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October 27, 2008

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“Superstrings” could raise cosmic clatter

Jan. 8, 2007
Courtesy University of Washington
and World Science staff

Al­bert Ein­stein the­o­rized long ago that mov­ing things would warp the fab­ric of space and time, which ac­cord­ing to his find­ings are unit­ed as a four-di­men­sional space-time. As the ob­jects trav­eled, they would al­so em­a­nate rip­ples of grav­i­ty called grav­i­ta­tion­al waves.

Cos­mic su­per­strings are the­o­rized to wig­gle and os­cil­late, pro­duc­ing grav­i­ta­tion­al waves, and then to slow­ly shrink as they lose en­er­gy un­til they dis­ap­pear. (Cour­te­sy Uni­ver­si­ty of Wash­ing­ton.)

No one has de­tected that yet, but some re­search­ers be­lieve they could find such waves com­ing from strange, wispy cos­mic struc­tures called su­per­strings.

Many phys­i­cists are in­ter­est­ed in a com­plex, contro­versial set of ideas called string the­o­ry, which casts the four bas­ic forc­es iden­ti­fied in na­ture as man­i­fes­ta­tion of one, un­der­ly­ing force. The four are called elec­tro­mag­netism, weak, strong and grav­i­ty.

String the­o­ry is some­times crit­i­cized for be­ing un­test­a­ble, even un­sci­en­tif­ic. But some ver­sions of it pre­dict the for­ma­tion of ex­ot­ic struc­tures that the re­search­ers say would have ob­serv­a­ble ef­fects: cos­mic su­per­strings. 

These are nar­row tubes of en­er­gy left over from the be­gin­ning of the uni­verse, and stretched to enor­mous lengths by the ex­pan­sion of the uni­verse, said cos­mol­o­gist Craig Ho­gan of the Uni­ver­si­ty of Wash­ing­ton in Seattle, Wash.

If the the­o­ry is cor­rect, there are count­less cos­mic su­per­strings stretched like galaxy-sized rub­ber bands, he added. They re­sem­ble ultra-thin tubes with some of the ear­ly uni­verse pre­served in­side, Ho­gan said. The strings can form in­to loops that flop around and emit grav­i­ta­tion­al waves. In the pro­cess, they give off their en­er­gy and eventually dis­ap­pear.

“They’re so light that they can’t have any ef­fect on cos­mic struc­ture, but they cre­ate this bath of grav­i­ta­tion­al waves just by de­cay­ing,” he said.

The­o­ry holds that eve­ry time some­thing moves it emits a grav­i­ta­tion­al wave. Col­lid­ing black holes would send out more waves than an­ything, typ­i­cal­ly a mil­lion times more pow­er than is pro­duced by all the galax­ies in the uni­verse. 

Some grav­i­ta­tion­al waves could the­o­ret­i­cally be heard, Ho­gan said. But most have a fre­quen­cy, or speed of vi­bra­tion, too low to hear—10 to 20 oc­taves, or full scales, be­low the range of hu­man hear­ing. “Big mass­es tend to take a long time to move about, so there are more sources at low­er fre­quen­cies,” he said. “Sens­ing these vi­bra­tions would add the sound­track to the beau­ti­ful im­age­ry of as­tron­o­my.”

A pro­posed or­bit­ing ob­serv­a­to­ry called the La­ser In­ter­fer­om­etry Space An­ten­na, be­ing de­vel­oped by NASA, could pro­vide the first mea­sure­ments of very low fre­quen­cy grav­i­ta­tion­al waves, per­haps the first such mea­sure­ments at any fre­quen­cy, Ho­gan said. In ad­di­tion to the ex­pected wave sources, these sig­nals al­so might come from su­per­strings—providing the first “real phys­i­cal ev­i­dence that these strings ex­ist,” he said.

Ho­gan and Matt De­Pies, a doc­tor­al stu­dent at the uni­ver­si­ty, were sched­uled to pre­s­ent cal­cu­la­tions for grav­i­ta­tion­al waves gen­er­at­ed by cos­mic strings, as well as the larg­er ra­tion­ale for the space an­ten­na mis­sion, on Mon­day at the Amer­i­can As­tro­nom­i­cal So­ci­e­ty na­tion­al meet­ing in Se­at­tle.

An Earth-based proj­ect called the La­ser In­ter­fer­om­etry Gravitational-Wave Ob­serv­a­to­ry al­so is try­ing to ob­serve grav­i­ta­tion­al waves. But it’s search­ing in high­er fre­quen­cies where Ho­gan be­lieves waves from su­per­strings would be much harder to de­tect, be­cause of back­ground noise.

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Albert Einstein theorized long ago that moving things would warp the fabric of space and time, which according to his findings are united as a four-dimensional space-time. As the objects traveled, they would also emanate ripples of gravity called gravitational waves. No one has detected that yet, but University of Washington researchers believe they could find such waves coming from strange wispy cosmic structures called superstrings. Many physicists are interested in a complex, sometimes-controversial set of ideas called string theory, which casts the four basic forces identified in nature as manifestations of one, underlying force. The four are called electromagnetic, weak, strong and gravity. String theory is sometimes criticized for being untestable, even unscientific. But some versions of it predict the formation of exotic structures that the University of Washington group said would have observable effects: cosmic superstrings. These are narrow tubes of energy left over from the beginning of the universe, and stretched to enormous lengths by the expansion of the universe, said cosmologist Craig Hogan of the university. If the theory is correct, there are countless cosmic superstrings stretched like galaxy-sized rubber bands, he added. They resemble ultra-thin tubes with some of the early universe preserved inside, Hogan said. The strings can form into loops that flop around and emit gravitational waves. In the process, they give off their energy and eventually disappear. “They’re so light that they can’t have any effect on cosmic structure, but they create this bath of gravitational waves just by decaying,” he said. Theory holds that every time something moves it emits a gravitational wave. Colliding black holes would send out more waves than anything, typically a million times more power than is produced by all the galaxies in the universe. Some gravitational waves could theoretically be heard, Hogan said. But most have a frequency, or speed of vibration, too low to hear—10 to 20 octaves, or full scales, below the range of human hearing. “Big masses tend to take a long time to move about, so there are more sources at lower frequencies,” he said. “Sensing these vibrations would add the soundtrack to the beautiful imagery of astronomy.” A proposed orbiting observatory called the Laser Interferometer Space Antenna, being developed by NASA, could provide the first measurements of very low frequency gravitational waves, perhaps the first such measurements at any frequency, Hogan said. In addition to the expected wave sources, these signals also might come from superstrings—providing the first “real physical evidence that these strings exist,” he said. Hogan and Matt DePies, a doctoral student at the university, were scheduled present calculations for gravitational waves generated by cosmic strings, as well as the larger rationale for the space antenna mission, on Monday at the American Astronomical Society national meeting in Seattle. An Earth-based project called the Laser Interferometer Gravitational-Wave Observatory also is trying to observe gravitational waves, but it is searching in higher frequencies where Hogan believes waves from superstrings would be much harder to detect, because of background noise. “The strings, if they exist, are part of that noise, but we want to listen in at lower frequencies and try to detect them,” he said.