CosmoCoffee

[YinZhe Ma]

Hi Guys,

I don't quite understand the meaning of drag epoch. In Komatsu 2009 paper, it says:

the drag epoch z_d and the decoupling epoch z_*:

They would be equal only when the energy density of photons and that of baryons were equal at the decoupling epoch – more precisely, they would be equal only when R=3ρ_b/4ρ_γ=1 at z=z_*?

What does this mean?

And why does the low density of baryon leads to later of drag epoch?

Thank you!

[Ben Gold]

The decoupling epoch is when the photon optical depth is one.

The drag epoch is when the baryon optical depth is one.

Since there are many more photons than baryons, the photons "decouple" (stop noticing the baryons) before the baryons stop noticing the photons.