On the presence of energy that is not related to the matter around
The existence of so-called dark energy not only follows from cosmological observations, but also arises theoretically. This could appear to be the energy of some quantum field unknown so far, which is referred as ‘quintessence’. Properties of this field may well be such that its energy in the volume of space changes almost synchronously with the increase of that volume due to expansion of the universe, so that the energy density of the ‘quintessence’ remains nearly constant. A similar effect can be achieved by introducing the so-called cosmological constant into the theory, which was proposed by Einstein, but still has not received the rationale. The contribution of the cosmological constant to the rate of expansion in the past had to differ significantly from the contribution of the ‘quintessence’, so detailed cosmological observations can reveal which version takes place in reality.