On anti-gravitational phenomenon of excited states in quantum systems

It is common belief that gravity is an attractive interaction between all things with mass or energy, affecting the motion of objects at the macroscopic scale and determining the large-scale structure of the universe. Contrary to the conventional cognition, here we reveal that gravitational repulsion is also ubiquitous in quantum systems -- the anomalous response of the position of the particles at (topologically) excited states to the gravitational field in confined systems. We prove that this anti-gravitational phenomenon results from a principle called `quantum-state exclusion' inherited from the orthogonality of quantum states. We further predict that, in an inflating space, this gravitational anomaly may cause quantum matter in excited states to expand even faster than space, leading to an observable accumulation of quantum matter near the boundary of the space. These unique phenomena can be simulated in ultracold atom experiments by using Bose-Einstein condensates with solitons. The accelerating expansion phenomenon in quantum systems also sheds new light on understanding the evolution of the universe, where the vacuum state may also be an excitation with topological defects.