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Other universes may be detectable, published study claims

Oct. 11, 2007
Special to World Science  

If there are oth­er un­iverses out there—as some sci­en­tists pro­pose—then one or more of them might be de­tect­a­ble, a new study sug­gests.

Such a find­ing, “while cur­rently spec­u­la­tive even in prin­ci­ple, and probably far-off in prac­tice, would surely con­sti­tute an ep­och­al dis­cov­ery,” re­search­ers wrote in a pa­per de­tail­ing their stu­dy. The work ap­pears in the Sep­tem­ber is­sue of the re­search jour­nal Phys­i­cal Re­view D.

Cos­mol­o­gists gen­er­ally hold that even if oth­er un­iverses ex­ist, a con­tro­ver­sial idea it­self, they would­n’t be vis­i­ble, and that test­ing for their ex­istence would be hard at best.

A half-sky map of slight tem­per­a­ture vari­a­tions in the cos­mic mi­cro­wave back­ground ra­di­a­tion, thought to map struc­tures in the very ear­ly uni­verse. Blue stands for colder ar­eas; red for hot­ter re­gions, where it's be­lieved mat­ter was dens­er. These dense re­gions are thought to have lat­er be­come ga­laxy-rich zones. The boxed ar­ea marks an un­u­su­al "cold spot" re­search­ers rec­og­nize in the da­ta. An un­ex­plained gi­ant cos­mic void has also been found in the di­rec­tion of that spot. In a new stu­dy, the­o­ret­i­cal phys­i­cists ar­gue that some sort of ir­reg­u­lar­ity in the mi­cro­wave back­ground, and in mat­ter dis­tri­bu­tion, might in­di­cate where our uni­verse once knocked in­to an­oth­er one. But the re­search­ers take no po­si­tion on wheth­er this cold spot could be the anom­a­ly they're look­ing for. Much more work is needed, they say. (Im­age cour­te­sy WMAP Sci­ence Team, NA­SA)

But the new stu­dy, by three sci­en­tists at the Un­ivers­ity of Cal­i­for­nia, San­ta Cruz, pro­poses that neigh­bor­ing un­iverses might leave a vis­i­ble mark on our own—if, per­chance, they have knocked in­to it. For such a scar to be de­tect­a­ble, they add, the col­li­sion might have had to take place when our un­iverse was very young. Just how the bruise might look re­mains to be clar­i­fied, they say.

“The ques­tion of what the af­ter­math of a col­li­sion might be is still quite open,” wrote Mat­thew C. John­son, one of the re­search­ers, in an e­mail. One the­o­ry even holds that a clash be­tween un­iverses could de­stroy the cos­mos we know. But John­son, now at the Cal­i­for­nia In­sti­tute of Tech­nol­o­gy in Pas­a­de­na, Calif., and col­leagues are ex­am­in­ing quite a dif­fer­ent sort of sce­nar­i­o.

Sev­er­al lines of rea­son­ing in mod­ern phys­ics have led to pro­pos­als that there are oth­er un­iverses. It’s a rath­er dodgy con­cept on its face, be­cause strictly speak­ing, “the un­iverse” means ev­ery­thing that ex­ists. But in prac­tice, cos­mol­o­gists of­ten loos­en the def­i­ni­tion and just speak of “a un­iverse” as some sort of self-en­closed whole with its own phys­i­cal laws.

Such a pic­ture, in con­cept, al­lows for oth­er un­iverses with dif­fer­ent laws. These realms are of­ten called “bub­ble un­ivers­es” or “pock­et un­ivers­es”—partly to side­step the awk­ward def­i­ni­tional is­sue, and partly be­cause many the­o­rists do in­deed por­tray them as bub­ble-like.

A key thread of rea­son­ing be­hind the idea of bub­ble un­iverses, which are some­times col­lec­tively called a “mul­ti­verse,” is the find­ing that seem­ingly emp­ty space con­tains en­er­gy, known as vac­u­um en­er­gy. Some the­o­rize that un­der cer­tain cir­cum­stances this en­er­gy can be con­vert­ed in­to an ex­plo­sively grow­ing, new un­iverse—the same pro­cess be­lieved to have giv­en rise to ours. The­o­ret­i­cal phys­i­cists in­clud­ing Mi­chio Kaku of ­city Col­lege of New York ar­gue that this might go on con­stant­ly—he has called it a “con­tin­ual gen­e­sis”—cre­at­ing many un­iverses, coex­isting not un­like bub­bles in a foamy bath.

How might one de­tect anoth­er un­iverse? John­son and his col­leagues rea­son that any col­li­sion be­tween bub­bles would, like all col­li­sions, pro­duce af­ter­ef­fects that prop­a­gate in­to both cham­bers. These ef­fects would probably take the form of some ma­te­ri­al ejected in­to both sides, John­son said, al­though just what is un­known. This would in turn af­fect the dis­tri­bu­tion of mat­ter in each pock­et un­iverse.

If such col­li­sions hap­pened re­cent­ly, they might be un­de­tect­a­ble be­cause our un­iverse might be too huge to be markedly af­fected; but not so if the events took place long enough ago, ac­cord­ing to the Un­ivers­ity of Cal­i­for­nia team, whose pa­per is al­so posted on­line. If a knock oc­curred when our ex­pand­ing un­iverse was still very small, they ar­gue, then the af­ter­math might still be vis­i­ble, blown up in size along with ev­er­ything else since then.

When the un­iverse was less than a thou­sandth its pre­s­ent size, it’s thought to have un­der­gone a trans­forma­t­ion. As it ex­pand­ed, it be­came cool enough for atoms to form. It then al­so be­came trans­par­ent. Be­fore that, ev­er­ything had been a thick fog, but with ti­ny varia­t­ions in its dens­ity at dif­fer­ent points; dens­er parts would eventually grow and co­a­lesce in­to ga­lax­ies.

This fog is still vis­i­ble, be­cause many of the light waves it gave off are just now reach­ing us: this is how as­tro­no­mers ex­plain a faint glow that per­me­ates space, called the cos­mic mi­cro­wave back­ground. It repre­s­ents the edge of our vis­i­ble un­iverse and is de­tected in all di­rec­tions of the sky.

A col­li­sion would lead to a re­ar­ranged pat­tern of dens­ity fluctua­t­ions in this back­ground, ac­cord­ing to the Un­ivers­ity of Cal­i­for­nia team. It’s un­clear just how this re­ar­range­ment would look, but it would probably ap­pear as some sort of ar­ea of ir­reg­u­lar­ity cen­tered sym­met­ric­ally on a patch of the sky—s­ince “each col­li­sion will af­fect a disc on our sky,” John­son wrote in an e­mail. An anal­o­gy: if you lived in a beach ball and it bounced off anoth­er beach ball, you’d see a change in a cir­cu­lar ar­ea of your wall.

“Noth­ing like this has pre­s­ently been ob­served, al­though no one has ev­er looked for this par­tic­u­lar sig­nal,” John­son added. 

On the oth­er hand, re­search­ers have found at least one strik­ing ir­reg­u­lar­ity in the back­ground glow—a “cold spot,” thought to be re­lat­ed to a vast and anom­a­lous void in the cos­mos. Could that be the mark of a sep­a­rate un­iverse? “I’m go­ing to re­main com­pletely non­com­mit­tal” on that, John­son said. “I can’t even tell you if it would be a hot spot or a cold spot.” Tem­per­a­ture varia­t­ions in the cos­mic mi­cro­wave back­ground are be­lieved to re­flect dens­ity varia­t­ions in the early un­iverse.

John­son and col­leagues stressed that their pro­pos­al may be only the be­gin­ning of a long, pains­tak­ing re­search pro­gram. “Con­nect­ing this pre­dic­tion to real ob­serva­t­ional sig­na­tures will en­tail both dif­fi­cult and com­pre­hen­sive fu­ture work (and probably no small meas­ure of good luck­),” they wrote. But “it ap­pears worth pur­su­ing.”

* * *

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If there are other universes out there—as some scientists propose—then one or more of them might be detectable, a new study suggests. Such a finding, “while currently speculative even in principle, and probably far-off in practice, would surely constitute an epochal discovery,” researchers wrote in a paper detailing their study. The work appears in the September issue of the research journal Physical Review D. Cosmologists generally hold that even if other universes exist, a controversial idea itself, they wouldn’t be visible, and that testing for their existence would be hard at best. But the new study, by three scientists at the University of California, Santa Cruz, proposes that neighboring universes might leave a visible mark on our own universe if they have knocked into it. For such a scar to be detectable, they add, the collision might have had to take place when our universe was very young. Exactly how the bruise might look is uncertain. “The question of what the aftermath of a collision might be is still quite open,” wrote Matthew C. Johnson, one of the researchers, in an email. One theory even holds that a clash between universes could destroy the cosmos we know. But Johnson, now at the California Institute of Technology in Pasadena, Calif., and colleagues are examining quite a different sort of scenario. Several lines of reasoning in modern physics have led to proposals that there are other universes. It’s a rather dodgy concept on its face, because strictly speaking, “the universe” means everything that exists. But in practice, cosmologists often loosen the definition and just speak of “a universe” as some sort of self-enclosed whole with its own physical laws. This picture, in concept, allows for other universes with different laws. They’re often instead called “bubble universes” or “pocket universes”—partly to sidestep the awkward definitional issue, and partly because many theorists do indeed portray them as something like bubbles. A key thread of reasoning behind the idea of bubble universes, which are sometimes collectively called a “multiverse,” is the finding that seemingly empty space contains energy, known as vacuum energy. Some theorize that under certain circumstances this energy can be converted into an explosively growing, new universe—the same process believed to have given rise to ours. Theoretical physicists including Michio Kaku of City College of New York argue that this might go on constantly—he has called it a “continual genesis”—creating many universes, coexisting not unlike bubbles in a foamy bath. How might one detect another universe? Johnson and his colleagues reason that any collision between bubbles would, like all collisions, produce aftereffects that propagate into both chambers. These effects would probably take the form of some material ejected into both sides, Johnson said, although just what is unknown. This would in turn affect the distribution of matter in each pocket universe. If such collisions happened recently, they might be undetectable because our universe might be too huge to be markedly affected; but not so if the events took place long enough ago, according to the University of California team, whose paper is also posted online. If a knock occurred when our expanding universe was still very small, they argue, then the aftermath might still be visible, blown up in size along with everything else since then. When the universe was less than a thousandth its present size, it’s thought to have undergone a transformation. As it expanded, it became cool enough for atoms to form. It then also became transparent. Before that, everything had been a thick fog, but with tiny variations in its density at different points; denser parts would eventually grow and coalesce into galaxies. This fog is still visible, because many of the light waves it gave off are just now reaching us: this is how astronomers explain a faint glow that permeates space, called the cosmic microwave background. It represents the edge of our visible universe and is detected in all directions of the sky. A collision would lead to a rearranged pattern of density fluctuations in this background, according to the University of California team. It’s unclear just how this rearrangement would look, but it would probably appear as some sort of area of irregularity centered on a patch of the sky—since “each collision will affect a disc on our sky,” Johnson wrote in an email. An analogy: if you lived in a beach ball and it bounced off another beach ball, you’d see a change in a circular area of your wall. “Nothing like this has presently been observed, although no one has ever looked for this particular signal,” Johnson added. On the other hand, researchers have found at least one striking irregularity in the background glow—a “cold spot,” thought to be related to a vast and anomalous void in the cosmos. Could that be the mark of a separate universe? “I’m going to remain completely noncommital” on that, Johnson said. “I can’t even tell you if it would be a hot spot or a cold spot.” Temperature variations in the cosmic microwave background are believed to reflect density variations in the early universe. Johnson and colleagues stressed that their proposal may be only the beginning of a long, painstaking research program. “Connecting this prediction to real observational signatures will entail both difficult and comprehensive future work (and probably no small measure of good luck),” they wrote. But “it appears worth pursuing.”