"Long before it's in the papers"
March 16, 2011

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Did the universe once have fewer dimensions?

March 16, 2011
World Science staff

The fa­mil­iar three di­men­sions of space—height, width and length—may have been just one or two when the un­iverse was formed, some phys­i­cists say.

In the March 11 is­sue of the re­search jour­nal Phys­i­cal Re­view Let­ters, a sci­en­tif­ic team pro­poses a test of the the­o­ry us­ing a planned, space-based ob­serv­a­to­ry for gravita­t­ional waves. Gravita­t­ional waves are rip­ples in space and time pre­dicted by Ein­stein’s the­o­ry of rel­a­ti­vity. 

TheLI­SA grav­i­ta­tion­al wave de­tec­tor con­sists of three satel­lites that would pick up grav­i­ta­tion­al waves from dis­tant, large-scale events, such as the Big Bang. (Cour­te­sy NA­SA)

The proposal is also sum­marized in an art­icle in Phys­i­cal Re­view Fo­cus, an on­line mag­a­zine of the Amer­i­can Phys­i­cal So­ci­e­ty, which also publishes Phys­i­cal Re­view Let­ters.

Al­though the the­o­ry is spec­u­la­tive, the re­port says, the re­search­ers think ex­ist­ing da­ta has al­ready shown hints of van­ish­ing di­men­sions at early times. 

According to the report, their pro­pos­al holds that the fa­mil­iar three di­men­sions of space could have been folded in­to two or just one at ex­tremely high en­er­gies and tem­pe­r­a­tures. Such cond­i­tions char­ac­ter­ized the un­iverse just af­ter the “Big Bang” that gave it birth. As the un­iverse cooled, in this view, more spa­tial di­men­sions would have ap­peared one by one. 

The the­o­ry al­so pro­poses that our cur­rent un­iverse has four spa­tial di­men­sions, but that we only ex­pe­ri­ence a three-di­men­sion­al “slice” of it, the re­port goes on. The ap­pear­ance of the fourth spa­tial di­men­sion would have giv­en rise to ex­tra en­er­gy, trig­ger­ing a boost to an on­go­ing ex­pan­sion of the un­iverse. This might in turn ex­plain an ac­cel­er­at­ing ex­pan­sion of the un­iverse that was dis­cov­ered in 1998 and is usu­ally as­cribed to a mys­te­ri­ous “dark en­er­gy” pe­r­vad­ing space. 

The the­o­ry al­so solves some prob­lems in par­t­i­cle phys­ics, said one of its pro­po­nents, phys­i­cist De­jan Sto­jkovic of the State Un­ivers­ity of New York at Buf­fa­lo, ac­cord­ing to Phys­i­cal Re­view Fo­cus.

Ev­i­dence of van­ish­ing di­men­sions may al­ready have been de­tected in cos­mic rays, high-en­er­gy par­t­i­cles from space that show­er the Earth’s at­mos­phe­re, the re­port ex­plains. A 2005 re­a­nal­y­sis of cos­mic ray da­ta showed that the jets of par­t­i­cles pro­duced by the most en­er­get­ic cos­mic rays were aligned un­ex­pectedly close to a flat plane, which, they ar­gue, could point to di­men­sion re­duc­tion. 

Oth­er re­search­ers plan to use the Large Had­ron Col­lider, a par­t­i­cle ac­cel­er­a­tor near Ge­ne­va, Switz­er­land, to test for van­ish­ing di­men­sions, the pub­li­cation said. The ma­chine smashes sub­a­tom­ic par­t­i­cles to re­veal what their com­po­nents are. If di­men­sions van­ish at very high en­er­gies, the think­ing goes, then the par­t­i­cles pro­duced in such col­li­sions would be con­fined to a two-di­men­sional plane, like a piece of pa­pe­r, in­stead of a three-di­men­sion­ vol­ume, like a box.

But in­ter­pret­ing the ac­cel­er­a­tor da­ta may be tricky be­cause dif­fer­ent mod­els will have dif­fer­ent pre­dic­tions, Sto­jkovic said in the re­port. So he and Jo­nas Mureika of Loy­o­la Mary­mount Un­ivers­ity in Los An­ge­les looked for a more de­fin­i­tive test. They set­tled on gravita­t­ional waves—rip­ples in space and time caused by vi­o­lent events in the cos­mos. These waves can’t ex­ist in few­er than three spa­tial di­men­sions, said Sto­jkovic. So in the van­ish­ing-di­men­sions hy­poth­e­sis, he told Fo­cus, “you can’t get around the non-ex­istence of gra­vity waves” at the ear­li­est times.

Gravita­t­ional waves reach­ing Earth are ex­pected to have orig­i­nat­ed at dif­fer­ent times, be­cause they can take a long while to get he­re. Some would pre­sumably come from the ear­li­est eras of the un­iverse, ac­cord­ing to sci­en­tists. Pro­po­nents of the van­ish­ing-di­men­sions hy­poth­e­sis say that the pri­mor­di­al gravita­t­ional waves with the high­est fre­quen­cies—that is, those that os­cil­late fastest—cor­re­spond to the high­est en­er­gies and the ear­li­est times. So there should be a max­i­mum fre­quen­cy of the ob­served waves; high­er fre­quen­cies could­n’t ex­ist be­cause they would come from the few­er-di­men­sion era. 

Sto­jkovic and Mureika have de­vel­oped an es­ti­mate of this fre­quen­cy cut­off: about 10,000 os­cilla­t­ions per sec­ond, de­pend­ing on cer­tain fac­tors. They say this is with­in the range ac­ces­si­ble to the La­ser In­ter­fer­om­eter Space An­ten­na, or LI­SA, a space-based gravita­t­ional wave de­tec­tor planned jointly by NASA and the Eu­ro­pe­an Space Agen­cy. Sto­jkovic and Mureika said they are work­ing with expe­rimen­tal­ists from seve­ral U.S. un­ivers­i­ties to pre­pare a test of their pro­pos­al, though this would be about a de­cade away.

This range of fre­quen­cies “is ex­tremely in­ter­est­ing to probe by ob­serva­t­ions, if that is pos­si­ble at al­l,” Mar­tin Bo­jowald, a the­o­rist at Penn­syl­va­nia State Un­ivers­ity in State Col­lege, told Fo­cus. He said the pa­pe­r is “promis­ing, if one tries to draw re­li­a­ble con­clu­sions in the ab­sence of a de­tailed un­der­ly­ing the­o­ry.”

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The familiar three dimensions of space—height, width and length, as we often think of them—may have been two or just one at the beginning of the universe, some physicists say. In the March 11 issue of the research journal Physical Review Letters, a scientific team proposes a test of the theory using a planned, space-based observatory for gravitational waves. Gravitational waves are ripples in space and time predicted by Einstein’s theory of relativity. A report in Physical Review Focus, an online magazine of the American Physical Society, also summarizes the new proposal. Although the theory is speculative, the report said, the researchers think existing data has already shown hints of vanishing dimensions at early times. Their proposal holds that the familiar three dimensions of space could have been folded into two or just one at extremely high energies and temperatures, such as characterized the universe just after the “Big Bang” that gave it birth. As the universe cooled, in this view, more spatial dimensions would have appeared one by one. The theory also proposes that our current universe has four spatial dimensions, but that we only experience a three-dimensional “slice” of it. The appearance of the fourth spatial dimension would have given rise to extra energy, triggering a boost to an ongoing expansion of the universe. This might in turn explain an accelerating expansion of the universe that was discovered in 1998 and is usually ascribed to a mysterious “dark energy” pervading space. The theory also solves some problems in particle physics, said one of its proponents, physicist Dejan Stojkovic of the State University of New York at Buffalo, according to Physical Review Focus. Evidence of vanishing dimensions may already have been detected in cosmic rays, high-energy particles from space that shower the Earth’s atmosphere, the report explains. A 2005 reanalysis of cosmic ray data showed that the jets of particles produced by the most energetic cosmic rays were aligned unexpectedly close to a flat plane, which, they argue, could point to dimension reduction. Other researchers are planning to use the Large Hadron Collider, a particle accelerator near Geneva, Switzerland, to test for vanishing dimensions. The machine smashes subatomic particles to reveal what their components are. If dimensions vanish at very high energies, the thinking goes, then the particles produced in these collisions would be confined to a two-dimensional plane, like a piece of paper, instead of a three-dimensional volume, like a box. But interpreting the accelerator data may be tricky because different models will have different predictions, Stojkovic said in the report. So he and Jonas Mureika of Loyola Marymount University in Los Angeles looked for a more definitive test. They settled on gravitational waves—ripples in space and time caused by violent events in the cosmos. These waves can’t exist in fewer than three spatial dimensions, said Stojkovic. So in the vanishing-dimensions hypothesis, “you can’t get around the non-existence of gravity waves” at the earliest times. Gravitational waves reaching Earth are expected to have originated at different times, because they can take a long while to get here. Some would presumably come from the earliest eras of the universe, according to scientists. Proponents of the vanishing-dimensions hypothesis say that the primordial gravitational waves with the highest frequencies—that is, those that oscillate fastest—correspond to the highest energies and the earliest times. So there should be a maximum frequency of the observed waves; higher frequencies couldn’t exist because they would come from the fewer-dimension era. Stojkovic and Mureika have developed an estimate of this frequency cutoff: about 10,000 oscillations per second, depending on certain factors. They say this is within the range accessible to the Laser Interferometer Space Antenna, or LISA, a future space-based gravitational wave detector that is a joint project between NASA and the European Space Agency. Stojkovic and Mureika are working with gravitational wave experimentalists from several U.S. universities to prepare a test of their proposal using computer simulations. This range of frequencies “is extremely interesting to probe by observations, if that is possible at all,” Martin Bojowald, a theorist at Pennsylvania State University in State College, told Physical Review Focus. He said the paper is “promising, if one tries to draw reliable conclusions in the absence of a detailed underlying theory.” Stojkovic acknowledges the lack of details in the theory and said he plans to clarify them in collaboration with others. Szabolcs Marka of Columbia University in New York told the publication that that vanishing dimensions may be one of many alternative theories for the observatory to investigate, but that it will take time. “LISA is probably about ten years in the future, so even if the idea works, we will not have observational evidence to assess it for a long time,” he added.