In the large$-N$ limit, no known no-go theorem rules out thermal time crystals that spontaneously break continuous time-translation, unlike in the large volume limit. If thermal time crystals exist in holographic CFTs, they would correspond to ensemble-dominating black holes with eternally time-varying exterior geometries. We point out that recent work on a conjectured non-linear instability of slowly rotating Kerr-AdS$_4$ produced viable candidates for such states. Then we show that the existence of holographic microcanonical time crystals would imply violations of the AdS Penrose inequality (PI). We proceed to look for violations of the PI in spherical symmetry, working with Einstein-scalar gravity with the most general possible boundary conditions compatible with boundary conformal invariance. We derive a set of ODEs for maximally PI-violating initial data. Solving these numerically, we find strong evidence that in the particular case of spherical symmetry, the PI holds iff the positive mass theorem (PMT) holds. This suggests that holographic CFT$_3$ time crystals can only possibly exist at non-zero angular momentum, at least in the absence of electric charge. We also discover neutral hairy black holes in a consistent truncation of M-theory that has a PMT and boundary conditions respecting conformal invariance, disproving an existing no-hair conjecture. Finally, we show that previous PI-violating solutions by the author all existed in theories where the PMT is violated. Unfortunately, our results imply that there currently are no known examples where the PI functions as a non-trivial Swampland constraint.

We study thermal two-point functions and four-point functions involving two heavy twisted operators and two light probes in symmetric product orbifolds. We identify cases where they are universal at large $N$, that is, they are only sensitive to the orbifold structure. Surprisingly, such observables mimic correlators obtained from the BTZ background, even though symmetric product orbifolds are not dual to semi-classical gravity. We discuss the interpretation of these results in light of the criteria for emergence of spacetime via Von Neumann algebras. Our analysis implies that a condition on the infinite $N$ thermal two-point functions cannot be stringent enough to define an emergent spacetime and the concept of a sharp horizon.

Dimensional reduction of gravity theories to $D=2$ along commuting Killing isometries is well-known to be classically integrable. The resulting system typically features a coset $\sigma$-model coupled to a dilaton and a scale factor of the dimensional reduction. In this article, we construct two families of deformations of dimensionally reduced gravity that preserve the Lax integrable structure. The first family is an extension of the Auxiliary Field Deformation recently introduced by Ferko and Smith, while the second family consists in the embedding of the Yang-Baxter $\sigma$-model into $D=2$ dimensionally reduced gravity. For both deformations we construct flat Lax representations. The Auxiliary Field Deformation, in particular, preserves the rich algebraic structure underlying the undeformed model and, leaving the canonical structure of the Lax connection's spatial components essentially unchanged, allows us to prove its integrability also in the Hamiltonian sense.

In this paper, we investigate the scattering of BPS magnetic monopoles through numerical simulations. We present an ansatz for various multi-monopole configurations suitable for analyzing monopole scattering processes. Our study includes planar scattering scenarios involving two, three, and four monopoles, as well as non-planar processes where three and four monopoles form intermediate tetrahedral and cubic states, respectively. Our observations align with the theoretical predictions of the moduli space approximation. Furthermore, we extend our analysis to relativistic velocities and explore parameters beyond the BPS limit.

Modes with zero longitudinal light-front momentum (zero modes) do have roles to play in the analysis of light-front field theories. These range from improvements in convergence for numerical calculations to implications for the light-front vacuum and beyond to fundamental issues in the connection with equal-time quantization. In particular, the discrepancy in values of the critical coupling for $\phi^4_{1+1}$ theory, between equal-time and light-front quantizations, would appear to be resolvable with the proper treatment of zero modes and near-zero modes. We provide a survey of these issues and point to open questions.

This article is a write-up of pedagogical lectures delivered at the Asia Pacific Center for Theoretical Physics online winter school held in January 2021, and also as part of the Quantum Information, Quantum Field Theory and Gravity program held at ICTS, Bengaluru, in August 2024. The topics covered include a brief derivation of Hawking radiation from the perspective of correlation functions, a description of entropy paradoxes in the eternal black hole, and details of the associated entanglement island computations in two-dimensional models.

In this work, we address the issue regarding the high-power behavior of power-expansion/OPE-expansion in supper-renormalizable theory. Using an $O(N)$-model with $N$-components scalars coupled through quartic interaction at the next-to-leading $\frac{1}{N}$ order in the large-$N$ expansion, we show that the IR subtractions cause addition factorial enhancements for high-power terms in the coefficient functions. Moreover, there are also factorial enhancements for the operator condensates, and the factorial enhancements cancel between coefficient functions and operators only {\it off-diagonally} across different powers. The factorial enhancements can be both alternating and non-alternating. The former are similar to ``UV renormalon'' of coefficient functions and cancel with factorial enhancements of operators at lower powers in diagrams with negative degrees of UV divergences. The later are similar to ``IR renormalon'' and cancel with factorial enhancements of renormalized operators at higher powers in diagrams with positive degrees of UV divergences. The factorial enhancement itself will render the momentum-space power expansion divergent.

In the framework of extended thermodynamics, where the cosmological constant $\Lambda$ plays the role of a dynamical pressure $p$, its conjugate variable $V$ arises naturally. This makes it possible to define $C_p$ and $C_V$, the heat capacities at constant pressure and volume, respectively. We extend our previous work on the heat capacities of the static ``quantum" version of the BTZ black hole defined on a braneworld model to the case where the black hole is rotating. The extra degree of freedom that rotation grants the system imparts it with infinite families of both $C_p$ and $C_V$. We find exact formulae for these heat capacities as functions of the three dimensionless parameters of the theory, and explore some special cases in detail. In all cases considered, at least two physically realizable branches were observed, including both positive and negative heat capacities, signaling both stable and unstable black holes, respectively. Though the critical point seen in the static case disappears, other interesting points arise where the heat capacities diverge. Finally, we discuss the conjectured connection in the literature between the super-entropicity of a black hole and its instability, though much like in the static case, the exact relationship, if any, remains unclear.

In this talk, I showcase models for fuzzy axion dark matter within the framework of type IIB string theory, focusing on axions originating from the Ramond-Ramond four-form in compactifications on Calabi-Yau orientifold hypersurfaces. These models are amenable to cosmological tests if a substantial relic abundance of fuzzy dark matter is produced. I present a topologically exhaustive ensemble of more than 350{,}000 Calabi-Yau compactifications with up to seven axions together with a systematic analysis of the misalignment production of fuzzy dark matter. The resulting dark matter composition is generally a mixture of fuzzy axions and heavier axions, including the QCD axion. Dark photons frequently emerge due to the orientifold projection. I will also comment on applications of optimisation strategies based on automatic differentiation for exploring the string axiverse. This talk is partially based on arXiv:2412.12012.

The notion of Courant algebroid relation is used to introduce a definition of relation between divergence operators on Courant algebroids. By introducing invariant divergence operators, a notion of generalised T-duality between divergences is presented through an existence and uniqueness result for related divergence operators on T-dual pairs of exact Courant algebroids, which naturally incorporates the dilaton shift. When combined with the notion of generalised isometry, this establishes circumstances under which generalised Ricci tensors are related, proving that T-duality is compatible with generalised string background equations. This enables an analysis of the compatibility between T-duality and generalised Ricci flow, showing that the T-dual of a solution of generalised Ricci flow is also a solution of generalised Ricci flow. Our constructions are illustrated through many explicit examples.

We continue our studies of the ghost condensate (GC) with sixth-order dispersion relation. Contrary to the GC with quartic dispersion relation, we find that the correction to the Newtonian potential explicitly depends on the space and time dependence of matter density. At late times when the Newtonian potential becomes time-independent, one obtains similar oscillatory behavior at the distance $\frac{M_\textrm{Pl}}{M^2}$, but this time at the time scale $\frac{M^4}{M_\textrm{Pl}^3}$, where $M^2$ is the ghost field velocity. We also show that the speed of gravitational wave is modified in a frequency dependent manner at momenta close to $\frac{M_\textrm{Pl}}{\sqrt{|\sigma_1|}}$, where $\sigma_1$ is the coefficient of $\gamma^{ij} \nabla_i K_{lr} \nabla_j K^{lr}$ operator in the unitary gauge action.

We discuss minimal covariant quantum space-time ${\cal M}^{1,3}_0$, which is defined through the minimal doubleton representation of $\mathfrak{so}(4,2)$. An elementary definition in terms of generators and relations is given. This space is shown to admit a semi-classical interpretation as quantized twistor space ${\mathbb C} P^{1,2}$, viewed as a quantized $S^2$-bundle over a 3+1-dimensional $k=-1$ FLRW space-time. In particular we find an over-complete set of (quasi-) coherent states, with a large hierarchy between the uncertainty scale and the geometric curvature scale. This provides an interesting background for the IKKT model, leading to a $\mathfrak{hs}$-extended gravitational gauge theory, which is free of ghosts due to the constraints on phase space arising from the doubleton representation.

``The unphysicality of Hilbert spaces'' by Carcassi, Calder\'on and Aidala (arXiv:2308.06669v3) is a thoughtful dissection of the mathematical structure of quantum mechanics that seeks to pinpoint difficulties inherent in postulating that the physical states are elements of a Hilbert space. Its pivotal charge against Hilbert spaces is that by a change of variables, which is a change-of-basis unitary transformation, one ``can map states with finite expectation values to those with infinite ones''. In the present comment it is shown that this statement is incorrect and the source of the error is spotted. In consequence, the purported example of a time evolution that makes ``the expectation value oscillate from finite to infinite in finite time" is also faulty, and the assertion that Hilbert spaces ``turn a potential infinity into an actual infinity'' is unsubstantiated. Two other objections to Hilbert spaces on physical grounds, both technically correct, are the isomorphism of separable Hilbert spaces and the unavoidable existence of infinite-expectation-value states. The former is of little relevance but the latter remains an issue without a fully satisfactory solution, although the evidence so far is that it is physically innocuous. All in all, while the authors' thesis that Hilbert spaces must be given up ought to be taken seriously, it seems insufficiently supported to be convincing.

In fault-tolerant quantum computing with the surface code, non-Clifford gates are crucial for universal computation. However, implementing these gates using methods like magic state distillation and code switching requires significant resources. In this work, we propose a new protocol that combines magic state preparation and code switching to realize logical non-Clifford operations with the potential for fault tolerance. Our approach begins with a special logical state in the $\mathbb{Z}_4$ surface code. By applying a sequence of transformations, the system goes through different topological codes, including the non-Abelian $D_4$ quantum double model. This process ultimately produces a magic state in a condensed $\mathbb{Z}_2$ surface code, which enables the implementation of a logical $T$ gate in the standard $\mathbb{Z}_2$ surface code. In our analysis, we employ a framework where the topological codes are represented by their topological orders and all the transformations are considered as topological manipulations such as gauging symmetries and condensing anyons. This perspective is particularly useful for understanding code switching between topological codes.

The present study focuses on the mesonic potential contributions to the Lagrangian of the extended linear-sigma model (eLSM) for scalar and pseudoscalar meson fields across various quark flavors. The present study focuses on the low-energy phenomenology associated with quantum chromodynamics (QCD), where mesons and their interactions serve as the pertinent degrees of freedom, rather than the fundamental constituents of quarks and gluons. Given that SU(4) configurations are completely based on SU(3) configurations, the possible relationships between meson states in SU(3) and those in SU(4) are explored at finite temperature. Meson states, which are defined by distinct chiral properties, are grouped according to their orbital angular momentum $J$, parity $P$, and charge conjugation $C$. Consequently, this organization yields scalar mesons with quantum numbers $J^{PC}=0^{++}$, pseudoscalar mesons with $J^{PC}=0^{-+}$, vector mesons with $J^{PC}=1^{--}$, and axialvector mesons with $J^{PC}=1^{++}$. We accomplished the derivation of analytical expressions for a total of seventeen noncharmed meson states and twenty-nine charmed meson states so that an analytical comparison of the noncharmed and charmed meson states at different temperatures becomes feasible and the contributions SU(3) and SU(4) configurations can be estimated, analytically.

We propose a framework for simulating the real-time dynamics of quantum field theories (QFTs) using continuous-variable quantum computing (CVQC). Focusing on ($1+1$)-dimensional $\varphi^4$ scalar field theory, the approach employs the Hamiltonian formalism to map the theory onto a spatial lattice, with fields represented as quantum harmonic oscillators. Using measurement-based quantum computing, we implement non-Gaussian operations for CQVC platforms. The study introduces methods for preparing initial states with specific momenta and simulating their evolution under the $\varphi^4$ Hamiltonian. Key quantum objects, such as two-point correlation functions, validate the framework against analytical solutions. Scattering simulations further illustrate how mass and coupling strength influence field dynamics and energy redistribution. Thus, we demonstrate CVQC's scalability for larger lattice systems and its potential for simulating more complex field theories.

In this work, we examine the implications of $q$-deformed theory on anisotropic Bianchi type-I cosmological model within the framework of Verlinde's entropic gravity. The $q$-deformed theory, rooted in quantum group structures, provides a natural generalization of classical particle dynamics and offers a useful framework for exploring deviations in standard cosmological models. By incorporating the principles of entropic gravity, which conceptualize gravity as an emergent entropic force, we derive the dynamical equations governing the Bianchi type-I anisotropic model influenced by $q$-deformation. Finally, we analyze the axisymmetric solution of Bianchi type-I model and these findings highlight the potential impact of $q$-deformation on the dynamics of the early universe and its subsequent evolution.

We renormalize the soft function entering the factorization and resummation of the $qg$ parton-scattering channel of the Drell-Yan process near the kinematic threshold $\hat{s}\to Q^2$ at next-to-leading power in the expansion around $z \equiv Q^2 / \hat{s} = 1$, and solve its renormalization-group equation.

Conformal field theory underlies critical ground states of quantum many-body systems. While conventional conformal field theory is associated with positive central charges, nonunitary conformal field theory with complex-valued central charges has recently been recognized as physically relevant. Here, we demonstrate that complex-valued entanglement entropy characterizes complex conformal field theory and critical phenomena of open quantum many-body systems. This is based on non-Hermitian reduced density matrices constructed from the combination of right and left ground states. Applying the density matrix renormalization group to non-Hermitian systems, we numerically calculate the complex entanglement entropy of the non-Hermitian five-state Potts model, thereby confirming the scaling behavior predicted by complex conformal field theory.

We study QCD at finite temperature and non-zero chemical potential to derive the critical temperature at the chiral phase transition (crossover). We solve a set of Dyson--Schwinger partial differential equations using the exact solution for the Yang--Mills quantum field theory based on elliptical functions. Assuming a Nambu-Jona--Lasino (NJL) model of the quarks, we obtain a very good agreement with recent lattice computations regarding the dependence of the critical temperature on the strong coupling scale. The solution depends on a single scale parameter, as typical for the theory and already known from studies about asymptotic freedom. The analysis is analytically derived directly from QCD.

We extend the previous work about the cosmological solutions with bounce without modifications of gravity or introducing an extra scalar field. The main finding was that the bounce is possible in the initially contracting Universe filled with matter. After a strong contraction, matter gains the equation of state close to the one of radiation, such that the effect on matter on the evolution of the FLRW metric disappears at the classical level. However, this effect comes back owing to the quantum trace anomaly in the matter/radiation sector. In the present contribution, we explore the weak impact of massive fields on the anomaly-driven bounce solution and discuss the role of the vacuum terms. The masses are assumed small and regarded as small perturbations, which enables using trace anomaly even in this case. On the other hand, by adding the $R^2$ term to the action, we arrive at the model with the trans-Planckian bounce and subsequent Starobinsky inflation. In such a framework, using the numerical analysis, we consider three scenarios providing bounce solutions.

The enigmatic phenomenon of dark energy (DE) is regarded as the elusive entity driving the accelerated expansion of our Universe. A plausible candidate for DE is the non-zero Einstein Cosmological Constant $\Lambda_{E}$ manifested as a constant energy density of the vacuum, yet it seemingly defies gravitational effects. In this work, we interpret the non-zero $\Lambda_{E}$ through the lens of scale-invariant cosmology. We revisit the conformal scale factor $\lambda$ and its defining equations within the Scale-Invariant Vacuum (SIV) paradigm. Furthermore, we address the profound problem of the missing mass across galactic and extragalactic scales by deriving an MOND-like relation, $g \sim \sqrt{a_0\,g_N}$, within the SIV context. Remarkably, the values obtained for $\Lambda_{E}$ and the MOND fundamental acceleration, $a_0$, align with observed magnitudes, specifically, $a_0 \approx 10^{-10} \, \mathrm{m} \, \mathrm{s}^{-2}$ and $\Lambda_{E} \approx 1.8 \times 10^{-52} \, \mathrm{m}^{-2}$. Moreover, we propose a novel early dark energy term, $\tilde{T}_{\mu\nu} \sim \kappa H$, within the SIV paradigm, which holds potential relevance for addressing the Hubble tension. Keywords: cosmology; theory; dark energy; dark matter; MOND; Weyl integrable geometry.

Self-gravitating horizonless ultra-compact objects that possess light rings have attracted the attention of physicists and mathematicians in recent years. In the present compact paper we raise the following physically interesting question: Is there a lower bound on the global compactness parameters ${\cal C}\equiv\text{max}_r\{2m(r)/r\}$ of spherically symmetric ultra-compact objects? Using the non-linearly coupled Einstein-matter field equations we explicitly prove that spatially regular ultra-compact objects with monotonically decreasing density functions (or monotonically decreasing radial pressure functions) are characterized by the lower bound ${\cal C}\geq1/3$ on their dimensionless compactness parameters.

The low energy effective theory of gravity comprises two elements of quantum theory joined to classical general relativity. The first is the quantum conformal anomaly, which is responsible for macroscopic correlations on light cones and a stress tensor that can strongly modify the classical geometry at black hole horizons. The second is the formulation of vacuum energy as $\Lambda_{\rm eff}\!\propto\! F^2$ in terms of an exact $4$-form abelian gauge field strength $F\!=\!dA$. When $A$ is identified with the Chern-Simons $3$-form of the Euler class, defined in terms of the spin connection, a $J\cdot A$ interaction is generated by the conformal anomaly of massless fermions. Due to the extreme blueshifting of local frequencies in the near-horizon region of a `black hole,' the lightest fermions of the Standard Model can be treated as massless there, contributing to the anomaly and providing a $3$-current source $J$ for the `Maxwell' equation $d\ast F = \ast J$. In this phase boundary region, torsion is activated, and $F$ can change rapidly. The Schwarzschild black hole horizon is thereby replaced by a surface, with a positive surface tension and $\mathbb{R}\otimes \mathbb{S}^2$ worldtube topology, separating regions of differing vacuum energy. The result is a gravitational vacuum condensate star, a cold, compact, horizonless object with a $p_{_V}\!=\! - \rho_{_V}$ zero entropy, non-singular de Sitter interior and thin quantum phase boundary layer at the Schwarzschild radius $2GM/c^2$.

Sudden violations of the slow-roll regime during inflation, a natural prediction of many UV-complete inflationary models, give rise to sharp features in the primordial power spectrum. At large scales, these features provide a unique window into the physics of inflation, with constraints primarily derived from Cosmic Microwave Background observations of linearly evolved primordial fluctuations. However, on smaller scales, it is less clear whether primordial features would survive the late-time nonlinear cosmological evolution, as they are expected to be washed out by mode coupling. In this paper, we run dedicated N-body simulations to tackle this question. We demonstrate that, while oscillatory-like patterns are erased over time by nonlinearities, signatures of primordial sharp features can persist through the nonlinear regime of structure formation. Those take the form of a localised power enhancement or decrease in the matter power spectrum, whose amplitude and position can in principle be used to recover the scale of the primordial feature, and an oscillatory pattern in the halo mass function. While these findings highlight the power for constraining inflationary physics at small scales, they also show the challenges posed by potential degeneracies with other physical processes relevant in the nonlinear regime of structure formation such as non-cold dark matter candidates. Our results open new avenues for probing inflationary physics in large scale structures and galactic physics and emphasise the need for refined theoretical tools to robustly constrain primordial features.