"Exotic" Distance Indicators
All of the above methods rely on a straightforward application of the inversesquare law or the
angular diameterdistance relation. There is also a range of techniques that use more involved or
indirect combinations of observables. Some examples are:
The Hubble time
: for simple bigbang models, ages of objects (stars, radioactive nuclei) set
bounds on
H
0
. The age of the universe is of order the Hubble time τ
H
=1/
H
0
, to within a factor of
order unity depending on the deceleration history of the expansion. For
H
0
=50 km/s Mpc, τ
H
= 2
x10
10
years; for 100 km/s Mpc, 10
10
years. This must be greater than the age determined from
geological and stellarevolutionary timescales, nuclear isotopic clocks like
235
U/
238
U, and
consistent with the dynamical status of galaxies and clusters. The small amount of evolution
observed in elliptical galaxies to about
z=1
favors smaller
H
0
in simple models (Hamilton 1985
ApJ 297, 371). One should beware subtly circular arguments  globularcluster ages were
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 Fall '10
 EmilyHoward
 Astronomy, Hubble's law, Hipparcos, angular diameterdistance relation, simple bigbang models, HST Cepheid results

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