The cosmological constant has turned out to be variable.

Modern cosmology is defined by the equations of Einstein's theory of relativity. When the physicist was developing his theory, he observed that these equations did not have a solution in which the Universe was static. At that time, scientists believed the Universe was unchanging. To allow for such a solution, Einstein introduced a so-called lambda term into the equation—a constant that ensured staticity.

In 1924, the Soviet meteorologist Alexander Friedmann demonstrated that the Universe is not static; rather, it is expanding. Initially, Einstein attempted to criticize Friedmann's findings but eventually acknowledged his mistake and, by the early 1930s, reluctantly accepted the idea of an expanding Universe. Following this, he referred to the lambda term as his greatest blunder.

However, in 1998, observations of distant supernovae unexpectedly revealed that Einstein was not mistaken after all: although the cosmos is not static, it is expanding at an accelerating rate. This acceleration cannot be reconciled with Einstein's equations without reintroducing the lambda term—albeit with a different value. This quantity is commonly referred to as the cosmological constant.

Research from the Dark Energy Spectroscopic Instrument collaboration (named after the instrument itself) has sparked a new chapter in this story. The researchers analyzed the redshift of a million galaxies. Their data suggest that, with a probability of 3.5 sigma, the lambda term in the early Universe differed quantitatively from its current value.

Meanwhile, the most popular hypothesis among many inflationary physicists regarding the nature of the cosmological constant is the so-called repulsive effect of vacuum, or negative pressure of vacuum. In such a scenario, the lambda term should remain strictly invariant, as the properties of vacuum do not change. The new result does not achieve the significance of five sigma, which would allow for a confident assertion of "closing" the negative pressure of vacuum, but it is relatively close to this threshold.

Therefore, at the next stage, theoretical physicists will need to propose alternative explanations for dark energy, that is, the accelerated expansion of the Universe. Since its cause is not rooted in the vacuum, other explanations must exist. Unfortunately, at the current stage of physics, the number of theories regarding dark energy is so vast that it will take considerable time to choose between them, even with new data.

Inflationary physicists have developed literally hundreds of different models of the Universe (and this is a conservative estimate). In many of these models, dark energy is not associated with the vacuum. The challenge lies in the fact that the existing inflationary hypotheses are quite difficult to validate or refute in practice: due to the overwhelming number of models, the predictions made by these models are extensive, and some of these predictions will always coincide with observations simply due to their sheer number.

Additionally, there is an alternative approach to the problem: cyclical cosmology of the Universe, with the most contemporary version presented in Nikolai Gor'kavy's theory. According to this theory, the cosmological constant was declared variable as early as 2018. This is because, in Gor'kavy's cyclical cosmology, this "constant" is inversely proportional to the radius of the Universe. Consequently, as the Universe expands, the acceleration of that expansion should decrease, aligning with the new data from the Dark Energy Spectroscopic Instrument.

If this scenario holds true, the Universe's expansion will eventually halt, after which it will begin to contract. According to calculations, there have been many (though not infinitely many) cycles of expansion and contraction, and the number of such cycles in the future will also be substantial.