Electromagnetic in cosmology

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?

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Answers and Replies

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

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
Science Advisor
Homework Helper
yes.