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Universe “forgets” its past

July 1, 2007
Courtesy PSU
and World Science staff

The cosmos may undergo ep­ic cy­cles of col­lapse and re-crea­t­ion—but some prop­er­ties of our pre­vi­ous un­iverse have left no mark on our own, a team of phys­i­cists has con­clud­ed. Two con­se­quences of this, they say, are that we can’t know our past un­iverse ex­actly, and suc­ces­sive un­iverses probably aren’t alike.

“An in­trin­sic cos­mic for­get­ful­ness” seems to pre­vent “the eter­nal re­cur­rence of ab­so­lutely iden­ti­cal un­ivers­es,” said team mem­ber Mar­tin Bo­jowald of Penn State Un­ivers­ity in Un­ivers­ity Park, Penn. 

For dec­ades, most phys­i­cists have agreed that our un­iverse was born in a “Big Bang,” an ex­plo­sion of what pre­vi­ously had been an in­fi­nitely com­pact point of ma­te­ri­al. One sign of this is the cos­mos is still found to be ex­pand­ing. But what caused the Big Bang, and what might have pre­ced­ed it? These ques­tions have posed stum­bling blocks, be­cause as tra­di­tion­ally de­scribed by Ein­stein’s The­o­ry of Gen­er­al Rel­a­ti­vity, the Big Bang is a non­sen­si­cal state: a vast amount of en­er­gy packed in­to a point of ze­ro size.

A grow­ing num­ber of sci­en­tists, though, are in­terest­ed in the idea that the un­iverse goes through end­less cy­cles in which the ex­pan­sion re­verses; then space col­lapses back to a point, and re-explodes. Thus the Big Bang would really be a “Big Bounce.”

Bo­jowald and col­leagues at Penn State are ex­plor­ing this no­tion us­ing a the­o­ry called Loop Quan­tum Gra­vity, which they say serves as sort of math­e­mat­i­cal time ma­chine. Their find­ings are to ap­pear in the July 1 early on­line issue of the re­search jour­nal Na­ture Phys­ics, and the Au­gust print edi­tion.

Ein­stein’s the­o­ries did­n’t in­clude the quan­tum physic­s—the mod­ern sci­ence of the fun­da­men­tal build­ing blocks of mat­ter—needed to de­scribe the ex­tremely high en­er­gies of the early cos­mos, Bo­jowald said. Loop Quan­tum Gra­vity, pi­o­neered at Penn State, does, he added. 

Loop Quan­tum Gra­vity is one of the more pop­u­lar the­o­ries that phys­i­cists have de­vised in at­tempts to un­ite na­ture’s var­i­ous forc­es, to de­scribe them as man­i­festa­t­ions of only one, un­der­ly­ing force.

Loop Quan­tum Gra­vity can al­so pro­duce cal­cula­t­ions that trace cos­mic his­to­ry, ac­cord­ing to Bo­jowald. Such work, he said, has found that the be­gin­ning was not in­fi­nitely small or dense after all; this in turn means the equa­t­ions can yield val­id re­sults for the pre-Big Bang era. The num­bers point to a pre­vi­ous un­iverse in which the ge­om­e­try of space and time was si­m­i­lar to that of ours, but with cer­tain prop­er­ties un­know­a­ble, the re­search­ers said.

Bo­jowald said his team re­vised pre­vi­ous equa­t­ions of Loop Quan­tum Gra­vity to create a sim­pler mod­el with more pre­cise re­sults. What tipped off re­search­ers that a sim­plifica­t­ion might ex­ist, he said, was that ear­li­er model was very com­pli­cat­ed, “but its so­lu­tions looked very clean.” 

The new equa­t­ions, though, con­tain some “free” param­e­ters that aren’t pre­cisely known, but which are needed to de­scribe cer­tain prop­er­ties. 

Bo­jowald and col­leagues found that two of these param­e­ters are com­ple­men­ta­ry: one is rel­e­vant al­most ex­clu­sively af­ter the Big Bounce, the oth­er al­most ex­clu­sively be­fore. Be­cause the lat­ter has es­sen­tially no in­flu­ence on cal­cula­t­ions of our cur­rent un­iverse, Bo­jowald con­cudes that its val­ue can’t be back-cal­culated from the oth­er. The param­e­ters rep­re­sent un­cer­tainty in the size of the cos­mos. 

“The pre­cise un­cer­tainty fac­tor for the vol­ume of the pre­vi­ous un­iverse nev­er will be de­ter­mined by... cal­culating back­wards from con­di­tions in our pre­s­ent un­iverse, even with most ac­cu­rate mea­sure­ments we ev­er will be able to make,” he said. The idea is “si­m­i­lar to the un­cer­tainty rela­t­ions in quan­tum physics,” equa­tions that show it’s in­her­ently im­pos­si­ble to know both the po­si­tion and ve­locity of a par­t­i­cle ex­act­ly. The dis­con­nect be­tween one cos­mos and the next al­so im­plies that the un­iverses probably can’t be iden­ti­cal, he added.


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The universe may go through epic cycles of collapse and re-creation—but some of the properties of our previous universe have left no mark on our own, a team of physicists has concluded. One consequence of this, they say, is that the successive universes probably aren’t alike. There seems to exist “an intrinsic cosmic forgetfulness,” which prevents “the eternal recurrence of absolutely identical universes,” said team member Martin Bojowald of Penn State University in University Park, Penn. For decades, most physicists have agreed that our universe was born in a “Big Bang,” an explosion of an infinitely compact point of material. One sign of this is the cosmos is still found to be expanding. But what caused the Big Bang, and what might have preceded it? These questions have posed stumbling blocks, because as traditionally described by Einstein’s Theory of General Relativity, the Big Bang is a nonsensical state: a vast amount of energy packed into a point of zero size. A growing number of scientists, though, have been investigating the idea that the universe goes through endless cycles in which the expansion reverses itself, collapses to a point again, and then re-explodes. Thus the Big Bang would really be a “Big Bounce.” Bojowald and colleagues at Penn State are exploring this using a theory known Loop Quantum Gravity, which they say serves as sort of mathematical time machine. The findings are to appear in the early on-line edition of the research journal Nature Physics on 1 July 2007, and in the August 2007 print edition. Einstein’s theories didn’t include the quantum physics—the modern science of the fundamental building blocks of matter—necessary to describe the extremely high energies that prevailed in the Big Bang era, Bojowald said. Loop Quantum Gravity, pioneered at Penn State, does, he added. Loop Quantum Gravity is one of the more popular theories that physicists have devised in an attempt to unite the various forces of nature, to describe them as manifestations of only one, underlying force. Loop Quantum Gravity can also serve to produce calculations that trace cosmic history. Such calculations have found that the beginning was not infinitely small or dense, Bojowald said, which in turn means the equations can valid results for the pre-Big Bang era. The numbers point to a previous universe in which the geometry of space and time is similar to that of our universe today, but with certain properties unknowable, researchers said. Bojowald said his team revised previous equations of Loop Quantum Gravity to arrive at a simpler model with more precise results. The earlier equations were very complicated, “but its solutions looked very clean”—tipping off researchers that a simplification might exist, he said. The new equations, though, contain some “free” parameters that aren’t precisely known, but which are needed to describe certain properties. Bojowald and colleagues found that two of these parameters are complementary: one is relevant almost exclusively after the Big Bounce, the other almost exclusively before. Because the latter has essentially no influence on calculations of our current universe, Bojowald concudes that its value can’t be back-calculated from the other. The parameters represent uncertainty in the size of the universe before and after the Big Bang. “The precise uncertainty factor for the volume of the previous universe never will be determined by a procedure of calculating backwards from conditions in our present universe, even with most accurate measurements we ever will be able to make,” he said. The idea is “similar to the uncertainty relations in quantum physics,” which indicate that it’s inherently impossible to know both the position and velocity of a particle exactly. The disconnect between one cosmos and the next also implies that the universes probably can’t be identical, he added.