Expansion of the Universe

The increase over time in distances between gravitationally unbound galaxies, caused by space itself expanding. It is described by the cosmic scale factor and does not imply expansion from a central point.

Scale factor: A dimensionless quantity describing how distances between comoving points in the universe change over time. Its growth directly represents the expansion of cosmic distances.
Alexander Friedmann: A Russian mathematician and physicist who derived dynamic cosmological solutions to Einstein's field equations. His solutions showed that general relativity permits an evolving universe.
Hubble constant: The present-day rate relating cosmic expansion to distance in the nearby universe. Its value sets the current expansion rate and affects estimates of cosmic age.
Peculiar velocity: An object's velocity relative to the average cosmic expansion, produced by local gravitational interactions. It can add to or subtract from the recession attributed to expansion.
Hubble's law: The relation that, at modest distances, a galaxy's recession velocity is proportional to its distance. It links observed galaxy distances and recession speeds to the expansion rate.
Georges Lemaître: A Belgian priest and physicist who proposed an expanding universe and connected it to galaxy redshifts. He developed an early physical interpretation of cosmic expansion before its broad acceptance.
Hubble tension: The persistent disagreement between early-universe and nearby-universe measurements of the present expansion rate. It tests whether expansion measurements or the standard cosmological model are incomplete.
Gravitationally bound system: A collection of objects whose mutual gravity keeps them together against cosmic expansion. Bound systems such as galaxies do not generally expand with the cosmic scale factor.
Cosmological redshift: The stretching of light's wavelength as the universe expands during its journey to an observer. Its measured value reveals how much the universe expanded while the light traveled.
Edwin Hubble: An American astronomer whose observations helped establish that galaxies lie beyond the Milky Way and recede with distance. His galaxy observations provided influential evidence for expansion.

Linked from 45 pages