Cosmic expansion is the large scale increase in the distances between gravitationally unbound regions of the universe as the universe evolves with time. It is commonly described by saying that space itself expands, although this phrase is a simplified description of the geometry represented by general relativity. Galaxies that are sufficiently separated and not gravitationally bound to one another generally become farther apart because the scale factor describing the universe increases. This does not mean that every object expands internally.
Atoms, people, planets, stars and gravitationally bound galaxies do not simply become larger in proportion to cosmic expansion because local forces maintain their structures. The expansion is instead most apparent on sufficiently large scales where the average distribution of matter follows the evolving geometry of spacetime. Cosmic expansion is one of the central observations supporting modern cosmology and is described mathematically by the Friedmann equations derived from general relativity.
Evidence for expansion comes especially from the relationship between distance and the redshift of distant galaxies. When light travels through an expanding universe, its wavelength is stretched along with the expansion of the cosmic scale factor. This produces cosmological redshift. Edwin Hubble and other astronomers established that more distant galaxies generally exhibit larger redshifts, although Georges Lemaitre had earlier derived and discussed the theoretical relationship between expansion and observed velocities.
The relationship is often expressed using the Hubble law for sufficiently nearby galaxies, where recession velocity is approximately proportional to distance. The proportionality constant is called the Hubble constant, although the measured value depends on the observational method and remains the subject of an important cosmological discrepancy called the Hubble tension. At greater distances, the simple linear relation must be replaced by a full cosmological model because the expansion rate has changed over cosmic history and because distances themselves can be defined in several different ways.
Cosmic expansion does not mean that galaxies are moving away from one central point through pre existing empty space in the same way that fragments of an explosion fly outward. In the standard cosmological description, there is no unique centre of the expansion within ordinary three dimensional space. Every sufficiently distant observer can see distant unbound galaxies receding in a pattern broadly consistent with expansion.
A useful mathematical analogy involves points drawn on the surface of an expanding sphere, where every point sees other points becoming farther away as the surface expands, without any point on the two dimensional surface being the centre. The analogy is limited because the universe is not necessarily a two dimensional spherical surface embedded in a higher dimensional space, and the actual geometry is described by spacetime rather than an ordinary balloon surface. The important concept is that cosmic distances evolve according to the geometry of the universe.
The expansion of the universe is governed by the contents of the universe and by the equations of general relativity. The Friedmann equations relate the expansion rate to the density of matter, radiation, spatial curvature and other contributions to the cosmic energy budget. Different components affect expansion differently because their energy densities change as the universe expands. Radiation becomes less important rapidly because both its density and photon energies decrease with expansion.
Matter becomes less dense as volume increases, but its individual particle rest masses do not decrease simply because of expansion. A cosmological constant or dark energy component behaves differently because its effective energy density remains approximately constant as space expands. The relative importance of these components has therefore changed over cosmic history. The early universe was radiation dominated, later became matter dominated, and the modern universe is undergoing accelerated expansion associated with dark energy in the standard cosmological model.
Cosmic expansion also explains cosmological redshift more accurately than the idea that galaxies simply emit light that loses energy while travelling through static space. As the universe expands, the wavelength of freely travelling light increases in proportion to the cosmic scale factor. A photon emitted when the scale factor was smaller therefore reaches an observer with a longer wavelength. The amount of stretching depends on the ratio of the scale factor at observation to the scale factor at emission.
This relationship allows cosmologists to use redshift as a measure of cosmic history, although converting redshift into distance requires assumptions about the expansion model. Cosmological redshift differs from ordinary Doppler redshift, although Doppler effects can also occur because individual galaxies have peculiar velocities relative to the average cosmic expansion. In nearby systems, these peculiar velocities can be significant compared with the expansion signal.
Expansion occurs even though many structures remain gravitationally bound. The Solar System does not expand appreciably because solar gravity maintains the orbital system. The Milky Way and Andromeda Galaxy are gravitationally interacting and are not simply carried apart by the average cosmic expansion. Galaxy clusters can also remain bound when their gravity is strong enough to overcome the tendency of large scale expansion to separate them. Beyond bound structures, however, the average separation between distant galaxies increases.
This distinction is essential because the expansion is a large scale property rather than a force that pulls apart every object. It also explains why the dimensions of atoms and planets do not continuously increase as the universe expands. Local physical interactions operate on scales where the cosmic expansion has negligible direct effect compared with electromagnetic, nuclear and gravitational binding forces.
Observations indicate that the expansion rate has not remained constant. The universe expanded extremely rapidly in its earliest phases, then experienced a period in which matter dominated the expansion dynamics and slowed the expansion relative to what would occur in an empty universe. Observations of distant type Ia supernovae in the late twentieth century provided evidence that the expansion has been accelerating during the relatively recent cosmic past.
This accelerated expansion is generally attributed to dark energy, although the precise physical nature of dark energy remains unknown. Measurements of the cosmic microwave background, baryon acoustic oscillations, galaxy distributions and supernova distances provide complementary constraints on the expansion history. The combined evidence supports a universe whose expansion has evolved through several distinct dynamical regimes rather than proceeding at one constant rate since the beginning.
Cosmic expansion is therefore a change in the large scale metric structure of the universe rather than a simple explosion of matter from a central location. It provides the framework for interpreting cosmological redshift, galaxy distances, the changing density of cosmic matter and the evolution of large scale structure. The expansion can be described through a scale factor and an expansion rate, and its history depends on the energy and matter content of the universe. Understanding expansion also requires distinguishing it from local motions, gravitationally bound systems and ordinary Doppler effects. The concept is central to modern cosmology because measurements of expansion provide information about the age, geometry, composition and future evolution of the universe.
