arXiv subjects

A. Shtennikova

Publications and source records attributed to A. Shtennikova.

6 recordsLinked to original sources

arXiv2025

Ghost-free, gauge invariant SVT generalizations of Horndeski theory

We analyze the generalizations of Kaluza--Klein compactifications of 5D Horndeski theory. They are Scalar--Vector--Tensor (SVT) theories with higher derivatives in the action, but with second order equations of motion. The vector field is invariant under a U(1) gauge transformation and the Scalar--Tensor sector corresponds to Horndeski theory. A subclass of these SVT theories is such that the Horndeski functions $G_4(π,X)$ and $G_5(π)$ remain free, while the speed of the tensor and vector modes is exactly the same. We show a subclass where the vector sector retains freedom through new functions of $π,\, X$ while the speed of the vector modes still tracks the speed of the tensor modes.

hep-th↗

arXiv2025

Luminal Scalar-Tensor theories for a not so dark Dark Energy

In general the speed of Gravitational Waves (GWs) in Scalar-Tensor modifications of Einstein's gravity is different from the speed of Light. Nevertheless, it has been measured that their speeds are nearly the same. For the most general Scalar-Tensor theories classified to date that do propagate a graviton -- DHOST, {\it including Horndeski and Beyond Horndeski (BH) theories} -- we show that, remarkably, up to 5 self-consistent couplings of the scalar of Dark Energy (DE) to the Photon are enough to make their GWs luminal in a wide set of cases. We find at least one Luminal Beyond Horndeski theory for which the GW decay into Dark Energy is suppressed in any cosmological background.

hep-th↗

arXiv2025

Higher derivative SVT theories from Kaluza-Klein reductions of Horndeski theory

It was recently pointed out that some precise Photon-Galileon couplings in four dimensions (4D) -- inspired by a higher dimensional reduction -- are enough to obtain a Horndeski theory that is less constrained by the stringent experimental bounds on the speed of Gravitational Waves. They imply the constancy of the ratio of speed of gravity to light throughout cosmic evolution. This holds even if we include the general scalar potentials $G_4 (π,X)$ and $G_5 (π)$. In this paper we go into the details of this 4D Luminal extension of Horndeski theory including its scalar sector. We also present the complete action including the general $G_5(π,X),\, G_6(π,X)$ scalar potentials. Thus we show all the $U(1)$ gauge invariant vector Galileons in 4D that result from a Kaluza-Klein dimensional reduction from 5D Horndeski. They provide a {\it consistent} coupling of a higher derivative vector to scalar modifications of gravity -- namely, without inducing Ostrogradsky ghosts and keeping gauge invariance -- in the aim to explore more universal couplings of dark energy to other matter, such as vectors and in particular the Photon.

hep-th↗

arXiv2024

Reviving Horndeski after GW170817 by Kaluza-Klein compactifications

The application of Horndeski theory/ Galileons for late time cosmology is heavily constrained by the strict coincidence in the speed of propagation of gravitational and electromagnetic waves. These constraints presuppose that the minimally coupled photon is not modified, not even at the scales where General Relativity (GR) may need modification. We find that the 4D Galileon obtained from a Kaluza-Klein compactification of its higher dimensional version is a natural simultaneous modification of GR and electromagnetism with automatically "luminal" gravitational waves. This property follows without any fine tuning of Galileon potentials for a larger class of theories than previously thought. In particular, the $G_4$ potential is not constrained by the speed test and $G_5$ may also be present. In other words, some Galileon models that have been ruled out since the event GW170817 are, in fact, not necessarily constrained if they arise in 4D from compactifications of their higher dimensional Galileon counterparts. Besides their compelling luminality, the resulting vector Galileons are naturally $U(1)$ gauge invariant. We also argue that the Vainshtein screening that allows to recover GR predictions for solar system tests is also at work for electrodynamics in the dense region of laboratory tests.

hep-th↗

arXiv2024

Perturbations in Horndeski theory above anisotropic cosmological background

Considering an anisotropic cosmological background is an interesting and simultaneously challenging problem of theoretical physics, since we not only assume a high degree of anisotropy in the early stages of the Universe, but also observe it to a small degree until now. In this paper we have constructed the unconstrained action for the perturbations above Bianchi I type background in the most general scalar-tensor theory of gravity, the Horndeski theory, and evaluate the effect of the deviation from the anisotropic background on the previously established stable solution obtained in previous works.

gr-qc↗

arXiv2023

Stable cosmological solutions in Horndeski theory

It is known that the construction of a completely stable solution in Horndeski theory is restricted very strongly by the so-called no-go theorem. Previously, various techniques have been used to avoid the conditions of the theorem. In this paper a new way of constructing stable solutions are shown in the general Horndeski theory. We considered the situation in which the unitary gauge studied earlier turns out to be singular. On this basis we construct a spatially flat, stable bouncing and genesis Universe solutions which are described by General Relativity with non-conventional scalar field.

gr-qc↗