We present a unified dark fluid model to describe the possible evolutionary behavior of $\Delta N_\mathrm{ eff}$ in dark radiation. This model can be viewed as an interacting model for the dark sectors, in which dark matter interacts with dark radiation. We show that the evolution of $\Delta N_\mathrm{ eff}$ can be nicely explained without some drawbacks, such as the blowup of $\Delta N_\mathrm{ eff}$ and the non-vanishing interaction at the late time.
Categories: astro-ph.CO hep-ph