When it became clear 20 years ago that the expansion of the universe was accelerating, scientists presented a complete, simple and testable explanation. But as more and more experimental and observational data came in, the reason for dark energy - presumably the reason for this acceleration - remained elusive. While it is technically equivalent to a "cosmological constant" (or energy inherent in space itself), there is no way to infer its value. But if we remember that placing certain forms of matter in empty space changes the forces acting on this matter, it is possible that dark energy arises according to a very simple principle: because matter as such exists in our Universe.
Clustering map of galaxies in our Universe. The presence of these structures can explain the presence and power of dark energy in its entirety.
Most forces and phenomena have origins that are easy to discover. Two massive objects experience the force of gravity due to the fact that spacetime is warped due to the presence of matter and energy. The universe has expanded because it has its own history of changes in the energy density in the universe and the initial conditions for expansion. And all particles in the Universe interact in a certain way due to the well-known rules of quantum field theory and the exchange of vector bosons. From the smallest subatomic particles to the largest scales, the same forces are at work that propel bosons and galaxies.
The strong interaction that occurs due to the presence of "color change" and the exchange of gluons holds the nuclei of atoms together
Even the most mysterious phenomena at their core contain explanations that are well understood. We do not know why there is more matter than antimatter in the Universe, but we do know that the conditions we need for this - baryon number violation, C and CP violation - take place. We do not know what the nature of dark matter is, but its general properties, where it is located and how it clusters, is all well understood. We also do not know whether black holes preserve information or not, but we understand the final and initial state of these objects, as well as how they are born and what happens to their event horizons over time.
An illustration of a black hole and its environment, an accelerating and inflating accretion disk. The initial and final states of black holes can be well predicted even if the loss or storage of information is not
But there is one thing we don't understand at all: dark energy. Of course, we can measure the acceleration of the universe and understand its magnitude accurately. But why do we even have a universe with non-zero dark energy? Why does empty space, in which there is nothing - no matter, no curvature, no radiation, nothing - has positive nonzero energy? And why did this amount of energy, which was incomprehensibly tiny and completely invisible during the first billion years of the history of the Universe, began to capture the Universe only by the moment the Earth appeared in it?
An illustration of a protoplanetary disk in which planets and planetesimals form, creating gaps in the disk. Four to five billion years ago, when our solar system was forming, dark energy simultaneously began to capture the expansion of the universe and the energy density
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There are many interesting things we can associate with dark energy and the universe as a whole. There is a lot of empty space that quantum fields permeate. There are no regions in the Universe where gravitational, electromagnetic, or nuclear forces penetrate; they are everywhere. If we try to calculate the so-called vacuum expected value (condensate) of various quantum fields, we will encounter an infinite number of terms and can only write it approximately. We will always deal with approximate values. And as far as we know, they are not balanced, and the universe is not slowing down - it is actually accelerating. Somehow, space itself has a little non-zero energy. And this energy makes distant galaxies in the Universe move away from us at an ever-increasing speed,though slowly, but constantly.
The question "why?" never ceases to torment theorists. Why is the universe expanding faster and faster? We cannot explain the presence of this dark energy in anything. Perhaps we have little understanding of 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 a reason to believe that this is possible, which is called the Casimir effect. We know him well.
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, no kidding. But if you place two metal plates at a certain distance between them, and then ask again what the electromagnetic force will be, it will cease to be zero. Due to the fact that some modes of quantum fluctuations are forbidden due to the boundaries of the plate, we not only predict, but also measure the non-zero force between these plates, arising literally from empty space. And what is most interesting, all forces, including gravitational forces, exhibit the Casimir effect.
Map of over a million galaxies in the Universe; there is a separate galaxy at each point. Different colors represent distances; red - further
What happens when we try to apply this effect to the whole universe and calculate the effect? The answer is simple: we get something that corresponds in some form to dark energy, only - again - of a different order. And this may be because 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. We may need a theoretical breakthrough. And, possibly, it will be associated with a trace anomaly, a change in dynamic magnitude, or even a trace of additional dimensions. We have only recently found its most difficult secret to explain. Perhaps the solution will lie in the physics we already know.
Ilya Khel