Accelerating expansion of the universe

The expansion of the universe has sped up over cosmic time, as inferred from distant supernovae and other observations. In the standard cosmological model, dark energy drives this acceleration.

Dark energy: An unknown component of the universe associated with its accelerated expansion. It is the name given to the component linked to the observed acceleration.
Type Ia supernova: A stellar explosion whose relatively consistent peak luminosity makes it useful for measuring astronomical distances. These explosions supplied the distant distance measurements behind the original discovery.
Standard candle: An astronomical object with known or inferable luminosity, used to estimate distance from apparent brightness. Supernova distances rely on treating Type Ia explosions as calibrated luminosity markers.
Dark energy equation of state: The ratio of dark energy pressure to its energy density, which may vary with cosmic time. Measuring whether this ratio changes could distinguish a cosmological constant from evolving dark energy.
Lambda-CDM model: The standard cosmological model combining a cosmological constant, cold dark matter, and ordinary matter. It incorporates accelerated expansion through its cosmological-constant component.
Cosmological constant: A constant energy density of empty space that appears in Einstein’s gravitational equations. It is the simplest standard-model explanation for sustained cosmic acceleration.
High-Z Supernova Search Team: An international collaboration that used distant supernovae to study cosmic expansion. Its observations provided one of the two landmark 1998 reports of acceleration.
Redshift: The increase in wavelength of light as its source recedes or space expands. Supernova spectra measure redshift, pairing cosmic distance with expansion history.
Hubble tension: A disagreement between measurements of the present expansion rate using early- and late-universe methods. It exposes unresolved tensions among precision measurements of cosmic expansion.
Cosmic microwave background: Relic radiation released when the early universe became transparent, now observed across the sky. Its measured pattern helps constrain cosmic composition alongside supernova evidence.

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