 /2 billion years ago from the plasma that gigantic proportions that one would be filled the early universe, and it has streamed enough to encompass the entire observ- toward the Milky Way ever since. (C) Early B able universe. observations hinting at the CMB’s smoothness In 1983 Alexander Vilenkin (Tufts Uni- surprised astronomers, since pre-inflationary versity) pointed out that new inflation cosmology didn’t allow regions now seen on and, indeed, almost all inflation models opposite sides of our sky to ever have been in are “eternal,” meaning that once the Milky WWayay thermal contact. (D) Inflation’s temporary expo- process starts, it never ends. Inflation, nential growth spurt made it possible for all C says Vilenkin, is like a chain reaction, os nd the parcels of cosmic real estate covering our m rou ic mi ckg stopping in one part of space only to ra crowave ba ns skies to have reached thermal equilibrium dia oto continue in another. By churning out an tion (CMBR) ph before being pulled out of one another’s reach. Diagrams are endless number of isolated bubble uni- not to scale verses, he adds, “eternal inflation totally changes the way we view the large-scale structure of space, C beyond our horizon.” As some cosmologists, Linde includ- Standard Cosmology ed, see things, eternal inflation also may provide a physical basis for the anthropic principle, since different “bubbles” can assume very different properties, with only a few being favorable to life (S&T: March 2004, page 42). Inflation’s First Tests Milky Way If eternal inflation sounds metaphysical to you, you’re not alone.

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