Building a house on sand is a bad idea. In physics, it was long believed that a quantum theory of gravity could not be built on a fixed "sand"—a pre-existing spacetime. Einstein's general relativity teaches that spacetime itself changes under the influence of matter, so any rigid background is an artificial simplification. It seemed that a fundamentally different, "background-free" approach was needed, like in loop quantum gravity.

But the authors of the new work show that the seemingly incompatible can be combined. They propose a method where quantum excitations of the gravitational field are described in the familiar way — via Fock space (a mathematical construct for particles). The main condition is that matter must be present in the system. Then the vacuum, the lowest-energy state, turns out to be not empty but a entangled tangle of geometry and matter. One cannot exist without the other.

This approach has an important technical advantage over loop quantum gravity. The state space here turns out to be separable — roughly speaking, it can be described by a countable set of basic elements, like notes in music. This greatly simplifies working with the main equation of quantum cosmology — the Wheeler-DeWitt equation, which in other approaches resembles an unsolvable puzzle. Now it can be formulated more rigorously and possibly bring us closer to calculations that show how our Universe was born.