Properties of perturbations in beyond Horndeski theories

We study whether the approach of Deffayet et al. (DPSV) can be adopted for obtaining a derivative part of quadratic action for scalar perturbations in beyond Horndeski theories about homogeneous and isotropic backgrounds. We find that even though the method does remove the second and higher derivatives of metric perturbations from the linearized Galileon equation, in the same manner as in the general Horndeski theory, it gives incorrect result for the quadratic action. We analyse the reasons behind this property and suggest the way of modifying the approach, so that it gives valid results

Genesis with general relativity asymptotics in beyond Horndeski theory

We suggest a novel version of a cosmological Genesis model within beyond Horndeski theory. It combines the initial Genesis behavior of Creminelli et al. [arXiv:1007.0027, arXiv:1209.3768] with complete stability property of the previous beyond Horndeski construction [arXiv:1705.06626]. The specific features of the model are that space-time rapidly tends to Minkowski in asymptotic past and that both asymptotic past and future are described by General Relativity (GR).Comment: 15 pages, 4 figures; v2: journal version with minor changes in Sec.

More about stable wormholes in beyond Horndeski theory

It is known that Horndeski theories, like many other scalar-tensor gravities, do not support static, spherically symmetric wormholes: they always have either ghosts or gradient instabilities among parity-even linearized perturbations. Here we address the issue of whether or not this no-go theorem is valid in "beyond Horndeski" theories. We derive, in the latter class of theories, the conditions for the absence of ghost and gradient instabilities for non-spherical parity even perturbations propagating in the radial direction. We find, in agreement with existing arguments, that the proof of the above no-go theorem does not go through beyond Horndeski. We also obtain conditions ensuring the absence of ghosts and gradient instabilities for all parity odd modes. We give an example of beyond Horndeski Lagrangian which admits a wormhole solution obeying our (incomplete set of) stability conditions. Even though our stability analysis is incomplete, as we do not consider spherically symmetric parity even modes and parity even perturbations propagating in angular directions, as well as "slow" tachyonic instabilities, our findings indicate that beyond Horndeski theories may be viable candidates to support traversable wormholes.Comment: 25 pages, 4 figures, journal version of the articl

Bounce beyond Horndeski with GR asymptotics and γ\gamma-crossing

It is known that beyond Horndeski theory admits healthy bouncing cosmological solutions. However, the constructions proposed so far do not reduce to General Relativity (GR) in either infinite past or infinite future or both. The obstacle is so called $\gamma$-crossing, which off hand appears pathological. By working in the unitary gauge, we confirm the recent observation by Ijjas [arXiv:1710.05990] that $\gamma$-crossing is, in fact, healthy. On this basis we construct a spatially flat, stable bouncing Universe solution whose asymptotic past and future are described by GR with conventional massless scalar field.Comment: 23 pages, journal version of the article. A minor comment has been introduced in Sec.3.

Subluminal cosmological bounce beyond Horndeski

We address the issue of potential superluminal propagation of gravitational waves in backgrounds neighboring the previously suggested bounce [arXiv:1807.08361] in beyond Horndeski theory. We find that the bouncing solution lies right at the boundary of the region where the gravitational waves propagate at speed exceeding that of light, i.e. that solution suffers superluminality problem. We suggest a novel version of a completely stable bouncing model where both scalar and tensor perturbations remain safely subluminal not only on the solution itself but also in its neighbourhood. The model remains free of superluminality when extra matter in the form of radiation or, more generally, ideal fluid with equation of state parameter $w\leq 1/3$ (and also somewhat higher) is added. Superluminality reappears when extra matter is added whose sound velocity is equal or close to 1 in flat space; an example is scalar field minimally coupled to metric. The latter property is characteristic of all beyond Horndeski cosmologies; we briefly discuss its significance.Comment: 24 pages, 12 figures. In v3: Major revision. A section on (the absence of) superluminality in the presence of additional matter added. Journal version, to appear in JCA

Cosmological bounce and Genesis beyond Horndeski

We study a "classical" bouncing scenario in beyond Horndeski theory. We give an example of spatially flat bouncing solution that is non-singular and stable throughout the whole evolution. The model is arranged in such a way that the scalar field driving the cosmological evolution initially behaves like full-fledged beyond Horndeski, whereas at late times it becomes a massless scalar field minimally coupled to gravity.Comment: 26 pages, 9 figure

Why Tc of MgB2 is the highest in a number of diborides?

It is shown, that the problem rising Tc diborides is a problem of nonstoichiometry in a plane of boron in diborides of heavy metals being compounds of variable composition.Comment: 8 pages, 2 figures. The paragraph 2.4.2. from the chapter L.M. Volkova, S.A. Polyshchuk, F.E. Herbeck, "Correlation of Tc with Crystal Chemical Parameters in High-Tc Cuprates, Diborides and Borocarbides: Concept of Arrangement and Function of Layered Superconductors" in "Focus on Superconductivity Research" Barry P. Martins., Ed., Nova Science Publisher, Inc. N.Y., 2003, (in print

Correlation of Tc with Crystal Chemical Parameters in High-Tc Cuprates, Diborides and Borocarbides: Concept of Arrangement and Function of Layered Superconductors

The problems of participation of crystal chemical factors in creating of conditions to occurrence of a superconducting state in layered superconductors are considered. On the basis of the experimental data, given in the literature, new correlations of Tc with relation of crystal chemical parameters in high-Tc cuprates, diborides and borocarbides are established, the critical crystal chemical parameters controlling Tc are selected, and their functions increating of prerequisites to the occurrence of superconductivity are determined. The similar character of the correlations allows to make conclusion about the common arrangement and function of layered superconductors irrespective of a nature of their superconductivity. The distinctions of crystal chemical characteristic of low-temperature superconductors from high-temperature ones are considered as the factors, which suppress superconductivity. It was shown, that the superconducting process is concentrated in an anisotropic three-dimensional fragment - sandwich, which arrangement and function allows to have stream of charge carriers and besides the space for its carry and resource of effect on this stream. If in a superconductor there are several such fragments, the Tc value of compound is defined by intrinsic Tc of that fragment, whose parameters are closer to optimal. The crystal chemical concept of arrangement and function of layered superconducting materials is advanced. It shows what conditions are necessary in addition to ones defining metal-insulator transition for appearance of superconductivity.Comment: 16 pages, 8 figures; the paragraphs 1-2.3 and 4 from the chapte

Crystal Chemical Concept of Arrangement and Function of Layered Superconducting Materials

The crystal chemical concept of arrangement and function of layered superconducting materials is supposed. The concept is based on results of our investigation of crystal chemistry of high-temperature superconductors (HTSC) cuprates, diborides AB2 and borocarbides of nickel RNi2B2C. According to these results: (1) the main role in appearance of superconductivity played by the structural fragments - sandwiches A2(CuO2) in HTSC cuprates, A2(B2) in diborides and RB(Ni) in nickel borocarbides but not the separate planes of CuO2, B2 or Ni; (2) correlations between Tc and crystal chemical parameters of these sandwiches have similar character in all three classes of compounds, despite of distinction of a nature of their superconductivity. The central idea of the concept consists in following: in contrast to metallic conduction, for which it is enough to provide only concentration and mobility of charge carriers, for occurrence of a superconductivity it is necessary to create in addition a space (channels) for stream of charge carriers, compression of stream of carriers and focusing to direction on path of motionComment: 4 pages, 2 figure

Crystal Chemical Estimation of Possible Exceeding of Tc of 77 K in Diborides

We establish the empirical correlation between Tc and crystal chemical parameters in diborides. Our results suggest that Tc of diborides can be higher 77K.Comment: 8 pages, 2 figures, 1 table, minor corrections, published in J. Superconductivity,14(6),693,(2001