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Mass density of radiation in Friedmann equation?
$$\frac{{\ddot a}}{a} = - \frac{{4\pi G}}{3}(\rho + 3P) + \frac{\Lambda }{3}$$
In the case of matter, pressure $P=0$,
In the case of radiation, pressure $P=(1/3)ρ$,
In the case of cosmological ...
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