It is remarkably difficult to reconcile unitarity and Vilenkin’s wave function. For example, the natural conserved inner product found in quantum unimodular gravity applies to the Hartle-Hawking wave function, but fails for its Vilenkin counterpart. We diagnose this failure from different angles (Laplace transform instead of Fourier transform, non-Hermiticity of the Hamiltonian, etc.) to conclude that ultimately it stems from allowing the connection to become imaginary in a section of its contour. In turn this is the unavoidable consequence of representing the Euclidean theory as an imaginary image within a fundamentally Lorentzian theory. It is nonetheless possible to change the underlying theory and replace the connection’s foray into the imaginary axis by an actual signature change (with the connection, action and Hamiltonian remaining real). The structural obstacles to unitarity are then removed, but special care must still be taken, because the Euclidean theory a priori has boundaries, so that appropriate boundary conditions are required for unitarity. Reflecting boundary conditions would reinstate a Hartle-Hawking-like solution in the Lorentzian regime. To exclude an incoming wave in the Lorentzian domain one must allow a semi-infinite tower of spheres in the Euclidean region, wave packets traveling through successive spheres for half an eternity in unimodular time. Such a “Sisyphus” boundary condition no longer even vaguely resembles Vilenkin’s original proposal.

We argue that the recent result of primordial helium-4 (${}^4\mathrm{He}$) abundance measurement by the EMPRESS Collaboration, which has reported a smaller ${}^4\mathrm{He}$ abundance than other measurements do, can be well fitted by assuming a time-varying fine structure constant $\alpha$, which takes a slightly smaller value than the present one during big bang nucleosynthesis (BBN). We find that the EMPRESS result in combination with deuterium abundance measurement indicates $-2.6\%<\mathrm{\Delta }\alpha /\alpha <-1.4\%$ (68% C.L.), where $\mathrm{\Delta }\alpha$ is the difference between the values of $\alpha$ at the BBN and present epochs, while $-1.2\%<\mathrm{\Delta }\alpha /\alpha <0.4\%$ (68% C.L.) is obtained from other previous ${}^4\mathrm{He}$ abundance data. We also investigate its effects in the framework where the effective number of neutrino species and the lepton asymmetry, which are other typical interpretations of the EMPRESS result, are allowed to vary. Once a smaller $\alpha$ is adopted, the EMPRESS result can be explained without assuming any nonstandard values for the effective number of neutrino species and lepton asymmetry.

We study the impact of pions in simulations of neutron star mergers and explore the impact on gravitational-wave observables. We model charged and neutral pions as a noninteracting boson gas with a chosen, constant effective mass. We add the contributions of pions, which can occur as a condensate or as a thermal population, to existing temperature- and composition-dependent equations of state. Compared with the models without pions, the presence of a pion condensate decreases the characteristic properties of cold, nonrotating neutron stars such as the maximum mass, the radius and the tidal deformability. We conduct relativistic hydrodynamical simulations of neutron star mergers for these modified equations of state models and compare to the original models, which ignore pions. Generally, the inclusion of pions leads to a softening of the equation of state, which is more pronounced for smaller effective pion masses. We find a shift of the dominant postmerger gravitational-wave frequency by up to 150 Hz to higher frequencies and a reduction of the threshold binary mass for prompt black-hole formation by up to $0.07M_{\odot }$. These quantitative changes compared with the equation of state model without pions are stronger for smaller effective pion masses and for underlying baryonic models which result in softer equations of state. We evaluate empirical relations between the threshold mass or the dominant postmerger gravitational-wave frequency and stellar parameters of nonrotating neutron stars. These relations are constructed to extract these stellar properties from merger observations and are built based on large sets of equation of state models which do not include pions. Comparing to our calculations with pions, we find that these empirical relations remain valid to good accuracy, which justifies their use although they neglect a possible impact of pions. Pions simultaneously modify merger characteristics and the properties of cold, nonrotating neutron stars. We also address the mass ejection by neutron star mergers and observe a moderate enhancement of the ejecta mass by a few ten percent. While such variations may be within statistical uncertainties of the simulations, the increase is slightly more pronounced than one would have expected from empirical relations predicting the ejecta mass based on stellar parameters of nonrotating neutron stars.

Tilt-to-length coupling was the limiting noise source in LISA Pathfinder between 20 and 200 mHz before subtraction in postprocessing. To prevent the adding of sensing noise to the data by the subtraction process, the success of this strategy depended on a previous direct noise reduction by test mass alignment. The exact dependency of the level of tilt-to-length coupling on the set points of LISA Pathfinder’s test masses was not understood until the end of the mission. Here, we present, for the first time, an analytical tilt-to-length coupling model that describes the coupling noise changes due to the realignments. We report on the different mechanisms, namely the lever arm and piston effect as well as the coupling due to transmissive components, and how they contribute to the full coupling. Further, we show that a pure geometric model would not have been sufficient to describe the coupling in LISA Pathfinder. Therefore, we model also the nongeometric tilt-to-length noise contributions. For the resulting coupling coefficients of the full model, we compute the expected error bars based on the known individual error sources. Also, we validated the analytical model against numerical simulations. A detailed study and thorough understanding of this noise are the basis for a successful analysis of the LISA Pathfinder data with respect to tilt-to-length coupling.

We consider extensions of the Standard Model by a hidden sector consisting of a gauge field coupled with a scalar field. In the absence of dimensionful parameters in the tree-level potential, radiative symmetry breaking will induce the electroweak scale of the Standard Model and generate mass for the hidden sector gauge field. We consider both $U(1{)}_{\mathrm{D}}$ and $SU(2{)}_{\mathrm{D}}$ dark sector gauge groups and focus on probing the models with a combination of direct detection experiments and gravitational wave observatories. We find that recent dark matter direct detection results significantly constrain the parameter space of the models where they can account for the observed dark matter relic density via freeze-out. The gravitational wave signals originating from strongly first-order electroweak phase transition in these models can be probed in future gravitational wave observatories such as the Laser Interferometer Space Antenna. We show how the projected results complement direct detection experiments and can help probe parameter space near the neutrino floor of direct detection.

A perturbative calculation of the three-nucleon ground-state energy shift in a finite volume is carried out within the nonrelativistic effective theory. The energy shift is evaluated up to and including $O(L^{-6})$, where $L$ is the size of a cubic box. The convergence of the perturbative series at physical values of the scattering lengths is studied numerically.

We investigate some general properties of linear gauge fixings and gauge-field correlators in lattice models with noncompact U(1) gauge symmetry. In particular, we show that, even in the presence of a gauge fixing, some gauge-field observables (like the photon-mass operator) are not well defined, depending on the specific gauge fixing adopted and on its implementation. Numerical tests carried out in the three-dimensional noncompact lattice Abelian-Higgs model fully support the analytical results and provide further insights. Apparently, only the hard Lorenz-gauge fixing provides a consistent definition of non-gauge-invariant quantities in three dimensions.

Simulations of lattice gauge theories with tensor networks and quantum computing have so far mainly focused on staggered fermions. In this paper, we use matrix product states to study Wilson fermions in the Hamiltonian formulation and present a novel method to determine the additive mass renormalization. Focusing on the single-flavor Schwinger model as a benchmark model, we investigate the regime of a nonvanishing topological $\theta$-term, which is inaccessible to conventional Monte Carlo methods. We systematically explore the dependence of the mass shift on the volume, the lattice spacing, the $\theta$-parameter, and the Wilson parameter. This allows us to follow lines of constant renormalized mass, and therefore to substantially improve the continuum extrapolation of the mass gap and electric field density. For small values of the mass, our continuum results agree with the theoretical prediction from mass perturbation theory. Going beyond Wilson fermions, our technique can also be applied to staggered fermions, and we demonstrate that the results of our approach agree with a recent theoretical prediction for the mass shift at sufficiently large volumes.

The contribution of axial-vector mesons to the muon’s anomalous magnetic moment through a light-by-light process is considered within a nonlocal quark model. The model is based on a four-quark interaction with scalar-pseudoscalar and vector–axial-vector sectors. While the transverse component of the axial-vector corresponds to a spin-1 particle, the unphysical longitudinal component is mixed with a pseudoscalar meson. The model parameters are refitted to the pion properties in the presence of $\pi -a_1$ mixing. The obtained estimation for the light-by-light contribution of $a_1+f_1$ mesons is $(3.6\pm 1.8)\times {10}^{-11}$.

We developed a detector signal characterization model based on a Bayesian network trained on the waveform attributes generated by a dual-phase xenon time projection chamber. By performing inference on the model, we produced a quantitative metric of signal characterization and demonstrate that this metric can be used to determine whether a detector signal is sourced from a scintillation or an ionization process. We describe the method and its performance on electronic-recoil (ER) data taken during the first science run of the XENONnT dark matter experiment. We demonstrate the first use of a Bayesian network in a waveform-based analysis of detector signals. This method resulted in a 3% increase in ER event-selection efficiency with a simultaneously effective rejection of events outside of the region of interest. The findings of this analysis are consistent with the previous analysis from XENONnT, namely a background-only fit of the ER data.

We forecast the ability of bispectrum estimators to constrain primordial non-Gaussianity using future photometric galaxy redshift surveys. A full-sky survey with photometric redshift resolution of ${\sigma }_z/(1+z)=0.05$ in the redshift range $0.2<z<2$ can provide constraints $\sigma (f_{\mathrm{NL}}^{\text{local}})=3.4$, $\sigma (f_{\mathrm{NL}}^{\text{equil}})=15$, and $\sigma (f_{\mathrm{NL}}^{\mathrm{orth}})=17$ for the local, equilateral, and orthogonal shapes, respectively, delivering constraints on primordial non-Gaussianities competitive to those from the cosmic microwave background. We generalize these results by deriving a scaling relation for the constraints on the amplitude of primordial non-Gaussianity as a function of redshift error, depth, sky coverage, and nonlinear scale cutoff. Finally, we investigate the impact that photometric calibration errors on the largest scales will have on the constraining power of future experiments. We show that peculiar velocities reconstructed via kinetic Sunyaev Zel’dovich tomography can be used to mitigate the impact of calibration errors on primordial non-Gaussianity constraints.

We introduce a gravitational lattice theory defined in an ${\mathrm{AdS}}_3$ black hole background that provides a holographic dual description of the linear-to-zigzag structural phase transition, characterized by the spontaneous breaking of parity symmetry observed in, e.g., confined Coulomb crystals. The transition from the high-symmetry linear phase to the broken-symmetry doubly degenerate zigzag phase can be driven by quenching the coupling between adjacent sites through the critical point. An analysis of the equilibrium correlation length and relaxation time reveals mean-field critical exponents. We explore the nonequilibrium phase transition dynamics leading to kink formation. The kink density obeys universal scaling laws in the limit of slow quenches, described by the Kibble-Zurek mechanism (KZM), and at fast quenches, characterized by a universal breakdown of the KZM.

In many cases, the near-horizon geometry encodes sufficient information to compute conserved charges of a gravitational solution, including thermodynamic quantities. These charges are Noether charges associated to asymptotic isometries that preserve appropriate boundary conditions at the future horizon. For isolated, compact horizons these charges turn out to be integrable, conserved and finite, and they have been studied in many examples of interest, notably in $3+1$ dimensions. In higher dimensions, where the variety of horizon structures is more diverse, it is still possible to apply the same method, although explicit examples have so far been limited to simple topologies. In this paper, we demonstrate that such computations can also be applied to higher-dimensional solutions with event horizons whose spacelike cross sections exhibit nontrivial topology. We provide several explicit examples, with particular focus on the 5-dimensional black ring.

We are interested in the charged dust solutions of the Einstein field equations in stationary and axially symmetric spacetimes and inquire if the naked singularities of the Israel-Wilson-Perjes (IWP) metrics can be removed. The answer is negative in four dimensions. We examine whether this negative result can be avoided by adding scalar or dilaton fields. We show that IWP metrics also arise as solutions of the Einstein-Maxwell system with a stealth dilaton field. We determine the IWP metrics completely in terms of one complex function satisfying the Laplace equation. With the inclusion of the stealth dilaton field, it is now possible to add a perfect fluid source. In this case, the field equations reduce to a complex cubic equation. Hence, this procedure provides interior solutions to each IWP metric, and it is possible to cover all naked singularities inside a compact surface where there is matter distribution.

This paper investigates the integrability properties of Einstein’s theory of gravity in the context of accelerating Newman-Unti-Tamburino (NUT) spacetimes by utilizing Ernst’s description of stationary and axially symmetric electrovacuum solutions. We employ Ehlers transformations, Lie point symmetries of the Einstein field equations, to efficiently endorse accelerating metrics with a nontrivial NUT charge. Under this context, we begin by rederiving the known C-metric NUT spacetime described by Chng, Mann, and Stelea in a straightforward manner and in the new form of the solution introduced by Podolský and Vrátný. Next, we construct for the first time an accelerating NUT black hole dressed with a conformally coupled scalar field. These solutions belong to the general class of type I spacetimes and, therefore, cannot be obtained from any limit of the Plebanśki-Demiański family whatsoever, and their integration needs to be carried out independently. Including Maxwell fields is certainly permitted; however, the use of Ehlers transformations is subtle and requires further modifications. Ehlers transformations not only partially rotate the mass parameter such that its magnetic component appears, but also rotate the corresponding gauge fields. Notwithstanding, the alignment of the electromagnetic potentials can be successfully performed via a duality transformation, hence providing a novel Reissner-Nordström-C-metric NUT black hole that correctly reproduces the Reissner-Nordström-C-metric and Reissner-Nordström-NUT configurations in the corresponding limiting cases. We describe the main geometric features of these solutions and discuss possible embeddings of our geometries in external electromagnetic and rotating backgrounds.

Employing the spherical collapse formalism, we investigate the linear evolution of the matter overdensity for energy-momentum-squared gravity (EMSG), which in practical phenomenological terms one may imagine as an extension of the $\mathrm{\Lambda }\mathrm{CDM}$ model of cosmology. The underlying model, while still having a cosmological constant, is a nonlinear material extension of the general theory of relativity and includes correction terms that are dominant in the high-energy regime, the early Universe. Considering the Friedmann-Robertson-Walker background in the presence of a cosmological constant, we find the effects of the modifications arising from EMSG on the growth of perturbations at the early stages of the Universe. Considering both possible negative and positive values of the model parameter of EMSG, we discuss its role in the evolution of the matter density contrast and growth function in the level of linear perturbations. While EMSG leaves imprints distinguishable from $\mathrm{\Lambda }\mathrm{CDM}$, we find that the negative range of the EMSG model parameter is not well behaved, indicating an anomaly in the parameter space of the model. In this regard, for the evaluation of the galaxy cluster number count in the framework of EMSG, we equivalently provide an analysis of the number count of the gravitationally collapsed objects (or the dark matter halos). We show that the galaxy cluster number count decreases compared to the $\mathrm{\Lambda }\mathrm{CDM}$ model. In agreement with the hierarchical model of structure formation, in EMSG cosmology the more massive structures are less abundant, meaning that they form at later times.

From prolonged x-ray and optical data of the ultracompact binary HM Cancri, two groups recently measured the second derivative of its orbital frequency. The space gravitational wave detector LISA will detect $\sim {10}^4$ Galactic binaries, and their second frequency derivatives will be interesting observational targets for LISA. Here, we forecast the gravitational wave signal analysis for HM Cancri, as an ideal reference system for these numerous binaries. We find that, in its nominal operation period $T\sim 4\mathrm{yr}$, LISA is unlikely to realize a sufficient measurement precision for the reported second frequency derivative of this binary. However, because of a strong dependence on the time baseline, the precision will be drastically improved by extending the operation period of LISA or combining it with other missions (e.g., Taiji and TianQin) in a sequential order.

Knowledge of the ionization and scintillation responses of liquid xenon (LXe) to nuclear recoils is crucial for LXe-based dark matter experiments. Current calibrations carry large uncertainties in the low-energy region below $\sim 3{\mathrm{keV}}_{\mathrm{nr}}$ where signals from dark matter particles of $<10\mathrm{GeV}/{\mathrm{c}}^2$ masses are expected. The coherent elastic neutrino-nucleus scattering ($\mathrm{CE}\nu \mathrm{NS}$) by solar ${}^8\mathrm{B}$ neutrinos also results in a continuum of nuclear recoil events below $3.0{\mathrm{keV}}_{\mathrm{nr}}$ (99% of events), which further complicates low-mass dark matter searches in LXe experiments. In this paper, we describe a method to quantify the uncertainties of low-energy LXe responses using published calibration data, followed by case studies to evaluate the impact of yield uncertainties on ${}^8\mathrm{B}$ searches and low-mass dark matter sensitivity in a typical ton-scale LXe experiment. We conclude that naively omitting yield uncertainties leads to overly optimistic limits by factor $\sim 2$ for a 6 GeV weakly interacting massive particle mass. Future nuclear recoil light yield calibrations could allow experiments to recover this sensitivity and also improve the accuracy of solar ${}^8\mathrm{B}$ flux measurements.

In the context of direct searches of sub-GeV dark matter particles with germanium detectors, the EDELWEISS collaboration has tested a new technique to tag ionizing events using NbSi transition edge athermal phonon sensors. The emission of the athermal phonons generated by the Neganov-Trofimov-Luke effect associated with the drift of electrons and holes through the detectors is used to tag ionization events generated in specific parts of the detector localized in front of the NbSi sensor and to reject by more than a factor 5 [at 90% confidence level (CL)] the background from heat-only events that dominates the spectrum above 3 keV. This method is able to improve by a factor of 2.8 the previous limit on spin-independent interactions of $1\mathrm{GeV}/c^2$ weakly interacting massive particles obtained with the same detector and dataset but without this tagging technique.

We use an algebraic approach to the calculation of Landau levels for a uniform magnetic field in the symmetric gauge with a vector potential $\overrightarrow{A}=\frac{1}{2}(\overrightarrow{B}\times \overrightarrow{r})$, where $\overrightarrow{B}$ is assumed to be constant. The magnetron quantum number constitutes the degeneracy index. An overall complex phase of the wave function, given in terms of Laguerre polynomials, is a consequence of the algebraic structure. The relativistic generalization of the treatment leads to fully relativistic bispinor Landau levels in the symmetric gauge, in a representation which writes the relativistic states in terms of their nonrelativistic limit, and an algebraically accessible lower bispinor component. Negative-energy states and the massless limit are discussed. The relativistic states can be used for a number of applications, including the calculation of higher-order quantum electrodynamic binding corrections to the energies of quantum cyclotron levels.