Collective excitations in quantum gravity condensates

A central open problem in quantum gravity is to understand how continuum spacetime emerges from quantum-geometric degrees of freedom in a background-independent setting. A many-body perspective suggests that spacetime emerges as a hydrodynamic phase of many atoms of quantum geometry. This idea underlies several approaches to quantum gravity, and it has been explicitly realised in the group field theory formalism. However, quantum fluctuations beyond the mean-field regime remain largely unexplored. We fill this gap by importing Bogolyubov theory to quantum gravity condensates, showing that leading beyond-mean-field effects manifest as collective excitations, in direct analogy with phonons in laboratory Bose-Einstein condensates. We implement the construction in a tractable group field theory model, where condensates of quantum-geometric atoms reproduce nonsingular expanding cosmologies, and derive the leading beyond-mean-field corrections to the emergent Friedmann dynamics. These results identify a new class of quantum-gravity excitations and establish a controlled bridge between microscopic quantum-gravitational dynamics, many-body collective phenomena, and signatures of spacetime emergence.