Expansion of the universe

The expansion of the universe is the increase in distance between galaxies and other distant objects in spacetime, driven by the geometry of space itself rather than motion through space. First observed by Hubble in the 1920s through redshifts of distant galaxies, this discovery revealed that the universe is dynamic and has a history.

The expansion is described mathematically by Einstein's equations, which relate the curvature of spacetime to the distribution of matter and energy. In the early universe, following the Big Bang, expansion was rapid. Around 1998, observations of distant supernovae revealed something startling: the expansion is accelerating, driven by a mysterious component called dark energy that makes up roughly 68% of the universe's total energy density.

The expansion does not mean galaxies move apart through stationary space—rather, space itself stretches. Crucially, the expansion has no center; from any vantage point, observers see themselves at the heart of the expansion. This is measured by the Hubble constant, which describes the current rate of expansion, though this value itself remains under debate as measurements diverge.

The universe's ultimate fate depends on the strength of dark energy and the total density of matter.

Related

Big Bang, Cosmology, Redshift, Dark matter, Cosmic microwave background, Universe