Astrophysics (Index) | About |

The **Hubble constant** (**H _{0}**) characterizes the rate
of expansion of the universe.

v = cd

where v is velocity, d is distance, and constant
c is H_{0}, 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

H_{0}= h × 100 km s^{-1}Mpc^{-1}

Currently, h calculates to 0.7.
Distances to galaxies are sometimes given in terms of the
Hubble parameter: e.g., a distance 30h^{-1}Mpc
which allows future readers to adjust the distance they assume
according to up-to-date values of the Hubble constant.
With an H_{0} of 70 km/s, 30h^{-1}Mpc 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.

http://en.wikipedia.org/wiki/Hubble_constant

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)