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The big­gest science news. PLUS: Hawk­ing’s last the­ory.

Our Uni­verse is fi­nite and is one of many sim­i­lar uni­verses, ac­cord­ing to Stephen Hawk­ing’s fi­nal pa­per on the na­ture of the cos­mos.

The pa­per, ti­tledA Smooth Exit From Eter­nal In­fla­tion?? is the end re­sult of Hawk­ing’ss long­stand­ing col­lab­o­ra­tion with Thomas Her­tog, a physi­cist based at the Catholic Univer­sity of Leu­ven in Bel­gium, and was sub­mit­ted for pub­li­ca­tion just days be­fore the physi­cist’s death ear­lier this year.

There are sev­eral com­pet­ing the­o­ries re­gard­ing ex­actly how the Uni­verse came to be, but most agree that in the frac­tions of a sec­ond fol­low­ing the Big Bang the Uni­verse ex­panded in­cred­i­bly rapidly in all di­rec­tions, much like a bal­loon does when it’s be­ing blown up. This is known as cos­mic in­fla­tion and ac­counts for the fact that the Uni­verse has a large-scale struc­ture that ap­pears to be the same in ev­ery di­rec­tion. There is far less of a con­sen­sus, how­ever, on what hap­pened next.

Ac­cord­ing to the the­ory of eter­nal in­fla­tion, one of the lead­ing the­o­ries, af­ter the Big Bang some pock­ets of space stopped ex­pand­ing while oth­ers con­tin­ued. This gave rise to many so- called bub­ble uni­verses, which are sep­a­rated from one an­other by vast ar­eas of ex­pand­ing space.

In our Uni­verse, ex­pan­sion ended. This en­abled the gal­ax­ies and stars to form, but it’s just one small pocket of space em­bed­ded in a much larger ex­pand­ing area within which there are count­less other bub­ble uni­verses. Ac­cord­ing to the the­ory, the laws of physics in these bub­ble uni­verses could be dif­fer­ent from ours, mak­ing them very strange worlds in­deed.

But this idea has never sat well with Hawk­ing. “The usual the­ory of eter­nal in­fla­tion pre­dicts that, glob­ally, our Uni­verse is like an in­fi­nite frac­tal, with a mo­saic of dif­fer­ent pocket uni­verses, sep­a­rated by an in­flat­ing ocean,” he said. “The lo­cal laws of physics and chem­istry can dif­fer from one pocket uni­verse to an­other, which to­gether would form a mul­ti­verse. But I have never been a fan of the mul­ti­verse. If the scale of dif­fer­ent uni­verses in the mul­ti­verse is large or in­fi­nite the the­ory can’t be tested.”


The new the­ory is based on the some­what ab­stract con­cept that the Uni­verse acts like a vast holo­gram. Phys­i­cal re­al­ity in cer­tain 3D spa­ces can, thanks to some very clever maths, be re­duced to 2D pro­jec­tions on a sur­face, much like a holo­gram can dis­play a 3D im­age on a 2D sur­face.

Af­ter hash­ing through some com­pli­cated equa­tions, Her­tog and Hawk­ing came to the con­clu­sion that our Uni­verse is fi­nite and far sim­pler than the in­fi­nite frac­tal struc­ture pre­dicted by the old the­ory of eter­nal in­fla­tion.

“We pre­dict that our uni­verse, on the largest scales, is rea­son­ably smooth and glob­ally fi­nite. So it’s not a frac­tal struc­ture,” said Hawk­ing. “We’re not down to a sin­gle, unique Uni­verse, but our find­ings im­ply a sig­nif­i­cant re­duc­tion of the mul­ti­verse, to a much smaller range of pos­si­ble uni­verses.” This means it’s pos­si­ble to test the the­ory through ex­per­i­ment. Her­tog be­lieves that pri­mor­dial grav­i­ta­tional waves, rip­ples made in space-time by some of the most vi­o­lent events in the cos­mos, may pro­vide a means of do­ing so.

ABOVE: Prof Stephen Hawk­ing worked with Thomas Her­tog (to his left) on his fi­nal pa­per

RIGHT: Eter­nal in­fla­tion en­ables the pos­si­bil­ity that many bub­ble uni­verses may ex­ist

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