Australian Sky & Telescope - 02.2019 - 03.2019

IT’S NATURAL TO WANT TO KNOW what happened before

the Big Bang. For years, cosmologists answered that it was

unknown, unknowable, or that there was nothing before the

Big Bang, not even time. As you extrapolate our expanding

universe backwards, you eventually reach a point of infinite

density where the known laws of physics break down. The Big

Bang theory doesn’t rule out the possibility that there was

some pre-existing universe from which ours sprang, but if

such a thing existed, it was beyond the reach of science.

But then something changed. Now, serious cosmological

theories posit that the Big Bang happened within a pre-

Redefining the Big Bang

What we do know from observation is that on large scales,

the galaxies sprinkled through the visible universe are

charging away from one another. The universe is expanding,

and if you extrapolate back in time, it looks like everything

was once a dense, trillion-degree soup of disembodied

particles. That’s the part of the Big Bang theory that remains

well-established. When it was first devised, the theory made

several sharp predictions, among them that the universe

would contain a specific ratio of hydrogen, helium and

lithium, and that radiation from the Big Bang would be

detectable today in the form of a pervasive cosmic microwave

background. Both of those were spectacularly confirmed.

But by the 1970s, problems appeared that made it clear

the theory had to be modified. For one thing, the Big Bang

failed to explain the relative homogeneity of the universe. On

very large scales, galaxies are distributed through the sky the

same way in all directions, as if they’d been stirred through

the heavens. But under the original Big Bang theory, it’s

physically impossible for them to have mixed together within

the finite age of the universe. There hasn’t been enough time.

In 1981, Guth hit on an adjustment to the Big Bang

that appeared to take care of the problem — a quick

burst of extremely fast expansion that would precede the

normal, more leisurely expansion of the universe. (Alexei

Starobinsky in the Soviet Union came up with a similar idea

independently.) Guth dubbed his idea inflation, but unlike

what was happening to the currency at the time, cosmological

inflation wouldn’t have diluted the cosmos. As other theorists

SBIG BANG COSMOLOGYCosmologists agree that the modern

observable universe arose from an extremely hot, dense state. As

things expanded and cooled, protons, neutrons and then atomic nuclei

formed. Eventually, the nuclei joined with electrons to make atoms,

and the universe became transparent, freeing the photons that we now

detect as the cosmic microwave background. But what came before

this hot, dense Big Bang state is debated.

existing space, universe or network of universes. Of course,

nothingcouldhavehappenedinourobservableuniverse

beforeitexisted,butscientiststodayareabletoconceiveof

events ‘before’ the Big Bang by widening their perspective.

Andforsome,thepushtouncoverourdeepestcosmic

originsistiedupwithanothergrandquest—tounderstand

thenatureoftime,andwhyitkeepspropellingusso

relentlessly into the future.

Inoticedthechangeofheartmorethanayearago,

when,afteralecture,IaskedpioneercosmologistAlanGuth

(MassachusettsInstituteofTechnology)whathappened

beforetheBigBang.Hedidn’tdismissthequestion.Instead,

he said that he’s working on it.

GuthreiteratedsomethingI’dheardhimsayinprevious

lectures—thattheBigBangtheorydoesn’ttellus“what

banged,whyitbanged,orwhathappenedbeforeitbanged”.

HealsosuggestedIspeakwithphysicistSeanCarroll

(California Institute of Technology). Carroll said the idea

thattheBigBangwasthebeginningoftimeisaplausible

hypothesis,butnottheonehethinksbestfitswiththe

universeasweobserveit.

So whatdidcomebefore?Therearealmostasmany

theoriesastherearetheorists,buttheyfallintoafewbroad

categories.Somepostulateaseaofrapidlyexpandingspace

thatgivesrisetonewuniverseslikebubblesinapotof

boilingwater.Othersfavourablandexpanseofemptyspace

that occasionally gives birth to baby universes full of energy

andmatter.Inonescenario,theBigBangwasmoreofa

BigBounce,thecomebackofacontractinguniverse.And

although these cosmic visions might sound more psychedelic

than scientific, the faint imprints of what came before might

notbeasunobservableasweoncethought.

?

HotBigBangphasebegins

Atomicnucleiform

CMBspectrumfixed

CMBphotonsreleased

Radiationbalancesmatter

(kelvin)

Time Temp

Present

13.8 billion years after

the Big Bang phase

10 –35 sec

100 sec

1 month

10,000 yrs

380,000 yrs

10 27

10 9

10 7

20,000

3,000

0

LEAH TISCIONE /

S&T

30 AUSTRALIAN SKY & TELESCOPE February | March 2019

At first, there was a general agreement that

the Big Bang happened first, and then a tiny

fraction of a second later, inflation began. But

now many cosmologists refer to inflation as

something that happened before the Big Bang.

COSMIC REVERIES