arXiv subjects

Astrid Eichhorn

Publications and source records attributed to Astrid Eichhorn.

At least 19 recordsLinked to original sources

arXiv2026

The fermion sector of the SMEFT from asymptotically safe gravity

Quantum gravity impacts Standard Model fields through effective interactions generated by renormalization. These appear as Standard Model Effective Field Theory (SMEFT) operators, where the effects of new physics are parametrized by the values of the higher-order SMEFT coefficients. As a step towards predicting these SMEFT coefficients from the asymptotically safe Standard Model with quantum gravity, we investigate the flow of a representative set of four-fermion operators under gravitational fluctuations. We strengthen the evidence for the near-perturbative nature of asymptotic safety by finding that these dimension-six-interactions remain irrelevant, resulting in the prediction of specific values for the dimensionless ratios of SMEFT coefficients in the IR. We also find a mechanism that can make a specific subset of these operators relevant. This subset is determined by symmetry considerations. On this basis, we provide a categorization of dimension-six-SMEFT interactions into two categories. Interactions in the first category are predicted to be non-zero, but Planck-scale suppressed. Interactions in the second category are predicted to be zero at small gravitational coupling, but may become free parameters of the theory at larger gravitational couplings.

hep-th↗

arXiv2026

Asymptotically safe quantum gravity and its phenomenology -- a review

Asymptotically safe quantum gravity is an approach to quantum gravity. It is based on the premise that quantum field theory can describe the quantum nature of gravity in our universe. At its core lies quantum scale symmetry. This review provides an introduction to the key ideas of the approach and surveys the current status of the field. Over the last years, the field has taken large strides towards an increasingly realistic setting: First, compelling evidence for quantum scale symmetry exists in four-dimensional, Euclidean, pure gravity, establishing the Reuter fixed point robustly. Second, matter fields, including the Standard Model as well as beyond-Standard-Model-candidates, have been studied in depth, with increasingly conclusive evidence for quantum scale symmetry. Most recently, the final gap to a realistic description of quantum gravity is being closed, because Lorentzian spacetime signature can now be accounted for. As a consequence of quantum scale symmetry in the ultraviolet, the approach is highly predictive at all scales. This review discusses the physics of asymptotic safety across all scales. Predictive power for particle physics, black holes and cosmology provides a clear pathway to confronting quantum gravity with current and near-future observations. The review closes by discussing the connection to other approaches to quantum gravity. It advocates the perspective that such connections between approaches may lead us to an understanding of universal physical features of quantum gravity.

hep-th↗

arXiv2026

Non-uniqueness of boundary-value problems in Renormalization Group flows

The Renormalization Group flow connects microscopic to macroscopic descriptions of a system and is therefore typically considered as an initial-value problem. Motivated by situations in which different couplings within a system of Renormalization Group equations are constrained at different scales, we instead consider boundary-value problems in Renormalization Group flows. We find that, unlike initial-value problems which provide $n$ conditions for $n$ couplings, boundary-value problems which provide $n$ conditions for $n$ couplings do not always have a unique solution. When the Jacobian matrix, i.e., the matrix of first derivatives of beta functions, has complex eigenvalues, boundary-value problems may be non-unique. We provide a diagnostic tool for non-uniqueness in systems with many couplings. We also provide two examples with potential relevance for physics, namely within the Standard Model as well as within the Einstein-Hilbert truncation of asymptotically safe quantum gravity.

hep-th↗

arXiv2026

Charting causal set configuration space with graph observables

The configuration space of causal sets is vast. It is a critical goal to map out this space. Here, we take a practical step towards this goal. We investigate nine classes of causal sets, most of them not studied before. These include manifoldlike causal sets with inhomogeneous Ricci curvature, both topologically trivial and nontrivial. We also study classes of non-manifoldlike causal sets, including lattices, layered orders as well as Lorentzian quasicrystals. Finally, we study classes of causal sets that are not manifoldlike, but are expected to become manifoldlike under a suitable coarse-graining process. We use this broad range of distinct classes of causal sets as a testbed for observables. Rather than focusing on continuum-geometry inspired observables, such as curvature invariants, which often exhibit large fluctuations and are computationally very expensive, we focus on graph observables, including some observables that constitute subgraph statistics and some that are global. We find that three observables, namely the link degree distribution, the eigenvalues of the graph Laplacian of the symmetrized Hasse diagram and the recently proposed abundance of causal intervals, can distinguish between the distinct classes of causal sets. This is made possible by the small fluctuations that these observables have in most classes.

gr-qc↗

arXiv2026

Towards black-hole horizons and geodesic focusing in causal sets

The event horizon of a black hole is arguably the most dramatic manifestation of the fact that in General Relativity, causal structure is dynamical and spacetimes can be separated into distinct regions by causal boundaries. Causal set quantum gravity is an approach to quantum gravity in which causal relations between spacetime points constitute the basic structure on which the theory is based. This raises the question how a discrete horizon can be identified in a causal set. In our paper, we first construct a local diagnostic to approximate a global concept, namely the event horizon, based on discrete timelike curves. We then turn to the concept of an apparent horizon, which is based on local properties of geodesics, rather than global properties of the entire spacetime. We undertake first steps towards detecting apparent horizons in causal sets, using so-called ladders as tracers of null geodesics. We find that a discrete counterpart of the expansion changes sign across the black-hole horizon, as it should. Finally, we introduce the notion of a fuzzy ladder, which enables us to track null geodesics for larger intervals of the affine parameter. Thereby, we construct a portion of a discrete horizon in a toy-model for a black-hole spacetime in 1+1 dimensions.

gr-qc↗

arXiv2026

Towards theory constraints on ultralight dark matter from quantum gravity

Ultralight scalar dark matter may couple to the Standard Model through dimension-five operators that contain the field-strength tensors of the gauge interactions. Recent progress in nuclear clocks is projected to increase the sensitivity to such couplings by several orders of magnitude. Future experimental constraints may even have Planck-scale sensitivity, calling for a study of such couplings in a framework that includes quantum gravity. We take a first step towards providing the theoretical constraints on such couplings that arise in asymptotically safe gravity. We find evidence that such couplings vanish in asymptotically safe gravity and are also not generated in a perturbative quantum-gravity regime that describes quantum gravity as an effective field theory.

hep-ph↗

arXiv2026

Regular black holes without mass-inflation instability and gravastars from modified gravity

We derive regular black-hole solutions, including the Hayward metric, from four-dimensional action principles involving vector fields in addition to the metric. These black holes possess additional hair associated with the vector fields, manifesting as free integration constants that regularize the geometry. These constants can be chosen such that regular black holes of all masses are extremal. As a result, they have vanishing surface gravity and are not susceptible to mass-inflation instability. We also discover another regular black-hole metric with these properties, which constitutes a gravastar for an appropriate choice of integration constant.

gr-qc↗

arXiv2026

Indications against dynamical CPT symmetry restoration in quantum gravity

CPT symmetry is at the heart of the Standard Model of particle physics and experimentally very well tested, but expected to be broken in some approaches to quantum gravity. It thus becomes pertinent to explore which of the two alternatives is realized: (i) CPT symmetry is emergent, so that it is restored in the low-energy theory, even if it is broken beyond the Planck scale, (ii) CPT symmetry cannot be emergent and must be fundamental, so that any approach to quantum gravity, in which CPT is broken, is ruled out. We explore this by calculating the Renormalization Group flow of CPT violating interactions under the impact of quantum fluctuations of the metric. We find that CPT symmetry cannot be emergent and conclude that quantum-gravity approaches must avoid the breaking of CPT symmetry. As a specific example, we discover that in asymptotically safe quantum gravity CPT symmetry remains intact, if it is imposed as a fundamental symmetry, but it is badly broken at low energies if a tiny amount of CPT violation is present in the transplanckian regime.

gr-qc↗

arXiv2026

Strong breaking of black-hole uniqueness from coexisting scalarization mechanisms

Black-hole uniqueness, i.e., the statement that all stationary vacuum black holes in the universe are described by the Kerr solution, is expected to break in theories beyond General Relativity. This breaking can take a particularly strong form, if several branches of black-hole solutions beyond the Kerr solution coexist. We find an example of a theory that exhibits such strong breaking. In this theory, a cubic coupling of a scalar field to the Gauss-Bonnet invariant triggers black-hole scalarization through a non-linear instability of the Kerr solution. At large spin, curvature-induced and spin-induced scalarization mechanisms compete at fixed sign of the coupling. This results in a rich phase structure of black-hole solutions and continuous as well as discontinuous transitions between the different branches of black holes.

gr-qc↗

arXiv2026

Towards a quantitative characterization of gravitational universality classes for order-4 random tensor models

Random tensor models can be used as combinatorial devices to generate Euclidean dynamical triangulations. A physical continuum limit of dynamical triangulations requires a suitable generalization of the double-scaling limit of random matrices. This limit corresponds to a fixed point of a pregeometric Renormalization Group flow in which the tensor size $N$ serves as the Renormalization Group scale. We search for corresponding fixed points in order-4 random tensor models associated to dynamical triangulations in 4 dimensions. In a $O(N)^{\otimes 4}$ symmetric setting, we discuss the resulting phase portrait as a function of the regulator parameters. We optimize our results, identifying parameter values for which the results are minimally sensitive to parameter changes. We find three fixed-point candidates: only one of them is real across the entire parameter range, but only has two relevant directions. This should be contrasted with the university class of the Reuter fixed point in continuum quantum gravity, very likely characterized by three relevant directions. We conclude that simple combinatorial models of Euclidean triangulations and the Reuter fixed point most likely lie in different universality classes.

gr-qc↗

arXiv2025

Non-minimal light-curvature couplings and black-hole imaging

Non-minimal couplings between the electromagnetic field strength and the spacetime curvature are part of the effective field theory of gravity and matter. They alter the local propagation of light in a significant way if the ratio of spacetime curvature to the non-minimal coupling is of order one. Spacetime curvature can become appreciable around black holes, and yet the effect of non-minimal couplings on electromagnetic observations of black holes remains underexplored. A particular feature of the non-minimal coupling between the electromagnetic field-strength and the Riemann tensor is that it generates two distinct photon rings for different polarizations. Working within the paradigm of lensing bands and focusing on the $n = 1$ lensing band, we illustrate by which diagnostics a modified light propagation may be distinguished from a modified spacetime geometry and how constraints on the value of the non-minimal coupling can be obtained

astro-ph.HE↗

arXiv2025

Visions in Quantum Gravity

To deepen our understanding of Quantum Gravity and its connections with black holes and cosmology, building a common language and exchanging ideas across different approaches is crucial. The Nordita Program "Quantum Gravity: from gravitational effective field theories to ultraviolet complete approaches" created a platform for extensive discussions, aimed at pinpointing both common grounds and sources of disagreements, with the hope of generating ideas and driving progress in the field. This contribution summarizes the twelve topical discussions held during the program and collects individual thoughts of speakers and panelists on the future of the field in light of these discussions.

hep-th↗

arXiv2025

Quark and lepton mixing in the asymptotically safe Standard Model

The quark mixing (CKM) matrix is near-diagonal, whereas the lepton mixing (PMNS) matrix is not. We learn that both observations can generically be explained within an ultraviolet completion of the Standard Model with gravity. We find that certain relations between CKM matrix elements should hold approximately because of asymptotically safe regimes, including $|V_{ud}|^2+|V_{us}|^2 \approx 1$ and $|V_{cd}|^2+|V_{cs}|^2\approx 1$. Theoretically, the accuracies of these relations determine the length of the asymptotically safe regimes. Experimental data confirms these relations with an accuracy of $10^{-5}$ and $10^{-3}$, respectively. This difference in accuracies is also expected, because the ultraviolet completion consists in a fixed-point cascade during which one relation is established already much deeper in the ultraviolet. This results in $|V_{ub}|^2 < |V_{cb}|^2$ and translates into measurable properties of $B$-mesons. Similar results would hold for the PMNS matrix, if neutrino Yukawa couplings were large. The ultraviolet complete theory therefore must -- and in fact can -- avoid such an outcome. It contains a mechanism that dynamically limits the size of neutrino Yukawa couplings. Below an upper bound on the sum of Dirac neutrino masses, this allows the PMNS matrix to avoid a near-diagonal structure like the CKM matrix. Thus, large neutrino mixing is intimately tied to small Dirac neutrino masses, $\sum m_ν \lesssim {\mathcal{O}} (1)\, \rm eV$ and a mass gap in the Standard Model fermion masses.

hep-ph↗

arXiv2025

Renormalization group flows in area-metric gravity

We put forward the first analysis of renormalization group flows in an area-metric theory, motivated by spin-foam quantum gravity. Area-metric gravity contains the well-known length-metric degrees of freedom of standard gravity as well as additional shape-mismatching degrees of freedom. To be phenomenologically viable, the shape-mismatching degrees of freedom have to decouple under the renormalization group flow towards lower scales. We test this scenario by calculating the renormalization group flow of the masses and find that these are in general even more relevant than dictated by their canonical scaling dimension. This generically results in masses which are large compared to the Planck mass and thereby ensure the decoupling of shape-mismatching degrees of freedom. In addition, the latter come in a left-handed and right-handed sector. We find that parity symmetry does not emerge under the renormalization group flow. Finally, we extract the renormalization group flow of the Immirzi parameter from this setup and find that its beta function features zeros at vanishing as well as at infinite Immirzi parameter.

gr-qc↗

arXiv2025

On the Renormalization Group flow of distributions

Renormalization Group flows relate the values of couplings at different scales. Here, we go beyond the Renormalization Group flow of individual trajectories and derive an evolution equation for a distribution on the space of couplings. This shift in perspective can provide new insights, even in theories for which the Renormalization Group flow of individual couplings is well understood. As a first application, we propagate errors under the Renormalization Group flow. Characteristic properties of an error distribution, such as its maximum or highest density region, cannot be propagated at the level of individual couplings, but require our evolution equation for the distribution on the space of couplings. We demonstrate this by calculating the most probable value for the metastability scale in the Higgs sector of the Standard Model. Our second application is the emergence of structure in sets of couplings. We discover that infrared-attractive fixed points do not necessarily attract the maximum of the distribution when the Renormalization Group is evolved over a finite range of scales. Instead, sharply peaked maxima can build up at coupling values that cannot be inferred from individual trajectories. We demonstrate this emergence of structure for the Standard Model, starting from a broad distribution at the Planck scale. The Renormalization Group flow favors the phenomenological ordering of third-generation Yukawa couplings and, strikingly, we find that the most probable values for the Abelian hypercharge and Higgs quartic coupling lie close to their observed values.

hep-th↗

arXiv2025

Towards a Non-singular Paradigm of Black Hole Physics

The study of regular black holes and black hole mimickers as alternatives to standard black holes has recently gained significant attention, driven both by the need to extend general relativity to describe black hole interiors, and by recent advances in observational technologies. Despite considerable progress in this field, significant challenges remain in identifying and characterizing physically well-motivated classes of regular black holes and black hole mimickers. This report provides an overview of these challenges, and outlines some of the promising research directions -- as discussed during a week-long focus programme held at the Institute for Fundamental Physics of the Universe (IFPU) in Trieste from November 11th to 15th, 2024.

gr-qc↗

arXiv2025

Neutrino mass generation in asymptotically safe gravity

There exist several distinct phenomenological models to generate neutrino masses. We explore, which of these models can consistently be embedded in a quantum theory of gravity and matter. We proceed by invoking a minimal number of degrees of freedom beyond the Standard Model. Thus, we first investigate whether the Weinberg operator, a dimension-five-operator that generates neutrino masses without requiring degrees of freedom beyond the Standard Model, can arise in asymptotically safe quantum gravity. We find a negative answer with far-reaching consequences: new degrees of freedom beyond gravity and the Standard Model are necessary to give neutrinos a mass in the asymptotic-safety paradigm. Second, we explore whether the type-I Seesaw mechanism is viable and discover an upper bound on the Seesaw scale. The bound depends on the mass of the visible neutrino. We find a numerical value of $10^{14}\, \rm GeV$ for this bound when neglecting neutrino mixing for a visible mass of $10^{-10}\, \rm GeV$. Conversely, for the most ``natural" value of the Seesaw scale in a quantum-gravity setting, which is the Planck scale, we predict an upper bound for the neutrino mass of the visible neutrino of approximately $10^{-15}\, \rm GeV$. Third, we explore whether neutrinos could also be Pseudo-Dirac-neutrinos in asymptotic safety and find that this possibility can be accommodated.

hep-ph↗

arXiv2025

The principled-parameterized approach to gravitational collapse

New physics beyond General Relativity impacts black-hole spacetimes. The effects of new physics can be investigated in a largely theory-agnostic way by following the principled-parameterized approach. In this approach, a classical black-hole metric is upgraded by following a set of principles, such as regularity, i.e., the absence of curvature singularities. We expect these principles to hold in many theories beyond General Relativity. In the present paper, we implement this approach for time-dependent spacetimes describing gravitational collapse. We find that the Vaidya spacetime becomes regular through the same modification of the spacetime metric as stationary black-hole spacetimes [1-3]. We investigate null geodesics and find indications that the modification is even sufficient to render null geodesics future complete. Finally, we find that the modification of the spacetime structure results in violations of the null energy condition in a finite region inside the apparent horizon of the black hole that forms. Null geodesics are attracted to the boundary of this region, such that the new-physics effects are shielded from asymptotic observers. An exception occurs, if the classical spacetime has a naked singularity. Then, the upgraded spacetime is singularity-free and null geodesics from the regular core can escape towards asymptotic observers.

gr-qc↗