sky and telescope - san francisco state universitysky & telescope november 200533s&t...

32 November 2005 Sky & Telescope

in january 1980, a young Stanfordphysicist named Alan Guth unveiled a brilliant idea that had just onedrawback: it didn’t work. At the time,Guth (now a professor at MIT) wasfully aware of this shortcoming, yethe was convinced of the idea’s impor-tance nevertheless. History shows hisfaith to have been well placed.

Unlike most 25-year-old ideas thatdon’t quite work, this one, whichGuth called “inflation,” was not dis-carded long ago. Instead, the notionof a fleeting yet explosive growthspurt in the universe’s earliest mo-ments has become a cornerstone ofmodern cosmology. University ofChicago astrophysicist Michael Turnergoes further, calling inflation “themost important idea in cosmologysince the Big Bang.”

Inflationupsizing

By Steve Nadis

Sky & Telescope November 2005 33

S&T illustration by Casey B. Reed

What put the bang in the Big Bang?A physical force whose natureremains cloaked in mystery.

When Guth first conceived of inflation, he doubted thatthe idea would be rigorously tested within his lifetime. Butinflation has already passed numerous observational hur-dles with flying colors. Now, with the age of “precision cos-mology” upon them, astronomers hope to see whether thispowerful idea holds up to even closer scrutiny.

Inflation stands at a critical threshold, claims MIT cos-mologist Max Tegmark. “For the first time, inflation theoryis bumping against data. We’re finally getting to the pointwhere we can kill off a lot of models.” But the cup is onlyhalf full, as the saying goes. Even if the idea withstands thechallenges posed by ever more stringent measurements,theorists still have to explain exactly how inflation works.

Birth of an IdeaGuth, of course, had no idea what he was getting into when,in the late 1970s, he embarked on the path that led to infla-tion. In fact, he knew little about cosmology at the time.The initial problem he took on, with help from CornellUniversity physicist Henry Tye, related to magnetic monopoles— hypothetical particles that carry lone north or southpoles. Guth and Tye’s calculations suggested that fantasti-cally large numbers of these particles should have beenproduced in the Big Bang. Yet none has ever been detected.

Guth and Tye showed that monopole production wouldbe suppressed if a phase transition in the early universe

were delayed by “supercooling,” so that it occurred at a lowertemperature than otherwise would have been the case (justas supercooled water turns to ice well below its normalfreezing point). Late one night in December 1979, Guthdiscovered another consequence of supercooling: it wouldpropel the universe into a state of exponential growth.Inflation was thus born.

The accelerated growth Guth proposed didn’t just dilutemagnetic monopoles to unobservably low densities. It alsosolved numerous cosmological puzzles, explaining why theuniverse is flat, as observed; why it’s so smooth; and evenwhy it produced the small deviations from complete bland-ness that eventually generated galaxies and galaxy clusters.

How does inflation accomplish these feats? Before an-swering that question, let’s first review some of the theory’sbasics. Before the universe was a tiny fraction of a secondold, the theory holds, it already had completed a rapidburst of exponential expansion lasting perhaps only 10–35

second, during which time its volume increased by a factorof 1090 or more. Fueling this outlandish growth was anexotic energy field — the inflaton (not inflation) field — thatturned gravity on its head. During the brief inflationaryepoch, the cosmos was filled with this invisible fog, whichpushed space apart and stretched it out.

This inflation-driving substance had another unusualproperty: it was hard to dilute, maintaining a constant ornearly constant density even as the volume of space it inhabited expanded like mad. Fortunately for life as weknow it, inflation’s gravity-defying energy field was unstable,and it eventually decayed into matter and the radiationnow seen as the cosmic microwave background (CMB). Itwas this transition that allowed the universe to follow a farmore leisurely expansion over the last 131/2 billion years.

Inflation made the observable universe geometrically“flat” in the same way that inflating a balloon flattens a smallpatch on the balloon’s surface. It also explains why today’suniverse is so remarkably smooth, yet not too smooth toform stars, galaxies, and galaxy clusters. The uniformity results from blowing up a tiny region — one small enoughto have achieved thermodynamic equilibrium — into a vastregion encompassing the visible realm. (This addresses theso-called horizon problem that arises in an inflation-free cos-mos, where energy would have had to travel 100 timesfaster than the speed of light in order to bring disparate regions into thermal equilibrium.)

Conversely, the seeds of today’s cosmic structures origi-nated when quantum fluctuations created lumps in theotherwise uniform tapestry of space-time and inflationthen blew them up to macroscopic proportions. Since theserandom, short-lived enhancements of mass and energy werecontinuously produced while space stretched outward, inflation generated fluctuations of roughly the same strengthacross a broad range of spatial scales, leading to a so-called“scale-invariant” spectrum — precisely what cosmologistsobserve today.

An Evolving TheoryAlthough Guth’s original inflation explained many mysteri-ous aspects of our universe, the idea was terminally flawed— as he noted himself in 1981, when he wrote his first paper


psizing up inflation

1026m

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Standardcosmology 1 mm

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Inflationperiod

Presentday

Before inflation entered the picture, most cosmologists believed that to-

day’s observable universe — the region within which light has had time

to reach us — was about 1 millimeter across when it was 10–35 second old.

Although small, this mm-wide region was far vaster than the distance that

light or heat could have traveled since the Big Bang. By contrast, inflation

posits that space expanded exponentially during the universe’s first

10–35 second (or thereabouts), allowing regions that once were in ther-

mal contact to temporarily be taken out of each other’s view. This graph

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-

rithmic: moving horizontally by 11 mm corresponds to multiplying the

unit of time by a factor of 1010 (10 billion), while 61/2 mm on the vertical

axis corresponds to a 10-billionfold increase in size.

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on the subject. How so? Bubbles generated randomly dur-ing the transition to a post-inflationary state would have destroyed the uniformity that inflation had established,producing a universe far more inhomogeneous than theone we see today.

“New inflation” — conceived in 1982 by Andrei Linde(now at Stanford University) and independently by PaulSteinhardt and Andreas Albrecht (now at Princeton Univer-sity and the University of California, Davis, respectively) —solved that problem by modifying the primordial phasetransition. Bubbles still formed, but they grew to suchgigantic proportions that one would beenough to encompass the entire observ-able universe.

In 1983 Alexander Vilenkin (Tufts Uni-versity) pointed out that new inflationand, indeed, almost all inflation modelsare “eternal,” meaning that once theprocess starts, it never ends. Inflation,says Vilenkin, is like a chain reaction,stopping in one part of space only tocontinue in another. By churning out anendless number of isolated bubble uni-verses, he adds, “eternal inflation totallychanges the way we view the large-scale structure of space,beyond our horizon.” As some cosmologists, Linde includ-ed, see things, eternal inflation also may provide a physicalbasis for the anthropic principle, since different “bubbles”can assume very different properties, with only a few beingfavorable to life (S&T: March 2004, page 42).

Inflation’s First TestsIf eternal inflation sounds metaphysical to you, you’re notalone. Even Vilenkin admits that the idea will not be sub-ject to empirical scrutiny anytime soon. Fortunately,though, many of inflation’s predictions are testable, andthey have been tested with exquisite precision. Just wheredoes the theory stand today in light of current data? “Sofar, the results are in beautiful agreement with inflation,”says Steinhardt, an inflation pioneer and occasional critic.

Inflation predicts that space should appear flat becauseany initial curvature in the region of the universe now visi-ble from Earth would have been stretched taut by the uni-verse’s rapid-fire expansion. This solved a problem naggingcosmologists in the 1970s. Preliminary data suggested that the universe was nearly flat — but not quite. Yet pre-inflation theory mandated that the slightest curvature wouldcause it to curl up like a ball (a “closed” geometry) or warplike a saddle (an “open” one). Consequently, cosmologistsreasoned, the universe had to be flat, or ours would be aninexplicably unusual time in cosmic history.

At the time, this vaguely Copernican argument was thebest evidence in favor of a flat cosmos. But data fromNASA’s Wilkinson Microwave Anisotropy Probe (WMAP)satellite and other CMB measurements now show the uni-verse to be flat with a precision of about 1 percent, accord-ing to Tegmark. (The European Space Agency’s Planckspacecraft, scheduled to fly in 2007, should improve uponthat accuracy tenfold.)

Inflation also predicts that the universe should be homo-

Edge of visible universe

Edge of visible universe

Milky Way

Inflationary Cosmology

Milky Way

Standard Cosmology

Diagrams arenot to scale

Milky Way

Cosmic microwave background

radiation (CMBR) photons

Milky Way

(A) Shown here in false color, this map of the

cosmic microwave background (CMB) from the

WMAP satellite dramatizes what actually are

tiny (parts per hundred thousand) deviations

from the microwave sky’s overall temperature

of 2.7° Kelvin. If the Earth were as smooth as

the microwave sky, its highest mountains

would be no taller than New York City’s sky-

scrapers. (B) The radiation emanated about

131/2 billion years ago from the plasma that

filled the early universe, and it has streamed

toward the Milky Way ever since. (C) Early

observations hinting at the CMB’s smoothness

surprised astronomers, since pre-inflationary

cosmology didn’t allow regions now seen on

opposite sides of our sky to ever have been in

thermal contact. (D) Inflation’s temporary expo-

nential growth spurt made it possible for all

the parcels of cosmic real estate covering our

skies to have reached thermal equilibrium

before being pulled out of one another’s reach.

The Horizon Problem COSMOLOGICAL PUZZLE NO. 1

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©2005 Sky Publishing Corp. All rights reserved.


genous on the largest observable scales. Data from WMAP,NASA’s earlier Cosmic Background Explorer, and variousground-based instruments all have borne this out, showingthat the CMB’s temperature varies across the entire sky byonly 1 part in 100,000. These measurements, says Guth,“are every bit as precise as data we get out of particlephysics experiments, and everything seems to be agreeingwith simple inflation.”

Yet another of inflation’s predictions is scale in-variance — the idea that the early universe had nopreferred scale. In such a cosmos, the relativenumbers and sizes of unusually dense or rarefiedregions should look roughly the same no matterhow closely you zoom into the cosmic tapestry. Anumber called the spectral index characterizes thedistribution of these density differences, with avalue of 1 implying perfect scale invariance.

Cosmologists expect a slight departure from 1,explains Tegmark, because if the universe were toremain perfectly scale-invariant, the density wouldnever change and inflation would go on forever. But weknow inflation ended because stars and planets would never have formed in an ever-inflating universe. So far,WMAP and galaxy maps from the Sloan Digital Sky Survey(SDSS) yield a spectral index of 0.97 plus or minus 0.03,which is encouraging, says Tegmark. If the index stays justbelow 1 after all the data from SDSS, WMAP, and Planckare in, that would be a “great triumph” for inflation. A value of exactly 1, on the other hand, would spell trouble,as would persuasive evidence (perhaps from Planck) thatthe universe is not flat after all.

Variations on a ThemeAs observers design ever more exacting tests, theorists aregrappling with their own set of challenges — the principalone being that inflationary theory is not really a theory atall. As many see it, inflation is a collection of scenariosrather than one compelling picture. “There are thousandsof models,” claims Turner, few of which have champions —“apart from the authors and their mothers.”

The abundance of models points to some remaining lee-way in the data as well as an incomplete understanding.“Inflation is still a vague idea that’s based on a vague infla-ton field,” concedes Guth. “What are the detailed dynamicsof this field? Right now we’re making them up.”

Physicists use the term scalar field to describe the gravity-countering substance that drives inflation, much as theydescribe photons (light “particles”) in terms of electromag-netic fields. Inflation’s scalar field, the inflaton field, is simply a number at every point in space, and that numbershould take on the same value everywhere to spur cosmic


Below: The observable universe’s geometry depends on Ω, the density of

all of the matter and energy it contains divided by a critical value. As it

turns out, if Ω differed even slightly from 1 in the universe’s infancy, it

would quickly take on extremely high or low values. Since 1980s-era

censuses of stars, galaxies, and other forms of matter suggested that Ω

was somewhere between 0.1 and 1 in today’s era, cosmologists reasoned

that it had to be exactly 1 to avoid implausible fine-tuning. Facing page(four panels): Inflation naturally explains why the observable universe

appears flat, just as a small patch on a balloon that expands trillions of

times over will look flat to an ant on its surface.

The Flatness Problem COSMOLOGICAL PUZZLE NO. 2

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Flat geometry (Ω = 1)

Critical density of mass/energy

Triangle corners = 180°

“For the first time, inflationtheory is bumping against data.We’re finally getting to the pointwhere we can kill off a lot ofmodels.” — Max Tegmark, MIT

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

expansion — though it must change with time so that infla-tion eventually can end.

According to the theory’s architects, the inflationaryprocess occurs when the universe is dominated by the scalarfield’s potential energy. Called a “false vacuum” by cosmol-ogists, this physical state is often compared to a ball perchedon a gentle hill. As the ball rolls downhill it picks up speed.Potential energy becomes kinetic energy, and acceleratedexpansion ceases. Researchers can concoct different infla-tionary scenarios by altering the slope of the potential-energy curve (the shape of the “hill”).

The whole notion of inflation is predicated on the exis-tence of the hard-to-dilute stuff described by the inflatonfield, says Tegmark. “Nobody knows what that stuff is,though it’s allowed by the laws of physics.” Indeed, the“dark energy” now thought to dominate our universe showsthat gravity can act repulsively — except that dark energy is expected to last a long time, maybe forever, rather thandecaying after a mere 10–35 second, and it is more than 10100

times weaker than inflation (S&T: March 2005, page 32).Guth believes that “quantum gravity” — a theory that

unites the physics of the large (general relativity) and small(quantum mechanics) — may be needed for all these ideasto make sense, with string theory being the leading candi-date. “The answer may indeed lie in a new kind of physics

we haven’t yet developed,” agreesUniversity of Chicago physicistRobert Wald. But quantum gravi-ty, he says, might revise the pic-ture so radically that the universeno longer goes through an early

period of exponential expansion.Even if the explanation for inflation resides in new

physics like string theory, Guth counters, the problems in-flation solves still have to be addressed. “We still need amechanism that makes a universe with 1090 particles,” the

approximate number within the visi-ble cosmos, he says. “For that, you almost certainly need exponentialgrowth. So I’m pretty well convincedthat any solution to those problemswill look a lot like inflation.”

What’s more, Guth adds, “recent advances in string theory make thewhole enterprise appear much moreplausible.” In 1999, for example, Tyeand New York University physicist GiaDvali showed how inflation mightarise through the gravitational attrac-tion of membranes, or “branes,”which, along with strings, serve asfundamental units of space-time(S&T: June 2003, page 38). In Tye andDvali’s model, inflation proceedswhen two stacks of three-dimensionalbranes drift toward each other withinhigher-dimensional space under thetug of gravity. Inflation is driven bythe gravitational potential energy ofthe separated branes, say Tye andDvali; it stops when the branes collideand melt, unleashing the energy ofthe hot Big Bang.

This geometric picture, which relieson the relative motion of branes to

The physics underlying inflation remain mysterious, but most cosmolo-

gists agree that the phenomenon occurred when the fabric of space-time

underwent a phase transition — an abrupt change vaguely akin to that

experienced by water freezing. In some theories, inflation occurred when

the strong force (which binds atomic nuclei) differentiated itself from

the electroweak force (which comprises electromagnetism and the weak

force governing nuclear decay). Adapted fom Michael Seeds, Astronomy:The Solar System and Beyond, 2nd ed.


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Above: Measured only recently, the CMB power spectrum tells

cosmologists the prevalence on the microwave sky of spots with

various angular sizes. Many observations show that the micro-

wave sky is blobbiest on angular scales of about 1/2°. This cor-

responds precisely to a flat cosmos, with the three angles of a

hypothetical intergalactic triangle adding up to 180°.

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Age of universe (second)10–43 10–35 10–12

Temperature (degrees Kelvin)1032 1027 1015

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drive inflation, is now central to string-theory models. Butbrane inflation cannot work without some mechanism forkeeping the six extra spatial dimensions of string theory,which are normally curled up in tight bundles, from un-wrapping during the process and spoiling everything. Abreakthrough came in 2003, when Linde and three coau-thors showed how to keep the extra dimensions clenchedtight. The approach has been utilized in almost everystring inflation model advanced since.

Yet Steinhardt, among others, finds string inflation unap-pealing because the theory predicts an enormous numberof possible universes (10500 or more), each shaped by differ-ent physical parameters and different brands of inflation.“We had hoped string theory would come in and tell us

what the inflaton is and clarify the whole story,” Steinhardtsays. “Instead we’re told that what we see is part of a muchmore complicated ‘landscape’ that may have an unboundednumber of versions of inflation.”

With inflation models growing increasingly “baroque”and “bizarre,” Steinhardt has turned to the “cyclic universe”— a competing paradigm he is developing with Neil Turok(Cambridge University). Steinhardt and Turok’s scenario islike brane inflation without inflation. Instead of two branescoming together and fusing, they bounce off each other,periodically moving apart and drawing together. Matterand radiation get smoothed out during expansion phases,while density fluctuations are created during contractions.Inflation never enters the picture.

Linde, for one, doubts that fluctuations could survive thecyclic universe’s bounce — a so-called “singularity” duringwhich matter and energy get squeezed to infinite densitiesand conventional physics breaks down. In 2004 Matias Zal-darriaga (Harvard-Smithsonian Center for Astrophysics)and his colleagues found that the density perturbationsproduced in the cyclic model are not scale-invariant andthus are incompatible with observations. But Steinhardtmaintains that the cyclic universe fits the data every bit aswell as inflation does.

Will Inflation Survive?While Steinhardt and Turok refine their calculations, somecosmologists see no real alternative to inflation at present.


COSMOLOGICAL PUZZLE NO. 3


Insofar as inflation predicts an essen-

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 Sloan Digital Sky Survey

BOOMERANG TEAM REIDAR HAND / FERMILAB

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

to macroscopic proportions.

Today’s Structured Cosmos

redshift=0Present era

redshift=1.49.1 billionyears ago


1 billionlight-years


Milky Way

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

But it’s too early for a “victory dance,” Turner cautions. Al-though inflation has withstood every attempt to disprove it,the idea has not yet been tested fully. Without some un-equivocal experimental validation, Turner adds, inflation“will remain just a convenient explanation for the observa-tions we see.”

There is, however, a smoking gun on the horizon: gravita-tional waves emitted during the same violent phase transi-tion that spawned inflation. The largest of these primordialspace-time ripples cannot be observed directly becausetheir wavelengths now span the entire visible universe. Butthey would leave a mark in the microwave background.While this signal would be hard to extract from CMB temperature maps, say theorists, gravitational waves wouldcreate a distinctive pattern in maps of theCMB’s polarization.

Although there is certain to be a gravi-tational-wave imprint in the CMB, saysTegmark, it may be too feeble to detect.For “classic inflation,” WMAP will proba-bly not be sensitive enough to see signsof gravitational waves, he says. “Planck,which will be an order of magnitude bet-ter, might be able to see it.” If not, hopeswill turn to the proposed Beyond Ein-stein Inflation Probe, which, if built, willprobe the CMB’s polarization with evengreater resolution than Planck’s.

Finding a gravitational-wave signatureon the CMB would be a monumentalbreakthrough for inflation, cosmologists agree. The ampli-tude of the waves would reveal inflation’s energy scale — theuniverse’s temperature during the exponential growth phase— thereby imposing tight constraints upon inflationary the-ory. But it’s anyone’s guess as to what will actually turn up.

Some of the simplest inflation models yield abundant,large-amplitude gravitational waves that could be spottedwithin a decade. Failure to detect those waves would ruleout a large class of models. But the overall notion of infla-tion would remain standing. Indeed, the gravitationalwaves produced in most string-theory models would be“unobservably small,” even with the best foreseeable tech-nology, according to theorist Juan Maldacena (Institute forAdvanced Study, Princeton).

Turner agrees that detecting inflation-era gravitationalwaves is not guaranteed. If Planck fails to see them, infla-tion’s ultimate corroboration may have to wait for anotherinstrument and another decade. Meanwhile, the status ofthis promising idea will remain up in the air until the nextmake-or-break test comes along.

Regardless of the final verdict, Guth was justified in af-firming inflation’s importance from the very outset, Turnermaintains. “An idea doesn’t have to be right to be important,so long as it gets people thinking in a new way.” By thatstandard alone, inflation has been a tremendous success.

Guth, for his part, takes pride in how well inflation hasheld up over the decades, though he appreciates its limita-tions. “Never have we had a model of the early universe that

worked so well in terms of fitting observations,” he claimedin a Santa Barbara, California, presentation last October.“But we are just as clueless as ever about how to describethe universe in terms of fundamental physics.”

Devising a full-fledged inflation theory will represent a bigstep toward that end, Guth says, though other mysteriesremain. Linde agrees. “Inflation is part of our past,” he says.“It shapes the universe and forms the galaxies, but it doesn’ttell us about the nature of dark matter or dark energy.Although inflation is an important part of the story, and apart I hope will stay with us, it’s not the whole story.” †

Science writer Steve Nadis covers cosmology and related fieldsfrom his Cambridge, Massachusetts, home office.


“Never have we had a model of theearly universe that worked so wellin terms of fitting observations. But we are just as clueless as everabout how to describe the universein terms of fundamental physics.”

— Alan Guth, MIT

Cosmic Background Imager ACBAR

ACBAR TEAMCBI / CALTECH / NSF

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

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