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.

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 “mul­ti­-verse,” 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.

Courtesy of World-Science