Cosmology studies the Universe as a dynamical spacetime governed by general relativity, filled with matter, radiation, and dark components whose statistics are inferred from sky surveys, the cosmic microwave background, large-scale structure, and distance indicators. The central achievement of modern cosmology is the $\Lambda$CDM model, a minimal framework in which the expansion history is controlled by the Friedmann equations and the growth of structure by linear and nonlinear gravitational instability.

For an FLRW spacetime with scale factor $a(t)$, the Hubble parameter is $H(t)=\dot a/a$. Light emitted at scale factor $a_e$ and observed today at $a_0$ is redshifted by $1+z=a_0/a_e$. Distances in cosmology are therefore not unique but depend on the observable being measured: luminosity distance $d_L$, angular-diameter distance $d_A$, comoving distance, and lookback time each probe different aspects of the expansion history.