Dynamical Variable

In physics a theory is usually described by a set of equations for some quantities called the dynamical variables of the theory. After writing a theory, the most important task is finding solutions of the equations. A solution of the equations describes a possible reality, according to the theory. Because an expanding universe is a solution of Albert Einstein’s gravitational equations, for example, it follows that an expanding universe is possible, according to this theory. A single theory may have many solutions, each describing a possible reality.