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cosmic structures

Jan 11, 2007 05:04 AM
by christinaleestemaker


"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 iden­ti­
fied 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 un­
sci­en­tif­ic. 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 Wash­ing­
ton 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 struc­
ture, but they cre­ate this bath of grav­i­ta­tion­al waves just by de­cay­
ing," he said.

The­o­ry holds that eve­ry time some­thing moves it emits a grav­i­ta­tion­
al wave. Col­lid­ing black holes would send out more waves than an­
ything, 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 hear­
ing. "Big mass­es tend to take a long time to move about, so there 
are more sources at low­er fre­quen­cies," he said. "Sens­ing these vi­
bra­tions would add the sound­track to the beau­ti­ful im­age­ry of as­tron­

A pro­posed or­bit­ing ob­serv­a­to­ry called the La­ser In­ter­fer­om­etry 
Space An­ten­na, be­ing de­vel­oped by NASA, could pro­vide the first mea­
sure­ments 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 su­per­
strings—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 an­
ten­na mis­sion, on Mon­day at the Amer­i­can As­tro­nom­i­cal So­ci­e­ty na­tion­
al 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 

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