### Hubble expansion

**(expansion of the universe)**
(ongoing expansion of the universe)

**Hubble expansion** is the **expansion of the universe**, space itself
expanding, which is currently thought to have occurred over the
life of the universe. At the scale of galaxies, being bound by
gravity, the expansion is resisted, but at very large scales,
it is evident, i.e., the distance between galaxy clusters is
growing. In the early 20th century, Edwin Hubble established that
the luminosity distance (determined from variable stars of
known luminosity) of galaxies is proportional to their
redshift, the latter of which indicates a receding
radial velocity, i.e., everything moving away from us (illustrated
with his Hubble diagram). This result is now termed **Hubble's law**,
which is thought to be approximately correct over the interval we've
been able to observe, but the expansion is now thought to have
increased over time, giving rise to the concept of dark energy,
to provide a cause for such acceleration. The Hubble constant (H_{0})
is the current rate of expansion and a reasonable approximation of
the expansion for much of the age of the universe, and
the **Hubble parameter** is the rate of expansion as a function of
time, i.e., accommodating the acceleration. Actual determination
of distance from redshift requires integration involving the Hubble
parameter, and the mapping of the universe's expansion over time
is of continuing interest. The apparent movement (increasing distances
between distant objects) is termed the **Hubble flow**, to distinguish
it from the peculiar velocity of the objects, e.g., the influence
on their motion due to nearby massive objects (the **local flow**).

(*cosmology*)
**Further reading:**

http://en.wikipedia.org/wiki/Hubble%27s_law

https://ui.adsabs.harvard.edu/abs/1929PNAS...15..168H/abstract

**Referenced by pages:**

Alcock-Paczyński effect 2 (AP effect)

alternative cosmologies

baryon acoustic oscillations (BAO)

Big Bang

chemical equilibrium (CE)

CMB anisotropies

comoving units

cosmological simulation

cosmological time dilation

critical density (ρ_{c})

dark energy

dark flow

Dark Energy Survey (DES)

Einstein-de Sitter model

freeze-out

general relativity (GR)

Hubble constant (H_{0})

Local Group (LG)

observable universe

particle horizon

peculiar velocity

redshift (z)

redshift-magnitude relation

relic

rest wavelength

scale factor (a)

star formation rate (SFR)

surface of last scattering

systemic velocity

tired light

Index