Cosmology Meets Quantum Fluids:
Researchers Simulate Expanding
Universe Using Bose-Einstein
Condensates
Cosmology is a scientific field that studies the universe as a whole. Unfortunately, there is only one universe, making it challenging to perform experiments in the same way as other scientific fields. However, researchers at Heidelberg University in Germany have found a solution to this problem by using a Bose-Einstein condensate (BEC) to simulate an expanding universe and certain quantum fields within it. This allows for the study of important cosmological scenarios.
Bose-Einstein condensates are a state of matter that occurs when a group of particles, typically atoms, are cooled to a very low temperature. At this point, the particles start to behave as a single entity, known as a condensate. These fluids can be the subject of experiments, allowing cosmology to be studied in the lab.
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| Albert Einstein Portrait |
To simulate an expanding universe, the researchers began with a flat droplet of BEC composed of potassium-39 atoms in an optical trap. This was the "universe" part of the simulator, and it had a spatial curvature related to the average density of the BEC. The quantum field part was played by phonons, quantized packets of sound energy moving through the fluid. These served as analogues to photons and other quantum fields fluctuating in the actual universe.
The phonons were created by firing a laser at the BEC. When the laser was switched off, a phonon vibration spread through the droplet. By studying the trajectory of these phonons, the researchers were able to confirm that the simulated universe had the spatial curvature they were aiming for. The expansion of space was cleverly instituted by adjusting the strength of interactions between the atoms in the BEC with magnetic fields. Decreasing the interaction strength also decreases the speed of sound, achieving the same effect as a corresponding expansion of space.
One particularly curious feature of quantum fields and dynamic space-time is that an expanding space can produce particles – an effect similar to the creation of Hawking radiation by black holes. By tuning the scattering length of the BEC, the scientists experimented with "ramping" up the size of their mini-universe in different ways, corresponding to uniform, accelerating, and decelerating expansions.
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| Universe Illustration |
What they observed did indeed correspond to the production of phonons, as expected. As these phonons interfered with one another, they produced patterns of random density fluctuations in the BEC. They had thus observed the same phenomenon predicted to be responsible for the seeding of large-scale structure in the early universe.
Even though the simulated universe differs greatly from our own – for example, it has only two spatial dimensions and a different overall curvature – these simple tools may help scientists solve difficult problems in the future.
“Already simplified cosmological models, like the one we considered, can contain some of the not-well-understood phenomena that are present in our universe,” explains Marius Sparn, one of the co-authors of the Nature paper.
This proof-of-principle experiment contained intriguing surprises. Not only were phonons produced by the expansionary ramps, but the characteristics of their collective oscillations depended on the type of ramp performed. The phonons contained information that revealed whether the expansion was constant, accelerating, or decelerating. This interesting feature, which Sparn says was only understood through the interplay between theory and experiment, demonstrates the possibilities of pursuing these lab-based studies.
In summary, this experiment shows that even simplified cosmological models can contain some of the not-well-understood phenomena present in our universe. The use of BECs to simulate the universe and certain quantum fields within it provides a powerful tool for studying cosmological scenarios in the lab. As scientists continue to study these simple models, they may gain insights into the mysteries of the universe that could not be obtained through observations alone.
