Sky & Telescope - February 2019 - 20

Cosmic Reveries

Inﬂation scenario

Bounce scenario

?
Inflationary expansion

Current expansion

Slow contraction

Bounce

Current expansion

ingly common outcome of elementary particle theories. In
Linde's theory of chaotic inﬂation, patches of inﬂating space
can emerge from an existing universe - one that's much less
orderly than our universe, a sort of hodgepodge of regions
dense and sparse.

Bouncing Universes to Baby Universes
Despite such radical variations on the theme, inﬂation
remains the mainstream view in cosmology. It ﬁts with new
observational evidence, much of it from that same leftover
radiation that helped conﬁrm the original Big Bang theory.
Over recent decades, scientists have studied this cosmic
microwave background in ever-ﬁner detail from ground-based
detectors and from the perch of a series of increasingly complex satellites.
According to the theory, if inﬂation happened, then tiny
density bumps in the early universe - inevitable thanks to
the laws of quantum mechanics - would have grown large
during the growth spurt and left an imprint of hot and cold
spots in the cosmic microwave background. That imprint
would survive as inﬂation ended and the hot Big Bang
started, with the denser regions seeding the formation of
galaxies. So far, observations show a pattern of hot and cold
spots that matches those predictions.
Still, other theorists found that they could explain these
same observations with a very different scenario - a series

FE B RUA RY 2 019 * SK Y & TELESCOPE

of "bouncing" phases of contraction and expansion. Back in
2001, a collaboration of physicists and cosmologists proposed
such 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 ﬁ xes, 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 favors 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 cream 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

LE AH TISCIONE / S&T

p INFLATION 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.

Sky & Telescope - February 2019

Table of Contents for the Digital Edition of Sky & Telescope - February 2019