When it was discovered 20 years ago that the expansion of the universe is accelerating, scientists presented this with a complete, simple and verifiable explanation. But as more and more data came from experiments and observations, the reason for the existence of dark energy – presumably the cause of this acceleration – remained elusive. Although technically it is equivalent to a "cosmological constant" (or energy inherent in the space itself), there is no way to derive its magnitude. But if we recall that the placement of certain forms of matter in an empty space is changing the forces that act on this matter, perhaps, the dark energy arises on a very simple principle: because in our universe there is matter in general as such
Map of clustering of galaxies in our Universe. The presence of these structures can explain the presence and strength of the dark energy entirely
Most forces and phenomena have origins that are easy to detect. Two massive objects experience the power of gravity because space-time is bent due to the presence of matter and energy. The universe has expanded, as it has its own history of changes in energy density in the universe and the initial expansion conditions. And all the particles in the universe interact in a certain way because of the well-known rules of quantum field theory and the exchange of vector bosons. From the smallest subatomic particles to the largest scale, the same forces act that drive bosons and galaxies.
The strong interaction that appears due to the presence of a "color change" and the exchange of gluons, keeps the nuclei of atoms together
Even the most mysterious phenomena in their basis store explanations that are well understood. We do not know why there is more matter in the Universe than antimatter, but we know that the conditions that we need for this are a violation of the baryon number, C and CP violation are taking place. We do not know what the nature of dark matter is, but its general properties, where it is located and how it is clustered, are all well understood. We also do not know whether black holes keep information or not, but we understand the final and initial state of these objects, and also how they are born and what happens to their event horizons with time.
Illustration of a black hole and its surroundings, an accelerating and inflating accretion disk. The initial and final state of black holes can be well predicted, even if the loss or preservation of information is not
But there is one thing that we do not understand at all: dark energy. Of course, we can measure the acceleration of the universe and accurately understand its magnitude. But why do we have a universe with a non-zero value of dark energy? Why does an empty space, in which there is nothing – no matter, no curvature, no radiation, nothing – has a positive non-zero energy? And why this amount of energy, which was inconceivably tiny and completely invisible during the first billions of years of the history of the universe, began to capture the universe only by the time the Earth appeared in it?
Illustration of a protoplanetary disk in which planets and planetesimals form, forming gaps in the disk. Four or five billion years ago, when our solar system was forming, the dark energy simultaneously began to capture the expansion of the universe and the energy density
There are many interesting things that we can associate with dark energy and the universe as a whole. There is a lot of empty space that permeates quantum fields. There are no regions in the universe, no matter where the gravitational, electromagnetic, or nuclear interaction has penetrated; they are everywhere. If we try to calculate the so-called vacuum expected value (condensate) of different quantum fields, we will encounter an infinite number of terms and can only write it down approximately. We will always deal with approximate values. And as far as we know, they are not balanced, and the universe does not slow down – it really accelerates. Somehow, the space itself has a small non-zero energy. And this energy makes distant galaxies in the Universe move away from us with ever-increasing speed, albeit slowly, but constantly.
The question "why?" Does not cease to torment the theorists. Why does the universe expand faster? We can not explain the presence of this dark energy in any way. Perhaps we do not understand the Universe itself. However, there is another option, which is rarely thought about: perhaps this property of empty space is determined by the presence of other things – like matter – in the universe.
And there is reason to believe that this is possible, which is called the Casimir effect. He is well known to us.
Illustration of the Casimir effect and how the forces outside the plates differ from the forces between them
What is the electromagnetic force of empty space? Zero, of course. In the absence of charges, currents and matter for interaction, it will be zero, without jokes. But if you put two metal plates a certain distance apart, and then ask again what the electromagnetic force is, it will stop being zero. Due to the fact that certain modes of quantum fluctuations are forbidden because of the boundaries of the plate, we not only predict, but also measure the nonzero force between these plates, which arises literally from empty space. And what is most interesting, all the forces, including the gravitational, manifest the effect of Casimir.
The map of more than a million galaxies in the universe; at each point a separate galaxy. Different colors mean distances; red – on
What happens when we try to apply this effect to an entire universe and calculate the effect? The answer is simple: we get something that in some form corresponds to a dark energy, only – again – of a different order. And this may be due to the fact that we do not fully know what the boundary conditions of the universe are, or how to correctly calculate the quantum-gravitational effect.
Mapping the universe can be the easiest part. It is unlikely that we will wait for an observed or experimental breakthrough that will lead us to an understanding of dark energy, the most elusive force in the universe. Perhaps we need a theoretical breakthrough. And, perhaps, it will be associated with a trace anomaly, a change in the dynamic value or even a trace of additional dimensions. We only recently found her most difficult to explain the secret. Perhaps the solution will lie in the physics already known to us.