This might be a silly question. If we're considering The closed Radiation dominated Friedmann-Lemaitre-Robertson-Walker universe It's common to consider the radiation as a perfect fluid and solve the Friedmann equations from there.
My issue is that the trace of the Stress energy Tensor of the free electromagnetic field is zero (or equivalently conformally invariant) implying zero Ricci curvature. (My understanding is this is true in a general spacetime) How can we then use this in such a model which supposes non-zero scalar Ricci curvature?
I'm sure I'm missing something simple. Thanks for reading!