# Why did dark energy play a subdominant role in the radiation and matter dominated era?

After inflation, the Universe passed through radiation dominated (RD) and matter dominated (MD) era during both of which the evolution of the scale factor (or the Hubble parameter) was decreasing with time as $$H\sim t^{-1}$$. These were phases of cosmic deceleration. However, the present Universe is undergoing an accelerated expansion. Whatever agent causes this expansion is termed as dark energy. It may be constant, or perhaps, it has been evolving with time.

If we assume that dark energy was always in the form of a constant energy density which doesn't dilute with expansion- a cosmological constant, why didn't it play a dominant role in the RD and MD era?

• The way you've phrased you question, it's a matter of definition. Dark energy played a subdominant role, because the era was dominated by other things. I get what you mean, of course :) – pela Aug 30 '19 at 13:54

The density of matter decrease with increasing volume $$V \propto a^3$$, where $$a$$ is the scale factor, or the "size", of the Universe. Radiation is also redshifted, and the redshift is linearly proportional to the expansion, so there's an extra factor of $$a$$; hence its energy density decreases as $$a^4$$.