Sky and Telescope

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 importance 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. Even Vilenkin admits that the idea will not be subject to empirical scrutiny anytime soon. Fortunately, though, many of inflation’s predictions are testable, and they have been tested with exquisite precision. Just where does the theory stand today in light of current data? “So far, the results are in beautiful agreement with inflation,” says Steinhardt, an inflation pioneer and occasional critic. Inflation predicts that space should appear flat because any initial curvature in the region of the universe now visible from Earth would have been stretched taut by the universe’s rapid-fire expansion. This solved a problem nagging D cosmologists in the 1970s. Preliminary data suggested Inflationary Cosmology that the universe was nearly flat — but not quite. Yet pre- inflation theory mandated that the slightest curvature would cause it to curl up like a ball (a “closed” geometry) or warp like a saddle (an “open” one). Consequently, cosmologists reasoned, the universe had to be flat, or ours would be an inexplicably unusual time in cosmic history. Milky Way At the time, this vaguely Copernican argument was the best evidence in favor of a flat cosmos. But data from NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) satellite and other CMB measurements now show the universe to be flat with a precision of about 1 percent, according to Tegmark. (The European Space Agency’s Planck spacecraft, scheduled to fly in 2007, should improve upon that accuracy tenfold.) CASEY B. REED; PANEL A: WMAP TEAM CASEY B. REED; PANEL &T:

Inflation also predicts that the universe should be homo- S

genous on the largest observable scales. Data from WMAP, Variations on a Theme NASA’s earlier Cosmic Background Explorer, and various As observers design ever more exacting tests, theorists are ground-based instruments all have borne this out, showing grappling with their own set of challenges — the principal that the CMB’s temperature varies across the entire sky by one being that inflationary theory is not really a theory at only 1 part in 100,000. These measurements, says Guth, all. As many see it, inflation is a collection of scenarios “are every bit as precise as data we get out of particle rather than one compelling picture. “There are thousands physics experiments, and everything seems to be agreeing of models,” claims Turner, few of which have champions — with simple inflation.” “apart from the authors and their mothers.” Yet another of inflation’s predictions is scale invariance — the idea that the early universe had no preferred scale. In such a cosmos, the relative “For the first time, inflation numbers and sizes of unusually dense or rarefied regions should look roughly the same no matter theory is bumping against data. how closely you zoom into the cosmic tapestry. A number called the spectral index characterizes the We’re finally getting to the point distribution of these density differences, with a where we can kill off a lot of value of 1 implying perfect scale invariance. Cosmologists expect a slight departure from 1, models.” — Max Tegmark, MIT explains Tegmark, because if the universe were to remain perfectly scale-invariant, the density would never change and inflation would go on forever. But we The abundance of models points to some remaining lee- know inflation ended because stars and planets would way in the data as well as an incomplete understanding. never have formed in an ever-inflating universe. So far, “Inflation is still a vague idea that’s based on a vague infla- WMAP and galaxy maps from the Sloan Digital Sky Survey ton field,” concedes Guth. “What are the detailed dynamics (SDSS) yield a spectral index of 0.97 plus or minus 0.03, of this field? Right now we’re making them up.” which is encouraging, says Tegmark. If the index stays just Physicists use the term scalar field to describe the gravity- below 1 after all the data from SDSS, WMAP, and Planck countering substance that drives inflation, much as they are in, that would be a “great triumph” for inflation. A describe photons (light “particles”) in terms of electromag- value of exactly 1, on the other hand, would spell trouble, netic fields. Inflation’s scalar field, the inflaton field, is as would persuasive evidence (perhaps from Planck) that simply a number at every point in space, and that number the universe is not flat after all. should take on the same value everywhere to spur cosmic

COSMOLOGICAL PUZZLE NO. 2 The Flatness Problem Below: The observable universe’s geometry depends on Ω, the density of was somewhere between 0.1 and 1 in today’s era, cosmologists reasoned all of the matter and energy it contains divided by a critical value. As it that it had to be exactly 1 to avoid implausible fine-tuning. Facing page turns out, if Ω differed even slightly from 1 in the universe’s infancy, it (four panels): Inflation naturally explains why the observable universe would quickly take on extremely high or low values. Since 1980s-era appears flat, just as a small patch on a balloon that expands trillions of censuses of stars, galaxies, and other forms of matter suggested that Ω times over will look flat to an ant on its surface.

1,000

100 Ω (1 second) = 1.01 Closed geometry (Ω > 1) 1.001 High density of mass/energy 10 1.0001 Triangle corners add up to > 180° Universe closed 1.0000... Ω 1 0.9999 0.1 0.999 Ω Open geometry ( < 1) Flat geometry (Ω = 1) Ω 0.01 Low density of mass/energy (1 second) = 0.99 Critical density of Triangle corners add up to < 180° mass/energy Universe open Triangle corners = 180° 0.001 1 102 104 106 Time (seconds)

1 second 1 minute 1 hour 1 day 1 week

SOURCE, ABOVE: 21ST CENTURY ASTRONOMY, JEFF HESTER ET AL.; SOURCE, FACING PAGE (FOUR PANELS): ALAN GUTH & DAVID KAISER / SCIENCE

36 November 2005 Sky & Telescope expansion — though it must change with time so that infla- Temperature (degrees Kelvin) tion eventually can end. 1032 1027 1015 According to the theory’s architects, the inflationary rce process occurs when the universe is dominated by the scalar nuclear fo Strong field’s potential energy. Called a “false vacuum” by cosmol- Grand-unified-theory ogists, this physical state is often compared to a ball perched (GUT) force k nuclear on a gentle hill. As the ball rolls downhill it picks up speed. El Wea ectro Potential energy becomes kinetic energy, and accelerated weak force forc expansion ceases. Researchers can concoct different infla- e E Planck lectr tionary scenarios by altering the slope of the potential- omagnetic

era Inflation energy curve (the shape of the “hill”). force G The whole notion of inflation is predicated on the exis- ravita tional forc tence of the hard-to-dilute stuff described by the inflaton e field, says Tegmark. “Nobody knows what that stuff is, though it’s allowed by the laws of physics.” Indeed, the 10–43 10–35 10–12 “dark energy” now thought to dominate our universe shows Age of universe (second) that gravity can act repulsively — except that dark energy is expected to last a long time, maybe forever, rather than The physics underlying inﬂation remain mysterious, but most cosmolo- decaying after a mere 10–35 second, and it is more than 1010 0 gists agree that the phenomenon occurred when the fabric of space-time times weaker than inflation (S&T: March 2005, page 32). underwent a phase transition — an abrupt change vaguely akin to that Guth believes that “quantum gravity” — a theory that experienced by water freezing. In some theories, inﬂation occurred when unites the physics of the large (general relativity) and small the strong force (which binds atomic nuclei) differentiated itself from (quantum mechanics) — may be needed for all these ideas the electroweak force (which comprises electromagnetism and the weak to make sense, with string theory being the leading candi- force governing nuclear decay). Adapted fom Michael Seeds, Astronomy: date. “The answer may indeed lie in a new kind of physics The Solar System and Beyond, 2nd ed. we haven’t yet developed,” agrees University of Chicago physicist period of exponential expansion. Robert Wald. But quantum gravi- Even if the explanation for inflation resides in new ty, he says, might revise the pic- physics like string theory, Guth counters, the problems in- ture so radically that the universe flation solves still have to be addressed. “We still need a no longer goes through an early mechanism that makes a universe with 1090 particles,” the approximate number within the visible cosmos, he says. “For that, you 80 almost certainly need exponential Predicted Predicted growth. So I’m pretty well convinced 70 peak if peak if that any solution to those problems universe universe will look a lot like inflation.” is closed is open 60 What’s more, Guth adds, “recent Actual advances in string theory make the data whole enterprise appear much more 50 plausible.” In 1999, for example, Tye and New York University physicist Gia 40 Flat geometry Dvali showed how inflation might arise through the gravitational attrac- 30 tion of membranes, or “branes,” which, along with strings, serve as Open geometry SCIENTIFIC AMERICAN 20 Closed geometry fundamental units of space-time (S&T: June 2003, page 38). In Tye and 10 Dvali’s model, inflation proceeds emperature fluctuations (microdegrees Kelvin) T when two stacks of three-dimensional

0 SOURCE: MAX TEGMARK / branes drift toward each other within 20 521 0.5 0.2 Angular scale (degrees) higher-dimensional space under the tug of gravity. Inflation is driven by Above: Measured only recently, the CMB power spectrum tells the gravitational potential energy of cosmologists the prevalence on the microwave sky of spots with the separated branes, say Tye and various angular sizes. Many observations show that the micro- Dvali; it stops when the branes collide 1 wave sky is blobbiest on angular scales of about /2°. This cor- and melt, unleashing the energy of responds precisely to a ﬂat cosmos, with the three angles of a the hot Big Bang. hypothetical intergalactic triangle adding up to 180°. This geometric picture, which relies on the relative motion of branes to

Sky & Telescope November 2005 37 psizing up inflation

COSMOLOGICAL PUZZLE NO. 3 Today’s Structured Cosmos

Right: This supercomputer simulation shows particles of matter attracting one another gravitationally while the universe expands. The lacy structures formed this way resemble those seen in three-dimensional maps of the present era’s galaxy distribution (below). To form such a structured cosmos today, the universe must have started out with some degree of small-scale irregularity in its infancy. Inflation provides the requisite seeds of large- scale structure by inflating microscopic quantum-mechanical fluctuations redshift=18.3 redshift=5.7 redshift=1.4 redshift=0 to macroscopic proportions. 13.4 billion 12.6 billion 9.1 billion Present era years ago years ago years ago

ABOVE: VIRGO CONSORTIUM / VOLKER SPRINGEL (MAX PLANCK INSTITUTE FOR ASTROPHYSICS); LEFT: MICHAEL A. STRAUSS / SDSS

what the inflaton is and clarify the whole story,” Steinhardt says. “Instead we’re told that what we see is part of a much more complicated ‘landscape’ that may have an unbounded number of versions of inflation.” With inflation models growing increasingly “baroque” 1 billion and “bizarre,” Steinhardt has turned to the “cyclic universe” light-years — a competing paradigm he is developing with Neil Turok (Cambridge University). Steinhardt and Turok’s scenario is like brane inflation without inflation. Instead of two branes coming together and fusing, they bounce off each other, periodically moving apart and drawing together. Matter and radiation get smoothed out during expansion phases, while density fluctuations are created during contractions. Milky Way Inflation never enters the picture. drive inflation, is now central to string-theory models. But Linde, for one, doubts that fluctuations could survive the brane inflation cannot work without some mechanism for cyclic universe’s bounce — a so-called “singularity” during keeping the six extra spatial dimensions of string theory, which matter and energy get squeezed to infinite densities which are normally curled up in tight bundles, from un- and conventional physics breaks down. In 2004 Matias Zal- wrapping during the process and spoiling everything. A darriaga (Harvard-Smithsonian Center for Astrophysics) breakthrough came in 2003, when Linde and three coau- and his colleagues found that the density perturbations thors showed how to keep the extra dimensions clenched produced in the cyclic model are not scale-invariant and tight. The approach has been utilized in almost every thus are incompatible with observations. But Steinhardt string inflation model advanced since. maintains that the cyclic universe fits the data every bit as Yet Steinhardt, among others, finds string inflation unap- well as inflation does. pealing because the theory predicts an enormous number of possible universes (10500 or more), each shaped by differ- Will Inﬂation Survive? ent physical parameters and different brands of inflation. While Steinhardt and Turok refine their calculations, some “We had hoped string theory would come in and tell us cosmologists see no real alternative to inflation at present.

Insofar as inflation predicts an essen- Boomerang Sloan Digital Sky Survey tially scale-free spectrum of primordial fluctuations and a visible universe that looks flat today, CMBR observations and galaxy-redshift maps from these instruments and others all support the theory while ruling out several alter- natives. However, they fall short of probing the physics of the inflationary era, when the universe was less than 10–35 second old. This achievement awaits progress in gravitational-wave astronomy, CMBR polarization, and high-energy particle physics.

BOOMERANG TEAM REIDAR HAND / FERMILAB 38 November 2005 Sky & Telescope But it’s too early for a “victory dance,” Turner cautions. Al- Turner agrees that detecting inflation-era gravitational though inflation has withstood every attempt to disprove it, waves is not guaranteed. If Planck fails to see them, infla- the idea has not yet been tested fully. Without some un- tion’s ultimate corroboration may have to wait for another equivocal experimental validation, Turner adds, inflation instrument and another decade. Meanwhile, the status of “will remain just a convenient explanation for the observa- this promising idea will remain up in the air until the next tions we see.” make-or-break test comes along. There is, however, a smoking gun on the horizon: gravita- Regardless of the final verdict, Guth was justified in af- tional waves emitted during the same violent phase transi- firming inflation’s importance from the very outset, Turner tion that spawned inflation. The largest of these primordial maintains. “An idea doesn’t have to be right to be important, space-time ripples cannot be observed directly because so long as it gets people thinking in a new way.” By that their wavelengths now span the entire visible universe. But standard alone, inflation has been a tremendous success. they would leave a mark in the microwave background. Guth, for his part, takes pride in how well inflation has While this signal would be hard to extract from CMB held up over the decades, though he appreciates its limita- temperature maps, say theorists, gravitational waves would tions. “Never have we had a model of the early universe that create a distinctive pattern in maps of the CMB’s polarization. Although there is certain to be a gravitational-wave imprint in the CMB, says “Never have we had a model of the Tegmark, it may be too feeble to detect. For “classic inflation,” WMAP will proba- early universe that worked so well bly not be sensitive enough to see signs in terms of ﬁtting observations. of gravitational waves, he says. “Planck, which will be an order of magnitude bet- But we are just as clueless as ever ter, might be able to see it.” If not, hopes will turn to the proposed Beyond Ein- about how to describe the universe stein Inflation Probe, which, if built, will in terms of fundamental physics.” probe the CMB’s polarization with even greater resolution than Planck’s. — Alan Guth, MIT Finding a gravitational-wave signature on the CMB would be a monumental breakthrough for inflation, cosmologists agree. The ampli- worked so well in terms of fitting observations,” he claimed tude of the waves would reveal inflation’s energy scale — the in a Santa Barbara, California, presentation last October. universe’s temperature during the exponential growth phase “But we are just as clueless as ever about how to describe — thereby imposing tight constraints upon inflationary the- the universe in terms of fundamental physics.” ory. But it’s anyone’s guess as to what will actually turn up. Devising a full-fledged inflation theory will represent a big Some of the simplest inflation models yield abundant, step toward that end, Guth says, though other mysteries large-amplitude gravitational waves that could be spotted remain. Linde agrees. “Inflation is part of our past,” he says. within a decade. Failure to detect those waves would rule “It shapes the universe and forms the galaxies, but it doesn’t out a large class of models. But the overall notion of infla- tell us about the nature of dark matter or dark energy. tion would remain standing. Indeed, the gravitational Although inflation is an important part of the story, and a waves produced in most string-theory models would be part I hope will stay with us, it’s not the whole story.” † “unobservably small,” even with the best foreseeable tech- nology, according to theorist Juan Maldacena (Institute for Science writer Steve Nadis covers cosmology and related fields Advanced Study, Princeton). from his Cambridge, Massachusetts, home office.

Cosmic Background Imager ACBAR

CBI / CALTECH / NSF ACBAR TEAM Sky & Telescope November 2005 39 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.