Ifentropyhasbeenincreasingforalltime,thenitmust
have been very low at the origin of the universe. Things don’t
naturally acquire low entropy, Carroll says. He realised that
neithertheBigBangnorinflationofferedanexplanation.So
hehadtoconsiderwhatcameevenearlier.
Sincethenaturalstateofthingsishighentropy,he
wondered if a universe could be born from the highest-
entropy-possible state of things. While it might seem
counterintuitive, theories of quantum gravity point to empty
space(withitsformlessvacuum energy)astheultimateand
finalequilibriumstateofaoncelivelyuniverse.That’sbecause
emptyspacehasmorewaystobedisorderlythanauniverse
with structure, or even one filled with a dense, soup-like
matter(see‘Entropy,’facingpage).Suchaplacewouldbevery
boring,mostofthetime.Buttheuncertaintyprincipleof
quantum mechanics allows interesting things to happen.
Justasradioactiveelementsemitparticlesinarandom
way,Carrollsays,there’saprobabilitythatthisempty
spacecangivebirthtotheoccasionalbabyuniverse.Sucha
newbornuniversecanpinchofftoformanewandseparate
regionofspaceandtime.Thisdoesn’tviolatethesecondlaw,
hesays.“Theentropyofthebabyuniverseissmall,butit’sanewadditiontotheentropyoftheuniverseasawhole,which
is therefore still increasing.”
Others conjecture different mechanisms for generating
anarrowoftime—increasingcomplexity,forexample,or
gravity. These ideas assume that time is what’s called an
emergent property—existingonlyundercertainconditions,
just as temperature and pressure as we experience them don’t
existindependentofmatter.Butsome,likephysicistLee
Smolin(PerimeterInstituteforTheoreticalPhysics,Canada),
arguethattimeisafundamentalfacetofrealitythat
transcendsthelawsofphysics.Looking for fingerprints
Inventing interesting, plausible theories, however, has
becomealoteasierthantestingthemwithobservations.
The abundance of untested ideas has led to accusations
thatcosmologyhasleftthepathofempiricalscience.But
cosmologists aver that they didn’t set out to transgress thebounds of experiment — the paradoxes of the Big Bang
as originally formulated forced them to enter seemingly
unobservable realms.
New hope for empiricism came in 2014, when it appeared
that a telescope called BICEP2, located at the South Pole, had
picked up a polarisation pattern in the cosmic microwaves.
This pattern would have ruled out the collision theory and
favored some versions of inflation over others. But it turned
out the observers had misinterpreted their signal — what they
saw had originated from mundane dust in our own galaxy.
Then, in early September 2018, Xingang Chen, Avi Loeb
and Zhong-Zhi Xianyu (Harvard), announced another possible
test: a faint signal that could rule out either all the inflation-
based scenarios or the contraction-then-bounce one.
The test rests on the premise that there was a fraction
of a second during which the space that would become our
universe existed before it became a hot, dense soup of particles
— the so-called hot Big Bang phase. At this stage, space was
permeated by high-energy fields that oscillated at regular time
intervals. The oscillations should have left different patterns
in the density variations that gave rise to cosmic structure,
depending on whether space was expanding exponentially,as in inflation, or contracting. And this, Chen says, might
be observed in the hot and cold regions of the microwave
background, or in the distribution of galaxies across the sky.
He and his colleagues posted the idea on arXiv.org, a site where
not-yet-published scientific papers go up for public review.
If cosmologists can determine whether in this instant
space was contracting or expanding, the result will point
to vastly different pictures of a much larger, or even
infinite, pre-existing physical reality. Thus, although these
cosmological theories might seem fanciful and far removed
from observations, many scientists are not content to let
them stay that way. As long as people keep thinking of tests
that might work, these scenarios remain tethered to the great
expansion of scientific knowledge. FAYE FLAM is a columnist for Bloomberg Opinion. She
writes on science, medicine and the environment. Find more
about her work at fayeflamwriter.com.XB MODESIf inflation happened,
then the resulting ripples in
spacetime should have left
polarisation patterns called B
modes(threeexamplesshown)in
thecosmicmicrowavebackground.
Conversely, a contraction and
bounce would not leave such
patterns.ANTONY LEWIS34 AUSTRALIAN SKY & TELESCOPE February | March 2019
COSMIC REVERIES