Bogolyubov Institute for Theoretical Physics
of the National Academy of Sciences of Ukraine

Department of Astrophysics and Elementary Particles
  • The properties of the radiation in the Lyman-alpha line of hydrogen in nine nearby galaxies with the lowest known content of chemical elements heavier than helium using Hubble Space Telescope observations were studied. These galaxies are considered as the best local analogues of dwarf galaxies that existed in the early Universe during the epoch of re-ionization when its age was less than 10 per cent of the present-day age. A strong and narrow Lyman-alpha emission line is observed in seven of the nine galaxies studied, which probably indicates a large loss of ionizing radiation that goes beyond these galaxies into the intergalactic medium and causes its ionization. The results of this study are a strong argument in favor of the fact that dwarf galaxies with active star formation were the main source of the re-ionization of the Universe.
    Y.I. Izotov, N.G. Guseva
  • It is shown that a large class of modified theories of gravity with the Lagrangian f(R) corresponds to models of a scalar field with a hilltop or tabletop potential in the Einstein frame. In such models, gravity can asymptotically disappear for infinite or large finite values of the scalar curvature. A universe evolving toward zero curvature is an observable universe. A universe evolving toward asymptotically vanishing gravity will either encounter the "Big-Rip" singularity or remain forever in the regime of inflation.
    Yu.V. Shtanov
  • The formalism of reduced quantum electrodynamics is generalized to the case of heterostructures composed of a few atomically thick layers, and the corresponding effective (2+1)-dimensional gauge theory is formulated. This dimensionally reduced theory describes charged fermions confined to N planes and contains N vector fields with Maxwell's action modified by non-local form-factors whose explicit form is determined. Taking into account the polarization function, the explicit formulae for the screened electromagnetic interaction are presented in the case of two and three layers. For a heterostructure with two atomically thick layers and charged fermions described by the massless Dirac equation, the quasiparticle gap generation of the excitonic type is studied. It is found that additional screening due to the second layer increases the value of the critical coupling constant for the gap generation compared to that in graphene.
    V.P. Gusynin, E.V. Gorbar
  • Yu.I. Izotov and N.G. Guseva as part of an international team of researchers from Ukraine, Switzerland, France, Denmark and the United States of America using data of the first observations of galaxies in the early Universe on the largest 6-meter James Webb space telescope determined the chemical composition of galaxies in the epoch of secondary ionization at high redshifts 6.7 - 8.4, when the age of the universe was less than 1 billion years. It was found that the studied galaxies have a chemical composition similar to the chemical composition of present-day galaxies. This implies the rapid chemical evolution of young galaxies over a short period of several tens of millions of years. As a result, galaxies quickly "forgot" about the physical conditions that existed during their formation.
    Y.I. Izotov, N.G. Guseva
  • We have elucidated the issue of the correspondence between the Jordan and Einstein conformal frames in the Standard Model interacting with modified gravity with action f(R). The scalaron-dependence of the vacuum Higgs-field expectation value and of the constant of strong interaction leads to a universal renormalization of masses of all particles in the Einstein system. We have analyzed possible potentially observable effects in such a theory, in which modified gravity plays the role of dark matter.
    Yu.V. Shtanov
  • We develop an approach to calculate the optical and Hall conductivities for models with arbitrary pseudospin and quasiparticle dispersion. The method is based on the evaluation of quasiparticle velocity correlators which also describe the phenomenon of Zitterbewegung. For the semi-Dirac model the obtained expressions allow us to analyze the role of spectrum anisotropy, Van Hove singularities, and Dirac cones in longitudinal conductivity.. For the dice model we emphasize the role of the spectral gap, which defines frequency thresholds related to transitions to and from a flat band.
    V.P. Gusynin
  • The gap generation in the dice model with local four-fermion interaction is studied. Due to the presence of two valleys with degenerate electron states, there are two main types of gaps. The intra and intervalley gap describes the electron and hole pairing in the same and different valleys, respectively. We found that while the generation of the intravalley gap takes place only in the supercritical regime, the intervalley gap is generated for an arbitrary small coupling. The physical reason for the absence of the critical coupling is the catalysis of the intervalley gap generation by the flat band in the electron spectrum of the dice model. The dispersionless band has a singular density of states that strongly enhances the intervalley gap generation leading to an extremely large gap proportional to the area of the Brillouin zone.
    Cor.-member of the NAS of Ukraine V.P. Gusynin, E.V. Gorbar
  • The observation with the Hubble Space Telescope of nine low-mass star-forming galaxies with stellar masses M★< 108 solar masses has been presented. According to theoretical predictions the escaping fraction of ionizing radiation should increase with decreasing galaxy stellar mass. However, contrary to these expectations, we find no increase of escape fraction of ionizing radiation capable of ionizing the intergalactic medium with decreasing galaxy stellar mass.
    Acad. of the NAS of Ukraine Y.I. Izotov, N.G. Guseva
  • A new cosmological scenario is proposed in which the scalaron of f (R) gravity after the electroweak crossover plays the role of dark matter. Its current energy density in this scenario matches the observed value for the scalaron mass m ≈ 4 meV. In this case, dark-matter perturbations of adiabatic type are automatically generated in an inhomogeneous universe.
    Yu.V. Shtanov
  • By using the effective low-energy Hamiltonian for pseudospin-1 fermions, the RKKY interaction for magnetic impurities placed on the same or different sublattices of dice lattice was calculated. It was shown that there are three types of interaction, which depend on the model parameter defining the relative strength of hoppings between sublattices, two of them can be reduced to graphene case while the third one is new and is due to the presence of a flat zero-energy band. The general analytical expressions for the RKKY interaction were derived in terms of Mellin-Barnes type integrals. It is shown that the interaction between impurities located at different rim sites displays a very strong temperature dependence at small doping being a direct consequence of the presence of the flat band.
    Cor. Member of the NAS of Ukraine V.P. Gusynin
  • It is shown for a one-dimensional system of point bosons that, at a weak coupling, the Lieb's "hole" excitation is a certain set of interacting phonons. This means that the holes are not a physically independent type of quasiparticles, although in literature it was considered the opposite for a long time.
    M.D. Tomchenko
  • We present observations with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope of eight nearby compact star-forming galaxies with low oxygen abundances aiming to study the properties of Lyman α emission in such conditions. In five galaxies Lyman α emission line is strong. In the remaining three galaxies, weak Lyman α emission is superposed on a broad Lyman α absorption line. We find an anticorrelation between Lyman α escape fraction and the distance between peaks of the Lyman α profile, though not as tight as the one found earlier between escape fraction of Lyman continuum and the distance between peaks of the Lyman α profile. This finding makes the distance between peaks of the Lyman α profile a promising indirect indicator of both the Lyman α and ionizing radiation leakage.
    Acad. of the NAS of Ukraine Y.I. Izotov, N.G. Guseva
  • We construct a perturbative chiral theory for quantum gravity in first-order formalism based on two-component spinors. In contrast to other approaches of this type, we succeeded in fixing gauge freedom so that the propagator of the spin connection vanishes. This significantly simplifies calculation of Feynman diagrams. The new formalism is the gravity analog of the well-known and powerful chiral description of Yang-Mills theory of gauge vector fields.
    Yu.V. Shtanov
  • The electron states of gapped pseudospin-1 fermions on a honeycomb lattice with additional atoms in the center of each hexagon (dice lattice) were studied in the presence of a charged impurity. The bound electron states are found in the case of the radially symmetric potential well and a regularized Coulomb potential. It is shown that as the charge of the impurity increases, bound-state energy levels descend from the upper and central continua and dive at certain critical charges into the central and lower continua, respectively, realizing the so-called phenomenon of atomic collapse for pseudospin-1 fermions. In the dice model, it is found that the flat band survives in the presence of a potential well, however, it is absent in the case of the Coulomb potential. The analytical results were obtained for the energy levels near continuum boundaries in the potential well. For the genuine Coulomb potential it is shown that analytical solutions exist for a countably infinite set of values of impurity charge at fixed couplings of atoms.
    Corr. Member of the NAS of Ukraine V.P. Gusynin, E.V. Gorbar
  • A method to describe the electric field in a spontaneously polarized isotropic nonpolar dielectric is developed. Helmholtz formula for the electric force acting on a volume element of a dielectric is generalized with regard for the contribution of the spontaneous polarization.
    M.D. Tomchenko
  • Observations with the Hubble Space Telescope of eight compact star-forming galaxies, aiming to study the properties of Lyman-alpha emission in such conditions, have been presented. The relation between the Lyman-alpha escape fraction and the Lyman-continuum escape fraction has been examined. It was found that shape the Lyman-alpha is a good measure of the escaping Lyman-continuum radiation and can be applied for a large number of the galaxies aiming to solve the problem of the reionization of the Universe.
    Acad. of the NAS of Ukraine Y.I. Izotov, N.G. Guseva
  • A simple scenario of inflationary magnetogenesis is proposed based on a helical coupling to electromagnetism. The helical coupling evolves linearly with conformal time interpolating between two constant values. This allows one to avoid the problem of strong gauge coupling and to generate helical magnetic fields of strength of order up to 10-7 G, when extrapolated to the current epoch, in a narrow spectral band centered at any physical wavenumber by adjusting the model parameters. Additional constraints on magnetic fields arise from the considerations of baryogenesis and, possibly, from the Schwinger effect of creation of charged particle-antiparticle pairs.
    Y.V. Shtanov
  • The possible dark matter signal has been searched in the X-ray astrophysical observation data. The surface brightness distribution of the 3.5 keV dark matter candidate line was obtained within the Galactic halo. This distribution was used to update the previous estimates of the sensitivity of the forthcoming dedicated mission Micro-X. The prediction for the width of the dark matter decay line in the future XRISM observations of the Perseus cluster was obtained with the use of cosmological hydrodynamical simulations.
    D.A. Iakubovskyi
  • From the combined analysis of the kinematic data of the dwarf spheroidal galaxies, a new lower bound of ~200 eV on the dark matter fermion mass was obtained. It was shown that it is impossible to constrain robustly the mass of dark matter particle from the cosmological absorption signal on frequencies 75-80 MHz ("21-cm absorption feature"). The detection of this signal was suggested by EDGES collaboration.
    A.V. Rudakovskyi, D.O. Savchenko, D.A. Iakubovskyi
  • Observations with the Hubble Space Telescope have detected the radiation capable to ionize hydrogen, the most abundant element in the Universe, in five unique dwarf galaxies with active star formation and extremely high ionization degree of their interstellar medium. These galaxies were selected from the largest survey of Sloan sky. It is obtained for the first time that the fraction of this radiation ranges between 2% and 72% of the radiation produced in these galaxies. This fraction is sufficient to explain the secondary ionization of the early Universe at the stage when its age was between 200 and 800 million years. The indirect method is proposed which links the fraction of ionizing radiation escaping the galaxy with the profile of the bright Lyman-alpha emission line, the latter being more easily observed in distant galaxies. For this, the data were used for all 11 galaxies in the Local Universe observed with the Hubble Space Telescope. This method can be applied for galaxies at any distances including those at large distances, for which observations of ionizing radiation are difficult or impossible.
    Acad. of the NAS of Ukraine Y.I. Izotov, N.G. Guseva
  • The term quadratic in curvature in the gravitational action in the Starobinsky model can be regarded as the leading quantum correction. We also assume the presence of parity-invariant and axial quantum corrections to the effective action that couple the space-time curvature to the electromagnetic strength tensor. We show that, in the Einstein frame of fields, there arises a nontrivial interaction between the scalar inflaton and electromagnetic field which previously was introduced in the theory artificially. Analysis of the inflationary magnetogenesis in this model shows that amplification of vacuum electromagnetic fields is possible no more than by five orders of magnitude, which is insufficient to explain the large-scale magnetic field in the Universe.
    Y. Shtanov, O. Savchenko
  • We propose a characterization tool for the studies of the band structure of new materials promising for the observation of topological phase transitions. We show that a specific resonant feature in the entropy per electron dependence on the chemical potential may be considered as a fingerprint of the transition between topological and trivial insulator phases. The entropy per electron in a honeycomb two-dimensional crystal of germanene subjected to the external electric field is obtained from the first-principles calculation of the density of electronic states and the Maxwell relation. In a wide energy range, the van Hove singularities in the electronic density of states manifest themselves as zeros in the entropy per particle dependence on the chemical potential.
    Corr. Member of the NAS of Ukraine V.P. Gusynin, S.G. Sharapov
  • We argue that the correct definition of the electric current in the chiral kinetic theory for Weyl materials should include the Chern-Simons contribution that makes the theory consistent with the local conservation of the electric charge in electromagnetic and strain-induced pseudoelectromagnetic fields. By making use of such a kinetic theory, we study the plasma frequencies of collective modes inWeyl materials in constant magnetic and pseudomagnetic fields, taking into account the effects of dynamical electromagnetism. We show that the collective modes are chiral plasmons. While the plasma frequency of the longitudinal collective mode coincides with the Langmuir one, this mode is unusual because it is characterized not only by oscillations of the electric current density, but also by oscillations of the chiral current density. The latter are triggered by a dynamical version of the chiral electric separation effect. We also find that the plasma frequencies of the transverse modes split up in a magnetic field. This finding suggests an efficient means of extracting the chiral shift parameter from the measurement of the plasma frequencies in Weyl materials.
    E.V. Gorbar
  • It is shown that the non-exponential form of the diffraction cone observed in the elastic scattering of protons by the LHC accelerator is a confirmation of the presence of a pion "coat" in protons associated with the analytic properties of the scattering amplitude.
    L. Jenkovszky
  • Using the dark matter density profiles from literature we have selected the 19 galaxy clusters with the largest expected dark matter decay flux. We processed all XMM-Newton public observation data for this objects. The emission spectra were modeled as sum of continuous plasma emission and known astrophysical lines. The fluxes of faint lines (e.g. K at 3.51 keV) were limited from above using fluxes of significantly detected lines. In 6 of spectra we identified >2σ positive residuals with average position 3.56±0.02 keV in the emitters frame. Their observed properties are unlikely to be explained by statistical fluctuations or astrophysical emission lines. Our detailed simulations showed that the combined significance of these detections equals to at least 2.6σ, raising to 3.2σ taking the line energy variation into account. Fitting the line position in observers frame prefers the astrophysical origin of line signal over instrumental one, with atomic line emission being included in the fitting models. Interpreting the new line due to decaying dark matter gives the radiative decay lifetime τDM≈(3.2−5.3)×1027 s consistent with previous detections.
    D.A. Iakubovskyi, D.O. Savchenko
  • The electron states in the field of a charged impurity in graphene in a magnetic field are studied. It is shown that a charged impurity removes the degeneracy of Landau levels converting them into bandlike structures. As the charge of impurity grows, the repulsion of sublevels of different Landau levels with the same value of orbital momentum takes place leading to the redistribution of the wave function profiles of these sublevels near the impurity. By studying the polarization effects, it is shown that the filling of Landau levels and screened charge of impurity can be very effectively tuned by gate voltage in agreement with the recent experiments. If the chemical potential is situated inside a Landau level, then the charge of impurity is strongly diminished. If the chemical potential lies between the Landau levels, then the screened potential does not change sign, the screening is minimal, and the charged impurity can strongly affect the electron spectrum.
    E.V. Gorbar, V.P. Gusynin
  • Sterile neutrino with mass of about 7 keV can explain the properties of the emission line at energy ~3.5 keV in the spectra of several dark-matter dominated cosmic objects. We investigate the influence of dark matter composed of such sterile neutrino on the process of reionization of the Universe and show that it causes a significantly sharper reionization, impossible to `imitate' within the cold-dark-matter scenario under any reasonable choice of model parameters. The future dedicated studies of reionization by 21-cm tomography of the Universe and investigations of the kinetic Sunyaev-Zeldovich effect will become essential for the reconstruction of particle candidate responsible for the 3.5 keV emission line.
    A. Rudakovskyi, D. Iakubovskyi
  • The process of generation and subsequent free evolution of the lepton chiral asymmetry and helical magnetic field in the early hot universe is studied analytically. For cosmologically relevant initial conditions, the chiral asymmetry and the energy density of helical magnetic field are shown to evolve by scaling laws. We derived an expression for the temperature at which this regime abruptly terminates as a function of the chiral asymmetry and energy density of magnetic field.
    M.Ye. Sydorenko, Y.V. Shtanov