Astrophysics (Index)About

Hubble constant

(rate of expansion of the universe)

The Hubble constant (H0) characterizes the rate of expansion of the universe. Hubble's law (aka the Hubble law) is a theory that the universe is expanding such that the velocity at which an object is moving away from another is proportional to the distance between them, i.e.,

v = cd

where v is velocity, d is distance, and constant c is H0, the Hubble constant. Estimates are in the range of 50-90 (km/s)/Mpc with current estimates very close to 70. This expansion is called the Hubble expansion. A Hubble diagram is a diagram mapping redshift to known distances to objects (e.g., galaxies), so a straight line demonstrates Hubble's law and the slope indicates Hubble's constant.

Observation has revealed evidence that the universe's expansion is not constant. One might say "the Hubble constant changes with time", but for much work, this issue can (or must) be set aside, and even when it is relevant, the term Hubble constant is often used specifically to mean the current rate of expansion.

The Hubble parameter (h) is a dimensionless variant on the constant, where

H0 = h × 100 km s-1Mpc-1

Currently, h calculates to 0.7. Distances to galaxies are sometimes given in terms of the Hubble parameter: e.g., a distance 30h-1Mpc which allows future readers to adjust the distance they assume according to up-to-date values of the Hubble constant. With an H0 of 70 km/s, 30h-1Mpc calculates to roughly 43Mpc.

The term Hubble flow is used for the apparent motion of astronomical objects due to this expansion.

The term Hubble time indicates the period of time since the universe was a point, assuming the expansion was constant at the current rate. Since determining the current rate is non-trivial, the Hubble time is subject to refinement. Other factors, including variation in the expansion rate are taken into account to produce a more accurate estimate of the age of the universe. Current estimates of Hubble time and the age of the universe differ by on the order of a percent. Timescales of processes are compared with the current Hubble time to see how likely it would be for an instance of the process to have completed, or equivalently, whether some observed phenomena seems likely be the result of an instance of the process.

The terms are named for Edwin Hubble, who in the early 20th century established that galaxies are more-distant Milky Way-like objects, and later that the distant ones can be seen to be receding from us, suggesting an expanding universe.

Further reading:

Referenced by pages:
Alcock-Paczyński effect
Big Bang
Calán/Tololo Survey
comoving units
cosmic distance ladder
cosmic time
dark flow
Hubble diagram
Hubble expansion
Lambda-CDM model (ΛCDM)
Local Group (LG)
Lick Observatory Supernova Search (LOSS)
luminosity function (LF)
peculiar velocity
redshift (z)
redshift survey
scale factor (a)
Supernova Cosmology Project (SCP)
star formation rate (SFR)
spectral feature
standard siren
systemic velocity
Sunyaev-Zel'dovich effect (SZ effect)