Einstein’s theory of relativity states that space and time are intertwined. In our universe, the curvature of space-time is relatively small and constant. However, researchers from the University of Heidelberg have succeeded in creating a laboratory experiment in which the structure of space-time can be manipulated. The researchers simulated a series of curved universes using ultracold quantum gases to explore various cosmological scenarios. They then compared these simulations with the predictions of a quantum field theory model. The results of the study were published in the journal Nature.
Curved spacetime is a concept from Einstein’s theory of general relativity that describes how gravity affects the shape of the universe. It suggests that the presence of matter or energy in the universe causes the structure of spacetime to bend.
The emergence of space and time on the time scale of the universe from the Big Bang to the present is the subject of current research and can only be based on observations of our single universe. The expansion and curvature of space is crucial to cosmological models. In a flat space such as our current universe, the shortest distance between two points is always a straight line. However, it is conceivable that our universe was curved in its early stages.
“Studying the consequences of curved spacetime is therefore a pressing issue in research,” says Professor Markus Oberthaler, a researcher at the Kirchhoff Institute of Physics at Heidelberg University. Together with his research group “Synthetic Quantum Systems”, he has developed a quantum field simulator for this purpose.
The quantum field simulator created in the laboratory consists of a cloud of potassium atoms that is cooled to just a few nanokelvin above absolute zero. This gives rise to a Bose-Einstein condensate – a special quantum mechanical state of atomic gas reached at extremely cold temperatures.
Professor Oberthaler explains that the Bose-Einstein condensate is a perfect background against which the smallest excitations, i.e. changes in the energy states of atoms, become visible. The form of the atomic cloud determines the dimensions and properties of spacetime, and these excitations operate like waves on it.
In our universe there are three dimensions of space as well as a fourth dimension: time. In the experiments carried out by Heidelberg physicists, atoms are trapped in a thin layer. As a result, the excitation waves could only travel in two spatial directions – space is two-dimensional. At the same time, the atomic clouds in the remaining two dimensions can be shaped in almost any way, so that curved spacetime is also possible. The interactions between atoms can be precisely tuned by magnetic fields, changing the speed of propagation of wave-like excitations on Bose-Einstein condensates.
“For waves on condensates, the propagation speed depends on the density and interaction of the atoms. This gives us the opportunity to create conditions similar to those in an expanding universe,” explains Professor Stefan Flörchinger. The researcher, who worked at the University of Heidelberg and joined the University of Jena earlier this year, developed the quantum field theory model for quantitatively comparing experimental results.
“Using the quantum field simulator, cosmic phenomena such as the production of particles based on the expansion of space and even the curvature of spacetime can be measured.” Cosmological problems usually occur on unimaginably large scales. Celia Viermann, lead author of the Nature article, says: “Being able to study them concretely in the laboratory allows us to test new theoretical models experimentally, thus opening up completely new possibilities for research.”
“Studying the interplay of curved spacetime and quantum mechanical states in the laboratory will occupy us for some time to come,” says Markus Oberthaler, whose research group is also part of Ruperto Carola’s STRUCTURES cluster of excellence.