US physicists have used ultra-cold atoms to model the universe’s expansion in a lab, a process that could be used to test cosmological theories right here on Earth.
Though you’re probably familiar with solids, liquids, gases and plasma, you might not know about the fifth state of matter: Bose-Einstein condensates (BECs). A BEC comprises an extremely dilute gas of subatomic particles called bosons — held at about one-hundred-thousandth the density of air — super-cooled to temperatures near absolute zero. At these temperatures, quantum effects become observable.
Since BECs were first experimentally produced in 1995, they have been used to mimic phenomena that are tricky to study in the wild, such as superconductivity, magnetism and black holes.
Now a new study, published in the journal Physical Review X, reports using a BEC to simulate the period of inflation, when our infant universe went through an explosive expansion. Inflation began about one trillion, trillion, trillionth of a second after the Big Bang, and saw the universe expand exponentially, and much faster than the speed of light, for a just an instant.
The team drastically cooled several hundred thousand sodium-23 atoms to form a condensate, and used lasers to trap it in a doughnut-shaped cloud. The ring’s radius was then quickly increased, causing the BEC to expand at supersonic speeds.
“From the atomic physics perspective, the experiment is beautifully described by existing theory,” says lead author Stephen Eckel, from the National Institute for Standards and Technology (NIST) in Maryland, US. “But even more striking is how that theory connects with cosmology.”
By studying snapshots of the process, the team observed how the BEC’s characteristics changed over time — including three features analogous to the expanding universe.
Firstly, the wavelengths of phonons stretched out — just like the astronomical redshift of photons during the expansion of the early universe.
Secondly, the team observed hints of “Hubble friction”, a damping effect used in cosmological models.
Thirdly, the BEC underwent an energy transfer process analogous to the “preheating” stage, during which, after it flash-froze during inflation, the universe had to heat back up again in order to form galaxies, stars, and, eventually, us.
“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,” says Gretchen Campbell, co-author of the study and also from NIST.
“And as theorists come up with models, it does give us a testbed where we could actually study those models and see what happens.”
They anticipate that expanding condensates might also help scientists understand other processes, including how particles were created in the early universe.