Sky & Telescope - February 2019 - 18

Cosmic Reveries

existing space, universe, or network of universes. Of course,
nothing could have happened in our observable universe
before it existed, but scientists today are able to conceive
of events "before" the Big Bang by widening their perspective. And for some, the push to uncover our deepest cosmic
origins is tied up with another grand quest - to understand
the nature of time, and why it keeps propelling us so relentlessly into the future.
I noticed the change of heart more than a year ago,
when, after a lecture, I asked pioneer cosmologist Alan Guth
(Massachusetts Institute of Technology) what happened
before the Big Bang. He didn't dismiss the question. Instead,
he said that he's working on it.
Guth reiterated something I'd heard him say in previous
lectures - that the Big Bang theory doesn't tell us "what
banged, why it banged, or what happened before it banged."
He also suggested I speak with physicist Sean Carroll
(California Institute of Technology). Carroll said the idea
that the Big Bang was the beginning of time is a plausible
hypothesis, but not the one he thinks best ﬁts with the universe as we observe it.
So what did come before? There are almost as many
theories as there are theorists, but they fall into a few
broad categories. Some postulate a sea of rapidly expanding
space that gives rise to new universes like bubbles in a pot
of boiling water. Others favor a bland expanse of empty
space that occasionally gives birth to baby universes full of
energy and matter. In one scenario, the Big Bang was more
of a Big Bounce, the comeback of a contracting universe.
And although these cosmic visions might sound more psychedelic than scientiﬁc, the faint imprints of what came
before might not be as unobservable as we once thought.
18

FE B RUA RY 2 019 * SK Y & TELESCOPE

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 wellestablished. 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 building on Guth's suggestion soon proposed, a peculiar kind of

0
10 -35 sec

Time

100 sec
1 month
10,000 yrs
380,000 yrs

Temp
ang phase begins (kelv
Hot Big B
in)
Atomic nuclei form

CMB spectrum ﬁxed

ion balances matter
Radiat
otons released
CMB ph

10 27

10 9
10 7
20,000
3,000

Present
13.8 billion years after
the Big Bang phase

p BIG BANG COSMOLOGY Cosmologists 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.

LE AH TISCIONE / S&T

t'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 inﬁ nite 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-

Sky & Telescope - February 2019

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