# Is dark energy just a cooling effect of CMB?

(1) Cosmic microwave background cooled down from about 3000K billions of years ago to a couple of Kelvin nowadays. (2) Adiabatic expansion causes gas (of photons in this case) to cool. (3) Dark energy acts by accelerating expansion of the Universe - the effect attributed to dark energy.

It seems like we can combine this statements as follows: CMB undergoes adiabatic cooling and expansion causing observed acceleration of the galaxies. Dark energy decreases to zero as CMB temperature approaches zero Kelvin.

Are there any experiments supporting or rejecting this hypothesis?

• This is an interesting question. I believe the answer is no, but I this is an interesting discussion nonetheless. – João Vítor G. Lima Nov 30 '18 at 4:14

Dark energy behaves in a very different way from the energy of the CMB. Radiation such as the CMB has the equation of state $$p=\rho/3$$ while dark energy has $$p=-\rho$$. (Here $$p$$ is pressure and $$\rho$$ is energy density.)