Assume a flat FRW universe, dominated by radiation throughout its history. Calculate the physical, cosmological horizon size at t=t_0 The initial conditions for the scale factor are a=a_0 at t=t0(today). Which asnwer is correct;  a)R_H=2a_0ct_0 b) R_H = 3a_0ct_0 c)R_H = ct_0 d) R_H=2ct_0

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