# I Particle horizon rewritten

1. Jul 25, 2016

### Lapidus

Here http://www.damtp.cam.ac.uk/user/db275/Cosmology/Lectures.pdf, I find on page 31 in (2.1.5)

I assume that it is childish calculus that connects both sides of this equation. But still, can someone help me why the integral can be rewritten like that?

thanks!

2. Jul 25, 2016

### Orodruin

Staff Emeritus
It is a standard change of variables.

$\int f(a(t)) dt = \int f(a) (dt/da) da$