arXiv subjects
Publications and source records attributed to I. Tkachev.
We propose a strategy of how to look for dark matter (DM) particles possessing a radiative decay channel and derive constraints on their parameters from observations of X-rays from our own Galaxy and its dwarf satellites. When applied to the sterile neutrinos in keV mass range, it allows a significant improvement of restrictions to its parameters, as compared with previous works.
We study Lorentz-violating models of massive gravity which preserve rotations and are invariant under time-dependent shifts of the spatial coordinates. In the linear approximation the Newtonian potential in these models has an extra ``confining'' term proportional to the distance from the source. We argue that during cosmological expansion the Universe may be driven to an attractor point with larger symmetry which includes particular simultaneous dilatations of time and space coordinates. The confining term in the potential vanishes as one approaches the attractor. In the vicinity of the attractor the extra contribution is present in the Friedmann equation which, in a certain range of parameters, gives rise to the cosmic acceleration.
We study numerically particle acceleration by the electric field induced near the horizon of a rotating supermassive (M \sim 10^9-10^{10} M_{\odot}) black hole embedded in the magnetic field B. We find that acceleration of protons to energy E > 10^{20} eV is possible only at extreme values of M and B. We also find that the acceleration is very inefficient and is accompanied by a broad band MeV-TeV radiation whose total power exceeds at least by a factor of 1000 the total power emitted in ultra-high energy cosmic rays (UHECR). This implies that if O(10) nearby quasar remnants were sources of proton events with energy E > 10^{20} eV, then each quasar remnant would overshine e.g. the Crab nebula by more than two orders of magnitude in the TeV energy band. Recent TeV observations exclude this possibility. A model in which O(100) sources are situated at 100-1000 Mpc is not ruled out and can be experimentally tested by present TeV gamma-ray telescopes. Such a model can explain the observed UHECR flux at moderate energies E \sim (4-5) 10^{19} eV.
We canonically quantize the dynamics of the brane universe embedded into the five-dimensional Schwarzschild-anti-deSitter bulk space-time. We show that in the brane-world settings the formulation of the quantum cosmology, including the problem of initial conditions, is conceptually more simple than in the 3+1-dimensional case. The Wheeler-deWitt equation is a finite-difference equation. It is exactly solvable in the case of a flat universe and we find the ground state of the system. The closed brane universe can be created as a result of decay of the bulk black hole.
In a series of papers we have found statistically significant correlations between arrival directions of ultra-high energy cosmic rays and BL Lacertae objects. Recently, our calculations were partly repeated by Evans, Ferrer and Sarkar with different conclusions. We demonstrate that the criticism of Evans, Ferrer and Sarkar is incorrect. We also present the details of our method.
Magnetic fields (MF) in the Local Supercluster (LSC) of galaxies may have profound consequences for the propagation of Ultra High Energy Cosmic Rays (UHECR). Faraday rotations measurements provide some informations about MF in compact clusters. However, very few is known about less dense regions and about the global structure of MF in the LSC. In order to get a better knowledge of these fields we are performing constrained magnetohydrodynamical simulations of the LSC magnetic field. We will present the results of our simulation and discuss their implications for the angular distribution of expected UHECR deflections.
We develop a formalism which allows to study correlations of charged UHECR with potential sources without using any Galactic Magnetic Field (GMF) model. The method is free of subjective chose of parameters on which the significance of correlations depends strongly. We show that correlations of the AGASA dataset with BL Lacs (found previously after reconstruction of particle trajectories in a specific GMF) are present intrinsically and can be detected without reference to a particular model of magnetic field.
After a short review of inlationary preheating, we discuss the development of equilibrium in the frameworks of massless $λΦ^4$ model. It is shown that the process is characterised by the appearance of Kolmogorov spectra and the evolution towards thermal equilibrium follows self-similar dynamics. Simplified kinetic theory gives values for all characteristic exponents which are close to what is observed in lattice simulations. This allows estimation of the resulting reheating temperature.
We analyze the existing evidence that BL Lacertae objects (BL Lacs) are sources of the highest-energy cosmic rays. We argue that three independent signatures observed in the real data -- (1) improvement of correlations with corrections of trajectories for the Galactic magnetic field; (2) connection between gamma-ray and UHECR emissions; (3) non-uniform distribution of correlating rays over the sky -- are consistent with the hypothesis that a substantial fraction of cosmic rays in the energy range 40-60 EeV are protons accelerated in BL Lacs.
A set of potentially gamma-ray--loud BL Lac objects is selected by intersecting the EGRET and BL Lac catalogs. Of the resulting 14 objects, eight are found to correlate with arrival directions of ultra--high-energy cosmic rays (UHECRs), with significance of the order of 5 sigma. This suggests that gamma-ray emission can be used as a distinctive feature of those BL Lac objects that are capable of producing UHECR.
Many models of supersymmetry breaking, in the context of either supergravity or superstring theories, predict the presence of particles with Planck-suppressed couplings and masses around the weak scale. These particles are generically called moduli. The excessive production of moduli in the early Universe jeopardizes the successful predictions of nucleosynthesis. In this paper we show that the efficient generation of these dangerous relics is an unescapable consequence of a wide variety of inflationary models which have a preheating stage. Moduli are generated as coherent states in a novel way which differs from the usual production mechanism during parametric resonance. The corresponding limits on the reheating temperature are often very tight and more severe than the bound of 10^9 GeV coming from the production of moduli via thermal scatterings during reheating.
The excessive production of gravitinos in the early universe destroys the successful predictions of nucleosynthesis. The thermal generation of gravitinos after inflation leads to the bound on the reheating temperature, T_{RH}< 10^9 GeV. However, it has been recently realized that the non-thermal generation of gravitinos in the early universe can be extremely efficient and overcome the thermal production by several orders of magnitude, leading to much tighter constraints on the reheating temperature. In this paper, we first investigate some aspects of the thermal production of gravitinos, taking into account that in fact reheating is not instantaneous and inflation is likely to be followed by a prolonged stage of coherent oscillations of the inflaton field. We then proceed by further investigating the non-thermal generation of gravitinos, providing the necessary tools to study this process in a generic time-dependent background with any number of superfields. We also present the first numerical results regarding the non-thermal generation of gravitinos in particular supersymmetric models.
Many models of supersymmetry breaking, in the context of either supergravity or superstring theories, predict the presence of particles with weak scale masses and Planck-suppressed couplings. Typical examples are the scalar moduli and the gravitino. Excessive production of such particles in the early Universe destroys the successful predictions of nucleosynthesis. In particular, the thermal production of these relics after inflation leads to a bound on the reheating temperature, T_{RH} < 10^9 GeV. In this paper we show that the non-thermal generation of these dangerous relics may be much more efficient than the thermal production after inflation. Scalar moduli fields may be copiously created by the classical gravitational effects on the vacuum state. Consequently, the new upper bound on the reheating temperature is shown to be, in some cases, as low as 100 GeV. We also study the non-thermal production of gravitinos in the early Universe, which can be extremely efficient and overcome the thermal production by several orders of magnitude, in realistic supersymmetric inflationary models.
We present a complete computation of the inflaton decay into very massive fermions during preheating. We show that heavy fermions are produced very efficiently up to masses of order 10^{17}-10^{18} GeV; the accessible mass range is thus even broader than the one for heavy bosons. We apply our findings to the leptogenesis scenario, proposing a new version of it, in which the massive right-handed neutrinos, responsible for the generation of the baryon asymmetry, are produced during preheating. We also discuss other production mechanisms of right-handed neutrinos in the early Universe, identifying the neutrino mass parameters for which the observed baryon asymmetry is reproduced.
We consider phase separation in nonequilibrium Bose gas with an attractive interaction between the particles. Using numerical integrations on a lattice, we show that the system evolves into a state that contains drops of Bose-Einstein condensate suspended in uncondensed gas. When the initial gas is sufficiently rarefied, the rate of formation of this quantum dew scales with the initial density as expected for a process governed by two-particle collisions.
We obtain restrictions on the universal baryon fraction, f_B = Omega_B/Omega_0, by assuming that the observed microlensing events towards the Large Magellanic Cloud are due to baryonic MACHOs in the halo of the Galaxy and by extracting a bound to the total mass of the Milky Way from the motion of tracer galaxies in the Local Group. We find a lower bound f_B > 0.29^{+0.18}_{-0.15}. Consistency with the predictions of primordial nucleosynthesis leads to the further constraint on the total mass density, Omega_0 < 0.2.
In a class of models designed to solve the cosmological constant problem by coupling scalar or tensor classical fields to the space-time curvature, the universal scale factor grows as a power law in the age, $a \propto t^α$, regardless of the matter content or cosmological epoch. We investigate constraints on such "power-law cosmologies" from the present age of the Universe, the magnitude-redshift relation, and from primordial nucleosynthesis. Constraints from the current age of the Universe and from the high-redshift supernovae data require "large" $α$ ($\approx 1$), while consistency with the inferred primordial abundances of deuterium and helium-4 forces $α$ to lie in a very narrow range around a lower value ($\approx 0.55$). Inconsistency between these independent cosmological constraints suggests that such power-law cosmologies are not viable.
We investigate nonthermal phase transitions that may occur after post-inflationary preheating in a simple model of a two-component scalar field with the effective potential $λ(ϕ_i^2 - {\rm v}^2)^2/4$, where $ϕ_1$ is identified with the inflaton field. We use three-dimensional lattice simulations to investigate the full nonlinear dynamics of the model. Fluctuations of the fields generated during and after preheating temporarily make the effective potential convex in the $ϕ_1$ direction. The subsequent nonthermal phase transition with symmetry breaking leads to formation of cosmic strings even for ${\rm v} \gg 10^{16}$ GeV. This mechanism of string formation, in a modulated (by the oscillating field $ϕ_{1}$) phase transition, is different from the usual Kibble mechanism.