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The **Hubble constant** (**H _{0}**) is the current rate of
expansion of the universe, which is presumed to be constant throughout
the current universe. It is expressed as a ratio of the relative velocity
between two points that results from the current expansion
to the current distance between the points,
typically expressed in km/s divided by megaparsecs.
Measurements put it in the in the range of 50-90 (km/s)/Mpc
with current determinations close to 70.
The zero subscript (H

Observation has revealed evidence that the universe's expansion
is not constant with time. The term **Hubble parameter** (**H** or
**H(t)**) is used to indicate a similar characterization of the
universe's expansion without specifying we're talking about right now.
Despite this variation, the Hubble constant (current value) is often
used as a good approximation of the expansion for nearly the entire
life of the universe.

The term **Hubble parameter** is also used for what could more
precisely be termed the **dimensionless Hubble parameter**,
indicated by **h**.

H = h × 100 km s^{-1}Mpc^{-1}

The currently determined Hubble constant places the current value
at about 0.7.
Distances to galaxies are sometimes given in terms of this
Hubble parameter: e.g., a distance of 30h^{-1} Mpc,
which allows future readers to adjust the distance they assume
according to up-to-date values of the Hubble constant.
Using a constant value of h = 0.7, 30h^{-1} Mpc
calculates to roughly 43 Mpc.

The terms are named for Edwin Hubble, who in the early 20th century
established that distance determinations to galaxies using
variable stars correlate with radial velocity
measurements of the galaxies by redshift (illustrated by his
**Hubble diagram**), thus, with one, you can reasonably
estimate the other.
This is termed **Hubble's law**,
a useful means of approximating distances to galaxies,
an approximation termed the galaxy's
**Hubble distance** (**D _{H}**).

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

https://www.astro.caltech.edu/~george/ay127/readings/FreedmanMadore2010.pdf

https://ui.adsabs.harvard.edu/abs/2020PhRvD.101d3533K/abstract

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

Alcock-Paczyński effect (AP effect)

Big Bang

Calán/Tololo Supernova Survey

comoving units

cosmic distance ladder

cosmic time

damped Lyman alpha absorber (DLA)

Doppler shift

Hubble diagram

Hubble expansion

Hubble time (t

Lambda-CDM model (ΛCDM)

Lick Observatory Supernova Search (LOSS)

luminosity function (LF)

peculiar velocity

redshift (z)

redshift space

redshift survey

scale factor (a)

Supernova Cosmology Project (SCP)

star formation rate (SFR)

supernova survey

spectral feature

standard ruler

standard siren

Sunyaev-Zel'dovich effect (SZ effect)