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That one sentence manages to be wrong in at least two ways. In the early radiation-dominated era there were all sorts of particles, since the temperature was high enough to allow particle-antiparticle pair creation. The particles were traveling at relativistic speeds and had temperature-dependent numbers, which gives a different relation between the pressure and the energy density than found later in the matter-dominated era.
Even if there were nothing but photons around, however, they would still attract gravitationally, just as light is attracted to other massive objects. As far as gravity is concerned, there's nothing much different between a photon and anything else traveling at or near the speed of light.
(published on 09/02/2011)
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