## Scientists simulate a whole family of universes with curvature in ultracold quantum gases

**According to Einstein’s general theory of relativity, space and time are inseparable. In our universe - which is hardly measurably curved - the structure of this spacetime is predetermined. Scientists at Heidelberg University have now succeeded in realizing an effective spacetime in a laboratory experiment that can be manipulated. In their research on ultracold quantum gases, they were thus able to simulate a whole family of curved universes in order to investigate different cosmological scenarios and compare them with the predictions of a quantum field theory model. The research results were published in ,,Nature."**

The origin of space and time on cosmic time scales from the Big Bang to the present is the subject of current research, which, however, can only refer to the observation of our one universe. An essential part of cosmological models is the expansion and curvature of space. In a flat space like our present universe the shortest distance between two points is always a straight line. However, it is conceivable that our universe was curved in its initial phase. Investigating the consequences of a curved spacetime is therefore a pressing research question," says Markus Oberthaler, a scientist 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 realized in the laboratory consists of a cloud of potassium atoms cooled to a few nanokelvin above absolute zero temperature. This produces a Bose-Einstein condensate - a special quantum mechanical state of atomic gas that is reached at very cold temperatures. As Prof. Oberthaler explains, the Bose-Einstein condensate acts as an ideal background on 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 the properties of the space-time on which these excitations move like waves. In the universe there are three dimensions of the space and a fourth - that of the time.

In the experiment conducted by the Heidelberg physicists, the atoms are trapped in a thin layer. Thus, excitations can only propagate in two spatial directions - space is two-dimensional. At the same time, the atom cloud can be shaped almost arbitrarily in the remaining two dimensions, making it possible to realize curved spacetimes. The interaction between the atoms can be precisely adjusted by a magnetic field, which changes the speed of propagation of the wave-like excitations on the Bose-Einstein condensate.

For the waves on the condensate, the propagation speed depends on the density and the interaction of the atoms. This gives us the possibility to create conditions like in an expanding universe," explains Stefan Flörchinger, previously a scientist at the University of Heidelberg and since the beginning of this year at the University of Jena. He worked out the quantum field theoretical model with which the experimental results were quantitatively matched.

With the quantum field simulator, cosmic phenomena, for example the production of particles due to the expansion of space, and spacetime curvature itself can be made measurable. Cosmological questions normally run on unimaginably large scales. Being able to study these in the laboratory in a very concrete way opens up completely new possibilities for research by allowing us to test new theoretical models experimentally," says Celia Viermann, who is first author of the "Nature" publication. "Exploring the interplay of curved spacetime and quantum mechanical states in the laboratory will keep us busy for some time to come," says Markus Oberthaler, who with his research group is a member of the STRUCTURES Cluster of Excellence at Ruperto Carola.

The work was carried out within the framework of the Collaborative Research Center 1225 ,,Isolated Quantum Systems and Universality under Extreme Conditions" (ISOQUANT) at Heidelberg University.

C. Viermann, M. Sparn, N. Liebster, M. Hans, E. Kath, Á. Parra-López, M. Tolosa-Simeón, N. Sánchez-Kuntz, T. Haas, H. Strobel, S. Stefan Flörchinger, M.K. Oberthaler: Quantum field simulator for dynamics in curved spacetime. Nature (9 November)