- Press Release
- August 18, 2022
Scientists zero in on why time flows in one direction
The big bang could be a normal event in the natural evolution of the
universe that will happen repeatedly over incredibly vast time scales
as the universe expands, empties out and cools off, according to two
University of Chicago physicists.
“We like to say that the big bang is nothing special in the history
of our universe,” said Sean Carroll, an Assistant Professor in
Physics at the University of Chicago. Carroll and University of
Chicago graduate student Jennifer Chen are scheduled to post their
paper describing their ideas at http://arxiv.org/ on Thursday evening,
Carroll and Chen’s research addresses two ambitious questions: why
does time flow in only one direction, and could the big bang have
arisen from an energy fluctuation in empty space that conforms to the
known laws of physics?
The question about the arrow of time has vexed physicists for a
century because “for the most part the fundamental laws of physics
don’t distinguish between past and future. They’re time-symmetric,”
And closely bound to the issue of time is the concept of entropy, a
measure of disorder in the universe. As physicist Ludwig Boltzmann
showed a century ago, entropy naturally increases with time. “You can
turn an egg into an omelet, but not an omelet into an egg,” Carroll
But the mystery remains as to why entropy was low in the universe to
begin with. The difficulty of that question has long bothered
scientists, who most often simply leave it as a puzzle to answer in
the future. Carroll and Chen have made an attempt to answer it now.
Previous researchers have approached questions about the big bang
with the assumption that entropy in the universe is finite. Carroll
and Chen take the opposite approach. “We’re postulating that the
entropy of the universe is infinite. It could always increase,” Chen
To successfully explain why the universe looks as it does today, both
approaches must accommodate a process called inflation, which is an
extension of the big bang theory. Astrophysicists invented inflation
theory so that they could explain the universe as it appears today.
According to inflation, the universe underwent a period of massive
expansion in a fraction of a second after the big bang.
But there’s a problem with that scenario: a “skeleton in the closet,”
Carroll said. To begin inflation, the universe would have encompassed
a microscopically tiny patch in an extremely unlikely configuration,
not what scientists would expect from a randomly chosen initial
condition. Carroll and Chen argue that a generic initial condition is
actually likely to resemble cold, empty space-not an obviously
favorable starting point for the onset of inflation.
In a universe of finite entropy, some scientists have proposed that a
random fluctuation could trigger inflation. This, however, would
require the molecules of the universe to fluctuate from a
high-entropy state into one of low entropy-a statistical longshot.
“The conditions necessary for inflation are not that easy to start,”
Carroll said. “There’s an argument that it’s easier just to have our
universe appear from a random fluctuation than to have inflation
begin from a random fluctuation.”
Carroll and Chen’s scenario of infinite entropy is inspired by the
finding in 1998 that the universe will expand forever because of a
mysterious force called “dark energy.” Under these conditions, the
natural configuration of the universe is one that is almost empty.
“In our current universe, the entropy is growing and the universe is
expanding and becoming emptier,” Carroll said.
But even empty space has faint traces of energy that fluctuate on the
subatomic scale. As suggested previously by Jaume Garriga of
Universitat Autonoma de Barcelona and Alexander Vilenkin of Tufts
University, these flucuations can generate their own big bangs in
tiny areas of the universe, widely separated in time and space.
Carroll and Chen extend this idea in dramatic fashion, suggesting
that inflation could start “in reverse” in the distant past of our
universe, so that time could appear to run backwards (from our
perspective) to observers far in our past.
Regardless of the direction they run in, the new universes created in
these big bangs will continue the process of increasing entropy. In
this never-ending cycle, the universe never achieves equilibrium. If
it did achieve equilibrium, nothing would ever happen. There would be
no arrow of time.
“There’s no state you can go to that is maximal entropy. You can
always increase the entropy more by creating a new universe and
allowing it to expand and cool off,” Carroll explained.