A new explanation of dark energy has been proposed by physicists. It may reveal the link between quantum field theory and general relativity theory, as well as a new view of the universe and its components.
What lies behind the term "dark energy" — and how does it relate to Albert Einstein's cosmological constant? Two physicists from the University of Luxembourg demonstrate the way to answer these open questions of physics.
The universe has a number of strange properties that are difficult to grasp with everyday experience. For example, the matter we know, which consists of elementary and composite particles that form molecules and materials, appears to make up only a tiny portion of the universe's energy. The greatest contribution, roughly two-thirds, comes from "dark energy," a hypothetical energy that physicists are still unsure about.
In a paper published in the journal Physical Review Letters, both characteristics appear to be connected. Moreover, dark energy is also considered a contributor to accelerated growth.
"Vacuum has energy. This is a fundamental result of quantum field theory," says Prof. Alexandre Tkatchenko, who is professor of theoretical physics at the University of Luxembourg's Department of Physics and Materials Science. Although quantum field theory is compatible with general relativity, it has one fundamental feature: it considers not only particles but also matter-free fields as quantum objects.
According to Tkatchenko, dark energy is a physical quantity that is created by a constant emergence and interaction of pairs of particles and their antiparticles in what is actually empty space.
Planck's cosmic microwave background
As vacuum or zero-point fluctuations, physicists refer to virtual particles' coming and going and their quantum fields. While the particle pairs quickly vanish into nothingness again, their existence leaves behind a certain amount of energy.
"This vacuum energy has a general relativity relevance," according to a Luxembourg researcher: "It manifests itself in Einstein's cosmological constant, which he has included into his equations for physical reasons."
The cosmological constant, unlike vacuum energy, may be determined directly by astrophysical experiments. Measurements with the Hubble space telescope and the Planck space mission have yielded close and reliable values for the fundamental physical quantity. On the other hand, quantum field theory calculations yield results that are up to 10120 times larger – a huge discrepancy, although in the world view of physicists today, both values should be equal.
Alexandre Tkatchenko claims to be one of the greatest inconsistencies in modern science.
Dr. Dmitry Fedorov, a Luxembourg researcher, has brought the answer to this long-standing problem a step closer. The two Luxembourg researchers propose a new interpretation of dark energy, which can be both measured and calculated.
Tkatchenko explains that the electrodynamic forces exerted by these particles during their extremely short existence give rise to a vacuum self-interaction. "It results in a power density that may be determined using a new model."
Together with Fedorov, they developed the basic model for atoms a few years ago and presented it for the first time in 2018. The model was originally used to describe atomic properties, in particular the relationship between the polarizability of atoms and the equilibrium properties of certain non-covalently bonded molecules and solids.
Fedorov adds that the two researchers examined the behavior of quantum fields, in particular the "coming and going" of electrons and positrons. These fluctuations may also be explained by an experimental geometry, which is already known.
The last step was then to quantum mechanically compute the energy density of the self-interaction between electrons and positrons. The obtained result is in line with the observed values for the cosmological constant, according to Alexandre Tkatchenko.
"Our investigation provides an elegant and unconventional approach to solving the mystery of the cosmological constant," says the scientist. "It also provides a verifiable proof that quantum fields such as those of electrons and positrons do indeed have a small but ever-present intrinsic polarization."
According to the two Luxembourg scientists, this finding opens the way for future experiments to detect this polarization in the laboratory as well. "Our aim is to obtain the cosmological constant from a rigorous quantum theory approach," says Dmitry Fedorov. "And our work contains a recipe for achieving this."
The new findings, which were discovered together with Alexandre Tkatchenko, are the first step toward a better understanding of dark energy and its connection to Albert Einstein's cosmological constant.
Tkatchenko is convinced that this article will help clarify the way quantum field theory and general relativity theory are interwoven as two approaches to understanding the universe and its components.
Alexandre Tkatchenko and Dmitry V. Fedorov, "Casimir Self-Interaction Energy Density of Quantum Electrodynamic Fields," Physical Review Letters, 24 January 2023. DOI: 10.1103/PhysRevLett.130.041601.
