Sky & Telescope - February 2019 - 21

irreversibly, even if temporary pockets of order can crop up
here and there.
If entropy has been increasing for all time, then it must
have been very low at the origin of the universe. Things don't
naturally acquire low entropy, Carroll says. He realized that
neither the Big Bang nor inﬂation offered an explanation. So
he had to consider what came even earlier.
Since the natural state of things is high entropy, he wondered if a universe could be born from the highest-entropypossible state of things. While it might seem counterintuitive,
theories of quantum gravity point to empty space (with its
formless vacuum energy) as the ultimate and ﬁnal equilibrium
state of a once lively universe. That's because empty space has
more ways to be disorderly than a universe with structure, or
even one ﬁlled with a dense, soup-like matter (see "Entropy,"
facing page). Such a place would be very boring, most of the
time. But the uncertainty principle of quantum mechanics
allows interesting things to happen.
Just as radioactive elements emit particles in a random
way, Carroll says, there's a probability that this empty
space can give birth to the occasional baby universe. Such a
newborn universe can pinch off to form a new and separate
region of space and time. This doesn't violate the second law,

that they didn't set out to transgress the bounds 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 polarization 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
(S&T: May 2015, p. 12).
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,

u B MODES If inflation happened,
then the resulting ripples in spacetime should have left polarization
patterns called B modes (three
examples shown) in the cosmic
microwave background. Conversely,
a contraction and bounce would not
leave such patterns.

he says. "The entropy of the baby universe is small, but it's a
new addition to the entropy of the universe as a whole, which
is therefore still increasing."
Others conjecture different mechanisms for generating an
arrow of time - increasing complexity, for example, or gravity. These ideas assume that time is what's called an emergent
property - existing only under certain conditions, just as
temperature and pressure as we experience them don't exist
independent of matter. But some, like physicist Lee Smolin
(Perimeter Institute for Theoretical Physics, Canada), argue
that time is a fundamental facet of reality that transcends
the laws of physics.

A N TO N Y L E WIS

Looking for Fingerprints
Inventing interesting, plausible theories, however, has become
a lot easier than testing them with observations. The abundance of untested ideas has led to accusations that cosmology
has left the path of empirical science. But cosmologists aver

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.
sk yandtele scope.com * FE B RUA RY 2 019

http://www.arXiv.org http://www.fayeflamwriter.com http://www.skyandtelescope.com

Sky & Telescope - February 2019

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