Expanding Cloud of Atoms May Offer Insight into Unresolved Cosmological Questions

Scientists exploring a cloud of ultracold atoms discovered a behavior that has a striking similarity to the universe in microcosm. Their research, which forges new relations between atomic physics and the quick expansion of the early universe, was reported in the April 19 issue of the Physical Review X and featured in Physics.

"From the atomic physics perspective, the experiment is beautifully described by existing theory," says Stephen Eckel, an atomic physicist at the National Institute of Standards and Technology (NIST) and the new paper’s lead author. "But even more striking is how that theory connects with cosmology."

In numerous sets of experiments, Eckel and his colleagues quickly expanded the size of a doughnut-shaped cloud of atoms, taking photographs during the process. The growth happens so rapidly that the cloud is left humming, and an associated hum may have appeared on cosmic scales during the fast expansion of the early universe - an era, which cosmologists term as the period of inflation.

The research brought together experts in atomic physics and gravity, and the authors say it is evidence to the versatility of the Bose-Einstein condensate (BEC) - an ultracold cloud of atoms that can be defined as a single quantum object - as a stage for testing ideas from other areas of physics.

"Maybe this will one day inform future models of cosmology," Eckel says. "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."

This is not the first time that scientists have related BECs and cosmology. Earlier studies imitated black holes and looked for analogs of the radiation predicted to spill forth from their mysterious boundaries. The new experiments concentrate instead on the BEC’s response to a fast expansion, a process that recommends several analogies to what may have transpired during the period of inflation.

The first and most straightforward analogy involves the way that waves travel through an expanding medium. Such a state doesn’t arise frequently in physics, but it occurred during inflation on a grand scale. During that expansion, space itself overextended any waves to much larger sizes and took energy from them through a process called Hubble friction.

The researchers detected similar features in their cloud of atoms in one set of experiments. They imprinted a sound wave onto their cloud—alternating regions of fewer atoms and more atoms around the ring, like a wave in the early universe—and observed its dispersion during expansion. Predictably, the sound wave stretched out, but its amplitude also diminished. The math showed that this damping resembled the Hubble friction, and the behavior was captured properly by numerical simulations and calculations.

"It's like we're hitting the BEC with a hammer," says Gretchen Campbell, the NIST co-director of the Joint Quantum Institute (JQI) and a paper’s coauthor, "and it’s sort of shocking to me that these simulations so nicely replicate what's going on."

The team exposed another, more hypothetical analogy in the second set of experiments. For these tests, they left the BEC free of any sound waves but triggered the same expansion, observing the BEC slosh back and forth until it relaxed.

In a way, that relaxation also bears a resemblance to inflation. Some of the energy that triggered the expansion of the universe eventually ended up forming all of the matter and light that surrounds man. Although there are a number of theories for how this occurred, cosmologists aren’t precisely sure how that unused energy got converted into all the stuff seen around today.

In the BEC, the energy of the expansion was rapidly moved to things like sound waves traveling around the ring. Some initial guesses for why this was taking place looked favorable, but they fell short of predicting the energy transfer perfectly. Therefore, the team now looked at numerical simulations that could capture a more comprehensive picture of the physics.

What materialized was a complex account of the energy conversion: After the expansion came to a standstill, atoms at the outer edge of the ring hit their new, expanded boundary and got reflected back toward the epicenter of the cloud. There, they interfered with atoms still moving outward, forming a zone in the middle where nearly no atoms could live. Atoms on either side of this hostile area had incompatible quantum properties, like two adjacent clocks that are not in sync.

The situation was extremely unstable and finally collapsed, resulting in the creation of vortices throughout the cloud. These vortices, or mini-quantum whirlpools, would break apart and produce sound waves that traveled around the ring, like the radiation and particles left over after inflation. A few vortices even escaped from the edge of the BEC, causing an imbalance that left the cloud rotating.

In contrast to the analogy to Hubble friction, the complicated story of how sloshing atoms can produce numerous quantum whirlpools may bear no similarity to what transpires during and after inflation. But Ted Jacobson, the new paper’s coauthor and a physics professor at the University of Maryland with expertise in black holes, says that his interaction with atomic physicists provided benefits beyond these technical results.

What I learned from them, and from thinking so much about an experiment like that, are new ways to think about the cosmology problem. And they learned to think about aspects of the BEC that they would never have thought about before. Whether those are useful or important remains to be seen, but it was certainly stimulating.

Ted Jacobson​​

Ted got me to think about the processes in BECs differently, and any time you approach a problem and you can see it from a different perspective, it gives you a better chance of actually solving that problem.

Stephen Eckel

Going forward, experiments may explore the complex transfer of energy during expansion more meticulously, or even search for more 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," Campbell says. "And as theorists come up with models, it does give us a testbed where we could actually study those models and see what happens."

The researchers acknowledge the contributions of two coauthors not cited in the text: Avinash Kumar, a graduate student at JQI; and Ian Spielman, a JQI Fellow and NIST physicist.