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The unification of quantum mechanics and general relativity has long been elusive. Only recently have empirical predictions of various possible theories of quantum gravity been put to test. The dawn of multi-messenger high-energy astrophysics has been tremendously beneficial, as it allows us to study particles with much higher energies and travelling much longer distances than possible in terrestrial experiments, but more progress is needed on several fronts. A thorough appraisal of current strategies and experimental frameworks, regarding quantum gravity phenomenology, is provided here. Our aim is twofold: a description of tentative multimessenger explorations, plus a focus on future detection experiments. As the outlook of the network of researchers that formed through the COST Action CA18108 "Quantum gravity phenomenology in the multi-messenger approach (QG-MM)", in this work we give an overview of the desiderata that future theoretical frameworks, observational facilities, and data-sharing policies should satisfy in order to advance the cause of quantum gravity phenomenology., Comment: Submitted to CQG for the Focus Issue on "Quantum Gravity Phenomenology in the Multi-Messenger Era: Challenges and Perspectives". Please contact us to express interesst of endorsement of this white paper

The double-scaled infinite temperature limit of the SYK model has been conjectured by Rahman and Susskind (RS) [1, 2, 3, 4], and independently by Verlinde [5] to be dual to a certain low dimensional de Sitter space. In a recent discussion of this conjecture Narovlansky and Verlinde (NV) [6] came to conclusions which radically differ from those of RS. In particular these conclusions disagree by factors which diverge as $N \to \infty$. Among these is a mismatch between the scaling of boundary entropy and bulk horizon area. In this note, we point out differences in two key assumptions made by RS and NV which lead to these mismatches, and explain why we think the RS assumptions are correct. When the NV assumptions, which we believe are unwarranted, are replaced by those of RS, the conclusions match both RS and the standard relation between entropy and area. In the process of discussing these, we will shed some light on: the various notions of temperature that appear in the duality; the relationship between Hamiltonian energy and bulk mass; and the location of bulk conical defect states in the spectrum of DSSYK$_{\infty}$., Comment: 48 + 24 pages, 3 figures

In the paper by Jo\~ao F. Melo "The propagator matrix reloaded" (SciPost Phys. Core 6, 019 (2023), arXiv:2112.09119) the author provides a derivation of a formalism which includes interactions in the initial conditions for non-equilibrium quantum field theory. The main statement of this paper is that one cannot ignore interactions in the initial conditions, so quantum field theories at finite temperature are not free in the infinite past. In this comment we revise in details calculations from the section 5 of the paper and demonstrate explicitly that the main statement of this paper should be reconsidered., Comment: 4 pages, no figures

Our limited understanding of the physical properties of matter at ultra-high density, high proton/neutron number asymmetry, and low temperature is presently one of the major outstanding problems in physics. As matter in this extreme state is known to only exist stably in the cores of neutron stars (NSs), complementary measurements from electromagnetic and gravitational wave astrophysical observations of NSs, combined with terrestrial laboratory constraints and further theoretical investigations, hold the promise to provide important insight into the properties of matter in a region of the quantum chromodynamics phase space that is otherwise inaccessible. This multidisciplinary endeavor imposes the following requirements for facilities and resources in the upcoming decade and beyond: * A next generation of gravitational wave detectors to uncover more double NS and neutron star-black hole mergers; * Sensitive radio telescopes to find the most massive and fastest spinning NSs; * Large-area, high-time-resolution and/or high angular resolution X-ray telescopes to constrain the NS mass-radius relation; * Suitable laboratory facilities for nuclear physics experiments to constrain the dense matter equation of state; * Funding resources for theoretical studies of matter in this regime; * The availability of modern large-scale high performance computing infrastructure. The same facilities and resources would also enable significant advances in other high-profile fields of inquiry in modern physics such as the nature of dark matter, alternative theories of gravity, nucleon superfluidity and superconductivity, as well as an array of astrophysics, including but not limited to stellar evolution, nucleosynthesis, and primordial black holes., Comment: Submitted to the Proceedings of the US Community Study on the Future of Particle Physics (Snowmass 2021) under Cosmic Frontier (CF07: Cosmic probes of fundamental physics); 30 pages, 8 figures

The baryon asymmetry of the Universe is one of the central motivations to expect physics beyond the Standard Model. In this Snowmass white paper, we review the challenges and opportunities in testing some of the central paradigms that predict physics at scales low enough to expect new experimental data in the next decade. Focusing on theoretical ideas and some of their experimental implications, in particular, we discuss neutron-antineutron transformations, flavor observables, next generation colliders, future neutron facilities, gravitational waves, searches for permanent electric dipole moments, $0\nu \beta \beta $ decay and some future large underground experiments as methods to test post-sphaleron baryogenesis, electroweak baryogenesis, mesogenesis and low scale leptogenesis. Finally, we comment on the cases where high scale physics can be probed through some of these same mechanisms., Comment: White paper to be submitted to the Snowmass Process' Rare Processes and Precision Measurements Frontier, Baryon and Lepton Number Violation Topical Group (This v2 version corrects a missing author and an author's name which was misspelled. Apologies to these authors for this oversight on the part of the submitter!)

Symmetry plays a central role in quantum field theory. Recent developments include symmetries that act on defects and other subsystems, and symmetries that are categorical rather than group-like. These generalized notions of symmetry allow for new kinds of anomalies that constrain dynamics. We review some transformative instances of these novel aspects of symmetry in quantum field theory, and give a broad-brush overview of recent applications., Comment: Contribution to 2022 Snowmass Summer Study. 10 pages + bibliography

We assess the state of naturalness in high-energy physics and summarize recent approaches to the three major naturalness problems: the cosmological constant problem, the electroweak hierarchy problem, and the strong CP problem., Comment: 43 pages, contribution to Snowmass 2021

Higher Spin Gravity refers to extensions of gravity including at least one field of spin greater than two. These extensions are expected to provide manageable models of quantum gravity thanks to the infinite-dimensional (higher spin) gauge symmetry constraining them. One of the key aspects of Higher Spin Gravity/Symmetry is the range and diversity of topics it embraces: (a) higher spin fields play a role in quantum gravity, AdS/CFT, string theory and are expected to have important consequences in cosmology and black hole physics; (b) higher spin symmetry finds applications in Conformal Field Theories, condensed matter systems and dualities therein; (c) these models often rely on tools developed in the study of the mathematical foundations of QFT or in pure mathematics: from deformation quantization and non-commutative geometry to conformal geometry, graded geometry (including BV-BRST quantization), and geometry of PDEs. Recent exciting applications also involve (d) modelling the coalescence of black hole binaries as scattering of massive higher spin particles., Comment: 16 pages + Refs = 37 pages; refs and comments added

This short review is intended as a colloquium-level summary, for the Snowmass 2021 process, on recent theoretical results on infrared observables in quantum gravity. We rely on simple physical arguments, most notably a random walk intuition, to show how effects of quantum gravity in the ultraviolet (at the Planck length $\ell_p \approx 10^{-35} \mbox{ m}$) may integrate into the infrared when the large measurement length scale $L$ enters into the observable. A quantum uncertainty at lightsheet horizons would give rise to an accumulated effect of size $\delta L^2 \simeq \ell_p L/4 \pi$. We discuss how the random walk intuition falls out from more formal calculations, such as from AdS/CFT, from the dimensional reduction of the Einstein-Hilbert action to dilaton gravity, from multiple gravitational shockwaves generated by vacuum energy fluctuations, as well as from an effective description of gravity as a fluid. We overview experimental prospects for measuring this effect with a simple Michelson interferometer utilizing many of the tools developed for gravitational wave observatories., Comment: 16 pages, 3 figures

The double copy is, in essence, a map between scattering amplitudes in a broad variety of familiar field and string theories. In addition to the mathematically rich intrinsic structure, it underlies a multitude of active research directions and has a range of interesting applications in quantum, classical and effective field theories, including broad topics such as string theory, particle physics, astrophysics, and cosmology. This Snowmass white paper provides a brief introduction to the double copy, its applications, current research and future challenges., Comment: 53 pages + refs, 4 figures, contribution to Snowmass 2021; feedback welcome, contact: JJ Carrasco

We review recent developments and outstanding questions regarding connecting the top-down UV complete physical framework of string theory with the observed physics of the Standard Model and beyond the Standard Model physics, emphasizing the global nonperturbative framework of F-theory and general lessons from UV physics. This paper, prepared for the TF01 conveners of the Snowmass 2022 process, provides a brief synopsis of this important area, focusing on ongoing developments and opportunities., Comment: 34 pages (17 pages text +17 pages references); v2: references added; Contribution to Snowmass 2022

High-energy physics is primarily concerned with uncovering the laws and principles that govern nature at the fundamental level. Research in this field usually relies on probing the boundaries of established physics, an undertaking typically associated with extreme energy and distance scales. It is therefore unsurprising that particle physics has traditionally been dominated by large-scale experimental methods often involving high energies, such as colliders and storage rings, cosmological and astrophysical observations, large-volume detector systems, etc. However, high-sensitivity measurements in smaller experiments, often performed at lower energies, are presently experiencing a surge in importance for particle physics for at least two reasons. First, they exploit synergies to adjacent areas of physics with recent advances in experimental techniques and technology. Together with intensified phenomenological explorations, these advances have led to the realization that challenges associated with weak couplings or the expected suppression factors for new physics can be overcome with such methods while maintaining a large degree of experimental control. Second, many of these measurements broaden the range of particle-physics phenomena and observables relative to the above set of more conventional methodologies. Combining such measurements with the conventional efforts above therefore casts both a wider and tighter net for possible effects originating from physics beyond the Standard Model (BSM). This paper argues that this assessment points at a growing impact of such methods and measurements on high-energy physics, and therefore warrants direct support as particle-physics research. Leveraging the recent rapid progress and bright outlook associated with such studies for high-energy physics, could yield high returns, but requires substantial and sustained efforts by funding agencies., Comment: Contribution to Snowmass 2021, 55 pages, 5 figures

The purpose of this White Paper is to review recent progress towards elucidating and evaluating string amplitudes, relating them to quantum field theory amplitudes, applying their predictions to string dualities, exploring their connection with gravitational physics, and deepening our understanding of their mathematical structure. We also present a selection of targets for future research., Comment: 10 pages, contribution to Snowmass 2021, v2: references added

The precision cosmological model describing the origin and expansion history of the universe, with observed structure seeded at the inflationary cosmic horizon, demands completion in the ultraviolet and in the infrared. The dynamics of the cosmic horizon also suggests an associated entropy, again requiring a microphysical theory. Recent years have seen enormous progress in understanding the structure of de Sitter space and inflation in string theory, and of cosmological observables captured by quantum field theory and solvable deformations thereof. The resulting models admit ongoing observational tests through measurements of the cosmic microwave background and large-scale structure, as well as through analyses of theoretical consistency by means of thought experiments. This paper, prepared for the TF01 and TF09 conveners of the Snowmass 2021 process, provides a synopsis of this important area, focusing on ongoing developments and opportunities., Comment: Contribution to Snowmass 2021

This white paper summarizes recent progress in the cosmological bootstrap, an approach to the study of the statistics of primordial fluctuations from consistency with unitarity, locality and symmetry assumptions. We review the key ideas of the bootstrap method, with an eye towards future directions and ambitions of the program. Focusing on recent progress involving de Sitter and quasi-de Sitter backgrounds, we highlight the role of singularities and unitarity in constraining the form of the correlators. We also discuss nonperturbative formulations of the bootstrap, connections to anti-de Sitter space, and potential implications for holography., Comment: 52 pages, 7 figures; Contribution to Snowmass 2021

Gravitational wave detectors are formidable tools to explore strong-field gravity, especially black holes and neutron stars. These compact objects are extraordinarily efficient at producing electromagnetic and gravitational radiation. As such, they are ideal laboratories for fundamental physics and have an immense discovery potential. The detection of black hole binaries by third-generation Earth-based detectors, space-based detectors and pulsar timing arrays will provide exquisite tests of general relativity. Loud "golden" events and extreme mass-ratio inspirals can strengthen the observational evidence for horizons by mapping the exterior spacetime geometry, inform us on possible near-horizon modifications, and perhaps reveal a breakdown of Einstein's gravity. Measurements of the black-hole spin distribution and continuous gravitational-wave searches can turn black holes into efficient detectors of ultralight bosons across ten or more orders of magnitude in mass. A precise monitoring of the phase of inspiralling binaries can constrain the existence of additional propagating fields and characterize the environment in which the binaries live, bounding the local dark matter density and properties. Gravitational waves from compact binaries will probe general relativity and fundamental physics in previously inaccessible regimes, and allow us to address fundamental issues in our current understanding of the cosmos., Comment: Contribution to Snowmass 2021 - Cosmic Frontier (CF07: Cosmic probes of fundamental physics). 24 pages, 4 figures. v2: added list of endorsers and references to related White Papers, minor corrections

During the last two decades many new insights into the dynamics of strongly coupled quantum many-body systems have been obtained using gauge/gravity duality, with black holes often playing a universal role. In this white paper we summarize the results obtained and offer some outlook for future developments, including the ongoing mutually beneficial feedback loop with the study of more general, not necessarily holographic, quantum many-body systems., Comment: 18+24 pages

The S-matrix Bootstrap originated on the idea that the S-matrix might be fully constrained by global symmetries, crossing, unitarity, and analyticity without relying on an underlying dynamical theory that may or may not be a quantum field theory. Recently this approach was revived from a somewhat different point of view. Using the same constraints, one numerically maps out the (infinite-dimensional) space of allowed S-matrices that should contain all consistent quantum field theories (and quantum theories of gravity). Moreover, in the best case scenario one finds special points in the space that can be identified with a certain quantum field theory of interest. In that case, the approach allows the numerical computation of the S-matrix without relying on the particular Lagrangian of the theory. In this white paper we summarize the state of the art and discuss the future of the topic., Comment: 9 pages, v2: added reference

Neutrinos are the Standard Model (SM) particles which we understand the least, often due to how weakly they interact with the other SM particles. Beyond this, very little is known about interactions among the neutrinos, i.e., their self-interactions. The SM predicts neutrino self-interactions at a level beyond any current experimental capabilities, leaving open the possibility for beyond-the-SM interactions across many energy scales. In this white paper, we review the current knowledge of neutrino self-interactions from a vast array of probes, from cosmology, to astrophysics, to the laboratory. We also discuss theoretical motivations for such self-interactions, including neutrino masses and possible connections to dark matter. Looking forward, we discuss the capabilities of searches in the next generation and beyond, highlighting the possibility of future discovery of this beyond-the-SM physics., Comment: Editors: Nikita Blinov, Mauricio Bustamante, Kevin J. Kelly, Yue Zhang. 29 pages, 16 figures, plus references. Contribution to Snowmass 2021

Superconformal field theories (SCFTs) occupy a central role in the study of many aspects of quantum field theory. In this white paper for the Snowmass process we give a brief overview of aspects of SCFTs in $3\leq D \leq 6$ space-time dimensions, including classification efforts and some of the vast current research trends on the physical and mathematical structures generated by this rich class of physical theories., Comment: References updated

The neutrino sector offers one of the most sensitive probes of new physics beyond the Standard Model of Particle Physics. The mechanism of neutrino mass generation is still unknown. The observed suppression of neutrino masses hints at a large scale, conceivably of the order of the scale of a Grand Unified Theory (GUT), a unique feature of neutrinos that is not shared by the charged fermions. The origin of neutrino masses and mixing is part of the outstanding puzzle of fermion masses and mixings, which is not explained in the SM. Flavor model building for both quark and lepton sectors is important in order to gain a better understanding of the origin of the structure of mass hierarchy and flavor mixing, which constitute the dominant fraction of the SM parameters. Recent activities in neutrino flavor model building based on non-Abelian discrete flavor symmetries and modular flavor symmetries have been shown to be a promising direction to explore. The emerging models provide a framework that has a significantly reduced number of undetermined parameters in the flavor sector. Model building based on non-Abelian discrete flavor symmetries and their modular variants enables the particle physics community to interpret the current and anticipated upcoming data from neutrino experiments. Pursuit of flavor model building based on such frameworks can also provide connections to possible UV completions, in particular to string theory. We emphasize the importance of constructing models in which the uncertainties of theoretical predictions are smaller than, or at most compatible with, the error bars of measurements in neutrino experiments., Comment: 24 pages, 7 figures, Contribution to Snowmass 2021

We review recent progress and future prospects for harnessing powerful tools from theoretical high-energy physics, such as scattering amplitudes and effective field theory, to develop a precise and systematically improvable framework for calculating gravitational-wave signals from binary systems composed of black holes and/or neutron stars. This effort aims to provide state-of-the-art predictions that will enable high-precision measurements at future gravitational-wave detectors. In turn, applying the tools of quantum field theory in this new arena will uncover theoretical structures that can transform our understanding of basic phenomena and lead to new tools that will further the cycle of innovation. While still in a nascent stage, this research direction has already derived new analytic results in general relativity, and promises to advance the development of highly accurate waveform models for ever more sensitive detectors., Comment: contribution to Snowmass 2021; 28+28 pages, 5 figures

In particle collisions at energies higher than the mass scale involved, the collinear splitting becomes the dominant phenomena. Suitable descriptions of the physics in this regime include the parton distribution functions, the initial state radiations, the final state radiations and the fragmentation functions. It is of fundamental importance to formulate those functions with consistent theoretical treatments, and to provide the adequate formalism for applications, as motivated by the recent discussions of partonic scatterings at the multi-TeV energy regime. In this report, we briefly present the resummation of the final-state logarithms as fragmentation functions for the electroweak processes. As an explicit example, we study an electroweak gauge boson splitting in the process $pp\to WZj$ and demonstrate the important effects from the fragmentation., Comment: 8 pages, 4 figures; Contribution to Snowmass 2021

We review recent progress and a number of future directions for applications of effective field theory methods to condensed matter systems broadly defined. Our emphasis is on areas that have allowed a fertile exchange of ideas between high energy physics and many-body theory. We discuss developments in the effective field theory of spontaneous symmetry breaking, of hydrodynamics and non-equilibrium dynamics more generally, fracton phases of matter, and dualities between 2+1 dimensional field theories. We furthermore discuss the application of effective field theory to non-Fermi liquids, the dynamics of entanglement entropy and to condensed matter aspects of cosmology., Comment: Contribution to Snowmass 2021; 13 + 20 pages; 1 figure

Small fluctuations around homogeneous and isotropic expanding backgrounds are the main object of study in cosmology. Their origin and evolution is sensitive to the physical processes that happen during inflation and in the late Universe. As such, they hold the key to answering many of the major open questions in cosmology. Given a large separation of relevant scales in many examples of interest, the most natural description of these fluctuations is formulated in terms of effective field theories. This was the main avenue for many of the important modern developments in theoretical cosmology, which provided a unifying framework for a plethora of cosmological models and made a clear connection between the fundamental cosmological parameters and observables. In this review we summarize these results in the context of effective field theories of inflation, large-scale structure, and dark energy., Comment: Contribution to Snowmass 2021

The next generation of gravitational-wave observatories can explore a wide range of fundamental physics phenomena throughout the history of the universe. These phenomena include access to the universe's binary black hole population throughout cosmic time, to the universe's expansion history independent of the cosmic distance ladders, to stochastic gravitational-waves from early-universe phase transitions, to warped space-time in the strong-field and high-velocity limit, to the equation of state of nuclear matter at neutron star and post-merger densities, and to dark matter candidates through their interaction in extreme astrophysical environments or their interaction with the detector itself. We present the gravitational-wave detector concepts than can drive the future of gravitational-wave astrophysics. We summarize the status of the necessary technology, and the research needed to be able to build these observatories in the 2030s., Comment: 31 pages, 5 figures, contribution to Snowmass 2021

We give a brief overview of the status of the numerical conformal bootstrap., Comment: Contribution to Snowmass 2021, 23 pages; v2: references added

This article provides a review of the literature on rigorous definitions and constructions in Quantum Field Theory, spanning the period of seven decades. Comparing with the ideas and constructions found in the modern physics literature, we conclude that none of the existing systems of QFT axioms can cover all the physical situations. Therefore, it is still an outstanding open problem to formulate a complete definition of QFT. We argue that the question is of relevance for both physicists and mathematicians., Comment: 12 pages + references; Please send your comments! v2: added more references, improved and expanded several statements, improved the discussion of CFT

Quantum gravity places important consistency conditions on low-energy effective field theory, such as the absence of global symmetries. These may have important consequences in the search for particle physics beyond the Standard Model. We review some of these conditions and their phenomenological implications for the strong CP problem, the weak scale, new gauge interactions, and cosmology. We also offer some general comments on how these ideas can guide model building., Comment: Contribution to Snowmass 2021; 11 pages + references

We present a summary of recent progress and remaining challenges in applying the methods and ideas of quantum information theory to the study of quantum field theory and quantum gravity. Important topics and themes include: entanglement entropy in QFTs and what it reveals about RG flows, symmetries, and phases; scrambling, information spreading, and chaos; state preparation and complexity; classical and quantum simulation of QFTs; and the role of information in holographic dualities. We also highlight the ways in which quantum information science benefits from the synergy between the fields., Comment: 24 + 21 pages; contribution to Snowmass 2021; v2: references added

Fundamental principles of local quantum field theory or of quantum gravity can enforce consistency requirements on the space of consistent low-energy effective field theories. We survey the various techniques that have been used to put UV constraints on IR physics, including those from causality considerations in the form of S-matrix positivity and bootstrap bounds, scattering time delays, conformal field theory and holographic methods, together with those that arise from landscape/swampland criteria such as the weak gravity conjecture. We review recent applications of these constraints to corrections to Standard Model physics, corrections to Einstein gravity, and cosmological theories and highlight promising future directions., Comment: Contribution to Snowmass 2021; 19 pages + references

The black-hole information paradox provides a stringent test of would-be theories of quantum gravity. String theory has made significant progress toward a resolution of this paradox, and has led to the fuzzball and microstate geometry programs. The central thesis of these programs is that only string theory has sufficiently many degrees of freedom to resolve black-hole microstructure, and that horizons and singularities are artifacts of attempting to describe gravity using a theory that has too few degrees of freedom to resolve the physics. Fuzzballs and microstate geometries recast black holes within string theory as horizonless and singularity-free objects that not only resolve the paradox but provide new insight into the underlying microstructure. We give an overview of this approach, summarize its current status and describe future prospects and insights that are now within reach., Comment: 11 pages + References

The analytic conformal bootstrap is an array of techniques to characterize, constrain, and solve strongly interacting quantum field theories using symmetries, causality, unitarity, and other general principles. In the last decade, bolstered by the development of new Lorentzian methods, it has been used to solve conformal field theories at large spin; to place bounds on energy distributions, event shapes, operator product coefficients, and other observables; and to understand aspects of quantum gravity in anti-de Sitter space. We review these advances and highlight several promising areas for future exploration. Targets include developing new methods to close the gap between numerical and analytic bounds, extending the bootstrap beyond conformal fixed points, applications to quantum gravity and cosmology, and building on ties to condensed matter theory and mathematics., Comment: Contribution to Snowmass 2022; 15 pages + references

We discuss progress and prospects in the application of bootstrap methods to string theory., Comment: 16 pages + refs

Black holes provide a window into the microscopic structure of spacetime in quantum gravity. Recently the quantum information contained in Hawking radiation has been calculated, verifying a key aspect of the consistency of black hole evaporation with quantum mechanical unitarity. This calculation relied crucially on recent progress in understanding the emergence of bulk spacetime from a boundary holographic description. Spacetime wormholes have played an important role in understanding the underpinnings of this result, and the precision study of such wormholes, in this and other contexts, has been enabled by the development of low-dimensional models of holography. In this white paper we review these developments and describe some of the deep open questions in this subject. These include the nature of the black hole interior, potential applications to quantum cosmology, the gravitational explanation of the fine structure of black holes, and the development of further connections to quantum information and laboratory quantum simulation., Comment: 16 + 17 pages. v2: references added

We present a brief overview of Moonshine with an emphasis on connections to physics. Moonshine collectively refers to a set of phenomena connecting group theory, analytic number theory, and vertex operator algebras or conformal field theories. Modern incarnations of Moonshine arise in various BPS observables in string theory and, via dualities, invariants in enumerative geometry. We survey old and new developments, and highlight some of the many open questions that remain., Comment: 8 pages + references; v2: references added

Strongly-coupled Quantum Field Theories (QFTs) are ubiquitous in high energy physics and many-body physics, yet our ability to do precise computations in such systems remains limited. Hamiltonian Truncation is a method for doing nonperturbative computations of real-time evolution in strongly coupled QFT in the continuum limit, and works by numerically solving the Schrodinger equation in a truncated subspace of the full Hilbert space. Recent advances in understanding this method have opened the door to progress in a range of applications, from gauge theories in $d\ge 2$ dimensions to relativistic nonequilibrium physics., Comment: 9+4 pages, 5 figures

It has been shown that entropy differences between certain states of perturbative quantum gravity can be computed without specifying an ultraviolet completion. This is analogous to the situation in classical statistical mechanics, where entropy differences are defined but absolute entropy is not. Unlike in classical statistical mechanics, however, the entropy differences computed in perturbative quantum gravity do not have a clear physical interpretation. Here we construct a family of perturbative black hole states for which the entropy difference can be interpreted as a relative counting of states. Conceptually, this paper begins with the algebra of mass fluctuations around a fixed black hole background, and points out that while this is a type I algebra, it is not a factor and therefore has no canonical definition of entropy. As in previous work, coupling the mass fluctuations to quantum matter embeds the mass algebra within a type II factor, in which entropy differences (but not absolute entropies) are well defined. It is then shown that for microcanonical wavefunctions of mass fluctuation, the type II entropy difference equals the logarithm of the dimension of the extra Hilbert space that is needed to map one microcanonical window to another using gauge-invariant unitaries. The paper closes with comments on type II entropy difference in a more general class of states, where the von Neumann entropy difference does not have a physical interpretation, but "one-shot" entropy differences do., Comment: 5 pages

Ever since its discovery by Roy Kerr in 1963, the geometry around rotating, electrostatically-neutral black holes, otherwise known as Kerr black holes, has significantly contributed to theoretical developments in the fields of general relativity, thermodynamics and beyond. Extremal Kerr black holes, in which the spin parameter of the rotating black hole is at a maximum without breaching certain postulates, has especially been of interest to the research community due to its sensitivity to higher order corrections to Einstein's theory of relativity. In this paper, we review some unresolved open questions regarding the discontinuity of certain properties of the Kerr black hole as it approaches the extreme limit. These include the form of the innermost stable circular orbit (ISCO), the nature of vanishing entropy, and the disappearance of the trapped surface as a Kerr black hole approaches extremality. We conclude that current theories are likely not suitable for studies of extremal black holes, and we posit that further development in theory may likely require a quantum approach to gravity. G=c=1 and Einstein summation convention are assumed throughout this paper., Comment: 2 figures, 9 pages. Literature review

This paper investigates Buchdahl transformations within the framework of Einstein and Einstein-Scalar theories. Specifically, we establish that the recently proposed Schwarzschild-Levi-Civita spacetime can be obtained by means of a Buchdahl transformation of the Schwarschild metric along the spacelike Killing vector. The study extends Buchdahl's original theorem by combining it with the Kerr-Schild representation. In doing so, we construct new vacuum-rotating black holes in higher dimensions which can be viewed as the Levi-Civita extensions of the Myers-Perry geometries. Furthermore, it demonstrates that the double copy scheme within these new generated geometries still holds, providing an example of an algebraically general double copy framework. In the context of the Einstein-Scalar system, the paper extends the corresponding Buchdahl theorem to scenarios where a static vacuum seed configuration, transformed with respect to a spacelike Killing vector, generates a hairy black hole spacetime. We analyze the geometrical features of these spacetimes and investigate how a change of frame, via conformal transformations, leads to a new family of black hole spacetimes within the Einstein-Conformal-Scalar system.

This paper combines the post-Minkowskian expansion of general relativity with the language of intersection theory. Due to the nature of the soft limit inherent to the post-Minkowskian expansion, the intersection-based approach is of enhanced utility in that theory compared to a generic QFT. In the language of intersection theory, Feynman integrals are rephrased in terms of twisted cocycles. The intersection number is a pairing between two such cocycles and its existence allows for the direct projection onto a basis of master integrals. In this paper we use this approach to compute the 2PM contribution to the scattering of two compact astronomical objects, getting results in agreement with previous findings., Comment: Ten pages, two figures

In a recent paper (Heras in $\href{https://doi.org/10.1140/epjp/s13360-023-03914-5}{{\rm Eur. Phys. J. Plus\, 138: 329, 2023}}$), we have demonstrated that when a dyon encircles an infinitely long solenoid enclosing electric and magnetic fluxes, its wave function accumulates a quantum phase invariant under electromagnetic duality transformations. In this paper, we show that this phase, in conjunction with the Witten effect, gives rise to a topological phase proportional to the vacuum angle $\theta$ and thereby connected with CP violation. We show that this phase becomes quantised in a vacuum state $\delta_{\theta}=n\theta$ and that the most general vacuum state associated with this quantisation identifies with an Abelian form of the $\theta$-vacua. We discuss two hypothetical interference effects in the vacuum where the angle $\theta$ could manifest., Comment: Published in The European Physical Journal Plus. See the related preceding paper, arXiv:1910.01117

Entanglement entropy, taken here to be geometric, requires a geometrically separable Hilbert space. In lattice gauge theories, it is not immediately clear if the physical Hilbert space is geometrically separable. In a previous paper we have shown that the physical Hilbert space in pure gauge abelian lattice theories exhibits some form of geometric scaling with the lattice volume, which suggest that the space is locally factorizable and, therefore, geometrically separable. In this paper, we provide strong evidence that indicates that this scaling is not present when the group is non-abelian. We do so by looking at the scaling of the dimension of the physical Hilbert space of theories with certain discrete groups. The lack of an appropriate scaling implies that the physical Hilbert space of such a theory does not admit a local factorization. We then extend the reasoning, as sensibly possible, to SU(2) and SU(N) to reach the same conclusion. Lastly, we show that the addition of matter fields to non-abelian lattice gauge theories makes the resulting physical Hilbert space locally factorizable.

Our recent works revisit the proof of chiral symmetry breaking in the confining phase of four-dimensional QCD-like theories, i.e. $SU(N_c)$ gauge theories with $N_f$ flavors of vectorlike quarks in the fundamental representation. The analysis relies on the structure of 't Hooft anomaly matching and persistent mass conditions for theories with same $N_c$ and different $N_f$. In this paper, we work out concrete examples with $N_c=3$ and $N_c=5$ to support and elucidate the results in the companion papers. Within the same examples, we also test some claims made in earlier works., Comment: 53 pages, many equations, companion paper of arXiv:hep-th/2212.02930 and another paper that appears simultaneously

The island paradigm asserts that after the Page time the operators in the interior of an AdS2 eternal black hole in equilibrium with a finite temperature non-gravitating bath can not be reconstructed using the operators in the black hole region outside the horizon. In a recent paper, we demonstrated this using the black hole interior reconstruction proposal due to Leutheusser and Liu, based on the half-sided translations. This was done by introducing a notion of the reduced half-sided translations associated with the algebra of operators restricted to the black hole region outside the horizon, and by showing that albeit the reduced half-sided translations translate operators in the black hole region outside the horizon to the black hole interior before the Page time, it fails to do so after the Page time. In this paper, we demonstrate the second assertion of the island paradigm, which states that after the Page time the operators in the black hole interior can be reconstructed using the operators in the bath. We show that even though before the Page time the reduced half-sided translations associated with the algebra of operators restricted to the bath do not translate operators in the bath to the black hole interior, after the Page time they take them to the black hole interior., Comment: 24 pages, 9 figures. arXiv admin note: text overlap with arXiv:2312.11085

In this paper, we show a local generalized second law (the generalized entropy is nondecreasing) in crossed product constructions for maximally extended static and Kerr black holes using modular theory. The new ingredient is the use of results from a recent paper discussing the entropy of the algebra of operators in subregions of arbitrary spacetimes. These results rely on an assumption which we show is true in our setting. However, we do assume as in that paper, that the gravitational constraints are implemented on each partial Cauchy slice. In this version, we have also added a discussion on inclusion of an observer in the calculation. In the last part of the paper, we look at modular Hamiltonians of deformed half-spaces in a class of static spacetimes, including the Schwarzschild spacetime. These are computed using path integrals, and we primarily compute them to investigate whether these non-local modular Hamiltonians can be made local by subtracting off pieces from the algebra and its commutant, as has been surmised in the literature. Along the way, the averaged null energy condition (ANEC) also follows in this class of spacetimes., Comment: 45 pages, typos fixed, Discussion enlarged to include an observer in the calculation

In this paper, we consider the effect of interactions on the local, average polarization of a quantum plasma of massless fermion particles characterized by vector, axial, and helical quantum numbers. Due to the helical and axial vortical effects, perturbations in the vector charge in a rotating plasma can lead to chiral and helical charge transfer along the direction of the vorticity vector. At the same time, interactions between the plasma constituents lead to the dissipation of the helical charge through helicity-violating pair annihilation (HVPA) processes and of the axial charge through the axial anomaly. We will discuss separately a QED-like plasma, in which we ignore background electromagnetic fields and thus the axial charge is roughly conserved, as well as a QCD-like plasma, where instanton effects lead to the violation of the axial charge conservation, even in the absence of background chromomagnetic fields. The non-conservation of helicity and chirality leads to a gapping of the Helical, Axial and mixed, Axial-Helical vortical waves that prevents their infrared modes from propagating. On the other hand, usual dissipative effects, such as charge diffusion, lead to significant damping of ultraviolet modes. We end this paper with a discussion of the regimes where these vortical waves may propagate., Comment: 56 pages, 14 figures. Part 2

This paper analyzes the effective field theory perspective on modern physics through the lens of the quantum theory of gravitational interaction. The historical part argues that the search for a theory of quantum gravity stimulated the change in outlook that characterizes the modern approach to the Standard Model of particle physics and General Relativity. We present some landmarks covering a long period, i.e., from the beginning of the 1930s until 1994, when, according to Steven Weinberg, the modern bottom-up approach to General Relativity began. Starting from the first attempt to apply the quantum field theory techniques to perturbatively quantize Einstein's theory, we explore its developments and interaction with the top-down approach encoded by String Theory. In the last part of the paper, we focus on this last approach to describe the relationship between our modern understanding of String Theory and Effective Field Theory in today's panorama. To this end, the non-historical part briefly explains the modern concepts of moduli stabilization and Swampland to understand another change in focus that explains the present framework where some string theorists move., Comment: Accepted for publication by European Physics Journal H for the Special Issue: History for Physics: Contextualizing Modern Developments in the Foundations of Quantum Theory

We show that the low-energy effective actions of two ten-dimensional supersymmetric heterotic strings are different by a $\mathbb{Z}_3$-valued discrete topological term even after we turn off the $E_8\times E_8$ and $Spin(32)/\mathbb{Z}_2$ gauge fields. This will be demonstrated by considering the inflow of normal bundle anomaly to the respective NS5-branes from the bulk. We also find that the $Spin(16)\times Spin(16)$ non-tachyonic non-supersymmetric heterotic string has the same non-zero $\mathbb{Z}_3$-valued discrete topological term. We will also explain the relation of our findings to the theory of topological modular forms. The paper is written as a string theory paper, except for an appendix translating the content in mathematical terms. We will explain there that our finding identifies a representative of the $\mathbb{Z}/3$-torsion element of $\pi_{-32}\mathrm{TMF}$ as a particular self-dual vertex operator superalgebra of $c=16$ and how we utilize string duality to arrive at this statement., Comment: 20 pages + an appendix, 2 figures

This research paper presents a black hole solution with a rational non-linear electrodynamics source in the Rastall gravity framework. The paper analyzes the thermodynamic properties of the solution in normal phase space and explores its critical behavior. The phase structure is examined using the extended first law of thermodynamics, with the cosmological constant $\Lambda$ serving as pressure $P$. The isotherms exhibit van der Waals behavior at small values of horizon $r_+$. The paper also investigates the Gibbs free energy behavior and finds two critical points with two pressures where the reentrant phase transition occurs and disappears. On the other hand, we explore the prevalent microstructure of black holes in Ruppeiner geometry, uncovering significant deviations in the nature of particle interactions from conventional practice. Moreover, the thermodynamic geometry is analyzed using the Ruppeiner formalism, with the normalized Ricci scalar indicating possible point-phase transitions of the heat capacity, and the normalized extrinsic curvature having the same sign as the normalized Ricci scalar. The three-phase transitions of the heat capacity are those that we find for the normalized Ruppeiner curvatures. Thus, there is an absolute correspondence., Comment: 23 pages, 7 figures