Stability of Black Holes and the Speed of Gravitational Waves within Self-Tuning Cosmological Models.

The gravitational wave event GW170817 together with its electromagnetic counterparts constrains the speed of gravity to be extremely close to that of light. We first show, on the example of an exact Schwarzschild-de Sitter solution of a specific beyond-Horndeski theory, that imposing the strict equality of these speeds in the asymptotic homogeneous Universe suffices to guarantee so even in the vicinity of the black hole, where large curvature and scalar-field gradients are present. We also find that the solution is stable in a range of the model parameters.

Constraints on shift-symmetric scalar-tensor theories with a Vainshtein mechanism from bounds on the time variation of G.

We show that the current bounds on the time variation of the Newton constant G can put severe constraints on many interesting scalar-tensor theories which possess a shift symmetry and a nonminimal matter-scalar coupling. This includes, in particular, Galileon-like models with a Vainshtein screening mechanism. We underline that this mechanism, if efficient to hide the effects of the scalar field at short distance and in the static approximation, can in general not alter the cosmological time evolution of the scalar field.

Recovering general relativity from massive gravity.

We obtain static, spherically symmetric, and asymptotically flat numerical solutions of massive gravity with a source. Those solutions show, for the first time explicitly, a recovery of the Schwarzschild solution of general relativity via the so-called Vainshtein mechanism.

Black hole mass decreasing due to phantom energy accretion.

Solution for a stationary spherically symmetric accretion of the relativistic perfect fluid with an equation of state p(rho) onto the Schwarzschild black hole is presented. This solution is a generalization of Michel solution and applicable to the problem of dark energy accretion. It is shown that accretion of phantom energy is accompanied by the gradual decrease of the black hole mass.

[Digital X-ray apparatus "Sibir'". Measurement of the value of a signal].

For objective measurement of digital detectors, the authors show it necessary to be limited to the quantum efficiency of recording for assessing the accuracy of measuring the value of a signal and to the frequency-contrast characteristics for estimating spatial accuracy. Since a computer is an integral part of the digital detector, this cannot be a basic obstacle to the application of the proposed procedure.