Papers

Localization effects are central to disordered electronics and photonics. In electronics, Anderson localization governs electron confinement in randomly perturbed lattices. Similarly, its photonic counterpart inhibits light transport via disorder – but with a unique exception: Brewster-anomaly delocalization, where the Brewster effect prevents multiple-scattering interference and counteracts the localization. Despite extensive research in electronics and photonics separately, the intricate role of localization effects in free-electron–light interactions – vital for lasers, accelerators, microscopy and spectroscopy, and quantum information – remains largely unexplored. At the same time, localization effects are widely regarded as a key factor limiting the efficient coupling between free electrons and light in random media. Here we overcome this key limitation via the unconventional interplay between Brewster-anomaly delocalization and free-electron radiation. In this way, free-electron radiation can be localization-free, intense and directional even in strongly disordered, unengineered multilayers. Essentially, this delocalization-mediated free-electron radiation is remarkably invariant not only to the random-medium configuration, but also to the light frequency and the electron velocity. Our findings unlock new opportunities for particle detectors and achromatic light sources operating in easy-to-fabricate complex media at previously inaccessible frequencies.

We investigate the hydrodynamics of expanding bubbles in first-order phase transitions with non-conformal thermodynamics. We analyze a broad class of equations of state interpolating between bag-model descriptions, commonly used for electroweak transitions, and QCD-like theories. As a concrete benchmark, we determine the bubble solutions for pure SU(3) Yang-Mills theory. We uncover a new set of hydrodynamic obstructions to bubble expansion. These obstructions arise both at the bubble wall and along the fluid flow, and can partially or completely eliminate otherwise allowed solutions. We also identify a new class of solutions, which we dub "shocked detonations", consisting of ordinary detonations with an additional shock inserted in the rarefaction wave. As a consequence of these obstructions, the space of admissible bubble wall velocities is significantly constrained, with gaps appearing between different expansion regimes. For example, for QCD-like theories, all detonation solutions, including shocked detonations, are excluded. We show that these effects can strongly impact the kinetic energy budget of the fluid and, therefore, the resulting gravitational-wave signal, potentially suppressing the most efficient configurations. Our results highlight the importance of non-conformal dynamics for accurately modeling phase transitions and the resulting gravitational-wave spectrum. The code used to construct the bubble solutions is publicly available. For completeness, we also show how the interpolation between the bag-model and QCD-like limits can be realized holographically by varying the backreaction of matter fields on the geometry.

The universal I-Love-Q relations for compact stars have significant applications in gravitational-wave astronomy, but thermal effects can break these relations in low-mass white dwarfs. In this work, we employ the stellar evolution code MESA to construct realistic models of $0.15 \, M_{\odot}$ helium-core and $0.6 \, M_{\odot}$ carbon-oxygen core white dwarfs at various temperatures. By utilizing the Clairaut-Radau equation, we quantitatively extract the radial variation of the eccentricity of internal isodensity surfaces. Our numerical results demonstrate that higher central temperatures amplify the eccentricity variation, causing the I-Love relations to deviate from the zero-temperature Chandrasekhar model, whereas subsequent cooling restores them. This confirms that the temperature-induced violation of the universal relations is fundamentally driven by the loss of self-similarity in isodensity surfaces, providing key insights into the applicability conditions of I-Love-Q relations in compact objects.

We study the degenerate limits of the scalar-tensor theory of gravity in its Hamiltonian connection dynamics formulation. The degenerate limit of the spatial triad is effectuated by a local scaling transformation of the triad, which leads to the degenerate limits of all other geometric quantities and relations in the connection dynamics. We derive the constraints for the degenerate scalar-tensor theory of gravity and discuss their physical implications.

In this work, we analyze the differences and similarities between two recent constructions, which are distinct in their methodologies for extending the Wald entropy of stationary black holes to non-stationary situations in general higher-derivative gravity. One of them, denoted by $S_\text{Wall}$, is constructed by exploiting the boost symmetry of the near-horizon geometry, whereas the other, denoted by $S_\text{dyn}$, is obtained from a covariant phase-space analysis based on the Wald-Iyer Noether charge formalism. While $S_\text{dyn}$ is, by construction, defined only for linearized fluctuations around a stationary black hole solution, $S_\text{Wall}$ does not require such a linearization for its construction. Although the linearization is necessary to interpret $S_\text{Wall}$ as a well-defined notion of entropy, the construction itself naturally contains terms that are higher order in the dynamical fluctuations. By comparing the technical structures underlying the two constructions, we clarify the fundamental differences between the methods on which they are based. We demonstrate that while the construction of $S_\text{dyn}$ given the $S_\text{Wall}$ is straightforward, the converse is more subtle. We develop an algorithm to obtain a local expression for $S_\text{Wall}$ from a known expression for $S_\text{dyn}$ in a generic diffeomorphism-invariant theory of gravity, provided certain technical conditions are satisfied. We justify our analytical findings with explicit demonstrations in a particular case: the Riemann-squared example of the higher-derivative theory of gravity.

We investigate the Casimir effect in rainbow space-time, focusing on leading-order corrections to the Casimir energy and force. Starting with the scalar field Lagrangian in rainbow space-time, with parallel plates introduced through delta-function potentials, we find the corresponding energy-momentum tensor. We obtain the vacuum expectation value of this energy-momentum tensor by expressing it as a quadratic operator acting on the Green's function. By solving the Euler-Lagrange equation of a scalar field in rainbow space-time, we obtain the Green's function solutions. Employing these Green's function solutions in the vacuum expectation value of the energy-momentum tensor, we obtain the modified Casimir energy and Casimir force expressions in rainbow space-time. We study the variation of the deformed Casimir force and energy with the distance between the plates for different choices of rainbow functions. Our results show that for two choices of rainbow functions, the absolute value of the Casimir energy and force is decreasing or increasing, whereas for one specific choice of rainbow functions, it remains the same as the standard result in Minkowski space-time. Comparing our result with experimentally measured value of Casimir force, we obtain the bound on the rainbow parameter dependent terms to be of the order of 10^-24.

We develop a formalism for studying Floquet dynamics for systems with non-relativistic conformal invariance in d spatial dimensions. Our analysis indicates the existence of two dynamical phases, hyperbolic and elliptic, separated by a parabolic transition surface. We demonstrate this by studying the fidelity of the driven state and the expectation value of a conformal generator in the many body ground state during the drive. Stroboscopically, they behave exponentially in the hyperbolic phase, show oscillatory behavior in the elliptic phase, and exhibit power-law on the transition surface. Our analysis is completely universal and can be directly applied to several systems including trapped fermions near unitarity and resonant anyons. The former can provide experimental signatures of these dynamical phases. We also comment on a holographic perspective of such driven non-relativistic CFTs and demonstrate that the hyperbolic phase is associated with a timelike stationary-limit surface, such as an ergosphere, in the bulk, while the parabolic phase corresponds to an extremal Killing horizon.

We investigate the magnetic dipole moments of doubly-bottom pentaquark states with spin-parities $J^P=\tfrac{1}{2}^-$ and $\tfrac{3}{2}^-$, interpreted as hadronic molecules in the $B\Sigma_b$, $B\Sigma_b^{*}$, and $B^{*}\Sigma_b$ configurations. The analysis is performed within the framework of QCD light-cone sum rules employing photon distribution amplitudes. Our results demonstrate a strong sensitivity of the magnetic dipole moments to the internal spin structure and quark composition of the states. In particular, the light-quark sector provides the dominant contribution in the $B\Sigma_b$ configuration, while the magnetic moment of $B\Sigma_b^{*}$ is largely governed by the heavy bottom quark. For the $B^{*}\Sigma_b$ molecular state, both light and heavy sectors contribute constructively, leading to a significantly enhanced magnetic dipole moment. These findings indicate that magnetic dipole moments constitute a sensitive probe of the internal structure of molecular-type doubly-bottom pentaquarks and provide testable predictions for future experimental studies.

In this paper, we calculate the tidal Love numbers of multi-state boson stars (MSBSs) composed of ground state and first excited state complex scalar fields. Under synchronized and nonsynchronized frequency conditions, the background solutions of MSBSs are classified into single-branch and double-branch types. The field functions, ADM mass, and binding energy of different solutions are discussed. We then calculate the quadrupolar ($\ell=2$) electric and magnetic tidal Love numbers for branches containing stable solutions. Our results show that the electric tidal Love numbers are initially positive and then suddenly transition to negative values. This phenomenon occurs when the parameters satisfy $\tilde{\mu}_1 > 0.891$ or $\tilde{\omega}_0 > 0.777$; for smaller values of these parameters, the electric Love numbers remain positive. The magnetic tidal Love numbers are always negative, with absolute values smaller than those of the electric tidal Love numbers.

We study the non-relativistic limit of Gaussian covariant hydrodynamics [1]. We argue that the condition of incompressibility provides additional symmetries matching relativistic hydrodynamics but incompressibility must break down at a “microscopic“ scale. We then develop the renormalization group equations for average and fluctuations w.r.t. that scale, to understand its effect on flows at intermolecular distances where hydrodynamics gives way to statistical mechanics. The resulting dynamics naturally incorporates spontaneous stochasticity as a macroscopic back reaction of statistical mechanics fluctuations, as well as features reminiscent of anomalous dissipation and “wild solutions“ as renormalization group counterterms. We frame these considerations into both a phenomenological discussion of the limits of applicability of fluid dynamics, and a discussion of where physics might shed some light on the mathematical issues associated with turbulence.

The Newtonian $N$-body problem in the zero-energy, zero-linear-momentum, and zero-angular-momentum sector provides a time-reversal-invariant setting in which generic complete solutions possess a Janus point and exhibit a gravitational arrow on both branches away from it. The dimensionless variety $V$, a global scale-invariant measure of clustering contrast, is not pointwise monotonic but fluctuates while growing between rising bounds away from the Janus region. Central configurations are critical shapes of the same scale-invariant landscape and therefore provide controlled probes of the structural information encoded by $V$. We investigate this question for two planar $N=5000$ central configurations using the numerical particle-coordinate data. Local morphology is quantified by the six-neighbour anisotropy $A_6$, which ranges from $0$ for an isotropic local environment to $1$ for an effectively one-dimensional one. Although the varieties of the two configurations differ by only $1.686\%$, their mean anisotropies differ by $156.8\%$, from $0.1331$ to $0.3419$. Moreover, $18.1\%$ of the particles in the higher-variety configuration satisfy $A_6>0.5$, whereas none do so in the lower-variety configuration. The anisotropy ordering persists for all tested neighbourhood sizes $4\le k\le12$, and the principal contrast survives a dimensionless close-pair robustness test. For this pair of critical shapes, nearby values of the global variety therefore coexist with markedly different local geometrical organization. Thus the scalar quantity whose long-term behavior characterizes the BKM gravitational arrow does not, by itself, uniquely specify morphology. This identifies local relational observables as a complementary level of description and provides a quantitative bridge between the static shape-space landscape and morphology along genuine Janus-point histories.

We construct the charge-generated intrinsic/canonical frame dictionary associated with the Kerr-selected soft dressing. Starting from the VV supertranslation, we formulate the higher-spin problem as an inverse problem at null infinity: the soft kernel ${\mathcal K}^{(s,0)}_{AB}[t]$, built from the parity-adapted maximally longitudinal scalar $\chi^{(s)}_t$, is matched to the Kerr-selected exponentiating projection of the universal soft contribution, thereby determining one parity component of the generator $t^{A_1\cdots A_s}$. Helicity conjugation fixes which one: the exponentiating source obeys $\overline{S^{(s)}_{+,{\rm exp}}}=(-1)^sS^{(s)}_{-,{\rm exp}}$, so the tower fixes the electric projection of the source at even levels and the magnetic projection at odd ones, matching the alternation of the Kerr mass and current moments; for aligned spin the projection is exhaustive and we solve the tower in closed form. The prescription reproduces the VV supertranslation at leading order and fixes the curl, not the divergence, of a smooth generalized-BMS vector at subleading order. The reason for the matching is physical: the same exponentiating soft factor is the classical limit of the Guevara–Ochirov–Vines spinning three-point operator and generates the Kerr multipole tower. We explain the corresponding hard flux charges and show how their external-state action gives the Ward representation of the soft theorem. The polynomial Poisson algebra on $T^\ast S^2$, with local $w_{1+\infty}$-type reductions, then acts on these frame-changing generators; it does not close on the Kerr-selected data alone. This gives the physical role of the $w_{1+\infty}$-like structure in Kerr black-hole scattering: it moves the intrinsic/canonical dictionary. Its observable imprint begins with displacement memory at $s=0$ and spin memory at $s=1$, followed by higher electric and magnetic memory moments.

According to Hohenberg's theorem, Bose-Einstein condensation (BEC) in two dimensions is impossible for any temperature $T>0$. By contrast, superfluidity does occur in two dimensions at finite temperatures; it emerges due to the breaking of Galilei invariance. Here we consider BEC and superfluidity on a compact two-dimensional space taking the form of a non-commutative ("fuzzy") sphere, where the scalar bosonic fields are promoted to $N\times N$ matrices. The dimension $N$ is related to the non-commutativity parameter of space and introduces an additional scale into the system. We find that non-commutativity favors ordered phases and so enhances BEC and superfluidity. We analyze BEC in ideal and weakly interacting Bose gases on a fuzzy sphere, finding in each case that the critical temperature of BEC is greater compared to that found in the case of a commutative sphere $S^2$. Then we investigate the superfluid response of weakly interacting Bose systems. To account for vortices in a superfluid, we show that, even on an ordinary sphere, the collective coordinates of vortices induce non-commutativity. With this in mind, we extend the definition of vortex defects to an inherently non-commutative sphere studied here, where the notion of a point is untenable. The non-commutativity is expected to be experimentally relevant to BEC and superfluidity since the fuzzy sphere has a thermodynamic limit distinct from the one defined over a plane, unlike the $S^2$ case. The significance of this difference is illustrated by the superfluid density calculation indicating that, in the large sphere limit, the normal fluid fraction on the fuzzy sphere yields a linear in $T$ dependence, while on a commutative $S^2$ it exhibits the usual two-dimensional $\sim T^3$ behavior. This linear dependence, arising directly from non-commutativity, is reminiscent of Uemura's law in cuprate high-$T_c$ superconductors.

The Veneziano–Vilkovisky supertranslation is the residual large diffeomorphism relating the canonical Bondi frame to the intrinsic frame of the scattering bodies. We show it leads a tower selected by the exponentiating soft expansion, the object generating the Kerr multipoles at three points. Since \(e^{\eta\omega a\cdot q}\) splits into even and odd parts, the tower alternates parity, and for aligned spin we solve it to all orders in hyperbolic integrals. After a chiral projection its composition law is the classical $w_{1+\infty}$ bracket: the physical content we assign to that algebra.

This paper illustrates the straightforward application of the Faddeev–Jackiw approach to several non-Abelian gauge theories, namely, the Freedman–Townsend, Yang–Mills, and non-Abelian BF models, as well as a nonlinear theory of BF type. For each theory, the procedure determines the phase space and its symplectic structure, the coisotropic constraint submanifold, the Hamiltonian, and the reducibility properties of the constraints. By combining these results with the Dirac conjecture, one reconstructs through direct computation the corresponding Lagrangian gauge transformations and their reducibility structure. The analysis provides a unified and efficient derivation of the principal Hamiltonian and Lagrangian gauge structures of these non-Abelian theories.

We develop a geometric theory of information processing in the Horizon-brightened acceleration radiation (HBAR) channel, in which the radiative horizon-area change provides an entropy budget for the information carried by the radiation field. Building on the quantum-optical description of atom–field interactions near the horizon and the resulting HBAR thermodynamic correspondence, we derive area-cost laws in the near-steady, thermally saturated regime. The accessible classical information and the mutual information generated between the radiation field and its environment are bounded by the associated radiative horizon-area budget. Reliability is incorporated through Fano's inequality, which translates a prescribed decoding error probability into an area requirement. We further derive Fisher-information speed limits that constrain the statistical evolution of the radiation field and place a lower bound on the duration required for correlation generation. Together, these results establish a bits-per-area principle linking black-hole thermodynamics, information geometry, and quantum information in the HBAR framework.

We study the impact of light spectator axions on the seeding of primordial black holes (PBHs) during inflation in string theory. Primordial black holes exhibit unique and novel phenomenology, and may constitute the observed dark matter. Cosmic inflation provides a mechanism for producing them, but such inflation models typically feature Planckian field excursions, necessitating an ultraviolet completion into quantum gravity. String theory provides a natural framework for doing so, and indeed Fibre Inflation has been shown to produce PBHs while satisfying constraints from cosmic microwave background data. In this work we study the dynamics of axions during Fibre Inflation, and find a diverse and rich set of possibilities, including turns in field space and enhancement of primordial perturbations. We find that across most of parameter space, notably an axion with a far sub-Planckian decay constant $f\ll M_{\rm Pl}$, there is a negligible impact on the power spectrum of curvature perturbations, indicating an overall robustness of the model. On the other hand, an axion with a larger but still sub-Planckian decay constant, $f\gtrsim {\cal O}(0.1) M_{\rm Pl}$, and an exponentially small prefactor of its non-perturbative potential, can enhance the growth of perturbations, making it easier to achieve the amplification needed to seed PBHs, effectively realizing axion-assisted PBHs in string theory.

In generally covariant theories, evolution in coordinate time is a gauge transformation, so that a symmetry made manifest in a gauge-fixed description need not be a symmetry of the physical dynamics. At the same time, deparametrisation removes gauge redundancies but may hide physical symmetries, in particular those that depend on the chosen physical clock. We investigate the relation between gauge and hidden physical symmetries in flat FLRW geometry coupled to an arbitrary number $n$ of free massless scalar fields. We show that conformal Killing vectors of the minisuperspace metric generate conserved charges that are Dirac observables and hence gauge-invariant. Their Poisson algebra realises the maximal conformal algebra $\mathfrak{conf}(n,1)\simeq\mathfrak{so}(n+1,2)$, extending previous single-field results to arbitrary $n$. We then revisit the Eisenhart–Duval lift in a family of gauges and show that the resulting symmetry algebra is gauge dependent. Only in the harmonic gauge does the algebra enlarge to the Schrödinger algebra, which is thus not a physical symmetry. Finally, we show that deparametrisation maps the lifted charges to gauge-invariant Dirac observables, which always realise a subalgebra of $\mathfrak{conf}(n,1)$, recovering it in full in the harmonic gauge. Our results provide a systematic framework for disentangling gauge from physical symmetries in minisuperspace models, recovering charges to which reduced phase-space descriptions are structurally blind, and extending naturally to models with potentials.

We revisit the well-known nonlinear sigma model (NLSM), an effective field theory describing the scattering of $\mathrm{SU}(N)$ Goldstone bosons and characterized by an infinite tower of two-derivative interactions. We introduce a local scalar Lagrangian involving two scalar fields $\psi^\pm$ carrying opposite "polarities", whose interacting part consists of a single polynomial quartic two-derivative operator, and prove that it reproduces planar NLSM amplitudes at all loop orders. This quartic two-derivative formulation reveals a previously hidden simplicity of the NLSM: its Feynman rules involve only a single quartic interaction vertex, making the Adler zero and the leading double-soft factor manifest at all loop orders on generalized cuts and significantly improving the efficiency of high-multiplicity computations. We also suggest a possible analogous description of the NLSM +$ \phi^3 $ theory in terms of a finite tower of local interactions.

We study inflation in the setting of unimodular loop quantum cosmology, where time evolution is defined in unimodular time rather than with respect to a free, massless scalar field as is standard in loop quantum cosmology. The unimodular setting leads to a natural Schrödinger time evolution in a time coordinate with clear geometric meaning, defined independently of any particular matter content; an inflaton can be included but is not needed as a clock. We review the unimodular version of loop quantum cosmology and comment on possible connections to full (unimodular) loop quantum gravity. Then, focusing on semiclassical effective equations, we derive analytical solutions in simple cases such as a constant potential, emphasising the use of a unimodular time coordinate. We also discuss numerical solutions for phenomenologically interesting cases such as a quadratic potential and Starobinsky inflation, comparing different possible choices of initial conditions. In particular, we show that choosing an $\alpha$-attractor potential allows for models of a bounce either dominated by kinetic or potential energy, which are compatible with observations while potentially including observable imprints of the quantum-gravity regime.

Perturbation theory is an essential tool to model and interpret gravitational dynamics, for example, binary black hole mergers. Therefore it is also crucial to precisely understand its regimes of validity. The collapse of a scalar field under its own self-gravity provides a clean laboratory to study these questions. By varying the field's initial amplitude we can transition smoothly between a perturbative regime, where the field scatters in an approximately flat spacetime; and a nonperturbative regime, where a black hole forms in finite time. In this work, we use numerical relativity simulations of this set-up to investigate the accuracy of a perturbative expansion around flat spacetime, including next-to-next-to-leading order effects. Our simulations show deviations from these perturbative predictions before black hole formation, once the maximum luminosity of the process is sufficiently large, $L_{peak} \sim 10^{-2} L_{Planck}$. We characterize these nonlinear effects including a redshift of the driving frequency and a power-law spectral amplitude, which we show is consistent with approximate discrete self-similarity. These results provide a step forward towards understanding the limits of perturbative expansions in more realistic strong-gravity phenomena such as non-spherical collapse and high-velocity black hole mergers.

Within the semi-classical regime of AdS/CFT correspondence, we consider the limit where the bulk dynamical field is linearized metric perturbations satisfying linearized Einstein equations over background pure AdS spacetime. AdS/CFT correspondence gives us a holographic map, which is an isometric embedding map of the GNS Hilbert space of linearized gravity in the bulk (w.r.t. the AdS-invariant vacuum) to the GNS Hilbert space of CFT in the boundary (w.r.t. the Minkowski-invariant vacuum). We assume that the map takes AdS-vacuum in the bulk to CFT-vacuum in the boundary and that it allows AdS-Rindler wedge reconstruction. Then using this map, we show that for a given ball-shaped region in the boundary $A$, the relative entropy of a bulk state w.r.t. the AdS vacuum in the algebra of causal wedge associated to $A$ matches with the relative entropy of the dual CFT state w.r.t. the CFT vacuum in the algebra of CFT observables in $A$ in the code subspace, which is known as Jafferis-Lewkowycz-Maldacena-Suh (JLMS) condition. Furthermore, for localized semi-classical coherent excitations in the causal wedge associated to $A$ which corresponds to perturbed bulk geometry, we show rigorously using modular crossed product construction that the state-dependent part of entropy of the dual CFT state in the dressed Type-II algebra associated to $A$ satisfies vacuum subtracted Hubeney-Rangamani-Takayanagi (HRT) formula.

We consider three families of semiclassical string solutions on AdS$_5\times S^5/\mathbb{Z}_L$ orbifold backgrounds which correspond to `twisted' single-trace operators in the SU$(2)$, SU$(3)$ and SL$(2)$ sectors, respectively, of the dual 4d $\mathcal{N}=2$ quiver gauge theory. The leading quantum correction to the classical energy of each solution is computed, and is matched, despite the lack of maximal supersymmetry, to the corresponding results from finite-size corrections to the twisted Bethe ansatz in the continuum $J\to\infty$ limit and the associated Landau-Lifshitz low-energy effective theory. The $\mathbb{Z}_L$ quotient allows the string to have fractional winding branches $\mu=\widetilde{m}+\frac{n}{L}$ along orbifolded directions. As such, stability is a property of the full winding branch $\mu$, rather than of the individual twisted sectors $n$. Restricting to the purely fractional branch ($\widetilde m=0$), we obtain new stable configurations which were absent in the analogous semiclassical string solutions on AdS$_5\times S^5$. The energies of these stable states connect smoothly to those of states in relevant BPS sectors as $\mu\to0$.

We apply the framework of Soft Collinear Effective Theory (SCET) to the theory of the geometric scalar field. The resulting “geoSCET” manifests an emergent geometry in the soft sector, mirroring the emergent soft gauge invariance of QCD SCET. We use geoSCET to derive the geometric soft theorems as straightforward consequences of effective-field-theory power counting, including extensions to multiple soft emissions and loop corrections. We demonstrate that theories without a potential satisfy universal geometric soft theorems for any number of soft emissions to all orders in perturbation theory. We also show that we can turn on a potential at the soft scale without spoiling the factorization of the soft physics. This work demonstrates the underlying field-space diffeomorphism origin of the geometric soft theorem.

We point out that observations of cosmic birefringence may provide direct experimental tests of Swampland conjectures. Ultralight scalar field birefringence mechanisms are constrained by Weak Gravity Conjecture, limiting their parameter space. Conversely, confirming that CMB birefringence is caused by ultralight scalars could spell problems for the Swampland framework. Resolving these difficulties without a revision of Swampland ideology points toward thin axionic domain walls, which can explain birefringence without propagating ultralight degrees of freedom. Importantly these conflicting scenarios for cosmic birefringence could be experimentally distinguished by a search for, or exclusion of, birefringence anisotropies, that could be measured by POLARBEAR, Simons Observatory and LiteBIRD. We also note that cosmic birefringence observables emerge via a Sakharov-like asymmetry, where linear polarization is first generated cosmologically and subsequently twisted by a parity-violating dynamics after last scattering.

A striking empirical observation about the CKM matrix is that the angles of the unitarity triangle $(\alpha, \beta, \gamma)$ are very close to $(\pi/2, \pi/8, 3 \pi/8) $, simple fractions of $\pi$ that are suggestive of an underlying theory linking flavor and spontaneous CP violation. However, relating this empirical observation to an underlying theory of flavor is challenging, since the unitarity triangle is a complicated function of the Yukawa matrices. In this letter we present a simple picture for the Yukawas where this direct link is possible. We begin by parametrizing the ten-dimensional space of flavor data via "nine-link textures", full-rank $Y_{u,d}$ matrices with a total of nine non-zero entries, with a single CP violating phase. Fitting the ten parameters of all such textures to the flavor data reveals a wonderful surprise: the CP phases cluster tightly around multiples of $\pi/8$! This happens because the entries of $Y_{u,d}$ naturally define a "Yukawa triangle", in most cases identical to the unitarity triangle at leading order in small flavor parameters. Most interestingly, these two triangles are not the same beyond leading order, yielding precise predictions for $(\alpha, \beta, \gamma)$ with calculable deviation from $(\pi/2, \pi/8, 3 \pi/8)$, which can be decisively excluded or strongly confirmed by the next generation of experimental measurements of the angles. The 9-link textures are sparse and their determinants are naturally real, which taken together with spontaneous CP violation can resolve the strong CP problem.

We explore the cosmological implications of combining dark dimension scenarios with an axiverse. If gauge sectors are realized on branes, towers of Kaluza-Klein (KK) excitations of closed string axions can propagate through the dark dimension in addition to the tower of graviton excitations. This modifies cosmology in two ways. First, if any of these axion towers interact with the standard model (SM) plasma, they can significantly alter the freeze-in production of the cosmological abundance of tower states. Freeze-in to graviton and axion towers can provide all of dark matter (DM) for an axion decay constant in $10^{12} \text{ GeV }\lesssim f_a\lesssim 10^{16}\,\text{ GeV }$ and reheating temperatures $5\text{ MeV }\lesssim T_{\rm RH} \lesssim O(1)\,\text{ GeV }$. Second, different towers fragment into each other and redistribute energy; each tower's fraction of energy at late times is fixed by their interactions. If there are $N\gg1$ axion towers, the energy visibly injected into the SM by any decaying tower is diluted by a factor of $N$. This suppression offers a simple realization of how dark dimension dark matter can avoid strong cosmological constraints which rule out the simplest models. In the process of this exploration we develop a continuum approach to evaluating tower fragmentation which offers insight and aids numerical calculations by reducing the problem to quadrature.

Precision experiments increasingly target weakly coupled waves, including axion dark matter and gravitational radiation. Such waves are commonly described as classical fields, yet they could exist in quantum states with no classical counterpart. We exhibit two severe obstructions to detecting nonclassical effects, both independent of the mode occupancy. First, realistic detectors cannot resolve the fundamental modes of a field; instead they couple to coarse-grained "effective" modes, which often washes out nonclassical effects. Second, all nonclassical effects are suppressed by extra powers of the weak coupling, making them much harder to detect than the waves themselves. We prove this in general, and explicitly show how the suppression arises for quadrature and number statistics, entanglement, and decoherence. The suppression can in principle be overcome given suitable quantum resources, such as highly squeezed detector states, but the required parameters are far beyond current experimental capabilities. We use the axion cavity haloscope as an explicit example, although our conclusions apply to many ultralight dark matter searches, and rule out proposals to establish the quantization of gravity from observations of gravitational waves.

We study the general form of meromorphic amplitudes that are compatible with unitarity, analyticity, crossing symmetry, polynomial boundedness and the hidden-zero and corresponding splitting conditions. These amplitudes are infinite-spin-tower (IST) amplitudes and are characterized by the distribution of poles. With infinitely many equidistant poles, the IST amplitudes reduce to Veneziano amplitudes. We construct such unitary amplitudes in a primal way (rule in) and find the bounds analytically. The allowed region we derive is smaller than, but close to, the region allowed by the positivity bounds (rule out). We argue that, with the conditions imposed, the analytic boundary we derive is the largest possible boundary in the primal construction of meromorphic amplitudes. This type of IST amplitude can also be extended to the fully crossing-symmetric case related to the Virasoro-Shapiro amplitude. We found from the IST amplitudes that graviton pole imposes unitarity constraints on UV spectrum.

What is a Faddeev–Popov ghost? The textbook answer is that it is a trick: a way of writing a functional determinant as a term in the action, and nothing more. Dougherty 2021 and Dougherty and Read 2026 have argued that this undersells the ghost, which on their account encodes classical structure of the configuration space, viz. the degree-one generators of the Chevalley–Eilenberg complex of the action Lie algebroid. I agree with them that the ghost encodes classical structure. But I think the structure is bigger than they say. The ghost, I argue, is a principal connection $\varpi$ on the bundle $F \to F/G$ of gauge potentials over physical configurations; the BRST operator is the exterior derivative along the orbits; and the Chevalley–Eilenberg ghost coincides with $\varpi$ restricted to the orbits (and every $\varpi$ has the same restriction). The restriction drops the horizontal complement, and I show that gauge-fixed quantisation needs that complement. A gauge-fixing installs it as a pairing of configurations across orbits (a counterpart relation, in Lewis's sense); dressed observables and local counterfactuals rely on this pairing, but it is discarded by the quotient $\F/\G$. In the non-Abelian case Singer's theorem forbids a flat $\varpi$, and there the pairing exists only as parallel transport. Finally, BRST survives gauge-fixing because it is vertical and rigid, and so leaves any such pairing unchanged.