Defining the universe

In physics, the universe may be defined as the smallest system containing the earth and not interacting with something outside it. This is in agreement with the common usage of the term in astronomy and cosmology. It has a precise meaning in any concrete mathematically formulated theory of physics. In each such theory, there is only a single such system.

General relativity is the study of the classical aspects of this universe (and variants of it that may be easier or more interesting to study), while quantum gravity is the study of its quantum aspects.

In a simplified version of the universe where gravitation is neglected, the standard model covers all aspects of the universe.

(In less conventional areas of physics, where one speculates about many worlds, a universe is something different, not really well-defined.)

Arnold Neumaier (
A theoretical physics FAQ