Why Are Primes Used in the Electromagnetic Signals Equation in Cosmology?

[trv]

Homework Statement

Title should be: ELectromagnetic signals in cosmology.

$\int\frac{d\rho}{\sqrt{1-k\rho^2}}>=-\int\frac{dt'}{a(t')}$

The integrals being from r1 to r and t1 to t respectively.

I can't understand the need for the prime here. Could someone explain to me the significance of the prime?

Homework Equations

The Attempt at a Solution

 

[nicksauce]

It's just because they don't want to have the same variable in the integrand and the bounds of integration. Nothing special.

 

[trv]

Ok so I could just as well write dt, and a(t), i.e. without the prime, as long as I had t1 to t2 or other such in the bounds of the integration?

 

[nicksauce]

yes.