arXiv subjects

Nick E. Mavromatos

Publications and source records attributed to Nick E. Mavromatos.

At least 19 recordsLinked to original sources

arXiv2026

Magnetic-monopole resummation justifies perturbatively calculated collider production cross sections

A one-loop resummation scheme, inspired by Dyson-Schwinger (DS) formalism of strongly coupled quantum field theories, is applied to spin-1/2 magnetic monopoles (MMs), in the context of an effective field theory (EFT), invariant under the gauge group U(1)_em x U(1)', where U(1)' is a dual strongly coupled Abelian interaction, associated with a "dark photon". The latter leads to a MM appearing as a limiting case of a "dyon", with a tiny "electric charge", compensated by a huge wave-function renormalization of the MM, thereby leading to a finite renormalized MM-photon coupling. An ultra-violet fixed point structure is found in the resummed theory, which is purely non-perturbative due to different boundary conditions of the resummation equations, compared to the weak coupling (perturbative) case. The renormalized coupling of the MM to the electromagnetic photon in the fixed-point theory is identified with the magnetic charge, compatible with the Dirac quantization condition. This provides for the first time a formal justification of the use of tree-level Drell-Yan and photon-fusion MM production processes in collider searches, and of the corresponding cross sections and MM mass bounds thereof. The latter provide a means to constrain the resummed-EFT parameters experimentally. The DS resummation applies here primarily to elementary MMs. However, this approach may also be applied to the last stage (collapse) of the formation of composite MM pairs at colliders, in case they behave as quantum excitations, with their core radius comparable to the Compton wavelength, thereby avoiding the extreme suppression of their production.

hep-ph↗

arXiv2026

Self-Gravitating Magnetic Monopoles and Dyons in String-Inspired Models: Structure and Stability

We present classical magnetic--monopole (MM) and dyon solutions induced by global monopoles in string-inspired gravitational models. Although several ingredients of the construction have appeared previously, we examine a new synthesis via their organised use in a self-gravitating finite-energy monopole/dyon system with explicit mass, charge, core and stability diagnostics. The global monopole provides the topological seed, while the dilaton, Kalb--Ramond (KR) axion and Born--Infeld (BI) electromagnetic (EM) sector supply the string-inspired dressing. Two related branches are analysed: (i) a dilaton--BI magnetic branch, distinct from earlier constructions, and (ii) a dyonic extension in which the KR axion couples electric and magnetic sectors through an axion-Pontryagin-anomaly interaction. In both cases BI dynamics regularises the EM self-energy within a self-gravitating global-monopole construction, and the solutions possess a de Sitter core, a positive ADM-mass parameter range and satisfy the standard energy conditions. A further novel feature is a limiting branch in which the ADM mass tends to zero while the magnetic charge remains finite, the configuration being supported by magnetic charge and de Sitter core pressure.We also examine stability diagnostics. The stress--energy tensor gives finite force components and an outward-pointing radial force, indicating absence of core collapse within the mechanical stability criterion. In the exterior analytic region we analyse gauge-invariant EM perturbations using the Gervalle-Volkov framework. The helicity modes $ψ_\pm$ obey a self-adjoint Sturm--Liouville system. MM helicities decouple, while dyons exhibit axion-induced helicity mixing and birefringent polarisation structure. Positivity of the EM helicity block provides a necessary long-range stability diagnostic; the full coupled Einstein-matter spectral problem is discussed

hep-th↗

arXiv2026

The $ω$-Effect from a Multimode Squeezed Graviton State

The $ω$-effect in entangled neutral-meson systems provides a sensitive probe of CPT violation induced by quantum-gravitational environments. In open quantum systems, interactions with inaccessible gravitational degrees of freedom can render the reduced meson dynamics non-unitary, causing the CPT operator to become ill-defined, even when the underlying microscopic Hamiltonian is CPT invariant. We present a microscopic derivation of the $ω$-effect arising from a multimode squeezed gravitational environment generated by an axion cloud around a Kerr black hole. Using the Takagi decomposition of the associated complex symmetric squeezing kernel, the graviton field is expressed in terms of independent squeezed supermodes possessing anomalous correlators. These correlators provide a microscopic quantum counterpart of the stochastic fluctuations that appear in earlier D-particle foam descriptions of the $ω$-effect, replacing phenomenological variances of flavour-changing D-particle recoil by calculable graviton correlation functions. After tracing over the graviton bath, the anomalous correlators and the weak-interactions-induced mixing combine to generate transitions between the antisymmetric and symmetric two-meson sectors. This results in a small exchange-symmetric admixture, parametrised by $ω$, in the otherwise antisymmetric EPR state. We obtain an explicit expression for $ω$ in terms of a sum over Takagi supermodes weighted by their squeezing amplitudes and phases together with the weak-interaction flavour-mixing matrix element. The resulting framework suggests that the $ω$-effect may be a generic signature of non-classical states of gravitational environments, extending beyond the specific axion-cloud scenario considered here. The observability of the $ω$-effect from other astrophysical and microscopic black-hole sources is discussed.

gr-qc↗

arXiv2026

Probing String-Theory-Inspired Topologies of the Early Universe through CMB Temperature and Polarization Anisotropies

TeV string-mass-scale strings have been excluded experimentally at colliders, as their effects have not been observed at the Large Hadron Collider (CERN). On the other hand, higher-scale string theory, with mass scales typically close to the Planck scale, is often regarded as experimentally inaccessible due to the enormous energies required for direct tests, and far beyond the reach of present or foreseeable particle accelerators. Nevertheless, the early Universe may provide an indirect observational window for high-string scale through imprints left on the Cosmic Microwave Background (CMB). In this work, building on previous studies, we reexamine temperature and polarization angular correlations as probes of the geometry and topology of the pre-inflationary Universe. We focus in particular on two-point correlation functions at large angular scales, where signatures of nontrivial spatial topology may survive as relics of primordial physics. We investigate the observational consequences of toroidal compactification and analyze their impact on the primordial power spectrum of the CMB provided by the Planck satellite. Within the current experimental and theoretical uncertainties, we identify a possible indication closely related to spatial-parity breaking, consistent with the presence of six spatial extra dimensions in the early Universe, compactified at the GUT epoch before the start of inflation. Finally, we extend our framework to B-mode polarization, highlighting its potential as a sensitive probe in forthcoming ground-based and space-borne experiments with unprecedented precision.

astro-ph.CO↗

arXiv2026

How Much Can Gravitons Be Squeezed?

Quantum Gravity remains elusive, largely because its observable effects are suppressed by powers of the Planck scale. Direct detection of single gravitons is widely believed to be impossible. Here we propose a concrete astrophysical mechanism that may overcome this suppression. We show that superradiant axion-like-particle clouds surrounding rotating black holes can generate multimode squeezed states of gravitons containing up to $10^6$ - $10^7$ correlated quanta. Such states exhibit distinctive polarization correlations and quantum-noise signatures that could be detectable in future gravitational-wave interferometers. Observation of these signatures would constitute direct evidence for the quantum nature of gravitational radiation. Conversely, their absence can place constraints on axion-cloud lifetimes. Our approach also provides a test of General Relativity as an effective field theory.

gr-qc↗

arXiv2026

The most general four-derivative Unitary String Effective Action with Torsion and Stringy-Running-Vacuum-Model Inflation: Old ideas from a modern perspective

The string-inspired running vacuum model (StRVM) of inflation is based on a Chern-Simons (CS) gravity effective action, in which the only four-spacetime-derivative-order term is a gravitational anomalous CS Pontryagin density coupled to an axion. In this work, we revisit curvature-squared string-inspired effective actions, from the point of view of appropriate local field redefinitions, leaving the perturbative string scattering matrices invariant. We require simultaneously unitarity and torsion interpretation of the field strength of the Kalb-Ramond antisymmetric tensor, features characterising the (3+1)-dimensional StRVM Cosmology. Unlike the higher dimensional case, the above feature is possible in the context of (3+1)-dimensional spacetimes, obtained after string compactification. We demonstrate that the unitarity and torsion-interpretation requirements lead to a single-type of extra four-derivative terms in the effective gravitational action, not discussed in the previous literature of StRVM, which however is shown to be subleading by many orders of magnitude, compared to the terms of the StRVM framework. Hence, its presence has no practical implications for the relevant inflationary (and, hence, postinflationary) physics of the StRVM. This demonstrates the phenomenological completeness of the StRVM cosmological scenario, which is thus fully embeddable in the UV complete (quantum-gravity compatible) string theory framework.

gr-qc↗

arXiv2026

On internal mechanical properties of Electroweak Magnetic Monopoles and their effects on stability

By considering properties of the energy-momentum tensor of the electroweak magnetic monopole and its Born-Infeld extension, we attempt to make comments on the stability of these configurations. Specifically, we perform a study of the behaviour of the so-called internal force and pressure of these extended field-theoretic solitonic objects, which are derived from the energy-momentum tensor. Our method is slightly different from the so-called Laue's criterion for stability of nuclear matter, a local form of which had been proposed and applied in the earlier literature to the `t Hooft-Polyakov (HP) magnetic monopole, and found to be violated.By applying our method first to HP monopole, we also observe that, despite its topological stability, the total (finite) internal force (which has only radial components) is directed inwards, towards the centre of the monopole, which would imply instability. Thus this mechanical criterion for stability is arguably violated in the case of the HP monopole, as is the local version of Laue's criterion. The criterion is satisfied for the short-range part of the energy momentum tensor, in which the long-range part, due to the massless photon of the U(1) subgroup, is subtracted. Par contrast, the total internal force of the Cho-Maison (CM) electroweak monopole has both radial and angular components, which diverge at the origin, leading to rotational instabilities. Finally, by studying finite-energy extensions of the CM, either with non-minimal Higgs couplings with the hypercharge sector or hypercharge Born-Infeld type models, we find that the total force, integrated over space, is finite, but it has also angular components in the Born-Infeld case. The latter feature is interpreted as indicating that the Born-Infeld-CM monopole might be subject to rotations upon the action of perturbations, but it does not necessarily imply instabilities of the configuration.

hep-th↗

arXiv2025

The axion coupling accelerates the Universe through PT-symmetric phases

The conjecture by two of the authors (N.E.M. and S.S.) that a \cPT-symmetric phase plays a role in understanding singular renormalisation group (RG) flows for a Chern-Simons (CS) gauge theory of axions, has been reexamined and significantly improved. We have used the more complete Wetterich equation, which includes gravitational couplings in a systematic way from the start, to understand the emergence of this phase. The singular structure of the RG flows has persisted on including gravitational-couplings, thereby offering further support to the conjecture that \cPT -symmetric phases of (repulsive) gravity characterise string-effective CS gravitational theories, where the axion is the massless string-model independent axion, which can also play a role of a totally-antisymmetric torsion degree of freedom. This has suggested a novel interpretation of the currently observed acceleration of the expansion of the Universe in terms of such a phase at large (cosmological) scales.

gr-qc↗

arXiv2025

Squeezed gravitons from superradiant axion fields around rotating black holes

We propose, in (3+1)-dimensional spacetimes, a novel astrophysical source of squeezed graviton states, due to superradiant axionic clouds surrounding rotating (Kerr-type) black holes (BH). The microscopic origin of these axions is diverse, ranging from the Kalb-Ramond (model-independent) axions and compactification axions in string theory, to \cm contorted geometries exemplified by a totally antisymmetric component of torsion in Einstein-Cartan theory. The axion fields couple to chiral gauge and gravitational Chern-Simons (CS) anomaly terms in the effective gravitational actions. In the presence of a Kerr BH background, such axions lead, upon acquiring a mass, to superradiance and the production of pairs of entangled gravitons in a squeezed state. The specific microscopic origin of the axions is not important, provided they are massive. This multimode squeezed-graviton state is examined through a Takagi-like decomposition, used in quantum optics. In the effective action it is shown that squeezing effects associated with conventional general relativity (GR) dominate, by many orders of magnitude, the corresponding effects due to the CS gravitational anomaly terms. For a sufficiently long lifetime of the axionic cloud of the BH, we find that significant squeezing (quantified through the average number of gravitons with respect to the appropriate vacuum) can be produced from the GR effects. It is also demonstrated explicitly that the structure of the entangled states (when the latter are expressed in a left-right polarization basis) depends highly on whether the GR or the anomalous CS effects produce the entanglement.

gr-qc↗

arXiv2025

Axions, Black Holes and the Detection of Gravitons: from Astrophysics to Cosmology

We review a novel scenario for the emergence of spin-polarisation entangled squeezed graviton states from superradiant axionic clouds in the neighborhood of astrophysical rotating black holes (BHs). The entangled squeezed graviton states are produced by both, conventional General-Relativity (GR) type axion-gravity interactions, and gravitational Chern-Simons (gCS) anomalous terms coupled to axions, which are non-trivial in the presence of rotating BHs. The two kinds of terms have different-symmetry contributions to the entangled squeezed states. The squeezing parameter is estimated in a weak-quantum-gravity framework. Some phenomenology with respect to current and future interferometric detection devices is discussed. Importantly, current data from LIGO/Virgo Experiments can impose upper-bound constraints on the value of the squeezing parameter and, thus, on the lifetime of the axionic clouds. In addition to the above rather direct-detection possibility of squeezed gravitons, there is also the possibility of indirect detection of quantum gravitons in Cosmology, given that chiral quantum gravitational-wave (GW) perturbations in the primordial Universe may imply condensation of gCS terms. This, in turn, leads to inflation of running vacuum type, with in principle observable patterns in the profile of the GW produced during the post-inflationary early radiation era, as well as the potential of alleviating cosmic tensions in the current era.

gr-qc↗

arXiv2025

On the Potential of Microtubules for Scalable Quantum Computation

We examine the quantum coherence properties of tubulin heterodimers arranged into the protofilaments of cytoskeletal microtubules. In the physical model proposed by the authors, the microtubule interiors are treated as high-Q quantum electrodynamics (QED) cavities that can support decoherence-resistant entangled states under physiological conditions, with decoherence times of the order of $\mathcal{O}(10^{-6})$ sec. We identify strong electric dipole interactions between tubulin dimers and ordered water dipole quanta within the microtuble interior as the mechanism responsible for the extended coherence times. Classical nonlinear (pseudospin) $σ$-models describing solitonic excitations are reinterpreted as emergent quantum-coherent-or possibly pointer-states, arising from incomplete collapse of dipole-aligned quantum states. These solitons mediate dissipation-free energy transfer along microtubule filaments. We discuss logic-gate-like behavior facilitated by microtubule-associated proteins, and outline how such structures may enable scalable, ambient-temperature quantum computation, with the fundamental unit of information storage realized as a quDit encoded in the tubulin dipole state. We further describe a process akin to decision making that emerges following an external stimulus, whereby optimal, energy-loss-free signal and information transport pathways are selected across the microtubular network. Finally, we propose experimental approaches-including Rabi-splitting spectroscopy and entangled surface plasmon probes-to validate the use of biomatter as a substrate for scalable quantum computation.

physics.bio-ph↗

arXiv2025

Superradiant Axionic Black-Hole Clouds as Seeds for Graviton Squeezing

It is shown that both, standard general relativity (GR) and Chern-Simons (CS) gravity, the latter containing chiral gravitational anomaly terms, seed the production of pairs of entangled gravitons in a multi-mode squeezed state. This involves the interaction of gravitons with the axionic cloud surrounding a superradiant Kerr (rotating) black hole background. The order of magnitude of the squeezing effect, specifically the number of graviton excitations in the squeezed vacuum, is estimated in the non-relativistic limit, relevant for the superradiance process. It is found analytically that the squeezing from the GR process of annihilation of two axions into two gravitons, dominates, by many orders of magnitude, that coming from the axion decay into two gravitons, induced by the higher-derivative CS term. It is also shown that significant squeezing effects are produced in the case of long-lived axionic clouds, whose lifetimes are much longer than the timescale for which superradiance is effective. A brief discussion on current exclusion (for the first time) of very-long axion-cloud lifetimes, through comparison of our results with current LIGO data, as well as potential detection of such effects in future interferometers is also given

gr-qc↗

arXiv2025

Quantum Gravity and Entanglement in Particle Physics and Gravitation

Some approaches to Quantum Gravity (QG) entail decoherence of quantum matter propagating in it, due to an ``environment'' of QG degrees of freedom inaccessible to low-energy observers. In the first part of this talk, I discuss potential, and rather unique, effects of QG-induced decoherence on entangled particle states, specifically an induced modification of Einstein-Podolsky-Rosen (EPR) correlations of entangled neutral-meson states in meson factories ($ω$-effect). In the second part, I summarise a recent work in which axion-like fields, forming a kind of condensate clouds surrounding rotating (Kerr-type) astrophysical black holes, can lead to superradiant instabilities, and, through these, to the production of EPR-like entangled states of gravitons, with the entanglement pertaining to (left, right) polarisation degrees of freedom. In the presence of axions and Kerr geometries, there are non-trivial gravitational Chern-Simons (gCS)-type anomalous terms in the respective low-energy gravitational effective actions. Depending on whether the graviton entanglement is due to the non-anomalous terms (of General-Relativity type) in the effective action, or to the gCS terms, one obtains different structures of the resulting entangled squeezed-graviton states, which resemble somewhat the $ω$-effect.

gr-qc↗

arXiv2025

Dynamical-System analysis of single-axion monodromy inflation with periodically-modulated potentials

In this work, we study field theoretic systems of a single axion-like field with linear potentials modulated by cosine terms, allegedly induced by non-perturbative instanton configurations. These systems are considered in expanding-Universe spacetime backgrounds (of Friedman-Lema$\hat{\rm i}$tre-Robertson-Walker type). Using a dynamical-system approach, we classify the various de-Sitter like (inflationary) vacua from the point of view of their stability, which depend on the values of the model parameters. In this respect, bifurcation points are found to be present for the various models under consideration. Part of the parameter space of the systems under consideration includes the running-vacuum (approximately) linear-axion monodromy potentials, considered in previous works by some of the authors, where inflation is induced by primordial gravitational-wave condensates. A particularly interesting case, corresponding to another part of the parameter space of the models, includes a series of stable de-Sitter vacua, which physically may correspond to a series of successive tunnelings of the system, via say non-perturbative effects, with a decreasing effective cosmological constant. Under certain values of the parameters, these successive tunnelings can reach a Minkowski spacetime, with zero value of the minimum of the axion potential. The situation is not dissimilar to the one of discrete inflation that arguably characterizes some minimal non-critical-string (Liouville) models of cosmology. Finally, for comparison, we also include in this article a dynamical-system study of standard axion-monodromy-modulated potentials characterizing some string/brane-compactification models of inflation.

gr-qc↗

arXiv2025

Inflation from Anomalies

We review a string-inspired model of inflation which is a consequence of condensates of chiral gravitational waves (GW) in the primordial Universe, leading in turn to a (approximately) constant condensate of a gravitational anomaly term of Chern-Simons (CS) type, present in the Lagrangian density that describes the dynamics of the very early Universe in the model. We discuss some mechanisms for the production of chiral GW, as well as the role of periodic modulations of the potential of the gravitational axion field, that couples to the CS anomaly term, in ensuring the correct inflationary slow-roll phenomenology of this model.

hep-th↗

arXiv2025

Impact of resummation on the production and experimental bounds of scalar high-electric-charge objects

A one-loop Dyson-Schwinger-like resummation scheme is applied to scalar High-Electric-Charge compact Objects (HECOs), extending previous work on spin-1/2 case. The electromagnetic interactions of HECOs are considered within the framework of strongly coupled scalar Quantun Electrodynamics. The resummation amounts to determining non-trivial ultraviolet (UV) fixed points, at which the effective Lagrangian, which will lead to the pertinent predictions on the cross sections, is computed. In contrast to the fermionic HECO case, in which the fixed point structure was determined solely by the interactions of the HECOs with the photon field, in the scalar case the existence of non-trivial UV fixed points requires the presence of additional strong self interactions among the HECOs. Our resummation scheme, which is notably different from a lattice strong-coupling approach, makes the computation of the pertinent scalar-HECO-production cross sections reliable, thus allowing revisiting the mass bounds obtained from searches for such objects in current or future colliders. Our MadGraph implementation of the results leads to enhanced (up to ~30%) lower bounds on the mass of scalar HECOs, as compared to those extracted from the tree-level processes typically used in LHC collider searches by ATLAS and MoEDAL experiments.

hep-ph↗

arXiv2025

Quantum-Ordering Ambiguities in Weak Chern-Simons 4D Gravity and Metastability of the Condensate-Induced Inflation

In this work, we elaborate further on a 4D cosmological Running-Vacuum-type Model (RVM) of inflation that characterises string-inspired Chern-Simons (CS) gravity. It has been shown that inflation in such models is caused by a condensation of the gravitational CS (gCS) terms induced by primordial gravitational waves (GW), which leads to a linear-axion potential, thus breaking the shift symmetry, and lifting its periodicity (monodromy). We demonstrate here that this inflationary phase may be metastable, due to the existence of non-trivial imaginary parts of the gCS condensate. These imaginary parts are quantum effects, proportional to appropriate commutators of GW perturbations. As we stress, their existence is quantum-ordering-scheme dependent. We argue here in favor of a physical importance of such imaginary parts, which we compute to second order in the GW (tensor) perturbations in the framework of a specific gauge-fixed effective Lagrangian, within a (mean field) weak-quantum-gravity path integral approach. We pay specific attention to the various space-time boundary terms. We thus provide an estimate of the life time of inflation. On matching our results with the relevant inflationary phenomenology, we fix the quantum-ordering ambiguities, and obtain an order-of-magnitude constraint on the ratio of the string energy scale $M_s$ in this model to the (four-spacetime-dimensional) reduced Planck mass $M_{\rm Pl}$, specifically, $M_s/M_{\rm Pl} = \mathcal{O}(10^{-1})$. This is consistent with the corresponding estimate obtained in previous analyses by the authors in this framework, based on a dynamical-system approach to linear-axion RVM inflation. Finally, we examine the role of periodic modulations in the axion potential induced by non-perturbative stringy effects on the slow-roll inflationary parameters, and find compatibility with the cosmological data.

gr-qc↗

arXiv2024

Do we owe our existence to a Gravitational Anomaly?

I review a mechanism for a potential explanation of the dominance of matter over antimatter in the Universe (``our existence''), based on a (3+1)-dimensional string-inspired cosmological model, characterised by (uncancelled) gravitational Chern-Simons (gCS) anomalies in the premordial epochs. The model is consistent with general covariance but entails spontaneous Lorentz (LV) and CPT Violation (CPTV), due to appropriate backgrounds of axion fields, which couple to the anomaly terms. Upon condensation of the gCS terms, due to primordial gravitational waves, these axions acquire a linear potential, which drives inflation of a Running-Vacuum-Model (RVM) type, while the effective action is also characterised by imaginary parts as well, which imply metastability of this RVM inflation. The axion backgrounds remain undiluted until the exit from inflation, and are responsible for yielding unconventional LV and CPTV Leptogenesis, which is then communicated to the baryon sector (Baryogenesis) via sphaleron processes. The model provides a phenomenologically consistent cosmological evolution from inflation till the modern era.

hep-ph↗