AUGUST 28, 2013
Prof. Cheng Chin and his U Chicago associates have simulated the impossibly hot conditions that followed the big bang within an ultracold vacuum chamber in his sub-basement laboratory in the Gordon Center for Integrative Science.
Photo by Jason Smith
Physicists have reproduced a
pattern resembling the cosmic microwave background radiation in a laboratory
simulation of the Big Bang, using ultracold cesium atoms in a vacuum chamber at
the University of Chicago.
“This is the first time an
experiment like this has simulated the evolution of structure in the early
universe,” said Cheng Chin, professor in physics. Chin and his associates
reported their feat in the Aug. 1 edition of Science Express, and
it will appear soon in the print edition of Science.
Chin pursued the project with lead author Chen-Lung
Hung, PhD’11, now at the California Institute of Technology, and Victor Gurarie
of the University of Colorado, Boulder. Their goal was to harness ultracold
atoms for simulations of the Big Bang to better understand how structure
evolved in the infant universe.
The cosmic microwave background is the echo of the Big
Bang. Extensive measurements of the CMB have come from the orbiting Cosmic
Background Explorer in the 1990s, and later by the Wilkinson Microwave Anisotropy
Probe and various ground-based observatories, including the UChicago-led South
Pole Telescope collaboration. These tools have provided cosmologists with a
snapshot of how the universe appeared approximately 380,000 years following the
Big Bang, which marked the beginning of our universe.
It turns out that under certain conditions, a cloud of
atoms chilled to a billionth of a degree above absolute zero (-459.67 degrees
Fahrenheit) in a vacuum chamber displays phenomena similar to those that
unfolded following the Big Bang, Hung said.
“At this ultracold temperature, atoms get excited
collectively. They act as if they are sound waves in air,” he said. The dense
package of matter and radiation that existed in the very early universe
generated similar sound-wave excitations, as revealed by COBE, WMAP and the
other experiments.
The synchronized generation of sound waves correlates
with cosmologists’ speculations about inflation in the early universe.
“Inflation set out the initial conditions for the early universe to create
similar sound waves in the cosmic fluid formed by matter and radiation,” Hung
said.
BIG BANG’S RIPPLING ECHO
The sudden expansion of the
universe during its inflationary period created ripples in space-time in the
echo of the Big Bang. One can think of the Big Bang, in oversimplified terms,
as an explosion that generated sound, Chin said. The sound waves began
interfering with each other, creating complicated patterns. “That’s the origin
of complexity we see in the universe,” he said.
These excitations are called Sakharov acoustic
oscillations, named for Russian physicist Andrei Sakharov, who described the
phenomenon in the 1960s. To produce Sakharov oscillations, Chin’s team chilled
a flat, smooth cloud of 10,000 or so cesium atoms to a billionth of a degree
above absolute zero, creating an exotic state of matter known as a
two-dimensional atomic superfluid.
Then they initiated a quenching process that
controlled the strength of the interaction between the atoms of the cloud. They
found that by suddenly making the interactions weaker or stronger, they could
generate Sakharov oscillations.
The universe simulated in Chin’s laboratory measured
no more than 70 microns in diameter, approximately the diameter as a human
hair. “It turns out the same kind of physics can happen on vastly different
length scales,” Chin explained. “That’s the power of physics.”
The goal is to better understand the cosmic evolution
of a baby universe, the one that existed shortly after the Big Bang. It was
much smaller then than it is today, having reached a diameter of only a hundred
thousand light years by the time it had left the CMB pattern that cosmologists
observe on the sky today.
In the end, what matters is not the absolute size of
the simulated or the real universes, but their size ratios to the
characteristic length scales governing the physics of Sakharov oscillations.
“Here, of course, we are pushing this analogy to the extreme,” Chin said.
380,000 YEARS VERSUS 10
MILLISECONDS
“It took the whole universe
about 380,000 years to evolve into the CMB spectrum we’re looking at now,” Chin
said. But the physicists were able to reproduce much the same pattern in
approximately 10 milliseconds in their experiment. “That suggests why the
simulation based on cold atoms can be a powerful tool,” Chin said.
None of the Science co-authors are cosmologists, but they
consulted several in the process of developing their experiment and
interpreting its results. The co-authors especially drew upon the expertise of
UChicago’s Wayne Hu, John Carlstrom and Michael Turner, and of Stanford
University’s Chao-Lin Kuo.
Hung noted that Sakharov oscillations serve as an
excellent tool for probing the properties of cosmic fluid in the early
universe. “We are looking at a two-dimensional superfluid, which itself is a
very interesting object. We actually plan to use these Sakharov oscillations to
study the property of this two-dimensional superfluid at different initial
conditions to get more information.”
The research team varied the conditions that prevailed
early in the history of the expansion of their simulated universes by quickly
changing how strongly their ultracold atoms interacted, generating ripples.
“These ripples then propagate and create many fluctuations,” Hung said. He and
his co-authors then examined the ringing of those fluctuations.
Today’s CMB maps show a snapshot of how the universe
appeared at a moment in time long ago. “From CMB, we don’t really see what
happened before that moment, nor do we see what happened after that,” Chin
said. But, Hung noted, “In our simulation we can actually monitor the entire
evolution of the Sakharov oscillations.”
Chin and Hung are interested in continuing this
experimental direction with ultracold atoms, branching into a variety of other
types of physics, including the simulation of galaxy formation or even the
dynamics of black holes.
“We can potentially use atoms to simulate and better
understand many interesting phenomena in nature,” Chin said. “Atoms to us can
be anything you want them to be.”
Citation: “From Cosmology to
Cold Atoms: Observation of Sakharov Oscillations in a Quenched Atomic
Superfluid,” by Chen-Lung Hung, Victor Gurarie and Cheng Chin. Originally
published in Science Express, Aug. 1, 2013
Funding: National Science
Foundation, Army Research Office and the Packard Foundation
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