arXiv subjects

S. Mironov

Publications and source records attributed to S. Mironov.

At least 19 recordsLinked to original sources

arXiv2026

Scalar-scalar-tensor interaction in DHOST theories

We derive the general cubic action for perturbations about a cosmological background in the quadratic subclass of degenerate higher-order scalar-tensor (DHOST) theories, focusing on scalar-scalar-tensor interactions. We adopt a fully covariant formulation and implement unitary gauge at the level of perturbations. This mixed sector provides the key ingredient for estimating the decay rate of a gravitational wave into two scalar excitations in the quadratic DHOST models of dark energy in the late-time Universe.

gr-qc↗

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

No-Go theorem in the cubic subclass of Horndeski theory for spherically symmetric dynamical background

We consider a general dynamical, spherically symmetric background in the cubic subclass of Horndeski theory and obtain the quadratic action for the perturbations using the DPSV approach. We analyse the stability conditions for high-energy modes and study the issue of the no-go theorem in the current subclass of Horndeski theory. We formulate the no-go theorem for weak dependence on one variable (time or radial) and derive its generalization to the cases which could be reduced by coordinate transformation to scenarios where the scalar field has weak dependence on one of the coordinates. Moreover we show that wide class of singular solutions are also prohibited within the cubic subclass of Horndeski theory.

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↗

arXiv2025

Complete stability for spherically symmetric backgrounds in beyond Horndeski theory

We consider a general static, spherically symmetric background in the quadratic beyond Horndeski theory and analyse the behaviour of linear perturbations in both parity odd and parity even sectors. We derive a full set of stability conditions for an arbitrary static, spherically symmetric solution which guarantees absence of ghosts, gradient instabilities, tachyons and superluminal modes in both sectors.

gr-qc↗

arXiv2025

Linear stability of a time-dependent, spherically symmetric background in beyond Horndeski theory and the speed of gravity waves

We address a dynamical, spherically symmetric background in beyond Horndeski theory and formulate a set of linear stability conditions for high energy perturbation modes above an arbitrary solution. In this general setting we derive speeds of propagation in both radial and angular directions for gravity waves and compare them with the speed of light in the case of minimally coupled photon. In particular, we find that the class of beyond Horndeski theories, which satisfy the equality of gravity waves' speed to the speed of light over a cosmological background, feature gravity waves propagating at luminal speeds above a time-dependent inhomogeneous background as well.

gr-qc↗

arXiv2025

Non-singular cosmological scenarios in scalar-tensor theories and their stability: a review

This article gives a concise overview of the development and current status of studies on healthy models of the early Universe without an initial singularity, namely the cosmological bounce and Genesis scenarios, constructed within a broad class of scalar-tensor theories, specifically Horndeski theories and their generalizations. The review focuses on the topics related to linear stability at the perturbation level over the non-singular background solutions: 1) the no-go theorem, valid for non-singular cosmologies within Horndeski theory, 2) the updates on possible approaches to evade the no-go theorem, 3) the role of disformal transformations relating the Horndeski subclasses with the generalized theories like DHOST, 4) the effects on stability caused by additional matter coupling and potential emergence of superluminal perturbation modes in the multi-component setting.

gr-qc↗

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

Time-dependent, spherically symmetric background in Kaluza-Klein compactified Horndeski theory and the speed of gravity waves

We revisit the models recently derived from a Kaluza-Klein compactification of higher dimensional Horndeski theory, where the resulting electromagnetic sector features non-trivial couplings to Horndeski scalar. In particular, this class of theories admits the electromagnetic waves propagating at non-unit speed, which in turn allows to relax the constraints on Horndeski theories following from multi-messenger speed test. In this work we prove that both gravitational wave and its electromagnetic counterpart propagate at the same, although non-unit, speed above an arbitrarily time-dependent, spherically symmetric background within the theories in question. Hence, we support the statement that several subclasses of Horndeski theories are not necessarily ruled out after the GW170817 event provided the photon-Galileon couplings are allowed. We also formulate the set stability conditions for an arbitraty solution within the discussed theoretical setting.

gr-qc↗

arXiv2024

Healthy Horndeski cosmologies with torsion

We show that the full Horndeski theory with both curvature and torsion can support nonsingular, stable and subluminal cosmological solutions at all times. Thus, with torsion, the usual No-Go theorem that holds in a curved spacetime is avoided. In particular, it is essential to include the nonminimal derivative couplings of the $\mathcal{L}_{5}$ part of the Horndeski action ($G^{μν}\,\nabla_μ\nabla_νϕ,$ and $(\nabla^2 ϕ)^3$). Without the latter a No-Go already impedes the eternal subluminality of nonsingular, stable cosmologies.

gr-qc↗

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↗

arXiv2024

Stability of nonsingular cosmologies in Galileon models with torsion. A no-go theorem for eternal subluminality

Generic models in Galileons or Horndeski theory do not have cosmological solutions that are free of instabilities and singularities in the entire time of evolution. We extend this No-Go theorem to a spacetime with torsion. On this more general geometry the No-Go argument now holds provided the additional hypothesis that the graviton is also subluminal throughout the entire evolution. Thus, critically different for Galileons' stability on a torsionful spacetime, an arguably unphysical although arbitrarily short (deep UV) phase occurring at an arbitrary time, when the speed of gravity $(c_g)$ is slightly higher than luminal $(c)$, and by at least an amount $c_g\geq \,\sqrt{2}\,c $, can lead to an all-time linearly stable and nonsingular cosmology. As a proof of principle we build a stable model for a cosmological bounce that is almost always subluminal, where the short-lived superluminal phase occurs before the bounce and that transits to General Relativity in the asymptotic past and future.

hep-th↗

arXiv2024

Quartic Horndeski-Cartan theories in a FLRW universe

We consider the Quartic Horndeski theory with torsion on a FLRW background in the second order formalism. We show that there is a one parameter family of Quartic Horndeski Cartan Lagrangians and all such theories only modify the dispersion relations of the graviton and the scalar perturbation that are usually found in the standard Horndeski theory on a torsionless spacetime. In other words, for the theories in this class torsion does not induce new degrees of freedom but it only modifies the propagation. This holds for first order perturbations in spite of a kinetic mixing between the Horndeski scalar with the torsion field in the action. We also show that for most Lagrangians within the family of Quartic Horndeski Cartan theories the dispersion relation of the scalar mode is radically modified. We find only one theory within the family whose scalar mode has a regular wave-like dispersion relation.

hep-th↗

arXiv2023

DPSV trick for spherically symmetric backgrounds

We discuss the approach suggested by Deffayet et al. (DPSV) to analysing the linearized perturbations in Horndeski theory in the case of a static, spherically symmetric background. In $\mathcal{L}_3$ subclass of Horndeski theories we prove the validity of the DPSV approach by showing that the original method corresponds to a specific gauge choice in the quadratic action for perturbations. We also show that in the case of a spherically symmetric background the DPSV trick does not work in a more general $\mathcal{L}_4$ Horndeski theory.

hep-th↗

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↗

arXiv2022

In hot pursuit of a stable wormhole in beyond Horndeski theory

We consider the issue of stability at the linearized level for static, spherically symmetric wormhole solutions within a subclass of scalar-tensor theories of beyond Horndeski type. In this class of theories we derive a set of stability conditions ensuring the absence of ghosts and both radial and angular gradient instabilities about a static, spherically-symmetric background. This set of constraints extends the existing one and completes the stability analysis for high energy modes in both parity odd and parity even sectors, while "slow" tachyonic instabilities remain unconstrained. We give an example of beyond Horndeski Lagrangian admitting a wormhole solution which complies with all stability constraints for the high energy modes.

gr-qc↗

arXiv2022

Ambiguity in mana and magic definition and knot states

We study the Mana and Magic for quantum states. They have a standard definition through the Clifford group, which is finite and thus classically computable. We introduce a modified Mana and Magic, which keep their main property of classical computability, while making other states classically computable. We also apply these new definitions to the studies of knot states of 2-strand knots.

hep-th↗