1,992 research outputs found
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
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
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.
Bounce beyond Horndeski with GR asymptotics and -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 -crossing, which off hand appears pathological.
By working in the unitary gauge, we confirm the recent observation by Ijjas
[arXiv:1710.05990] that -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
(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
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