# Primodial Curvature Perturbation Equation

1. Apr 16, 2007

This equation gives us (delta (rho))/rho (which I understand is the fractional perturbation in the energy density), at the time of "horizon entry" (which I'm unsure about). Does this mean the time that decoupling occured?

2. Apr 17, 2007

### Dick

"Horizon entry" has nothing to do with decoupling. Almost by the definition of inflation, during inflation the scale factor is growing faster than the hubble radius - so a given perturbation wavelength will eventually become larger than the hubble radius. Conversely, after inflation the scale factor is growing more slowly than the hubble radius - so that same scale will once again be smaller than the hubble radius. That's 'horizon entry'.