Metric Expansion
Metric expansion is a fundamental property of the universe wherein the spatial distance between two comoving points increases over time. This phenomenon is not driven by the motion of objects through space, but rather by the expansion of space itself. Consequently, galaxies that are sufficiently distant from one another recede at speeds that can exceed the speed of light, a result that does not violate special relativity because the objects are not moving locally through their immediate spacetime.
The mathematical framework for this expansion is provided by the Friedmann–Lemaître–Robertson–Walker (FLRW) metric, which serves as a solution to Einstein’s field equations in general relativity. This model assumes the universe is homogeneous and isotropic on large scales, allowing for the description of cosmic evolution through the scale factor. The rate of this expansion is quantified by the Hubble parameter, which relates the recession velocity of galaxies to their distance from the observer.
Observational evidence for metric expansion includes the redshift of light from distant galaxies, known as Hubble’s Law, and the cosmic microwave background radiation. These observations support the Big Bang theory, which posits that the universe began in a hot, dense state and has been expanding ever since. The nature of this expansion, including its acceleration driven by dark energy, remains a central topic of modern cosmological research.