Function lookbackTime( z, H0, omegaM, omegaLambda )

Description:
Lookback time. This returns the difference between the age of the universe at time of observation (now) and the age of the universe at the time when photons of redshift z were emitted.
Parameters:
z (floating point)
redshift
H0 (floating point)
Hubble constant in km/sec/Mpc
omegaM (floating point)
density ratio of the universe
omegaLambda (floating point)
normalised cosmological constant
Return Value (floating point):
lookback time in Gyr
Signature:
double lookbackTime(double, double, double, double)