It has been conjectured and proven that entanglement island is not consistent with long-range (massless) gravity in a large class of spacetimes, including typical asymptotically anti-de Sitter spacetimes, in high spacetime dimensions. The conjecture and its proof are motivated by the observation that existing constructions of entanglement islands in high dimensions are all in gravitational theories where the graviton is massive for which the standard gravitational Gauss’ law doesn’t apply. In this letter, we show that this observation persists to lower dimensional cases. We achieve this goal by providing a unified description of the gravitational Gauss’ law violation in island models that can work in any dimensions. This unified description teaches us new lessons on entanglement islands and subregion physics in quantum gravity. We focus on the case of the (1+1)-dimensional Jackiw-Teitelboim (JT) gravity for the purpose of demonstration.

Motivated both by classical physics problems associated with “Newton’s bucket” and recent developments related to QCD in rotating frames of reference relevant to heavy ion collisions, we discuss the difference between “active” and “passive” rotations in quantum systems. We examine some relevant potentials and give general symmetry arguments to give criteria where such rotations give the same results. We close with a discussion of how this can be translated to problems of current interest in quantum field theory and quantum gravity.

Abstract Our present contribution sets out to investigate a scenario based on the effects of the Loop Quantum Gravity (LQG) on the electromagnetic sector of the Standard Model of Fundamental Interactions and Particle Physics (SM). Starting then from a post-Maxwellian version of Electromagnetism that includes LQG effects, we work out and discuss the influence of LQG parameters on classical quantities, such as the components of the stress-tensor. Furthermore, we inspect the propagation of electromagnetic waves and study optical properties of the QED vacuum in this scenario. Among these, we contemplate the combined effect between the LQG parameters and a homogeneous background magnetic field on the propagation of electromagnetic waves, considering in detail issues like group velocities and refractive indices of the QED vacuum. Finally, with the help of the LQG-extended photonic dispersion relations previously analyzed, we re-discuss the kinematics of the Compton effect and conclude that there emerges an interesting nonlinear profile in the wavelengths of both the incoming and the deflected photons. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Article funded by SCOAP3 and published under licence by Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Science and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd.

A bstract When states in a tower like the Kaluza-Klein or the string tower couple to another state through the irrelevant operators of the same type, their contributions to the loop corrections of the relevant or the marginal operators are not negligible, threatening the perturbativity. This can be avoided provided the cutoff scale is lower than the species scale associated with the irrelevant operator. We apply this to towers of states associated with the neutrino which couple to the Higgs through the Weinberg operator, the dimension-5 irrelevant operator generating the Majorana neutrino mass. Requiring the ‘Majorana species scale’, the species scale associated with the Weinberg operator, to be below the gravitational species scale, one finds the lower bound on the Majorana neutrino mass determined by the species number. The Festina-Lente bound also gives the lower bound on the Majorana neutrino mass, but it is not so stringent. Meanwhile, even if the neutrino mass is of the Dirac type at the renormalizable level, the Majorana mass term still can be written in the effective field theory action so far as the Weinberg operator is not forbidden. Even if the Majorana neutrino mass is larger than the Dirac one, so far as there are sufficient degrees of freedom with mass smaller than the scale of the cosmological constant, the observation of the Majorana nature of the neutrino may not contradict to quantum gravity constraints which rules out the neutrino mass purely given by the Majorana type.

Quantum gravity corrections to the spontaneous excitation of an accelerated atom interacting with a quantum scalar field

Abstract We argue that a scalar field in de Sitter spacetime should feel explicit thermal effects associated with its curvature. Starting from the Bunch-Davies vacuum and a scalar field with small mass compared to the de Sitter curvature, we use the thermo-field dynamics formalism in order to expose these thermal effects. We compute the thermal Wightman function connecting two spacetime points and from it, via the point-splitting regularization technique, the renormalized thermal energy-momentum tensor. We then examine how these corrections affect the de Sitter geometry by solving for the semi-classical backreaction and find that their sign depends on the initial conditions. In order to place our results in context, we compare them to the corresponding 2-loop quantum gravity correction to the cosmological constant derived in [1].

Abstract In the context of the general effort to model black hole dynamics, and in particular their return-to-equilibrium through quasi-normal modes, it is crucial to understand how much test-field perturbations deviate from physical perturbations in modified gravity scenarios. On the one hand, physical perturbations follow the modified Einstein equations of the considered extension of general relativity. The complexity of those equations can quickly escalate with extra fields and non-linear couplings. On the other hand, test-field perturbations, with negligible back-reaction on the space-time geometry, describe the propagation of both matter fields and spin s=2 gravitational waves on the black hole geometry. They are not subject to the intricacies of the modified Einstein equations, and only probe the background spacetime metric. If their physics were to not deviate significantly from physical perturbations, they would be especially useful to investigate predictions from quantum gravity scenarios which lack explicit detailed Einstein equations.
Here we focus on a specific modified gravity solution -- Babichev-Charmousis-Leh\'ebel (BCL) black holes in scalar-tensor theories -- for which physical perturbations and related QNM frequencies have already been studied and computed numerically. We compute the test-field QNM frequencies and compare the two QNM spectra. This provides a concrete example of the significant differences arising between test-fields and physical perturbations, and flags unphysical deviations related to the test-field framework.

Abstract It is well-known that a Lagrangian resembling scalar–tensor gravity naturally emerges from string theory in the low-energy limit. This suggests that a scalar–tensor theory intrinsically inhabits precious information about quantum gravitational effects. With this motivation, we quantize the scalar–tensor theory $$R+ \lambda R\phi ^2$$ R + λ R ϕ 2 in order to unfold the black hole physics in quantum gravity. We are interested in the behavior of the wave function near the interior singularity of a black hole, which would give a pathway to resolving the singularity. Consequently, we employ canonical quantization in a Kantowski–Sachs minisuperspace representation for the black hole interior. The ensuing Wheeler–DeWitt equation turns out to be intricately coupled among the three minisuperspace variables. Upon a suitable transformation, one of the variables could be easily separated – however, the scalar field $$\phi $$ ϕ remains coupled intricately and cannot be separated by any obvious transformation. Consequently, we employ a perturbative approach in powers of $$\lambda $$ λ . As a result, the zeroth order equation is immediately separable, and the first order equation still remains coupled in two variables. We adopt a Green function approach to solve this first order (inhomogeneous) equation. Our detailed calculation reveals that the Green function is well-defined in the vicinity of the black hole singularity, an important result for admitting the DeWitt criterion for singularity resolution. Furthermore, our calculations illustrate that both the zeroth order and first order wave functions are well-defined at the singularity as they are consistent with the DeWitt criterion, paving the way to black hole singularity resolution in scalar–tensor quantum gravity.

The paper proposes a method for high-precision testing of the equivalence principle for neutrons by calculating their weakly bound quantum states in the weak gravitational field of the Earth. Using the nonrelativistic Schrödinger equation, the energy spectrum of a neutron was obtained taking into account a possible violation of the equality between its gravitational and inertial masses. It is shown that, with a relative violation of the equivalence principle at the level of current experimental constraints 10 −4 , the relative shift of highly excited quantum levels can reach an order of 10 −3 . This method demonstrates the potential of precision spectroscopy of neutron quantum states for testing the equivalence principle and searching for manifestations of quantum gravity.

Abstract It is well known that, making the Abelian projection of Einstein’s theory one can obtain the restricted gravity which is simpler than Einstein’s theory but describes the core dynamics of Einstein’s gravity. In this paper we present the Lagrangian formalism of the restricted gravity which makes the restricted gravity a self consistent field theory by itself, independent of Einstein’s theory. With this we present interesting solutions of the restricted gravity, in particular the gravitational cosmic string, the Bertotti–Robinson spacetime, the axisymmetric pp-wave, and the conformally flat waves with flat wavefront. Moreover, we show that in the restricted gravity the Rosen–Bondi gravitational plane wave could be described by two Maxwellian potentials. This could play important role in quantum gravity. We discuss the physical implications of the restricted gravity.

Abstract We provide a reply to the Argument from Intimacy on behalf of defenders of emergent spacetime in theories of quantum gravity. We argue that if one accepts that spacetime regions are nowhere in the sense that they are locations but do not have locations, then the Argument from Intimacy can be resolved. We go on to consider a problem with this response, namely that it is unavailable to super-substantivalists. We argue that this is right for identity but not priority super-substantivalists. We then suggest that there is no cost for our solution here, since identity versions of super-substantivalism face severe challenges in the context of spacetime emergence and so should be rejected anyway.

In this paper, we study the quantum information dynamics of spin-1/2 particles in a spinning conical Gödel-type spacetime. Our focus is on calculating the Shannon and Fisher information measures to understand the influence of spacetime parameters on quantum information. By analyzing these information metrics, we explore the physical implications of the connection between the spin-1/2 particles and the geometric properties of the spacetime such as curvature, rotation, and conical structure. This investigation provides deeper insights into how these spacetime features affect quantum information, offering potential applications in quantum gravity and cosmology.

Abstract In this study, we explore the combined effects of quantum gravity induced by non-commutativity and scale-dependent gravitational coupling on the thermal properties of the thin accretion disks around a Schwarzschild black hole. We consider a $\kappa$-deformed Renormalization Group Induced (RGI) Schwarzschild black hole, where the classical Schwarzschild black hole geometry is modified by the $\kappa$-deformation of space-time and the running Newton's coupling constant $G(r)$. Using the modified metric, we derive the geodesic motion of massive particles, the effective potential, and the thermal properties such as the radiated energy flux, luminosity, and the temperature profile of the accretion disk around the $\kappa$-deformed RGI-Schwarzschild black hole. Our study shows that when non-commutativity is combined with the RGI framework, the effects produce a noticeable deviation from the classical Schwarzschild case. In particular, for small values of the deformation parameter, we observe an increase in the peak energy flux and the temperature of the accretion disk. This suggests that quantum gravity corrections enhance the disk's radiative efficiency, especially in the inner regions closer to the black hole.

The canonical quantization of gravity in general relativity is greatly simplified by the artificial decomposition of space time into a 3 + 1 formalism. Such a simplification appears to come at the cost of general covariance. This quantization procedure requires tangential and perpendicular infinitesimal diffeomorphisms generated by the symmetry group under the Legendre transformation of the given action. This gauge generator, along with the fact that Weyl curvature scalars may act as “intrinsic coordinates” (or a dynamical reference frame) that depend only on the spatial metric (gab) and the conjugate momenta (pcd), allows for an alternative approach to canonical quantization of gravity. In this paper, we present the tensorial solution of the set of Weyl scalars in terms of canonical phase-space variables.

Abstract In this letter, we propose a relationship between the so-called Hubble-Lema\^{i}tre constant $H_{0}$ and the holographic complexity related to the emergence of spacetime in quantum gravity. Such a result can represent an important step to understanding the Hubble tension by introducing a quantum gravity perspective for cosmological observations, regarding the degree of quantum complexity we measure around us.

A bstract Berends and Giele derived the Parke-Taylor formula for Yang-Mills MHV amplitudes by computing Berends-Giele currents involving gluons of all-plus and all-but-one-plus helicities. Remarkably, the all-plus current already encodes much of the Parke-Taylor formula structure. The all-but-one-plus current satisfies a more intricate recursion relation than the all-plus case, but one that can still be solved explicitly. This current turns out to be proportional to the all-plus current, which explains why the essential features of the MHV formula are already present at the all-plus level. In this paper, we carry out an analogous program for gravity. The all-plus graviton Berends-Giele current satisfies a recursion relation that is more involved than in the Yang-Mills case, but whose explicit solution is known: a sum over spanning trees of the complete graph on n vertices. We derive and solve the recursion relation for the all-but-one-plus graviton current. The solution is again given by a sum over spanning trees, where each tree contributes a term proportional to the corresponding all-plus current, multiplied by a factor given by a sum over subtrees. Only a small subset of these terms contributes to the MHV amplitude, which we recover explicitly. This provides a direct derivation of the gravity MHV formula from the gravitational Feynman rules — achieving what Berends, Giele, and Kuijf in their 1987 paper regarded as “hard to obtain directly from quantum gravity”.

Quantum gravity is expected to impose constraints on the moduli spaces of massless fields that can arise in effective quantum field theories. A recent proposal asserts that the asymptotic volume growth of these spaces is severely restricted, and related to the existence of duality symmetries. In this work we link this proposal to a tameness criterion, by suggesting that any consistent moduli space should admit a tame isometric embedding into Euclidean space. This allows us to promote the volume growth constraint to a local condition, and give the growth coefficient a geometric interpretation in terms of complexity. We study the implications of this proposal for the emergence of dualities, as well as for the curvature and infinite distance limits of moduli spaces.

In loop quantum gravity (LQG), states of the gravitational field are represented by labeled graphs called spin networks. Their dynamics can be described by a Hamiltonian constraint, which acts on the spin network states, modifying both spins and graphs. Fixed-graph approximations of the dynamics have been extensively studied, but its full graph-changing action so far remains elusive. The latter, alongside the solutions of its constraint, are arguably the missing features in canonical LQG to access phenomenology in all its richness. Here, we discuss a recently developed numerical tool that, for the first time, implements graph-changing dynamics via the Hamiltonian constraint. We explain how it is used to find new solutions to that constraint and to show that some quantum geometric observables behave differently than in the graph-preserving truncation. We also point out that these new numerical methods can find applications in other domains.

Starting from the realization that the theory of quantum gravity (QG) cannot be deterministic due to its intrinsic quantum nature, the requirement is posed that QG should fulfill a suitable Heisenberg Generalized Uncertainty Principle (GUP) to be expressed as a local relationship determined from first principles and expressed in covariant 4-tensor form. We prove that such a principle places also a physical realizability condition denoted as "quantum covariance criterion", which provides a possible selection rule for physically-admissible spacetimes. Such a requirement is not met by most of current QG theories (e.g., string theory, Geometrodynamics, loop quantum gravity, GUP and minimum-length-theories), which are based on the so-called multiverse representation of space-time in which the variational tensor field coincides with the spacetime metric tensor. However, an alternative is provided by theories characterized by a universe representation, namely in which the variational tensor field differs from the unique "background" metric tensor. It is shown that the latter theories satisfy the said Heisenberg GUP and also fulfill the aforementioned physical realizability condition.

A bstract Leading quantum effects in perturbative quantum gravity are captured by functional determinants of kinetic operators. We study such 1-loop determinants in three-dimensional Euclidean (anti-) de Sitter gravity evaluated using two seemingly disparate tools, the Selberg zeta function and the Wilson spool. For the Euclidean BTZ black hole, we demonstrate the Wilson spool for massive bosons of arbitrary spin directly equates to a representation-theoretic version of the Selberg zeta function. In the case of Euclidean de Sitter, we show a new trace formula associated with the Fredholm determinant for the scalar Laplacian on the three-sphere reproduces the Wilson spool. Generalizing the trace formula, we comment on how to extend this Wilson spool construction to lens space quotients and higher-dimensional spheres.