In this paper , we obtain luminosity distances by using ages of 32 old passive galaxies distributed over the redshift interval 0.11 < z < 1.84 and test the cosmic conservation of photon number by comparing them with 580 distance moduli of type Ia supernovae ( SNe Ia ) from the so-called Union 2.1 compilation .
Our analyses are based on the fact that the method of obtaining ages of galaxies relies on the detailed shape of galaxy spectra but not on galaxy luminosity .
Possible departures from cosmic conservation of photon number is parametrized by \tau ( z ) = 2 \epsilon z and \tau ( z ) = \epsilon z / ( 1 + z ) ( for \epsilon = 0 the conservation of photon number is recovered ) .
We find \epsilon = 0.016 ^ { +0.078 } _ { -0.075 } from the first parametrization and \epsilon = -0.18 ^ { +0.25 } _ { -0.24 } from the second parametrization , both limits at 95 % c.l .
In this way , no significant departure from cosmic conservation of photon number is verified .
In addition , by considering the total age as inferred from Planck ( 2015 ) analysis , we find the incubation time t _ { inc } = 1.66 \pm 0.29 Gyr and t _ { inc } = 1.23 \pm 0.27 Gyr at 68 % c.l .
for each parametrization , respectively .