surprisingly common outcome of elementary particle
theories.InLinde’stheoryofchaotic inflation,patchesof
inflating space can emerge from an existing universe —
onethat’smuchlessorderlythanouruniverse,asortof
hodgepodge of regions dense and sparse.Bouncing universes to baby universes
Despitesuchradicalvariationsonthetheme,inflation
remains the mainstream view in cosmology. It fits with new
observational evidence, much of it from that same leftover
radiationthathelpedconfirmtheoriginalBigBangtheory.
Over recent decades, scientists have studied this cosmic
microwave background in ever-finer detail from ground-
baseddetectorsandfromtheperchofaseriesofincreasingly
complex satellites.
According to the theory, if inflation happened, then tiny
densitybumpsintheearlyuniverse—inevitablethanksto
thelawsofquantummechanics—wouldhavegrownlarge
duringthegrowthspurtandleftanimprintofhotandcold
spotsinthecosmicmicrowavebackground.Thatimprint
wouldsurviveasinflationendedandthehotBigBang
started, with the denser regions seeding the formation of
galaxies.Sofar,observationsshowapatternofhotandcold
spotsthatmatchesthosepredictions.
Still, other theorists found that they could explain these
sameobservationswithaverydifferentscenario—aseries
of ‘bouncing’ phases of contraction and expansion. Back in
2001,acollaborationofphysicistsandcosmologistsproposedsuch a scenario in which the Big Bang was really a collision of
two existing universes floating in a higher-dimensional space.
The idea took the notion of higher dimensions from string
theory, which predicts the existence of 11 dimensions —
seven spatial ones beyond our familiar three of space and one
of time. While string theory posits that the extra dimensions
are curled up in a way that makes them impossible to observe,
some physicists have proposed that one or more of these
dimensions stretch out, so that our universe might float
within a higher-dimensional space the way a sheet of paper
might float through a three-dimensional room.
One of the inventors of this scenario, Paul Steinhardt
(Princeton University), says that in the past decade he and
his collaborators have streamlined the bouncing universe
idea so that they no longer need the collision or the extra
dimensions. All that’s required is an existing universe, which
contracts slowly until it ‘bounces’ and starts expanding. This,
he says, could happen once or in cycles.
The contraction of an existing universe can solve all the
puzzles that inflation fixes, Steinhardt argues. It smooths out
variations and gives rise to the same large-scale uniformity
and cosmic structure, all without requiring any more
assumptions than inflation does. The cause of the contraction
isn’t well understood, but neither is the cause of inflation.
And the bouncing scenario has the advantage of not
predicting an infinite number of universes, he adds.
Caltech’s Sean Carroll favours another possible prequel
to the Big Bang. He came to thinking about the origin of the
universe while trying to answer a question about everyday
life: Why do we have an apparent arrow of time? The laws of
physics are symmetrical forwards and backwards, and yet, we
can stir milk into coffee and scramble eggs, but can’t un-stir
or unscramble them. Time streams along indifferently into
the future, toward disorder, death and decay.
As Carroll explains in his 2010 book, From Eternity to
Here, scientists of the 1800s finally made some serious
headway on this ancient problem when they discovered the
second law of thermodynamics. A property called entropy,
which is something like disorder, increases relentlessly and
irreversibly, even if temporary pockets of order can crop up
here and there.SINFLATION OR BOUNCE?To explain why the observable universe is geometrically flat and its contents well mixed, astronomers think it began
almost infinitely compact, then grew in a brief, exponential spurt called inflation before continuing to expand more slowly. However, it’s unclear what
came before inflation. One alternative idea is that instead of inflating, the universe existed earlier in a contracting state, then bounced.Inflationary expansion Current expansion?
Slow contraction Bounce Current expansionInflation scenario Bounce scenarioLEAH TISCIONE /S&TENTROPY
We usually shorthand the concept of entropy to ‘disorder’ or
‘randomness’. More specifically, it’s related to the number
of microscopic ways that a macroscopic system can be in
a particular state — the higher the number, the greater the
entropy. For example, a shiny new car has a relatively small
number of ways that its molecules can be arranged such
that the car still qualifies as shiny and new. But there’s a far
greater number of ways that the car can look old — myriad
possibilities for dents, rust spots and so forth. The ‘old’ state
thus has a higher entropy than the ‘new’ state.32 AUSTRALIAN SKY & TELESCOPE February | March 2019
COSMIC REVERIES