Inflationary Universe

The equations of the Standard Model must be consistent with Einstein's principle of relativity, which states that the laws of Nature take the same form in every inertial frame of reference. An inertial frame is one in which a free body moves without acceleration. An earth-bound frame approximates to an inertial frame if the gravitational field of the earth is introduced as an external field. The time and space coordinates of an event measured in different inertial frames of reference are related by a Lorentz transformation.