In a question, I am given the current densities of dark energy, dark matter and normal matter and am asked to find the ratio of density of dark energy at the time of CMB and now.
The answer is 1. Is this a simple calculation or is it something we are expected to know?
Assuming dark energy is a cosmological constant and not quintessence or something more complicated, it is a constant and does not change as the universe expands. Well, if it changed it would be a cosmological variable not a constant!
For more on this type of calculation you should read Pulsar's answer to Equation for Hubble Value as a function of time as this describes the principles involved.
