What is exactly the drag epoch?

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YinZhe Ma
Posts: 11
Joined: October 09 2008
Affiliation: University of KwaZulu-Natal

What is exactly the drag epoch?

Post by YinZhe Ma » December 02 2011

Hi Guys,

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

the drag epoch zd 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=3ρb/4ργ=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
Posts: 81
Joined: September 25 2004
Affiliation: University of Minnesota
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What is exactly the drag epoch?

Post by Ben Gold » December 07 2011

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.

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