Big Bang or Big Bounce

8/12/2019 Big Bang or Big Bounce

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Big Bang or Big Bounce?: New Theory on the Universe's Birth

Our universe may have started not with a big bang but with a big bouncean implosion

that triggered an explosion all driven by exotic !uantum"gravitational e##ects

By $artin Bo%owald

&toms are now such a commonplace idea that it is hard to remember how radical theyused to seem (hen scientists #irst hypothesi)ed atoms centuries ago they despaired o#

ever observing anything so small and many !uestioned whether the concept o# atoms

could even be called scienti#ic *radually however evidence #or atoms accumulated andreached a tipping point with &lbert +instein,s -./0 analysis o# Brownian motion the

random %ittering o# dust grains in a #luid +ven then it too1 another 2/ years #or

physicists to develop a theory explaining atomsnamely !uantum mechanicsand

another 3/ #or physicist +rwin $4ller to ma1e the #irst microscope images o# themToday entire industries are based on the characteristic properties o# atomic matter

5hysicists, understanding o# the composition o# space and time is #ollowing a similarpath but several steps behind 6ust as the behavior o# materials indicates that they consist

o# atoms the behavior o# space and time suggests that they too have some #ine"scale

structureeither a mosaic o# spacetime 7atoms8 or some other #iligree wor1 $aterialatoms are the smallest indivisible units o# chemical compounds9 similarly the putative

space atoms are the smallest indivisible units o# distance They are generally thought to

be about -/30 meter in si)e #ar too tiny to be seen by today,s most power#ul

instruments which probe distances as short as -/-; meter uch an interpretation however goes too

#ar because the in#inite values indicate that general relativity itsel# brea1s down To

explain what really happened at the big bang physicists must transcend relativity (e

must develop a theory o# !uantum gravity which would capture the #ine structure o#spacetime to which relativity is blind

The details o# that structure came into play under the dense conditions o# the primordialuniverse and traces o# it may survive in the present"day arrangement o# matter and

radiation =n short i# spacetime atoms exist it will not ta1e centuries to #ind the evidence

as it did #or material atoms (ith some luc1 we may 1now within the coming decade

5ieces o# >pace

5hysicists have devised several candidate theories o# !uantum gravity each applying


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!uantum principles to general relativity in a distinct way $y wor1 #ocuses on the theory

o# loop !uantum gravity 7loop gravity8 #or short@ which was developed in the -../s

using a two"step procedure Airst theorists mathematically re#ormulated general relativity

to resemble the classical theory o# electromagnetism9 the eponymous 7loops8 o# thetheory are analogues o# electric and magnetic #ield lines >econd #ollowing innovative

procedures some that are a1in to the mathematics o# 1nots they applied !uantumprinciples to the loops The resulting !uantum gravity theory predicts the existence o#spacetime atoms see 7&toms o# >pace and Time8 by Cee >molin9 >cienti#ic &merican

6anuary 2//DE

Other approaches such as string theory and so"called causal dynamical triangulations do

not predict spacetime atoms per se but suggest other ways that su##iciently short distances

might be indivisible see 7The *reat cienti#ic

&merican 6ulyE The di##erences among these theories have given rise to controversy but

to my mind the theories are not contradictory so much as complementary >tring theory#or example is very use#ul #or a uni#ied view o# particle interactions including gravity

when it is wea1 Aor the purpose o# disentangling what happens at the singularity where

gravity is strong the atomic constructions o# loop gravity are more use#ul

The theory,s power is its ability to capture the #luidity o# spacetime +instein,s great

insight was that spacetime is no mere stage on which the drama o# the universe un#olds =t

is an actor in its own right =t not only determines the motion o# bodies within theuniverse but it evolves & complicated interplay between matter and spacetime ensues

>pace can grow and shrin1

Coop gravity extends this insight into the !uantum realm =t ta1es our #amiliar

understanding o# particles o# matter and applies it to the atoms o# space and time

providing a uni#ied view o# our most basic concepts Aor instance the !uantum theory o#electromagnetism describes a vacuum devoid o# particles such as photons and each

increment o# energy added to this vacuum generates a new particle =n the !uantum

theory o# gravity a vacuum is the absence o# spacetimean emptiness so thorough wecan scarcely imagine it Coop gravity describes how each increment o# energy added to

this vacuum generates a new atom o# spacetime

The spacetime atoms #orm a dense ever shi#ting mesh Over large distances theirdynamism gives rise to the evolving universe o# classical general relativity Under

ordinary conditions we never notice the existence o# these spacetime atoms9 the mesh

spacing is so tight that it loo1s li1e a continuum But when spacetime is pac1ed withenergy as it was at the big bang the #ine structure o# spacetime becomes a #actor and the

predictions o# loop gravity diverge #rom those o# general relativity

&ttracted to Fepulsion

&pplying the theory is an extremely complex tas1 so my colleagues and = use simpli#ied

versions that capture the truly essential #eatures o# the universe such as its si)e and


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simpli#ied are #ounded on general principles and avoid introducing new ad hoc

assumptions

Using the di##erence e!uations we can try to reconstruct the deep past One possiblescenario is that the initial high"density state arose when a preexisting universe collapsed

under the attractive #orce o# gravity The density grew so high that gravity switched tobeing repulsive and the universe started expanding again impli#ied though this model was

understanding it initially re!uired a set o# numerical simulations that were completed

only in 2//K by &bhay &shte1ar Tomas) 5awlows1i and 5arampreet >ingh all at5ennsylvania >tate University They considered the propagation o# waves representing

the universe both be#ore and a#ter the big bang The model clearly showed that a wave

would not blindly #ollow the classical tra%ectory into the abyss o# a singularity but wouldstop and turn bac1 once the repulsion o# !uantum gravity set in

&n exciting result o# these simulations was that the notorious uncertainty o# !uantummechanics seemed to remain #airly muted during the bounce & wave remained locali)ed

throughout the bounce rather than spreading out as !uantum waves usually do Ta1en at

#ace value this result suggested that the universe be#ore the bounce was remar1ably

similar to our own: governed by general relativity and perhaps #illed with stars andgalaxies =# so we should be able to extrapolate #rom our universe bac1 in time through

the bounce and deduce what came be#ore much as we can reconstruct the paths o# two

billiard balls be#ore a collision based on their paths a#ter the collision (e do not need to1now each and every atomic"scale detail o# the collision

Un#ortunately my subse!uent analysis dashed this hope The model as well as the

!uantum waves used in the numerical simulations turned out to be a special case =n

general = #ound that waves spread out and that !uantum e##ects were strong enough to berec1oned with >o the bounce was not a brie# push by a repulsive #orce li1e the collision

o# billiard balls =nstead it may have represented the emergence o# our universe #rom an

almost un#athomable !uantum statea world in highly #luctuating turmoil +ven i# the

preexisting universe was once very similar to ours it passed through an extended periodduring which the density o# matter and energy #luctuated strongly and randomly

scrambling everything

The #luctuations be#ore and a#ter the big bang were not strongly related to each other The

universe be#ore the big bang could have been #luctuating very di##erently than it did

a#terward and those details did not survive the bounce The universe in short has atragic case o# #orget#ulness =t may have existed be#ore the big bang but !uantum e##ects

during the bounce wiped out almost all traces o# this prehistory


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>ome >craps o# $emory

This picture o# the big bang is subtler than the classical view o# the singularity (hereas

general relativity simply #ails at the singularity loop !uantum gravity is able to handle

the extreme conditions there The big bang is no longer a physical beginning or amathematical singularity but it does put a practical limitation on our 1nowledge

(hatever survives cannot provide a complete view o# what came be#ore

Arustrating as this may be it might be a conceptual blessing =n physical systems as in

daily li#e disorder tends to increase This principle 1nown as the second law o#

thermodynamics is an argument against an eternal universe =# order has been decreasing#or an in#inite span o# time the universe should by now be so disorgani)ed that structures

we see in galaxies as well as on +arth would be all but impossible The right amount o#

cosmic #orget#ulness may come to the rescue by presenting the young growing universe

with a clean slate irrespective o# all the mess that may have built up be#ore

&ccording to traditional thermodynamics there is no such thing as a truly clean slate9

every system always retains a memory o# its past in the con#iguration o# its atoms see7The ean $ cienti#ic &merican 6uneE

But by allowing the number o# spacetime atoms to change loop !uantum gravity allows

the universe more #reedom to tidy up than classical physics would suggest

&ll that is not to say that cosmologists have no hope o# probing the !uantum"gravitational

period *ravitational waves and neutrinos are especially promising tools because they

barely interact with matter and there#ore penetrated the primordial plasma with minimalloss These messengers might well bring us news #rom a time near to or even be#ore the

big bang

One way to loo1 #or gravitational waves is by studying their imprint on the cosmic

microwave bac1ground radiation see 7+choes #rom the Big Bang8 by Fobert F

cienti#ic &merican 6anuary 2//-E =# !uantum"gravitational repulsive gravity drove cosmic in#lation these observations might #ind some

hint o# it Theorists must also determine whether this novel source o# in#lation could

reproduce other cosmological measurements especially o# the early density distributiono# matter seen in the cosmic microwave bac1ground

&t the same time astronomers can loo1 #or the spacetime analogues o# random Brownian

motion Aor instance !uantum #luctuations o# spacetime could a##ect the propagation o#light over long distances &ccording to loop gravity a light wave cannot be continuous9 it

must #it on the lattice o# space The smaller the wavelength the more the lattice distorts

it =n a sense the spacetime atoms bu##et the wave &s a conse!uence light o# di##erentwavelengths travels at di##erent speeds &lthough these di##erences are tiny they may add

up during a long trip Mistant sources such as gamma"ray bursts o##er the best hope o#

seeing this e##ect see 7(indow on the +xtreme Universe8 by (illiam B &twood 5eterA $ichelson and >teven Fit)9 >cienti#ic &merican Mecember 2//HE

=n the case o# material atoms more than 20 centuries elapsed between the #irst


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speculative suggestions o# atoms by ancient philosophers and +instein,s analysis o#

Brownian motion which #irmly established atoms as the sub%ect o# experimental science

The delay should not be as long #or spacetime atoms

Aor charts and graphs related to this story clic1 the images below to enlarge

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Big Bang or Big Bounce