In the expanding universe, light is subject to redshift.
Does redshift also affect dark energy - and why?
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2 Answers
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Particles with and without mass are affected by cosmological redshift - e.g. both the photon and neutrino energy densities of the universe are reduced by both expansion and a redshift effect.
Dark energy is somewhat different. In its vanilla form dark energy is hypothesised to have a fixed energy density; as the universe grows, so does the amount of dark energy.
There are other models though, grouped under the term quintessence. These are scalar fields with time-varying energy density, but they don't simply reduce with the volume implied by the scale factor of the universe.
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There is a correlation between dark energy and redshift. The dark energy itself doesn't redshift. If space expands with constant rate over time, the redshift will show similar behavior. The farther we look, the more light will be redshifted. That's pretty clear, I gues. If dark energy is constant then the relation will be linear. If, on the other hand, the dark energy is not constant, then this will show in the redshift measurements at various distances. This showed up in measuring the redshift of faraway supernovae, called standard candles of the universe as they are considered to burn with equal intensity everywhere at all times. It turned out, maybe surprisingly, that the expansion is not constant, nor decelerating. It's accelerating since about some billion years ago. It was decelerating first, because the mass/energy density contribute more than the dark energy density. The ME contribution lowered because of the volume increasing. The dark energy contribution stayed the same so there was a moment it overtook. Acceleration took off.
So, the dark energy doesn't redshift over time, but it causes a non-trivial redshift. It's value remains constant, it's the matter-energy contribution that varies.
