(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?

  • $\begingroup$ This is an interesting question. I believe the answer is no, but I this is an interesting discussion nonetheless. $\endgroup$ – 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.)

The different sign is hugely important. Radiation gravitates attractively, but dark energy causes a kind of cosmic repulsion.

The two make the universe expand in very different ways. In a radiation-dominated universe, the scale factor increases as the square root of time (so the expansion gets slower and slower), but in a dark-energy-dominated universe, the scale factor increases exponentially with time (so the expansion gets faster and faster).

Thus the red-shifted CMB does not have the properties necessary to explain astronomical observations that the expansion of the universe is getting faster and faster, not slower and slower. Furthermore, the amount of energy density in the CMB today is a completely negligible part of the total energy density of the universe.

  • $\begingroup$ How can we tell if expansion slows down or not? Observation of the speed of remote galaxies? $\endgroup$ – Stepan Nov 30 '18 at 4:32
  • $\begingroup$ Yes. And we determine their speed from the red-shift of their light. $\endgroup$ – G. Smith Nov 30 '18 at 4:35

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