It is valuable when thinking about distant galaxies to be calibrated on
the timescales
involved. Based on a reasonable set of numbers, i.e., an open cosmology
with *H*_{0} = 65 and *q*_{0} = 0.05,
*t*_{0} = 15 Gyr, with the lookback time at *z* ~ 1
is about 50% of *t*_{0},
while the lookback time to *z* = 3 is about 11 Gyr. Or, in a way
that is better for thinking about
formation timescales, the time since recombination to *z* = 10 is
about 1.4 Gyr, to *z* = 5 is about 2.6 Gyr
and to *z* = 3 is about 3.9 Gyr. The timescales grow somewhat
longer with a lambda-universe,
with _{} = 0.7 and
_{m} = 0.3, and dramatically
shorter for an Einstein-de
Sitter _{m} = 1
cosmology (giving a rather unrealistically
short timescale for galaxy development by *z* ~ 5 of < 1 Gyr!).