Can absorb dark energy light

Particle cosmology - interface of particle physics and cosmology

As the latest findings show, only four percent of the universe consists of conventional matter - the rest is dark matter and dark energy. Answers to the question of what is hidden behind this riddle could come from the ranks of particle physics. Because these cosmological questions are closely linked to the knowledge of particle physics. Theoretical physicists are also working on this at the DESY research center.

Enigmatic universe

An outstanding success of modern cosmology is the determination of the energy density of the universe on the basis of measurements of the light emitted by supernovae and the analysis of the cosmic background radiation. The composition of this energy density is completely surprising: The matter known from planets, stars and interstellar gas - which we humans are made of - only contributes four percent, while 96 percent is "dark", i.e. it neither absorbs nor emits light. This dark part is revealed in previous observations only indirectly through its gravitational effect. 23 percent behave like dark matter, which forms spatial structures similar to visible matter. The vast majority of 73 percent, the dark energy, is spatially homogeneous - it does not form structures, but evenly penetrates the entire space - and, due to its negative pressure, leads to the accelerated expansion of the universe (see also article "Dark matter and dark energy" ).

Cosmic background radiation

The question of the origin of visible matter and the nature of dark matter and dark energy is closely linked to particle physics and its theoretical basis, quantum field theory. Interactions that violate certain conservation laws and symmetries - as discovered in experiments on accelerators - are a prerequisite for the creation of a tiny excess of matter compared to the antimatter in the early universe. We owe it to this in turn that matter even exists in today's universe. Supersymmetrical extensions of the standard model of particle physics predict the existence of new elementary particles - such as neutralino, axino or gravitino - which are the main constituents of dark matter. The dark energy, on the other hand, could be generated by quantum effects of the gravitational field or other fields.

The connection between particle physics and cosmology, which arouses so much interest today, has long been part of the DESY theory group's research program. As early as the late 1980s, ideas were developed here whose far-reaching significance only became clear a decade later - for example leptogenesis to explain the matter-antimatter imbalance in the early universe or certain extensions of the theory of gravity, which today plays an important role in play the discussion of the nature of dark energy.

More matter than antimatter

In the early universe, the density of quarks, antiquarks, leptons, antileptons, and photons were about the same. Today, however, one observes an imbalance between matter and antimatter, the so-called baryon asymmetry. At that time, this corresponded to a tiny excess of quarks compared to antiquarks and a corresponding tiny excess of leptons compared to antileptons. Such an asymmetry can be created by the decay of heavy neutrinos, which, due to their quantum mechanical mixture with light neutrinos, produce their very small masses - observed in experiments on neutrino oscillations. The decisive factor here is that the decay of these heavy neutrinos violates the CP symmetry, whereby different frequencies of leptons and antileptons are generated (see also articles "Antimatter in the Universe" and "Neutrinos").

Matter-antimatter asymmetry calculations

The size of the generated baryon asymmetry depends on the properties of the neutrinos, their masses and mixtures. The DESY theory group is also conducting detailed studies on this. For example, theoretical analyzes show that the matter-antimatter asymmetry in the early universe was created for certain typical masses of neutrinos \ (10 ​​^ {- 26} \) seconds after the Big Bang. This opens up a fascinating connection between neutrino physics and the early stages of the universe. It is most remarkable that the experimental evidence for the existence of neutrino masses obtained from experiments on neutrino oscillations and the leptogenesis mechanism elaborated in theoretical studies are quantitatively consistent. This has led to a large number of investigations which, especially in supersymmetric theories, give hope for the discovery of further processes that could contribute to the understanding of the matter-antimatter imbalance in the universe.

Dark matter

A mathematical concept that goes beyond the standard model of particle physics is supersymmetry, which assigns a supersymmetrical partner particle to each particle (see article “The Particle Doubler: Supersymmetry”). In many supersymmetric extensions of the Standard Model, the lightest of these new superparticles (Lightest Supersymmetric Particle, LSP for short) electrically neutral and stable. A popular candidate for the LSP is the neutralino, a super partner of the photon, Z boson and Higgs boson. In this case, in the experiments at the LHC, characteristic events should be observed which apparently violate the law of energy conservation, since part of the total energy escapes unobserved from the detectors in the form of neutralinos. Neutralinos of dark matter could also scatter on normal matter via the weak force and thus be directly detected in laboratory experiments.

The investigation of the leptogenesis mechanism points to another possibility: Dark matter could also consist of gravitinos, the supersymmetrical partner particles of gravitons, which mediate the force of gravity, just as photons are the carriers of electromagnetic force. Under certain conditions, the gravitinos could decay into ordinary particles, especially photon-neutrino pairs. Experimental evidence for this hypothesis could come from gamma-ray telescopes and the LHC: Satellite experiments can be used to measure the flow of photons generated inside and outside the Milky Way, which has a characteristic energy spectrum. The satellite-based gamma-ray telescope EGRET observed an anomaly in the photon flux at the end of the 1990s. This can be explained by the effect predicted by the Gravitino hypothesis. If this hypothesis is actually true and the dark matter consists of gravitinos, the space telescope Fermi Gamma-ray Space Telescope (formerly called GLAST), which was launched in June 2008, should also observe a signal in the next few years; and characteristic decays of other heavy super-particles should be discovered at the LHC. So it could be that the mystery of dark matter will be cleared up within the next few years. In the search for the “world formula” an important milestone would have been reached.