As everyone knows, the universe is constantly expanding. But many do not even realize that the process is accelerating, and that physicists have no sane explanation for this phenomenon. A group of theorists suggested that a mysterious "dark energy" was involved, and now we will tell you in an accessible form what it is.
For almost two decades, astronomers have known that the expansion of the universe is accelerating, as if a mysterious "dark energy" inflates it from the inside, like a balloon. This energy remains one of the biggest mysteries in physics today. Now a trio of theorists argue that dark energy arises from an amazing source. As creepy as it may sound, in their opinion, it goes against the fundamentals of physics that everyone learned in school: the amount of total energy in the Universe is not fixed and unchanged, it can gradually disappear.
According to scientists, dark energy may be a special field, a bit like electric, that fills space. On the other hand, it can be part of the cosmos itself, which is called the cosmological constant (otherwise the lambda term). The second scenario looks like a mockery of Einstein's theory of relativity, which claims that gravity occurs when mass and energy bend space and time. In fact, the cosmological constant is also an invention of Einstein, and he invented it literally by adding a constant to his equations to explain how the universe resists destruction by its own gravity. However, he abandoned the idea when astronomers discovered in the 1920s that the universe was not static, but expanding, as if it had been born from an explosion.
Upon closer observation, it became clear that the expansion of the universe was accelerating, and the cosmological constant returned again. Within the framework of quantum mechanics, however, it becomes much more cunning. Quantum mechanics suggests that the vacuum itself must oscillate imperceptibly. In general relativity, these tiny quantum fluctuations produce energy that will serve as the cosmological constant. However, all other things being equal, it must be 120 orders of magnitude larger to destroy the universe. So the explanation of why the cosmological constant, although it exists, but in a very modest form, is a big mystery for physicists. When it was not yet necessary, physicists simply assumed that some as yet unknown effect would simply reduce it to zero.
Now Thibault Josette and Alejandro Perez of the University of Aix-Marseille in France and Daniel Sudarski of the National Autonomous University of Mexico in Mexico City claim they have found a way to derive a reasonable value for the cosmological constant. They began with a version of general relativity, which Einstein himself invented, called unimodular gravity. General relativity assumes mathematical symmetry, general covariance, which implies that no matter how you determine the position of a coordinate in space and time, the theoretical prediction will remain the same. This symmetry requires conservation of energy and momentum. Unimodular gravity has a more limited version of this mathematical symmetry.
This system reproduces most of the assumptions of general relativity. However, according to it, quantum fluctuations of the vacuum do not create gravity or affect the cosmological constant (which, after all, is just a mathematical constant, and its value can be anything). But this comes at a price: unimodular gravity does not require energy to conserve, so theorists have to limit it arbitrarily.
A trio of scientists have shown that unimodular gravity, if accepted and allowed to violate the law of conservation of energy and momentum, actually sets the value of the designated constant. The argument is mathematical, but in fact even a small part of the energy that disappears in the Universe leaves a trace in the form of a change in the cosmological constant. “The dark energy in our model is precisely the result of how much energy and momentum has been lost in the universe over its entire existence,” says Perez.
To prove that their theory is reasonable and applicable to reality, the scientists looked at two scenarios of how breaking the law of conservation of energy would theoretically affect the underlying problems of quantum mechanics. For example, the theory of continuous spontaneous localization (CSL) tries to explain why subatomic particles like electrons can literally be in two places at the same time, but such large objects as cars or people cannot. CSL assumes that such states of matter spontaneously arise and disintegrate in dependence, which increases with an increase in the volume of an object, which means that a large object simply cannot be "double" in the conditions of the Earth. Against this theory is the fact that it does not take into account the conservation of energy. However, theorists have shown that the sum of violations of the provisions on conservation of energy is just that,to give a cosmological constant of the desired size.
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Nevertheless, according to some scientists, theorists are simply playing with math. They still have to assume that the cosmological constant starts at some small value, but they do not explain this aspect. However, modern physics is full of inexplicable constants, like the charge of an electron or the speed of light, so this is just another constant on a long list.