If you point a sensitive microwave detector at the sky, you will see a background spectrum which depends on the direction in which you look.
It is believed that this “cosmic microwave background anisotropy” is caused by cosmological particles which spontaneously appeared during the big bang. In that sense, the sky is a giant microscope looking at the early universe. The spontaneous production of the cosmological particles, and their effect on the cosmic microwave background, can be simulated in a laboratory experiment. Professor Jeff Steinhauer and collaborators at the Sorbonne University have done just that, using a fluid of photons [J. Steinhauer et al., Nature Communications 13, 2890 (2022)].
Usually, photons do not feel the presence of one another, but as a laser beam traverses a gas of rubidium atoms, there is an effective interaction between photons. Rapidly turning off this interaction is analogous to the rapid expansion of the early universe which created the cosmological particles. The researchers found that the sudden change in the interactions resulted in the quantum mechanical production of sound waves propagating in the fluid of light. These sound waves were analogous to the cosmological particles, and their spectrum was very similar to that of the cosmic microwave background.
Visit Professor Jeff Steinhauer's homepage: https://phsites.technion.ac.il/atomlab/
Image: The anisotropies of the cosmic microwave background (CMB) as observed by Planck. Credit: ESA, Source: https://www.esa.int/ESA_Multimedia/Images/2013/03/Planck_CMB.