The rapid expansion of a Bose-Einstein condensate (BEC), a cloud of atoms cooled to temperatures very near to absolute zero in which all the atoms occupy the same quantum state, can mimic the expansion of the early Universe, according to a team of physicists from the University of Maryland, the National Institute of Standards and Technology (NIST) and the Joint Quantum Institute. Their work is published in the journal Physical Review X.
An expanding, ring-shaped Bose-Einstein condensate shares several striking features with the early Universe. Image credit: Emily Edwards, University of Maryland.
“From the atomic physics perspective, our experiments are beautifully described by existing theory. But even more striking is how that theory connects with cosmology,” said Dr. Stephen Eckel, an atomic physicist at NIST.
In several sets of experiments, Dr. Eckel and co-authors rapidly expanded the size of a doughnut-shaped BEC, taking snapshots during the process. The growth happens so fast that the cloud is left humming, and a related hum may have appeared on cosmic scales during the rapid expansion of the early Universe — an epoch that cosmologists refer to as the period of inflation.
“Maybe this will one day inform future models of cosmology. Or vice versa. Maybe there will be a model of cosmology that’s difficult to solve but that you could simulate using a cold atomic gas,” Dr. Eckel said.
It’s not the first time that physicists have connected BECs and cosmology. Prior studies mimicked black holes and searched for analogs of the radiation predicted to pour forth from their shadowy boundaries.
The new experiments focus instead on the BEC’s response to a rapid expansion, a process that suggests several analogies to what may have happened during the period of inflation.
The first and most direct analogy involves the way that waves travel through an expanding medium. Such a situation doesn’t arise often in physics, but it happened during inflation on a grand scale. During that expansion, space itself stretched any waves to much larger sizes and stole energy from them through a process known as Hubble friction.
In one set of experiments, Dr. Eckel and colleagues spotted analogous features in their BEC.
Their imprinted a sound wave onto their cloud — alternating regions of more atoms and fewer atoms around the ring, like a wave in the early Universe — and watched it disperse during expansion. Unsurprisingly, the sound wave stretched out, but its amplitude also decreased.
The math revealed that this damping looked just like Hubble friction, and the behavior was captured well by calculations and numerical simulations.
“It’s like we’re hitting the BEC with a hammer and it’s sort of shocking to me that these simulations so nicely replicate what’s going on,” said co-author Dr. Gretchen Campbell, the NIST co-director of the Joint Quantum Institute.
In a second set of experiments, the scientists uncovered another, more speculative analogy.
For these tests they left the BEC free of any sound waves but provoked the same expansion, watching the BEC slosh back and forth until it relaxed.
In a way, that relaxation also resembled inflation. Some of the energy that drove the expansion of the Universe ultimately ended up creating all of the matter and light around us. And although there are many theories for how this happened, cosmologists aren’t exactly sure how that leftover energy got converted into all the stuff we see today.
In the BEC, the energy of the expansion was quickly transferred to things like sound waves traveling around the ring. Some early guesses for why this was happening looked promising, but they fell short of predicting the energy transfer accurately. So the authors turned to numerical simulations that could capture a more complete picture of the physics.
What emerged was a complicated account of the energy conversion: after the expansion stopped, atoms at the outer edge of the ring hit their new, expanded boundary and got reflected back toward the center of the cloud.
There, they interfered with atoms still traveling outward, creating a zone in the middle where almost no atoms could live.
Atoms on either side of this inhospitable area had mismatched quantum properties, like two neighboring clocks that are out of sync.
The situation was highly unstable and eventually collapsed, leading to the creation of vortices throughout the cloud.
These vortices, or little quantum whirlpools, would break apart and generate sound waves that ran around the ring, like the particles and radiation left over after inflation.
Some vortices even escaped from the edge of the BEC, creating an imbalance that left the cloud rotating.
Unlike the analogy to Hubble friction, the complicated story of how sloshing atoms can create dozens of quantum whirlpools may bear no resemblance to what goes on during and after inflation.
Future experiments may study the complicated transfer of energy during expansion more closely, or even search for further cosmological analogies.
“The nice thing is that from these results, we now know how to design experiments in the future to target the different effects that we hope to see. And as theorists come up with models, it does give us a testbed where we could actually study those models and see what happens,” Dr. Campbell said.
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S. Eckel et al. 2018. A Rapidly Expanding Bose-Einstein Condensate: An Expanding Universe in the Lab. Phys. Rev. X 8 (2); doi: 10.1103/PhysRevX.8.021021
