arXiv subjects

Archil Kobakhidze

Publications and source records attributed to Archil Kobakhidze.

At least 19 recordsLinked to original sources

arXiv2026

On the $\uptheta$-vacua and CP violation

Recent claims have suggested the absence of CP violation in theories with a $θ$-vacuum structure, particularly in quantum chromodynamics. We highlight several key points, from a perspective that is not widely discussed in the literature, which clarify why such conclusions are incorrect. In particular, an open boundary in a finite-volume theory must be accompanied by boundary degrees of freedom, the edge modes, in order to preserve large gauge invariance and faithfully capture the topological features of the theory. In the infinite-volume limit, these edge states become non-dynamical, leaving the standard $\uptheta$-vacuum structure intact, irrespective of whether this limit is taken before or after summing over topological sectors. Consequently, the $\uptheta$-vacuum structure does give rise to observable CP violation once the theory is consistently quantised.

hep-th↗

arXiv2026

Electromagnetic instantons and asymmetric Hawking radiation of black holes

We argue that the topological structure of Abelian gauge theories, such as Maxwell electrodynamics, in the background of a Euclidean Schwarzschild black hole manifests itself through an asymmetry in Hawking radiation. In particular, the topology of the black hole manifold, characterised by a non-contractible 2-sphere and Euler characteristic $χ= 2$, admits non-trivial gauge-field configurations. These take the form of 2-form field strengths that are closed but not exact. From a topological perspective, such configurations are classified by the second cohomology group, which is isomorphic to $\mathbb{Z} \oplus \mathbb{Z}$, and are labelled by integer electric ($n$) and magnetic ($m$) charges, $(n,m)$. Self-dual ($n = m$) and anti-self-dual ($n = -m$) dyonic configurations carry vanishing Euclidean energy and are fully compatible with the Euclidean Schwarzschild geometry. More general dyonic configurations, by contrast, are interpreted as off-shell Euclidean field configurations. Nevertheless, both classes contribute to the thermal equilibrium vacuum and to finite-temperature correlation functions in the corresponding Lorentzian framework. Furthermore, because of the non-trivial topology, the electromagnetic $θ_{\rm EM}$-term contributes to the physical observables. In particular, it sources $CP$-asymmetric Hawking radiation, observable as an imbalance between left- and right-polarised photons in the emission spectrum. We briefly discuss some implications of this phenomenon.

hep-th↗

arXiv2026

QCD and electroweak phase transitions with hidden scale invariance: implications for primordial black holes, quark-lepton nuggets and gravitational waves

We study the cosmological implications of the minimal non-linear realisation of scale invariance within the Standard Model (SM). This framework provides a technically natural explanation for the hierarchy between the Planck scale and the electroweak scale and introduces only a light, feebly coupled dilaton field beyond the SM particles. Although the model is almost indistinguishable from the minimal SM at low energies, its cosmological consequences differ dramatically. In particular, the electroweak Higgs field remains trapped in the symmetric phase until the Universe cools to very low temperatures, $T_c^{(χ)}\sim 28$ MeV, where the first-order QCD chiral symmetry-breaking phase transition triggers the electroweak phase transition. This scenario offers intriguing possibilities for the production of primordial black holes, low-frequency gravitational waves, and multi-quark and lepton nuggets, which we explore in some detail using simplified approximations.

hep-ph↗

arXiv2025

Emergent Dark Matter

We entertain the possibility that the phenomena typically attributed to dark matter may have a fundamentally emergent nature, rather than arising from new particle degrees of freedom. To illustrate this idea, we consider a field-theoretic model of a three-form gauge field coupled to a cosmological fluid composed of ordinary matter and radiation. In the absence of interactions, the 3-form gauge theory describes only a global, non-propagating state, which can be associated with dark energy. However, when coupled to the cosmic fluid, the theory gives rise to an emergent, dynamical in-medium state. We identify this emergent state of the 3-form gauge field with dark matter. Thus, our proposal provides a unified framework for the dark sector of the universe within the context of an interacting three-form gauge theory. Furthermore, we speculate that the three-form field may have a gravitational origin, potentially supported by the lepton-number asymmetry in the primordial plasma. If this scenario is correct, conventional direct and indirect searches for dark matter would likely be futile.

hep-ph↗

arXiv2025

Comments on the gauge dependence of the effective potential and the utility of the Vilkovisky-DeWitt formalism

We provide some additional comments on the long-lived discussions surrounding an effective action and potential plagued by a number of ambiguities. We reinforce the importance of an extra condition on the gauge-fixing function, namely the vanishing of its vacuum expectation value in the absence of external sources, when concluding gauge-independence of the effective action and potential at an extremum. We advocate for the alternative construction of the effective action and potential based on the Vilkovisky-DeWitt approach, and demonstrate its independence from the gauge-fixing parameter. We also exhibit a high-temperature generalisation of this alternative construction in the specific case of the Abelian-Higgs model.

hep-th↗

arXiv2025

$η_{\rm w}$-meson from topological properties of the electroweak vacuum

We further scrutinize the evidence for a recently suggested pseudo-scalar particle, the electroweak $η_{\rm w}$-meson. Its existence is demanded by matching the removal of the weak vacuum angle $θ_{\rm w}$ by the anomalous $B+L$ - symmetry with a massive pole in the topological susceptibility of the vacuum. We specifically focus on the possibility of the emergence of $η_{\rm w}$ as a collective excitation of the phase of the condensate of the 't Hooft fermion determinant, generated by the electroweak instantons, which breaks the $B+L$ - symmetry spontaneously. We argue that the generation of the 't Hooft vertex is in one-to-one correspondence with its non-zero vacuum expectation value which is cutoff insensitive. We outline certain puzzles about the nature of the emergent $η_{\rm w}$ which require further investigations.

hep-th↗

arXiv2025

Electroweak $η_{\rm w}$ meson

We argue that the Standard Model is accompanied by a new pseudo-scalar degree of freedom, $η_{\rm w}$-meson, which cancels the topological susceptibility of the electroweak vacuum and gets its mass from this effect. The prediction is based on the analyticity properties of the Chern-Simons correlator combined with the basic features of gravity. Depending on the quality level of the $U(1)_{B+L}$-symmetry, $η_{\rm w}$ emerges as a $B+L$ pseudo-Goldstone boson or as a Stückelberg $2$-form of the electroweak gauge redundancy. An intriguing scenario of the first category is the emergence of $η_{\rm w}$ in the form of the phase of a $U(1)_{B+L}$-violating fermion condensate triggered by the instantons, somewhat similar to $η'$-meson in QCD. Regardless of its origin, the presence of $η_{\rm w}$-meson in the theory appears to be a matter of consistency.

hep-th↗

arXiv2024

Hint to Supersymmetry from GR Vacuum

The $S$-matrix formulation of gravity suggests that the $θ$-vacuum structure must not be sustained by the theory. We point out that, when applied to the vacuum of general relativity, this criterion hints to supersymmetry. The topological susceptibility of gravitational vacuum induced by Eguchi-Hanson instantons can be eliminated neither by spin-$1/2$ fermions nor by an axion coupled via them since such fermions do not provide instanton zero modes. Instead, the job is done by a spin-$3/2$ fermion, hence realizing a local supersymmetry. This scenario also necessitates the spontaneous breaking of supersymmetry and predicts the existence of axion of $R$-symmetry which gets mass exclusively from the gravitational instantons. The $R$-axion can be a viable dark matter candidate. Matching between the index and the anomaly imposes a constraint that spin-$1/2$ fermions should not contribute to the chiral gravitational anomaly.

hep-th↗

arXiv2024

Renormalization group flows and emergent symmetries

We discuss the following proposition: Renormalization Group flow of quantum theory with a biased symmetry exhibits a fixed hypersurface at which the symmetry is exact. Such emergent symmetries may have important phenomenological implications, including supersymmetric models, gauge theories, and massive gravity. Most interesting example is an emergent supersymmetry in non-abelian gauge theories with appropriate field content, in the IR limit i.e. strong coupling regime.

hep-th↗

arXiv2022

Coloured gravitational instantons, the strong CP problem and the companion axion solution

Quantum gravity introduces a new source of the combined parity (CP) violation in gauge theories. We argue that this new CP violation gets bundled with the strong CP violation through the coloured gravitational instantons. Consequently, the standard axion solution to the strong CP problem is compromised. Further, we argue that the ultimate solution to the strong CP problem must involve at least one additional axion particle.

hep-ph↗

arXiv2021

Apparent horizons of the Thakurta spacetime and the description of cosmological black holes

We discuss the validity of the Thakurta metric to describe cosmological black holes by analysing the nature of its horizon. By adopting the preferred foliation of the Thakurta spacetime associated with the Kodama time, we demonstrate that the Thakurta horizon is indeed a future outer trapping horizon. Therefore, the respective observers see it as a cosmological black hole, contrary to some claims in the literature.

gr-qc↗

arXiv2021

Cosmology of the companion-axion model: dark matter, gravitational waves, and primordial black holes

The companion-axion model introduces a second QCD axion to rescue the Peccei-Quinn solution to the strong-CP problem from the effects of colored gravitational instantons. As in single-axion models, the two axions predicted by the companion-axion model are attractive candidates for dark matter. The model is defined by two free parameters, the scales of the two axions' symmetry breaking, so we can classify production scenarios in terms of the relative sizes of these two scales with respect to the scale of inflation. We study the cosmological production of companion-axion dark matter in order to predict windows of preferred axion masses, and calculate the relative abundances of the two particles. Additionally, we show that the presence of a second axion solves the cosmological domain wall problem automatically in the scenarios in which one or both of the axions are post-inflationary. We also suggest unique cosmological signatures of the companion-axion model, such as the production of a $\sim$10 nHz gravitational wave background, and $\sim 100\,M_\odot$ primordial black holes.

hep-ph↗

arXiv2021

Phenomenology of the companion-axion model: photon couplings

We study the phenomenology of the 'companion-axion model' consisting of two coupled QCD axions. The second axion is required to rescue the Peccei-Quinn solution to the strong-CP problem from the effects of colored gravitational instantons. We investigate here the combined phenomenology of axion-axion and axion-photon interactions, recasting present and future single-axion bounds onto the companion-axion parameter space. Most remarkably, we predict that future axion searches with haloscopes and helioscopes may well discover two QCD axions, perhaps even within the same experiment.

hep-ph↗

arXiv2021

Probing Quadratic Gravity with Binary Inspirals

In this paper, we study gravitational waves generated by binary systems within an extension of General Relativity which is described by the addition of quadratic in curvature tensor terms to the Einstein-Hilbert action. Treating quadratic gravity as an effective theory valid in the low energy/curvature regime, we argue that reliable calculations can be performed in the early inspiral phase, and furthermore, no flux of additional massive waves can be detected. We then compute the massive dipole (-1PN) leading corrections to the post-Newtonian (PN) expansion of the standard waveform. By confronting these theoretical calculations with available experimental data, we constrain both unknown parameters of quadratic gravity to be $0 \leq γ\, \lesssim 5.7\cdot 10^{76}$, and $-\fracγ{4} \leq β\, \lesssim - 4.2\cdot 10^{75}$.

gr-qc↗

arXiv2021

Non-Schwarzschild Primordial Black Holes as Dark Matter in Quadratic Gravity

One-loop renormalised quantum effective action for gravity contains quadratic in curvature terms. We have found an approximate analytic black hole solution in quadratic gravity by keeping only the radial spherically symmetric fluctuations and dimensionally reducing the 4-dimensional (4D) theory down to the 2-dimensional (2D) dilaton gravity with a potential. The solution reduces to the Schwarzschild black hole in the limit of Einstein's gravity, but otherwise admits non-negative Arnowitt-Deser-Misner (ADM) and positive quasi-local Misner-Sharp masses that can differ significantly. We then study the thermodynamics of such quantum corrected black holes and compute their lifetime under Hawking evaporation. We note that for some range of parameters, black holes increase in mass while emitting Hawking radiation. This pathological behaviour is related to the negative energy states that are present in quadratic gravity. We also find that the micro-lensing of non-Schwarzschild black holes could significantly deviate from the micro-lensing of their Schwarzschild counterparts. These findings have important ramifications for the phenomenology of primordial black holes (PBHs) as dark matter. In particular, the quoted constraints on PBH dark matter from micro-lensing data can be completely evaded, thus making PBHs in the mass range $\sim 10^{-12} - 10~M_{\odot}$ viable dark matter candidates.

gr-qc↗

arXiv2021

Comment on: Cosmological black holes are not described by the Thakurta metric

Recently, Hütsi et al.[arXiv:2105.09328] critiqued our work that reconsidered the mathematical description of cosmological black holes. In this short comment, we highlight some of the conceptual issues with this criticism in relation to the interpretation of the quasi-local Misner-Sharp mass, and the fact that our description of cosmological black holes does not impose any assumptions about matter accretion.

astro-ph.CO↗

arXiv2021

Eliminating the LIGO bounds on primordial black hole dark matter

Primordial black holes (PBHs) in the mass range $(30$--$100)~M_{\odot}$ are interesting candidates for dark matter, as they sit in a narrow window between microlensing and cosmic microwave background constraints. There are however tight constraints from the binary merger rate observed by the LIGO and Virgo experiments. In deriving these constraints, PBHs were treated as point Schwarzschild masses, while the more careful analysis in an expanding universe we present here, leads to a time-dependent mass. This implies a stricter set of conditions for a black hole binary to form and means that black holes coalesce much more quickly than was previously calculated, namely well before the LIGO/Virgo's observed mergers. The observed binaries are those coalescing within galactic halos, with a merger rate consistent with data. This reopens the possibility for dark matter in the form of LIGO-mass PBHs.

astro-ph.CO↗

arXiv2020

Topologically induced black hole charge and its astrophysical manifestations

Quantum corrected effective action for gravity contains massive spin-2 ghost degrees of freedom and admits a topological term which couples longitudinal vector degrees of freedom of the massive spin-2 to Maxwell's electromagnetism. We argue that in the presence of otherwise unobservable gravimagnetic poles this topological term induces an electric charge on a black hole which can be probed through the associated electric and magnetic fields. In particular, we discuss the electromagnetic follow up from the LIGO-sensitive charged black hole binary coalescence due to the synchrotron radiation from the surrounding plasma and the shadow of super-massive charged black holes.

gr-qc↗