Hubble Constant
The Hubble constant H is one of the most important numbers in
cosmology
because it may be used to estimate the size and
age of the Universe.
It indicates the rate at which the Universe
is expanding. The Hubble "constant" is not really constant because it
changes with time (and therefore should probably more properly be called the "Hubble
parameter"). It is often
written with a subscript "0" to denote explicitly
that it refers to the value of H at the present time, but
we shall not do so.