  This work is devoted to study the behavior of massless particles within the
context of curved spacetime. In essence, we investigate the consequences of the
scale factor $C(\eta)$ of the Friedmann-Robertson-Walker metric in the
Einstein-aether formalism to study photon-like particles. To do so, we consider
the system within the canonical ensemble formalism in order to derive the
following thermodynamic state quantities: spectral radiance, Helmholtz free
energy, pressure, entropy, mean energy and the heat capacity. Moreover, the
correction to the Stefan-Boltzmann law and the equation of states are also
provide. Particularly, we separate our study within three distinct cases, i.e.,
$s=0,p=0$; $s=1,p=1$; $s=2,p=1$. In the first one, the results are derived
numerically. Nevertheless, for the rest of the cases, all the calculations are
accomplished analytically showing explicitly the dependence of the scale factor
$C(\eta)$ and the Riemann zeta function $\xi(s)$. Furthermore, our analyses are
accomplished in general taking into account three different regimes of
temperature of the universe, i.e., the inflationary era ($T=10^{13}$ GeV), the
electroweak epoch ($T=10^{3}$ GeV) and the cosmic microwave background
($T=10^{-13}$ GeV).
