Researchers manipulate the speed of sound in an ultracold gas to mimic the properties of a curved spacetime and to mimic the behavior of quantum fields predicted by early Universe theories.
The large-scale structure of the universe has led cosmologists to conclude that space was expanding rapidly just after the big bang. Except for quantum fields in vacuum, the Universe was empty during this expansion. How these fields evolved in the curling spacetime of an expanding universe is still unclear, but finding an answer could shed light on the origins of particles.
What is Space-Time Curvature?
To explore these areas, researchers at the University of Heidelberg in Germany have now created an analog system. An ultra-cold atom In Bose-Einstein condensation (BEC) They present a new method for simulating the evolution of quantum fields in an expanding universe, exhibiting properties that match curved space and time.
Recreating curved spacetime is not an easy task, as both the space direction and the temporal direction of the system must be bent. The geometry of the system (whether it is flat, spherical, or hyperbolic) affects the spatial curvature. Time curvature, on the other hand, considers the evolution of the system as stationary, expanding, or both. William Unruh of the University of British Columbia in Canada made the observation in 1980 that light waves propagating in curved space-time can be compared to sound waves propagating in a flowing liquid. Since then, other simulators have been created that mimic the effects of curvature using fluid flow.
Based on these ideas, Markus Oberthaler and his colleagues at the University of Heidelberg have developed a curved space-time analog in a stationary BEC fluid. They did a robbery at BEC by slowing down the sound. For example, they can increase the effective distance between two points by slowing the speed of sound because it will take longer for sound to travel between two points.
The effective "bending" of spacetime can be achieved by controlling the velocity.
The technology was created by researchers by trapping ultra-cold potassium-39 ions in an optical device. The trap changed the BEC intensity distribution, allowing for spatial sound velocity control. For example, the researchers showed that a particular trap structure produces an intensity profile with a smooth peak in the middle, resulting in a high sound velocity distribution in the center and low in the periphery. The model predicted for a 2D projection of a hyperbolic universe should be seen in the way sound waves traveling through this gas bend away from the center.
The researchers concentrated a laser beam near the center of their BEC and looked for intensity vibrations corresponding to quantized sound waves known as phonons to confirm that their BEC approximated a spatially curved geometry. When they tracked the positions of the phonons as they radiated from the focal point, they discovered that the orbits were in line with what would be expected given a spatially curved geometry. These tests were also repeated for spherical and hyperbolic geometries.
The scientists exposed the gas to a coherent magnetic field that changes the tiny strengths of interaction between the potassium atoms and allows for temporal control of the speed of sound. The BEC acted as if it was expanding by gradually reducing these connections. According to Oberthaler, the ability to apply expansion without actively growing the system or changing the density distribution makes this a crucial step.
The researchers examined how the intensity distribution of BEC changed after lowering the interaction intensity; which corresponded to a universe that had nearly doubled in size. They discovered that, as would be expected in the expanding space, there are large-scale amplified density changes after the ramp. These increased density fluctuations were associated with "particle generation" within the phonon field, in line with predictions of an expanding universe made by quantum field theory.
The researchers modified the ramp behavior and monitored the evolution of intensity fluctuations under acceleration, deceleration, and uniform expansion scenarios. For the most part, intensity contrast correlations grew following the ramp, as would be expected for an expanding cone of sound. The evolution of density fluctuations over time also matched Sakharov oscillations, as predicted by the expanding universe theories.
According to physicist Chen-Lung Hung, who studies ultracold quantum gases at Purdue University in Indiana, this demonstration brings together decades of theory and principles of experimentation. While the sources are available, this is the first example in which curved spacetime and important experimental markers have been effectively combined to show that it can be created.
According to Hung, it may be possible to simulate acoustic black holes in a tabletop experiment and see how quantum fields change in these new geometries.
The versatility of the BEC simulator is what makes it so interesting, according to Stefano Liberati, professor of quantum and classical gravity at the International Institute for Advanced Studies in Italy.
He claims that they can independently change the curvature and time dependence, allowing simulation of unusual physics in addition to normal model simulations.