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In this paper, we obtain luminosity distances by using ages of 32 old passivegalaxies distributed over the redshift interval $0.11 < z < 1.84$ and test thecosmic conservation of photon number by comparing them with 580 distance moduliof type Ia supernovae (SNe Ia) from the so-called Union 2.1 compilation. Ouranalyses are based on the fact that the method of obtaining ages of galaxiesrelies on the detailed shape of galaxy spectra but not on galaxy luminosity.Possible departures from cosmic conservation of photon number is parametrizedby $\tau(z) = 2 \epsilon z$ 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 limitsat 95\% c.l. In this way, no significant departure from cosmic conservation ofphoton number is verified. In addition, by considering the total age asinferred from Planck (2015) analysis, we find the incubation time$t_{inc}=1.66\pm0.29$ Gyr and $t_{inc}=1.23\pm0.27$ Gyr at 68\% c.l. for eachparametrization, respectively.}

Testing the cosmic conservation of photon number with type Ia supernovae and ages of old objects