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I read that at the end of inflation, while the inflaton is oscillating about the bottom of it's potential (before it decays), the universe may be treated as matter dominated (e.g. $w=0$; $a\propto t^{2/3}$).

From what I read, this was stated without justification. Is there an intuitive way to see this?

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  • $\begingroup$ Good question. I'm not sure there is a justification other than that matter seems to dominate now. $\endgroup$ – Lewis Miller Jul 14 '16 at 2:58
  • $\begingroup$ True, but between now (MD) and the end of inflation, there was also the radiation dominated phase. It seems natural for radiation domination to begin at reheating (which is after inflation). Rhetorical Question: Why not state the universe immediately after inflation as radiation dominated then? $\endgroup$ – Bob Jul 14 '16 at 16:45
  • $\begingroup$ before it decays, there is no radiation. A massive scalar inflaton field behaves more like matter than radiation. The math supports this as well, but that's a nice conceptual way to remember it $\endgroup$ – Jim Aug 3 '16 at 12:11
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For the case of free massive scalar field, one can derive a(t) and in the leading order term the universe behaves like a matter-dominated universe with zero pressure. You may find the derivation in "Physical Foundations of Cosmology" by Viatcheslav Mukhanov, chapter 5, pages 236-240

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