In the first moments after the Big Bang, the Universe underwent rapid expansion: inflation. But perhaps it was contracting before that, and then bounced. Both scenarios leave nearly identical imprints on the cosmic microwave background, and cosmologists have debated for decades which one is correct. Distinguishing them would require physics at extremely small scales, the Planck scale, which is inaccessible to direct experiments. However, a group of physicists has proposed a workaround: recreate the early Universe in the laboratory, inside a cloud of ultracold gas.

The key idea is to use not the usual contact interactions between atoms, but dipole-dipole forces arising between their magnetic moments. These forces can be precisely tuned using a magnetic field (Feshbach resonance). If the gas is squeezed into a thin layer, its collective oscillations become analogous to perturbations of spacetime in the early Universe. Moreover, anisotropic squeezing (different in the longitudinal and transverse directions) generates a non-adiabatic correction that breaks the duality between inflation and bounce. Previously, it was thought that these two scenarios yield identical perturbation spectra; now it turns out that during contraction, a noticeable spectral tilt appears at small momenta.

This finding means that a laboratory quantum simulator can provide a criterion that astronomical observations cannot. If an experiment with cold gas can measure the density fluctuation spectrum in the contracting phase and see a red tilt, that would be a strong argument in favor of the bounce model. Moreover, this approach allows testing predictions of quantum gravity without reaching Planck energies: the imprints of high-energy physics show up on large scales through the spectral tilt.

Experimental realization is possible today with dysprosium or erbium condensates. One just needs to tune the contact interaction via a Feshbach resonance so that the relative dipolar strength approaches the critical value. Then, squeezing the gas in a trap will reproduce trans-Planckian physics, and we can literally see how the Universe was born.