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Cosmologists aim to reveal first moments of time

Feb. 16, 2009
Courtesy University of Chicago
and World Science staff

With­in a dec­ade, a del­i­cate meas­ure­ment of pri­mor­di­al light might re­veal ev­i­dence for the pop­u­lar cos­mic infla­t­ion the­o­ry, which pro­poses that a ran­dom, mi­cro­scop­ic dens­ity fluctua­t­ions in the fab­ric of space and time spawned the uni­verse. 

Such fluctua­t­ions would have led to a hot “Big Bang,” as as­tro­no­mers call the sort of ex­plo­sion be­lieved to have giv­en birth to the cos­mos some 13.7 bil­lion years ago.

The South Pole Tel­e­scope takes ad­van­tage of the clear, dry skies at the Na­tion­al Sci­ence Foun­da­tion's South Pole Sta­tion to study the cos­mic back­ground ra­di­a­tion, the af­ter­glow of the big bang. The tel­e­scope meas­ures eight me­ters (26.4 feet) in di­am­e­ter. (Pho­to by Jeff Mc­Mah­on)

Among cos­mol­o­gists search­ing for ev­i­dence of these events will be John Carl­strom, a Uni­ver­s­ity of Chica­go cos­mol­o­gists who op­er­ates the South Pole Tel­e­scope with sci­en­tists from nine in­sti­tu­tions.

They plan to put cos­mic infla­t­ion the­o­ry to its most strin­gent ob­serva­t­ional test so far by de­tect­ing faint “gra­vity waves”—an ex­ot­ic phe­nom­e­non that Ein­stein’s gen­er­al rel­a­ti­vity the­o­ry pre­dicts cos­mic infla­t­ion should pro­duce.

“If you de­tect gra­vity waves, it tells you a whole lot about infla­t­ion for our uni­verse,” Carl­strom said. It al­so would rule out var­i­ous com­pet­ing ideas for its or­i­gin. “There are few­er than there used to be, but they don’t pre­dict that you have such an ex­treme, hot big bang, this quan­tum fluctua­t­ion, to start with,” he said. Nor would they pro­duce gra­vity waves at de­tecta­ble lev­els.

Carl­strom and col­league Scott Do­del­son were on pan­el of cos­mol­o­gists dis­cussing these and re­lat­ed is­sues on Feb. 16 at the Amer­i­can As­socia­t­ion for the Ad­vance­ment of Sci­ence an­nu­al meet­ing in Chica­go. Fel­low pan­elists in­clud­ed Al­an Guth of the Mas­sa­chu­setts In­sti­tute of Tech­nol­o­gy. In 1979, Guth pro­posed the cos­mic infla­t­ion the­o­ry, which pre­dicts the ex­ist­ence of an in­fi­nite num­ber of uni­verses. Un­for­tu­nate­ly, cos­mol­o­gists have no way of test­ing this pre­diction.

“S­ince these are sep­a­rate uni­verses, by def­i­ni­tion that means we can nev­er have any con­tact with them. Noth­ing that hap­pens there has any im­pact on us,” said Do­del­son, a sci­ent­ist at Fer­mi Na­tional Ac­cel­er­a­tor Lab­o­r­a­to­ry and the Uni­ver­s­ity of Chica­go.

But there is a way to probe the val­id­ity of cos­mic infla­t­ion. The phe­nom­e­non would have pro­duced two clas­ses of per­turba­t­ions, cos­mol­o­gists say. The first, fluctua­t­ions in the dens­ity of sub­a­tom­ic par­t­i­cles, hap­pen con­tin­u­ously eve­ry­where; sci­en­tists have de­tected them. Infla­t­ion would have in­stan­ta­ne­ously stretched some of these per­turba­t­ions in­to cos­mic pro­por­tions. “We can cal­cu­late what those per­turba­t­ions should look like, and it turns out they are ex­actly right to pro­duce the ga­lax­ies we see,” Do­del­son said.

A sim­u­la­tion (click on im­age) of dis­tor­tions in space and time at the sub­a­tom­ic scale, the re­sult of quan­tum fluc­tu­a­tions oc­cur­ring con­tin­u­ous­ through­out the uni­verse. Near the end of the sim­u­la­tion, cos­mic in­fla­tion be­gins to stretch space-time to the cos­mic pro­por­tions of the uni­verse. (Cour­te­sy S. Do­del­son, Fer­mi­l­ab/U. Chi­ca­go)

The sec­ond class of per­turba­t­ions would be gra­vity waves—E­in­steinian dis­tor­tions in space and time. Gra­vity waves al­so would get pro­mot­ed to cos­mic pro­por­tions, per­haps even strong enough for cos­mol­o­gists to de­tect them with sen­si­tive tele­scopes. “We should be able to see them if John’s in­stru­ments are sen­si­tive enough,” Do­del­son said.

Carl­strom and col­leagues are build­ing a spe­cial in­stru­ment, a po­lar­im­e­ter, as an at­tach­ment to the South Pole Tel­e­scope, to search for gra­vity waves. The tel­e­scope is built to de­tect light waves from the mi­cro­wave to the in­fra­red range.

Cos­mol­o­gists al­so use the tel­e­scope in their quest to solve the mys­tery of dark en­er­gy. A re­pul­sive force, dark en­er­gy is pro­posed to push the uni­verse apart and over­whelm gra­vity, the at­trac­tive force ex­erted by all mat­ter. Dark en­er­gy is in­vis­i­ble, but as­tro­no­mers see its ap­par­ent in­flu­ence on clus­ters of ga­lax­ies that formed with­in the last few bil­lion years.

The South Pole Tel­e­scope de­tects the cos­mic mi­cro­wave back­ground radia­t­ion, the af­ter­glow of the “Big Bang.” Cos­mol­o­gists have mined a for­tune of da­ta from the mi­cro­wave back­ground da­ta, which rep­re­sent the force­ful drums and horns of the cos­mic sym­pho­ny. But now the sci­en­tif­ic com­mun­ity has its ears cocked for the tones of a sub­tler in­stru­ment—gravita­t­ional waves—that un­der­lay the mi­cro­wave back­ground.

“We have these key com­po­nents to our pic­ture of the uni­verse, but we really don’t know what phys­ics pro­duces any of them,” said Do­del­son of infla­t­ion, dark en­er­gy and the equally mys­te­ri­ous dark mat­ter. “The goal of the next dec­ade is to iden­ti­fy the phys­ics.”

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Within a decade, a delicate measurement of primordial light might reveal evidence for the popular cosmic inflation theory, which proposes that a random, microscopic density fluctuations in the fabric of space and time spawned the universe. Such fluctuations would have led to a hot “Big Bang,” as astronomers call the sort of explosion believed to have given birth to the cosmos some 13.7 billion years ago. Among cosmologists searching for evidence of these events will be John Carlstrom, a University of Chicago cosmologists who operates the South Pole Telescope with scientists from nine institutions. They plan to put cosmic inflation theory to its most stringent observational test so far by detecting faint “gravity waves”—an exotic phenomenon that Einstein’s general relativity theory predicts cosmic inflation should produce. “If you detect gravity waves, it tells you a whole lot about inflation for our universe,” Carlstrom said. It also would rule out various competing ideas for its origin. “There are fewer than there used to be, but they don’t predict that you have such an extreme, hot big bang, this quantum fluctuation, to start with,” he said. Nor would they produce gravity waves at detectable levels. Carlstrom and colleague Scott Dodelson were on panel of cosmologists discussing these and related issues on Feb. 16 at the American Association for the Advancement of Science annual meeting in Chicago. Fellow panelists included Alan Guth of the Massachusetts Institute of Technology. In 1979, Guth proposed the cosmic inflation theory, which predicts the existence of an infinite number of universes. Unfortunately, cosmologists have no way of testing this prediction. “Since these are separate universes, by definition that means we can never have any contact with them. Nothing that happens there has any impact on us,” said Dodelson, a scientist at Fermi National Accelerator Laboratory and a Professor in Astronomy & Astrophysics at the University of Chicago. But there is a way to probe the validity of cosmic inflation. The phenomenon would have produced two classes of perturbations, cosmologists say. The first, fluctuations in the density of subatomic particles, happen continuously everywhere; scientists have detected them. Inflation would have instantaneously stretched some of these perturbations into cosmic proportions. “We can calculate what those perturbations should look like, and it turns out they are exactly right to produce the galaxies we see in the universe,” Dodelson said. The second class of perturbations would be gravity waves—Einsteinian distortions in space and time. Gravity waves also would get promoted to cosmic proportions, perhaps even strong enough for cosmologists to detect them with sensitive telescopes. “We should be able to see them if John’s instruments are sensitive enough,” Dodelson said. Carlstrom and colleagues are building a special instrument, a polarimeter, as an attachment to the South Pole Telescope, to search for gravity waves. The telescope is built to detect light waves from the microwave to the infrared range. Cosmologists also use the telescope in their quest to solve the mystery of dark energy. A repulsive force, dark energy pushes the universe apart and overwhelms gravity, the attractive force exerted by all matter. Dark energy is invisible, but astronomers are able to see its influence on clusters of galaxies that formed within the last few billion years. The South Pole Telescope detects the cosmic microwave background radiation, the afterglow of the “Big Bang.” Cosmologists have mined a fortune of data from the microwave background data, which represent the forceful drums and horns of the cosmic symphony. But now the scientific community has its ears cocked for the tones of a subtler instrument—gravitational waves—that underlay the microwave background. “We have these key components to our picture of the universe, but we really don’t know what physics produces any of them,” said Dodelson of inflation, dark energy and the equally mysterious dark matter. “The goal of the next decade is to identify the physics.”