'Superstrings' could raise cosmic clatter
Jan 12,2007 00:00 by World-science.net

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 some re­search­ers be­lieve they could find such waves com­ing from strange, wispy cos­mic struc­tures called su­per­strings.

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.)
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. “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.