Quantum simulator to create the universe
What happened at the beginning of the universe has always puzzled scientists, in part because we can't go back far enough in time to see what happened in this ancient era. Scientists believe the universe must have undergone a huge expansion after the Big Bang, but it's unclear how this rapid expansion phase unfolded.
A study published Wednesday in the journal Nature reports [1] that a team of physicists at Heidelberg University created a tiny expanding universe using a "quantum field simulator" made of ultracold atoms. The experiment is able to simulate different versions of curved spacetime that correspond to models of the universe with a spherical or hyperbolic geometry. These tunable curvatures of spacetime affect how particles are created, among many other factors.
The goal of the experiment is to explore dynamics that might resemble the early universe in different scenarios in the lab, being able to pause the whole system and analyze it more closely - something you can't do in the real universe.
The success of the experiment suggests that similar simulators "offer the possibility of entering unexplored territory" in quantum physics, which is the study of matter and energy at the tiny scale of the atom, the team said in the study. While no experiment has been able to produce conditions directly comparable to those of the early universe, the new study explores physical mechanisms that may be somewhat similar to the creation of space-time and particles after the Big Bang.
Nikolas Liebster, an experimental physicist at the University of Heidelberg in Germany, said, "We're certainly not the first to perform some kind of expansion or to demonstrate the production of such particles. But we are the first to put it in this particular context, where these different types of expansion histories - such as an accelerating universe, a decelerating universe, or an expanding universe - can change the particles you produce."
Liebster noted, "The general role of these simulated cosmology and simulated gravity experiments is that if I have a system that resembles some sort of cosmological model - either a hydrodynamic model or a quantum model - -what experiments can I do to learn more about what might have happened in the cosmological context of the history of our universe? How can experiments further the theory and deepen our understanding of how we got to where we are today?"
Liebster and his colleagues explored these questions by cooling about 20,000 potassium-39 atoms to a temperature slightly above absolute zero (about -273.15 degrees Celsius). At low temperatures, the atoms formed Bose-Einstein condensation, a state of matter that can be used to model various exotic physical phenomena that occur around black holes or in the early universe.
The Bose-Einstein condensation (BEC) in this experiment is a superfluid, meaning a fluid without viscosity, shaped like a two-dimensional pancake. The setup can be adjusted to simulate different theories of the expansion of the universe and different types of spacetime curvature, such as the planar model, the spherical model, and the hyperbolic model.
By having sound waves (an analog of light traveling through the universe) travel through the BEC - Liebster and his colleagues were able to examine the strange physics of each model, which may be similar to those that appeared in the early universe. The sound waves in the experiment play the role of light waves in the real universe, as their path through the BEC is affected by the different models. "In the past, our universe may have had different types of spatial curvature, and that's what we can tune in the system," Liebster explained. "We can control those parameters."
He continues, "How sound waves travel through your system is a very effective way to check the shortest path between two points, because sound waves always take the shortest path. Sound waves are like light waves in real cosmology. They have the same properties, which is why we use them to detect our space-time."
In this way, the team was able to simulate models of the expanding universe that can be stopped to examine the dynamics behind them, which Liebster called "a dream in cosmology. In short, the experiment matches theoretical predictions of different curvatures in time and space, validating the simulation, although it does not currently confirm or refute any particular model of the early universe.
Our work is mostly about benchmarking our simulator, and there are a lot of very interesting theoretical questions you can ask about the different types of curvature in space-time and curvature in space, and their effects," Liebster said. But there are still many hurdles to overcome before we can make a direct one-to-one comparison with the real universe."
Liebster continued: "This is only an approximation at the end of the day. I must be careful to say that cosmology has very specific results. But we know that for these specific assumptions of this model system, it fits the theory very well, and now we can ask questions beyond what the current theory can answer."
[1]https://www.nature.com/articles/s41586-022-05313-9
