Aims. The Galileon model is a modified gravity theory that may provide an explanation for the accelerated expansion of the Universe. This model does not suffer from instabilities or ghost problems ...(normally associated with higher-order derivative theories), restores local General Relativity – thanks to the Vainshtein screening effect – and predicts late-time acceleration of the expansion. Methods. We derive a new definition of the Galileon parameters that allows us to avoid having to choose initial conditions for the Galileon field. We tested this model against precise measurements of the cosmological distances and the rate of growth of cosmic structures. Results. We observe a weak tension between the constraints set by growth data and those from distances. However, we find that the Galileon model remains consistent with current observations and is still competitive with the ΛCDM model, contrary to what was concluded in recent publications.
We study a black hole in an expanding Universe during the radiation-dominated stage. In particular, such a black hole may be of the primordial origin. In the case when the black hole radius is much ...smaller than the cosmological horizon, we found a self-consistent solution for the metric and the matter distribution and its velocity far from the black hole. At distances much smaller than the cosmological horizon our solution coincides with the previously obtained solution for quasi-stationary accretion. Our results can be applied, in particular, for the formation of dark matter density spikes around primordial black holes, and for the evolution of dark matter clumps during the radiation-dominated stage.
A change in the law of attraction in some regimes is predicted in the modified gravity models being actively developed at present. The set of up-to-date observational data leaves a wide range of ...admissible parameters for the theory. In this paper, we consider the possibility that the signal recorded by the Geograv resonant gravitational-wave detector in 1987 during the explosion of SN 1987A was produced by an abrupt change in the metric during the passage of a powerful neutrino flux through the detector. Such an impact on the detector is possible, in particular, in extended scalar–tensor theories in which the local matter density gradient affects the gravitational force. The first short neutrino pulse emitted at the initial stage of stellar core collapse before the onset of neutrino opacity could exert a major influence on the detector, because it could produce the detector response at the first resonance frequency. In contrast, the influence of the subsequent broad pulse (with a duration of several seconds) in the resonant detector is exponentially suppressed, despite the fact that the second pulse carries an order-of-magnitude more neutrino energy, and it could generate a signal in the LSD neutrino detector. This explains the time delay of 1.4 s between the Geograv and LSD signals. The consequences of this effect of modified gravity for future LIGO/Virgo observations are discussed.
We describe the spherically symmetric steady-state accretion of perfect fluid in the Reissner-Nordström metric. We present analytic solutions for accretion of a fluid with linear equations of state ...and of the Chaplygin gas. We also show that under reasonable physical conditions, there is no steady-state accretion of a perfect fluid onto a Reissner-Nordström naked singularity. Instead, a static atmosphere of fluid is formed. We discuss a possibility of violation of the third law of black hole thermodynamics for a phantom fluid accretion.
Aims. The Galileon model is a modified gravity model that can explain the late-time accelerated expansion of the Universe. In a previous work, we derived experimental constraints on the Galileon ...model with no explicit coupling to matter and showed that this model agrees with the most recent cosmological data. In the context of braneworld constructions or massive gravity, the Galileon model exhibits a disformal coupling to matter, which we study in this paper. Methods. After comparing our constraints on the uncoupled model with recent studies, we extend the analysis framework to the disformally coupled Galileon model and derive the first experimental constraints on that coupling, using precise measurements of cosmological distances and the growth rate of cosmic structures. Results. In the uncoupled case, with updated data, we still observe a low tension between the constraints set by growth data and those from distances. In the disformally coupled Galileon model, we obtain better agreement with data and favour a non-zero disformal coupling to matter at the 2.5σ level. This gives an interesting hint of the possible braneworld origin of Galileon theory.
The scintillation crystal bismuth germanate (BGO) is widely used in many applications. The unique technique developed in the Nikolaev Institute of Inorganic Chemistry, Novosibirsk, Russia, allows ...routine production of top quality large-sized BGO crystals. One of the important properties of a scintillation crystal is its radiation hardness. The intensive study of the radiation hardness of BGO crystals has been carried out by the collaboration of the Institutes of Siberian Branch of the Russian Academy of Science, Novosibirsk. The influence of the raw material purity and growth procedure on radiation hardness has been studied. The final tests of the crystal radiation hardness were performed by irradiating it with gamma rays from the radioactive source. The degradation of a light output of the best crystals is less than 10% after irradiation with doses of 10-100 krad expected in astrophysics experiments at a satellite. Based on the results of this study, the radiation hard BGO crystals for the "INTEGRAL" and "ASTRO-H" satellite missions have been made. A selfrecovery of the crystal light output is clearly seen. The time scale of the self recovery strongly depends on a dose of irradiation. It is days for 1 krad irradiation, weeks - for a 10 krad dose and a much longer period is required for 100 krad irradiation. All the crystals irradiated with a dose of 100 krad completely recover after annealing. The study reveals that there will be no essential degradation if this dose will be integrated over years of operation.
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