When computing the mass and energy content of the Universe, it becomes clear that a large amount other matter, different from the ordinary (baryonic) matter, is needed to agree with standard cosmological models. Baryonic matter is made up of so called baryons, which includes, most importantly, protons and neutrons. In addition to the baryonic matter, two other components are required for the observations to match the theories: dark matter and dark energy.
While ordinary matter is assumed to make up only 5% of the Universe, dark matter makes up about 27% and dark energy approximately 68% of the mass-energy content of the Universe.
Dark matter (DM) is a hypothetical type of matter that differs from ordinary (baryonic) matter. The term "dark" refers to the fact that dark matter has no known interaction other than gravitational interaction and thus remains invisible in the electromagnetic spectrum.
The following indirect evidence for the existence of dark matter has been found:
- The flat rotational curves of galaxies. Computation of the observed rotation of stars with respect to their distance from the center makes clear that additional matter is needed to explain the obtained curves, see image below.
- The mass/luminosity ratio of galaxies. The ratio of the observed mass of galaxies and their brightness per star is too high compared to what is expected (e.g. in comparison with the Sun).
- Distribution of hot gas in clusters of galaxies. A much larger gravitational potential well than is expected from the visible mass, is needed to keep the gas inside the clusters.
- Weak gravitational lensing. Graviational lensing is a light bending effect caused by a distribution of matter between a distant light source (such as a galaxy) and the observer. When the mass distribution of a cluster is measured via the weak gravitational lensing effect, it completely exceeds visible mass estimations.
- The fluctuations of the Cosmic Microwave Background (CMB). The CMB is remnant radiation from the early universe, about 380 000 years after the Big Bang, see image below. Cosmological models can explain the fluctuations of the CMB only when including both dark matter and dark energy.
This observational evidence unavoidably requires the modification of familiar physics. There are two ways for modification:
- Modify the theory of gravity (e.g. via MOdefied Newtonian Dynamics (MOND)) or
- Modify the Standard Model of elementary particles by adding a "dark matter particle".
Modification of the theory of gravity, however, faces certain issues with explanations of the CMB fluctuations and observations of visible and dark matter in distant astrophysical objects, such as the Bullet cluster of galaxies (see image below).
If the modification of the theory of gravity is excluded, the natural explanation for dark matter is that there is a particle not included in the Standard Model of elementary particles which is very weakly or not interacting with other Standard Model particles. Interestingly, among the vast variety of possible additions to the Standard Model with dark matter candidate particles, only 3 additions are able to also solve other open questions of modern physics and astrophysics - axions, sterile neutrinos and WIMPs, all of which will be explained below.
Astrophysics can not provide us a way to directly measure the parameters (e.g. mass) of DM particles. The only option is to deduce these parameters indirectly from observations which disagree with other astrophysical explanations.
If there is a DM particle, the most straightforward way to detect it is through the observation of its decay or annihilation to photons. This should leave a corresponding feature in DM-dominated objects' electromagnetic spectra.
However, DM profiles can have large uncertainties, especially near the center of DM dominated objects, from where most of the DM signal is naturally expected. Moreover, DM decay/annihilating spectral features can be easily confused with regular astrophysical lines/features or instrumental artifacts. Furthermore, the expected feature is generally weak and requires the analysis of large data sets. Having said that, a detected weak feature in a large data set may also be caused by the instruments and devices used and might therefore be attributed to a systematic error and not to an actual feature.
At the IAAT, indirect DM search is conducted for three of the most promising candidates for DM: axions, sterile neutrinos and WIMPs. The basic idea is to find an electromagnetic signature for one of the candidates by analysing different types of astrophysical objects and by conducting numerical studies.
Axions are hypothetical particles, which were first postulated to explain the existence of certain symmetries of the strong nuclear force. Interestingly, they are also good candidates of DM due to their very low mass and very weak interaction. Some theories predict that it should be detectable via two photon decays and by the conversion to photons in a magnetic field (the Primakoff effect), see image below.
Consequently, in astrophysics, axions could be detected e.g. by features in pulsars' or magnetars' spectra and/or additional polarization of spectra of these sources. Researchers at the IAAT are investigating the presence of such features in a variety of astrophysical sources.
All particles in the Standard Model of elementary particles except the neutrino can have positive ("right-handed") and negative ("left-handed") projection of their spin onto their momentum. Standard neutrinos can be only left-handed. However, some theories propose to add right-handed "sterile" companions to the neutrinos, which could also be good candidates for DM thanks to their very small mass. In addition, the theory of sterile neutrinos helps to solve other problems of the standard model, which make them a very elegant solution to both fields. Decays of sterile neutrinos provide an observable signature which could be detected in X-ray spectra.
At the IAAT, researchers are performing long exposure observations to find the photon-decay features of sterile neutrinos in a broad (keV-MeV) energy range in a variety of astrophysical objects (from dwarf spheroidal galaxies to clusters of galaxies) with data from all modern X-ray missions (XMM-Newton, Chandra, NuStar, INTEGRAL).
Additional efforts have been made to study the nature of an unexplained 3.55 keV line in spectra of clusters of galaxies. This line was thought to have possibly DM-decay origin. IAAT researchers performed long exposure observations of other DM-dominated objects (e.g. dwarf spheroidal galaxies) and did not find evidence for the presence of such a line, which makes the DM-decay origin of 3.55 keV doubtful.
The scientists are also exploring possible future European missions to detect the DM-decay line.
Weakly Interacting Massive Particles (WIMPs)
Another popular candidate for DM particles are WIMPs. These hypothetical particles are also proposed as a solution to problems in the standard model through the theory of supersymmetry. They are natural candidates for both DM and supersymmetry and are expected to produce an electromagnetic signal from the annihilation with their anti-particles in the GeV to TeV range.
At the IAAT, researchers are systematically analyzing gamma-ray FERMI/LAT spectra of clusters of galaxies, the galactic center regions and diffuse background in order to find a possible hint for WIMP-annihilation spectral feature in any of these data.