Researchers are simulating a whole family of curved universes in ultracold quantum gases

In a laboratory experiment, researchers at the University of Heidelberg have succeeded in realizing an effective and manipulable space-time. In their research on ultracold quantum gases, they were able to simulate a whole family of curved universes to study different cosmological scenarios and compare them with the predictions of a quantum field theoretical model.

According to Einstein’s theory of relativity, space and time are inextricably linked. In our universe, whose curvature is hardly measurable, the structure of this space-time is fixed. In a laboratory experiment, researchers at the University of Heidelberg have succeeded in realizing an effective and manipulable space-time. In their research on ultracold quantum gases, they were able to simulate a whole family of curved universes to study different cosmological scenarios and compare them with the predictions of a quantum field theoretical model. The research results were published in Nature.

The emergence of space and time on cosmic time scales from the Big Bang to the present is the subject of current research that can only be based on the observation of our only universe. The expansion and curvature of space are essential to cosmological models. In a flat space like our current universe, the shortest distance between two points is always a straight line. “But it is conceivable that our universe was curved in its early phase. Investigating the consequences of a curved space-time is therefore an urgent research question,” says Prof. Dr. Markus Oberthaler, researcher at the Kirchhoff Institute for Physics at Heidelberg University. With his research group “Synthetic Quantum Systems” he has developed a quantum field simulator for this purpose.

The quantum field simulator created in the lab consists of a cloud of potassium atoms cooled to a few nanokelvins above absolute zero. This creates a Bose-Einstein condensate – a special quantum mechanical state of the atomic gas that is reached at very low temperatures. Prof. Oberthaler explains that the Bose-Einstein condensate is a perfect background against which the smallest excitations, i.e. changes in the energy state of the atoms, become visible. The shape of the atomic cloud determines the dimensionality and properties of the space-time on which these excitations ride like waves. In our universe there are three spatial dimensions and a fourth: time.

In the experiment conducted by the Heidelberg physicists, the atoms are trapped in a thin layer. The excitations can therefore only propagate in two spatial directions – the space is two-dimensional. At the same time, the atomic cloud can be shaped almost arbitrarily in the remaining two dimensions, which means that curved spacetimes can also be realized. The interaction between the atoms can be precisely adjusted using a magnetic field, which changes the propagation speed of the wave-like excitations on the Bose-Einstein condensate.

“The propagation speed of the waves on the condensate depends on the density and the interaction of the atoms. This gives us the opportunity to create conditions like in an expanding universe,” explains Prof. Dr. Stefan Floerchinger. The researcher, who previously worked at the University of Heidelberg and came to the University of Jena earlier this year, developed the quantum field theoretical model that is used to quantitatively compare the experimental results.

With the quantum field simulator, cosmic phenomena such as the generation of particles due to the expansion of space and even the curvature of space-time can be made measurable. “Cosmological problems usually play out on unimaginably large scales. Being able to examine them in a targeted manner in the laboratory opens up completely new possibilities in research, as we can test new theoretical models experimentally,” says Celia Viermann, first author of the book “Nature” article. “Investigating the interaction of curved space-time and quantum mechanical states in the The laboratory will keep us busy for some time,” says Markus Oberthaler, whose research group is also part of the STRUCTURES cluster of excellence at Ruperto Carola.

The work was carried out as part of the Collaborative Research Center 1225 “Isolated Quantum Systems and Universality in Extreme Conditions” (ISOQUANT) at Heidelberg University.

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