**1.2. The Hubble Parameter h**

Determining the Hubble constant, *H*_{0}, requires that we
have a way of
getting the distance to galaxies independently of their redshifts.
The history of determining the extragalactic distance scale is in
itself a fascinating subject
(Rowan-Robinson, 1986)
and even today
there is considerable uncertainty. There seems to be two distinct
bodies of opinion, one clustering its estimates of *H*_{0}
around 50 km
s^{-1} Mpc^{-1} and the other around 80 km
s^{-1} Mpc^{-1}. We shall absorb this
ignorance into a "Hubble parameter" *h* defined so that

So all distances quoted will contain the quantity *h*, and the reader
is invited to substitute her/his favourite value.

It is probably safer in practise to use radial velocity to express
distances. This reflects the Hubble law and so when we say a galaxy is
at a distance of 30*h*^{-1} Mpc. we could equally well say
it is at a
distance of 3000 km s^{-1}. This is fine, but it may look a bit
strange to
say that a void has a diameter of 5000 km s^{-1}, or to say that the
galaxy clustering correlation function drops to unity on a scale of
500 km s^{-1}.
The present value of the Hubble Constant, *H*_{0}, and the
density parameter,
_{0},
together determine the present age of the universe. In the case of an
_{0} < 1
universe:

It is certain that there should not be any objects older than this in
the Universe, so determining ages is an important way of constraining
the values of *H*_{0} and
_{0}. It
seems that the oldest known stellar
systems for which we can determine ages have ages in excess of 16
Gyr. (Sandage and Cacciari,
1990).
If we accept this value, then we see that an
_{0} = 1
universe is always too young unless *H*_{0} is
considerably lower than any of the values so far put forward. An open
universe with
_{0} <
0.1 can work provided *H*_{0} is at the lower
end of the suggested range of values.

What are we to make of this? That neither age determinations of star clusters nor the extragalactic distance scale can be relied on, with the latter probably being the most uncertain. Introducing a cosmological constant would of course help.