Quantum effects are expected to modify the cosmological dynamics of the early universe while maintaining some (potentially discrete) notion of space-time structure. In one approach, loop quantum cosmology, current models are shown here to either be incompatible with a consistent space-time structure, or to have physical singularities. The latter happens in spite of a nonzero scale factor in the isotropic background dynamics. A new effective Friedmann equation shows that a bounce is obtained at sub-Planckian densities, preceded by a physical singularity at infinite scale factor that resembles a time-reversed big rip, or a big fix. The entire phase is accompanied by rapid changes of the Hubble radius. In addition, a new version of perturbative inhomogeneity in loop quantum cosmology is introduced that maintains a consistent space-time structure and has a nonsingular background dynamics.

In this work I use a frequency-domain Regge-Wheeler-Zerilli approach to compute the gravitational-wave energy radiated by a compact body moving along a hyperbolic or parabolic geodesic of a Schwarzschild black hole. I compare my results with the latest post-Minkowskian (PM) calculations for the radiated energy and find agreement for hyperbolic orbits with large impact parameters and characterized by a velocity at infinity, $v_{\infty }$, as large as $v_{\infty }/c=0.7$. I also find agreement between my results and the leading-order PM expansion for the radiation absorbed by the black hole. I make further comparisons with post-Newtonian (PN) theory and show the effectiveness of a simple PN-PM hybrid model. Finally, I make a first comparison of the radiated energy between self-force and numerical relativity.

We study the bound states of a massive scalar field around a topological star and show that these are strictly normal modes. This yields a genuine gravitational atom, sharply distinguishing horizonless objects from black holes. We show that the modes are controlled by the field’s Compton wavelength compared to the size of the star. When the Compton wavelength is large, the field forms a cloud with a hydrogenlike spectrum, while in the opposite regime it is localized along timelike trajectories. When the two scales are comparable the spectrum becomes richer, and we characterize it in detail allowing the field to carry electric charge and Kaluza-Klein momentum.

The Planck measurement of the cosmic microwave background (CMB) has established the $\mathrm{\Lambda }$ cold dark matter ($\mathrm{\Lambda }\mathrm{CDM}$) model as the concordant model along with other observations. However, recent measurements of baryon acoustic oscillations (BAO) from the Dark Energy Spectroscopic Instrument (DESI) have renewed the matter fraction ${\mathrm{\Omega }}_{\mathrm{m}}$ tension between Planck-$\mathrm{\Lambda }\mathrm{CDM}$ and DESI-$\mathrm{\Lambda }\mathrm{CDM}$. Directly reconciling this CMB-BAO tension with a dynamical dark energy in Chevallier-Polarski-Linder parametrization seems to imply a crossing of the equation of state (EOS) through $w=-1$ at low redshifts. In this paper, we resolve this ${\mathrm{\Omega }}_{\mathrm{m}}$ tension by allowing for the dark matter nonminimally coupled to gravity via a quintessence field. This nonminimal coupling is preferred over $3\sigma$ confidence level. Consequently, even though the usual effective EOS of the coupled quintessence apart from the standard CDM part never crosses but always is above $w=-1$, a misidentification with the $w_0{w}_a\mathrm{CDM}$ model would exactly fake such a crossing behavior, and the tensions on neutrino mass and growth rate in the $\mathrm{\Lambda }\mathrm{CDM}$ model are also relieved in our model as a result of the resolved ${\mathrm{\Omega }}_{\mathrm{m}}$ tension.

We derive a general procedure for calculating the gravitational wave background (GWB) from cosmic string loops whose typical shape evolves over time, as in gravitational backreaction. Using the results of a large-scale study of numerical gravitational backreaction on Nambu-Goto cosmic string loops, we construct GWBs of backreacted cosmic strings for a range of tensions and frequencies of cosmological interest and compare them to current and upcoming gravitational wave detectors. The GWBs are lower than prior predictions by anywhere from a few percent to around 30%, depending on the frequency and tension in question.

Little red dots (LRDs) are enigmatic, compact, red galaxies at high redshift, $z\sim 4–7$, discovered by the James Webb Space Telescope. Broad emission lines in the absence of x-ray and radio counterparts suggest that they host accreting supermassive black holes embedded in dense gaseous envelopes. This black-hole-envelope configuration facilitates efficient photohadronic interactions and neutrino production. Remarkably, their observed source number density and luminosity are compatible with the energetics of the diffuse neutrino background. We consider that relativistic jets and outflows are launched from the black hole and propagate through low-density polar funnels within envelopes, where particle acceleration and neutrino emission occur. This leads to LRDs being effectively hidden sources. Our analytic and numerical calculations show that, in an optimistic scenario, LRDs can contribute $\sim 30\%$ of the observed diffuse background at TeV–sub-PeV energies, predominantly through photomeson production. At high neutrino energies, $\gtrsim {10}^{5.5}\mathrm{GeV}$, inverse-Compton cooling of muons modifies the resulting flavor ratio, providing a distinctive diagnostic for IceCube-Gen2 and other upcoming neutrino telescopes.

We consider electric flux tube solutions in SU(3) gauge theory with scalar fields in the fundamental representation. Such solutions can possibly be constructed in two classes, corresponding to the two maximally commuting generators ${\lambda }_3$ and ${\lambda }_8$ of SU(3). We successfully construct the 3-type solution with two scalar fields and show that it is immune to decay through Schwinger pair production of gauge bosons, but we have not been able to construct an 8-type solution.

Motivated by the observation of nonzero neutrino masses and the potential for discovering physics beyond the Standard Model, numerous experiments are actively searching for neutrinoless double beta $(0\nu \beta \beta )$ decay. In all of these searches, a substantial amount of data on two-neutrino double beta $(2\nu \beta \beta )$ decay has been collected. In this work, we explore the sensitivity of current and future double beta decay experiments to a massive Majoron-like scalar particle coupled to neutrinos and potentially dark sector fermions, and compare their reach to the relevant cosmological constraints. On- and off-shell production of such a scalar leads to characteristic distortions in the emitted electron spectrum. We investigate how these distortions manifest in current and future double beta decay experiments, deriving the sensitivity to such a scenario. We project the reach of future experiments which can probe scalar-neutrino couplings of $|a_{\nu }|\approx 2\times {10}^{-6}$ for sub-MeV scalar particles and remain sensitive to off-shell production above the $Q$-value of double beta isotopes.

We explore the fundamental but untested possibility that the photon is a continuous spin particle (CSP) with a small but nonzero spin Casimir $\rho$. When $\rho \ne 0$, the familiar polarization modes of the photon transform nontrivially under Lorentz boosts, leading to deviations from familiar QED. Surprisingly, these deviations are strongest at low energy, but smoothly vanish in the $\rho \to 0$ limit. In this letter, we compute corrections to the hydrogen 21-cm transition rate, which is expected to be particularly sensitive given the small hyperfine energy splitting $\omega$. We find deviations from $\mathrm{QED}\propto {\rho }^2{\alpha }^2/{\omega }^2$ at leading order, suggesting experimental constraints $\rho \lesssim 1\mathrm{meV}$. Building on this work, we expect that a range of other atomic, molecular, or condensed matter systems could be used to provide even more stringent tests of $\rho$ in electromagnetic interactions.

We develop a systematic Hamiltonian formulation of minimally doubled lattice fermions in ($3+1$) dimensions, derive their nodal structures (structures of zeros), and classify their symmetry patterns for both four-component Dirac and two-component Weyl constructions. Motivated by recent single-Weyl proposals based on Bogoliubov-de Gennes representation, we argue that the corresponding single-Weyl Hamiltonians are obtained from the minimal-doubling Hamiltonians supplemented by an appropriate species-splitting mass term, and we reexamine the non-on-site symmetry protecting the physical Weyl node in terms of a Ginsparg-Wilson-type relation. We then construct a one-parameter family of deformations that preserves all the symmetries and demonstrate that, once the parameter exceeds a critical value, additional Weyl nodes emerge and the system exits the single-node regime. This indicates that in interacting theories radiative corrections can generate symmetry-allowed counterterms, so maintaining the desired single-Weyl phase generically requires “moderate” parameter tuning.

We present an algorithm to include the correlation between the production and decay planes of gluons in a parton-shower simulation. The technique is based on identifying the charge currents responsible for the creation and annihilation of the vector field. It is applicable in both the hard-collinear and the soft wide-angle region. As a function of the number of particles, the algorithm scales linear in computing time and memory. We demonstrate agreement with fixed-order perturbative calculations in the relevant kinematical limits, and present a new observable that can be used to probe correlations beyond current-current interactions.

This study examines the spectral functions of neutral (${\pi }_0$) and charged (${\pi }_{\pm }$) pions under a uniform magnetic field using the SU(2) Nambu–Jona-Lasinio model with the Ritus method. The analysis highlights the complex interplay of magnetic field effects, thermal influences, and chiral symmetry on meson properties in extreme QCD environments. For ${\pi }_0$, whose properties are governed by the behavior of its constituent quarks, magnetic field-induced Landau levels lead to a multipeak structure in its spectral function, reflecting stable and resonance solutions that evolve with temperature, showing shifts and critical enhancements near chiral restoration. For ${\pi }_{\pm }$, cross terms that come from the asymmetry between the constituent quarks introduce Landau cuts alongside unitary cuts, indicating damping effects, with decay widths narrowing at higher temperatures, suggesting increased stability.

The correct identification of charged hadrons plays a crucial role in flavor-physics measurements. The final detector configurations at the proposed Future Circular Collider are yet to be determined and this study aims to contribute to this discussion by benchmarking the particle-identification (PID) performance of the proposed CLD and IDEA detectors using fully simulated events. At present, neither detector proposal includes dedicated PID systems, relying instead on information from the tracking subsystems. We estimate the expected level of contamination due to misidentified charged hadrons for $b$-flavor tagging, rare $b\to s$ transitions, and $s$-jet tagging. The PID information provided by silicon trackers, namely time-of-flight and energy-deposit measurements, leads to significant background suppression with high signal efficiency for the low-momentum hadrons considered for same-side $b$-flavor tagging. In order to improve the contamination in rare decays where momenta are in the medium range, only good timing resolution of 30 ps and below can yield an improvement of one order of magnitude below the level achieved by kinematic criteria alone. Light-quark jet-flavor tagging requires identification of particles with very large momentum, which is not possible using only time-of-flight or energy-deposit information in silicon. Access to the number of clusters in a drift-chamber setup, as proposed for the IDEA detector, however, results in strong background suppression in every case. This suppression can be further improved in some scenarios by time-of-flight resolution of 30–50 ps or better. The PID quality generally exhibits only a small dependence on the cluster-counting efficiency. Whether dedicated PID detectors could further enhance flavor-physics sensitivity should be the subject of future study.

Based on $(10087\pm 44)\times {10}^{6}J/\psi$ events collected by the BESIII detector at the center-of-mass energy $\sqrt{s}=3.097\mathrm{GeV}$, the first absolute measurement of the branching fractions for the decays ${\mathrm{\Sigma }}^{+}\to p{\pi }^0$ and ${\mathrm{\Sigma }}^{+}\to n{\pi }^{+}$ is performed. The branching fractions are determined to be $B_{{\mathrm{\Sigma }}^{+}\to p{\pi }^0}=(49.79\pm 0.06\pm 0.22)\%$ and $B_{{\mathrm{\Sigma }}^{+}\to n{\pi }^{+}}=(49.87\pm 0.05\pm 0.29)\%$, where the first uncertainties are statistical and the second systematic. Our results show significant deviations from the Particle Data Group values for both decays, with differences of $4.4\sigma$ for ${\mathrm{\Sigma }}^{+}\to p{\pi }^0$ and $3.4\sigma$ for ${\mathrm{\Sigma }}^{+}\to n{\pi }^{+}$. These measurements will play important roles in precise studies of the decay properties of strange, charmed, and beauty baryons with ${\mathrm{\Sigma }}^{+}$ in the final states. Furthermore, the $\mathrm{\Delta }I=1/2$ rule is tested in nonleptonic ${\mathrm{\Sigma }}^{\pm }$ decays. The observed results deviate from zero by more than $5\sigma$, indicating the presence of the $\mathrm{\Delta }I=3/2$ transition amplitude in the $\mathrm{\Sigma }$ hyperon decays.

We apply the wavelet formalism of quantum field theory to investigate nonperturbative dynamics within the Hamiltonian framework. In particular, we employ Daubechies wavelets in momentum space, whose basis functions are labeled by resolution and translation indices, providing a natural nonperturbative truncation of both infrared and ultraviolet truncation of quantum field theories. As an application, we compute the energy spectra of a free scalar field theory and the interacting $1+1$-dimensional ${\varphi }^4$ theory. This approach successfully reproduces the well-known strong-coupling phase transition in the $m^2>0$ regime. Our results indicate that with increasing resolution, the extracted critical coupling can approach toward its expected value, providing numerical evidence for the plausibility of the wavelet-based Hamiltonian formulation for nonperturbative field-theoretic calculations.

We investigate composite models of gravity and explore how dynamical tensor fields can emerge within the functional renormalization group framework. We consider two prototype models: a fermionic theory and a scalar theory. In both cases, an auxiliary tensor field is introduced via a Hubbard-Stratonovich transformation, corresponding to the composite channel associated with the energy-momentum tensor. We derive the flow equations for the field renormalization factors of the auxiliary tensor field and demonstrate that finite kinetic terms are dynamically generated in the infrared regime. Agreement with the diffeomorphism-invariant quadratic Einstein-Hilbert structure can be established in the transverse-traceless spin-2 sector, while the remaining contributions reside in the longitudinal and trace sectors. Although these terms can be cast into a form reminiscent of gauge-fixed presentations of the quadratic Einstein-Hilbert action, we do not interpret them as originating from a genuine gauge-fixing procedure within the present truncation.

Gravitational lensing of gravitational waves (GWs) can encode valuable information about the properties of the intervening lens, but most existing studies remain restricted to the small-deflection, weak-field regime. To bridge this crucial gap, this work presents the first systematic analysis of strong-field, wave-optical GW lensing by a Kerr black hole (BH), extending recent results obtained in [Phys. Rev. D 112, 064009 (2025)] for nonrotating lens to the astrophysically more relevant case of spinning-lens. Using the Mano-Suzuki-Takasugi formalism, we define and compute the strong-field scattering factor (SFSF) and show that the spin produces characteristic modifications to the lensed waveform, and high-frequency incident radiation is not strongly absorbed by the BH lens, contrary to earlier claims in the abovementioned paper. We further derive an explicit expression for the observed waveform for the general source-lens-observer configuration, showcasing the distortions produced by the scattering and quantifying their departure from the Schwarzschild case. Specializing to on-axis scattering, a mismatch analysis for a GW150914-like source lensed by a Kerr BH of mass $M={10}^2{M}_{\odot }$ situated $100GM/c^2$ away from the source reveals percentage-level deviations from the direct (unscattered) wave at scattering angles near $\pi /6$ radians, across a range of lens spin values. The mismatch generally decreases as the scattering angle increases, but this behavior can change substantially when polarization mixing induced by scattering becomes significant. In such cases, components that are absent or suppressed in the direct signal may become appreciable once scattering effects are taken into account. For a fixed scattering angle, however, the mismatch shows only a weak dependence on the BH spin in the case of on-axis scattering, which may improve for more general off-axis considerations. The framework developed here offers a unified treatment of strong-field GW scattering in Kerr spacetime and provides tools for interpreting future high-precision observations of compact-object lenses in the wave-optics regime.

We present the frequency-domain quasicircular precessing binary-black-hole model phenomxpnr. This model combines the most precise available post-Newtonian description of the evolution of the precession dynamics through inspiral with merger-ringdown model informed by numerical relativity. This, along with a phenomenological model of the dominant multipole asymmetries, results in the most accurate and complete representation of the physics of precessing binaries natively in the frequency-domain to date. All state-of-the-art precessing models show bias when inferring binary parameters in certain regions of the parameter space. We demonstrate that the developments presented here ensure that phenomxpnr shows the least degree of bias for some high mass systems with a large degree of precession. Further, as a phenomenological, frequency-domain model, phenomxpnr is one of the most computationally efficient models available and is therefore well suited to the era of gravitational-wave astronomy with its ever growing rate of detected signals.

We develop a new technique for studying the perturbations of dRGT-type massive gravity theories around arbitrary background spacetimes. Built initially from the vielbein formulation of the theory, but switching back to the metric formulation afterward, our approach bypasses many of the complications that arise in previous metric formulation approaches to linearizing massive gravity around generic backgrounds, naturally elucidates the ghost-free structure of the interactions, and readily generalizes to higher orders in perturbation theory, as well as to multiple interacting metric tensor fields. To demonstrate the power of our technique, we apply our formalism to a number of commonly occurring example backgrounds—proportional, cosmological, and black hole—recovering and extending many known results from the literature at linear order. Lastly, we provide, for the first time, the cubic order multigravity potential around a generic background spacetime.