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The first moments after the birth of the universe are shrouded in mystery, in part because we cannot peer far enough back in time to observe what unfolded in this ancient era. Scientists think that the universe must have undergone an enormous expansion right after the Big Bang, but it’s not clear at all how this rapid inflationary stage unfolded.
Now, a team of physicists has created a kind of tiny expanding universe with a “quantum field simulator” made of ultracold atoms, reports a study published on Wednesday in Nature. The experiment was able to simulate different versions of curved spacetime that correspond to models of the universe as spherical or hyperbolic in its geometry, for example. These adjustable spacetime curvatures influence how particles are produced, among many other factors.
The goal of the experiment was to explore dynamics that might be similar to the early universe under different scenarios in a lab, with the ability to pause the whole system and analyze it more closely—something you can’t do with the real universe.
The success of the experiment suggests that similar simulators “offer the possibility to enter unexplored regimes” in quantum physics, which is the study of matter and energy on the tiny scales of atoms, the team said in the study. While no experiment can produce conditions that are directly comparable to the conditions of the early universe, the new research probes mechanisms that might be somewhat analogous to the physics governing spacetime and particle production in the moments after the Big Bang.
“We're definitely not the first experiment to do some kind of expansion or to show this particle production,” said Nikolas Liebster, an experimental physicist at Heidelberg University in Germany who co-authored the study, in a call with Motherboard. “But we're the first to put it in this specific context of how these different kinds of inflationary histories—like an accelerated universe, a decelerated universe, or a constantly expanding universe—can change the particles that you produce.”
“The general role of these analog cosmology and analog gravity experiments is to be able to see, if I have a system that's analogous to some kind of cosmological model—whether it's hydrodynamic model, or a quantum model, or all of these kinds of things—what experiments can I do to learn more about what could have happened in the cosmological setting in our universe’s history?” Liebster noted. “And how can the experiments push the theory further to deepen our understanding of how we got to where we are now?”
Liebster and his colleagues explored these questions by cooling about 20,000 potassium-39 atoms down to temperatures barely above absolute zero (roughly around -400°F). In this frigid environment, the atoms form what’s known as a Bose–Einstein condensate, which is a state of matter that can be used to simulate the kinds of exotic physical phenomena that occur around black holes or in the early universe.
The condensate in this experiment was a superfluid, meaning a fluid that exhibits no viscosity, that was shaped like a two-dimensional pancake. The setup could be adjusted to simulate different theories of cosmic inflation as well as different types of spacetime curvature, such as flat, spherical, and hyperbolic models.
By running sound waves through the condensate—an analog for light shining through the universe—Liebster and his colleague were able to examine the strange physics of each model, which may be similar to those that arose in the early universe. The sound waves in the experiment played the role of light waves in the real universe, since their path through the condensate was influenced by different configurations.
“It could be that in the past, our universe had different kinds of spatial curvature, and that's what we can tune in our system,” Liebster explained. “We have control over those kinds of parameters.”
“How the sound wave moves through your system is a very efficient way of checking what is the shortest path between two points, because the sound wave will always take the shortest path,” he continued. “The sound waves are like light waves in real cosmology. They have the same properties and that's why we use them to probe our spacetime.”
In this way, the team could simulate models of cosmic inflation that could be halted to examine the dynamics underlying them, which Liebster called “a dream in cosmology.” Overall, the experiment matched theoretical predictions for different curvatures in time and space, validating this simulator approach, though it does not confirm or refute any particular models of the early universe at this time.
“Our work is mainly a benchmarking that our simulator works at all,” Liebster said. “There's a lot of very interesting theoretical questions that you can ask about different kinds of spacetime curvature, and spatial curvature, and what the effects of that are” though he added that “there's a number of hurdles to overcome before we can make direct one-to-one comparisons” to the real universe.
“It's all an approximation at the end of the day,” Liebster continued. “I would be careful to say that there are very specific concrete outcomes for cosmology. But we know that for these specific assumptions for this model system, it agrees very well with theory, and now we can ask questions that go beyond what the current theory can answer.”
To that end, the researchers outlined a host of fundamental questions in quantum physics that could be explored with future versions of the simulator. Experts from many fields of physics will be needed to unravel the answers to these longstanding questions about this strange universe we find ourselves in, but at least the roadmap is becoming more clear
“I wouldn't necessarily think we're close to discovering the secrets of the Big Bang,” Liebster concluded. “But even just this collaboration between theory and experiments—and asking what questions can we answer that you can't, and what questions can you answer that we can’t—is very motivating,”