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## Homework Statement

Title should be: ELectromagnetic signals in cosmology.

[itex]

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

[/itex]

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

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