We study the gravitational bremsstrahlung owing to collisions mediated by a $1/r$ potential. We combine classical and first order Born approximation results in order to construct an approximate gravitational “Gaunt factor” for the total emitted energy. We also obtain the cross section with an angular momentum cutoff and hence the cross section for emission via close hyperbolic encounters in a gravitating cluster. These effects are the dominant source of very high frequency gravitational noise in the Solar System. The total gravitational wave power of the Sun is $76\pm 20\mathrm{MW}$.

We present results of a follow-up search for continuous gravitational waves (CWs) associated with subthreshold candidates from the LIGO-Virgo-KAGRA (LVK) all-sky all-frequency (ASAF) directed radiometer analysis, using Advanced LIGO data from the third observing run (O3). Each ASAF candidate corresponds to a $1/32\mathrm{Hz}$ frequency band and $\sim 13{\mathrm{deg}}^2$ sky pixel. Assuming they represent possible CW sources, we analyze all 515 ASAF candidates using a semi-coherent, $\mathcal{F}$-statistic-based matched filter search. The search algorithm incorporates a hidden Markov model (HMM), expanding the signal model to allow frequency spin-wandering, as well as unmodeled frequency evolution of less than ${10}^{-5}\mathrm{Hz}$ per day that is not captured by the searched range of $\pm {10}^{-9}\mathrm{Hz}/\mathrm{s}$ in frequency derivative. Significance thresholds with a 5% probability of false alarm per ASAF candidate are determined empirically by searching detector noise at various off-target sky positions. We obtain 14 outliers surviving a set of vetoes designed to eliminate instrumental artifacts. Upon further investigation, these outliers are deemed unlikely to represent astrophysical signals. We estimate the sensitivity of our search to both isolated and binary sources with orbital period greater than one year by recovering simulated signals added to detector data. The minimum detectable strain amplitude at 95% confidence for isolated (long-period binary) sources is $h_0^{95\%}=8.8\times {10}^{-26}$ ($9.4\times {10}^{-26}$) at a frequency of 222.6 Hz. While this study focuses on ASAF subthreshold candidates, the method presented could be applied to follow up candidates from future all-sky CW searches, complementing currently existing methods.

We present a calculation of the spin alignment for unflavored vector mesons in thermalized quark-gluon plasma based on the Kubo formula in linear response theory. This is achieved by expanding the system to the first order of the coupling constant and the spatial gradient. The effect strongly relies on the vector meson’s spectral functions which are determined by the interaction and medium properties. The spectral functions are calculated for the one-quark-loop self-energy with meson-quark interaction. The numerical results show that the correction to the spin alignment from the thermal shear tensor is of the order ${10}^{-4}\sim {10}^{-5}$ for the chosen values of quark-meson coupling constant, if the magnitude of thermal shear tensor is ${10}^{-2}$.

Neutrons and neutrinos are natural probes for new physics. Since they carry no conserved gauge quantum numbers, both can easily mix with the fermions from hidden sectors. A particularly interesting effect is the oscillation of a neutron or a neutrino into a fermion propagating in large extra dimensions. In fact, such a mixing has been identified as the possible origin of small neutrino mass. In this paper, we study neutron oscillations into an extradimensional fermion and show that this effect provides a resonance imaging of the Kaluza-Klein tower. The remarkable feature of this phenomenon is its generic nature: because of a fine spacing of the Kaluza-Klein tower, neutrons at a variety of energy levels, both free or within nuclei, find a bulk oscillation partner. In particular, the partner can be a Kaluza-Klein mode of the same species that gives mass to the neutrino. The existence of bulk states matching the neutron energy levels of nuclear spectra gives rise to tight constraints as well as to potentially observable effects. For a free neutron, we predict recurrent resonant oscillations occurring with the values of the magnetic field correlated with the KK levels. We derive bounds on extra dimensions from ultracold neutron experiments and suggest signatures for refined measurements, which, in particular, can probe the parameter space motivated by the hierarchy problem. Ultracold neutron experiments offer a unique way of Kaluza-Klein spectroscopy.

One of the methods proposed in recent years for studying nonperturbative gauge theory physics is quantum simulation, where lattice gauge theories are mapped onto quantum devices which can be built in the laboratory, or quantum computers. While being very promising and already showing some experimental results, these methods still face several challenges related to the interface between the technological capabilities and the demands of the simulated models; in particular, one such challenge is the need to simulate infinitely dimensional local Hilbert spaces, describing the gauge fields on the links in the case of compact Lie gauge groups, requiring some truncations and approximations which are not completely understood or controllable in the general case. This work proposes a way to obtain arbitrarily large such local Hilbert spaces by using coarse graining of simple, low-dimensional qubit systems, made of components available on most quantum simulation platforms, and thus opening the way to new types of lattice gauge theory quantum simulations.

We study the renormalization group (RG) running of the nonsinglet tensor operator, for $N_{\mathrm{f}}=3$ QCD with Wilson fermions in a mixed action setup, with standard Schrödinger functional (SF) boundary conditions for sea quarks and chirally rotated Schrödinger functional ($\chi \mathrm{SF}$) boundary conditions for valence quarks. Based on a recursive finite-size scaling technique we compute nonperturbatively the tensor step-scaling function for an energy range between a hadronic scale and an electroweak scale, above which perturbation theory may be safely applied. Our result is expressed as the RG-running factor $T^{\mathrm{RGI}}/[T({\mu }_{\mathrm{had}}){]}_{\mathrm{R}}$, where the numerator is the scale independent (renormalization group invariant—RGI) tensor operator and the denominator is its renormalized counterpart at a hadronic scale ${\mu }_{\mathrm{had}}=233(8)\mathrm{MeV}$ in a given scheme. We determine the step-scaling function in four distinct renormalization schemes. We also compute the renormalization parameters of these schemes at ${\mu }_{\mathrm{had}}$ which, combined with the RG-running factor, gives the scheme-independent quantity $Z_{\mathrm{T}}^{\mathrm{RGI}}(g_0^2)$ in four schemes and for a range of bare gauge couplings in which large volume hadronic matrix element simulations are performed by the CLS consortium in $N_{\mathrm{f}}=2+1$ QCD. All four results are compatible and also agree with a recent determination based on a unitary setup for Wilson quarks with Schrödinger functional boundary conditions [arXiv:2309.04314]. This provides a strong universality test.

The single-particle inclusive fragmentation function and the particle multiplicity are observables of fundamental importance in studying properties of quantum chromodynamics at colliders. It is well known that at high energies the multiplicity distribution satisfies Koba-Nielsen-Olesen (KNO) scaling in which all moments are proportional to powers of the mean multiplicity. We prove that, under weak assumptions, the leading dependence of the fragmentation function on multiplicity is itself a kind of KNO scaling in which all moments are inversely proportional to powers of the mean multiplicity. This scaling with multiplicity additionally accounts for the dominant dependence on collision energy in the fragmentation function. The proof relies crucially on properties of the fragmentation function conditioned on the total multiplicity and application of the Stieltjes moment problem. In the process, we construct a novel basis of the fragmentation function expressed as an overall exponential suppression times a series of Laguerre polynomials. We study this scaling of the fragmentation function in experimental electron-position collision data and observe that residual scale violations are significantly reduced.

Starting with the polarization dependent Wigner function of vector mesons, we derive an expression for the 00–component (${\rho }_{00}$) of spin density matrix in terms of the second order gradients of the vector meson distribution functions. We further apply a thermal model to analyze the transverse momentum and the azimuthal angle dependence of ${\rho }_{00}$ for $\varphi$ and $K^{*0}$ mesons resulting from distribution gradients in Au-Au collisions with $\sqrt{s_{NN}}=130\mathrm{GeV}$ at midrapidity. Our results for the transverse momentum dependence indicate that the deviations of ${\rho }_{00}$ from $1/3$ as the signal for spin alignment are greatly enhanced at large transverse momenta and have a strong centrality dependence while analysis of the azimuthal angle (${\varphi }_q$) dependence suggest that such deviations have a $\cos (2{\varphi }_q)$ structure with opposite sign for $\varphi$ and $K^{*0}$. Our finding may be considered as a baseline for probing spin-alignment mechanisms beyond hydrodynamic gradients.

We present a detailed study of the finite momentum dynamics of the $O(4)$ critical point of QCD, which lies in the dynamic universality class of “model G.” The critical scaling of the model is analyzed in multiple dynamical channels. For instance, the finite momentum analysis allows us to precisely extract the pion dispersion curve below the critical point. The pion velocity is in striking agreement with the predictions relation and static universality. The pion damping rate and velocity are both consistent with the dynamical critical exponent $\zeta =3/2$ of model G. Similarly, although the critical amplitude for the diffusion coefficient of the conserved $O(4)$ charges is small, it is clearly visible both in the restored phase and with finite explicit symmetry breaking, and its dynamical scaling is again consistent with $\zeta =3/2$. We determine a new set of universal dynamical critical amplitude ratios relating the diffusion coefficient to a suitably defined order parameter relaxation time. We also show that in a finite volume simulation, the chiral condensate diffuses on the coset manifold in a manner consistent with dynamical scaling, and with a diffusion coefficient that is determined by the transport coefficients of hydrodynamic pions. Finally, the amplitude ratios (together with other nonuniversal amplitudes also reported here) compile all relevant information for further studies of model G both in and out of equilibrium.

We derive the dynamics of (isotropic) scalar perturbations from the mean-field hydrodynamics of full Lorentzian quantum gravity, as described by a two-sector (timelike and spacelike) Barrett-Crane group field theory model. The rich causal structure of this model allows us to consistently implement in the quantum theory the causal properties of a physical Lorentzian reference frame composed of four minimally coupled, massless, and free scalar fields. Using this frame, we are able to effectively construct relational observables that are used to recover macroscopic cosmological quantities. In particular, small isotropic scalar inhomogeneities emerge as a result of (relational) nearest-neighbor two-body entanglement between degrees of freedom of the underlying quantum gravity theory. The dynamical equations we obtain for geometric and matter perturbations show remarkable agreement with those of classical general relativity for sub-Planckian modes. Quantum gravity effects produce important deviations from the classical general relativistic dynamics for trans-Planckian modes, which we show to be associated to subhorizon scales in the physical reference frame we are employing.

This paper studies the reduced phase space formulation (relational formalism) of gravity coupling to the Brown-Kuchař dust for asymptotic flat spacetimes. A set of boundary conditions for the asymptotic flatness are formulated for Dirac observables on the reduced phase space. The physical Hamiltonian generates the time translation of the dust clock. We compute the boundary term of the physical Hamiltonian, which is identical to the Arnowitt-Deser-Misner mass. We construct a set of the symmetry charges on the reduced phase space, which are conserved by the physical Hamiltonian evolution. The symmetry charges generate transformations preserving the asymptotically flat boundary condition. Under the reduced-phase-space Poisson bracket, the symmetry charges form an infinite dimensional Lie algebra ${\mathcal{A}}_G$ after adding a central charge. A suitable quotient of ${\mathcal{A}}_G$ is analogous to the Bondi-Metzner-Sachs algebra at spatial infinity by Henneaux and Troessaert.

An interesting idea, dating back to Feynman [Report from Chapel Hill Conference, edited by C. M. DeWitt and D. Rickles (1957)], argues that quantum mechanics may break down for large masses if one entertains the possibility that gravity can be “classical,” thereby leading to predictions different from conventional low-energy quantum gravity. Despite the technical difficulty in testing such deviations, a large number of experimental proposals have been put forward due to the high level of fundamental interest. Here, we consider the Schrödinger-Newton (SN) theory and the correlated worldline (CWL) theory, and show that they can be distinguished from conventional quantum mechanics, as well as each other, by performing pulsed optomechanics experiments. For CWL specifically we develop a framework resembling the commonly used “Heisenberg-picture” treatment of coupled oscillators, allowing one to perform simple calculations for such systems without delving into the deeper path-integral formalism. We find that discriminating between the theories will be very difficult until experimental control over low frequency quantum optomechanical systems is pushed much further. However, the predicted departures of SN and CWL from quantum mechanics occur at the same scale, so both alternative models could in principle be probed by a single experiment.

Adiabatic binary inspiral in the small mass ratio limit treats the small body as moving along a geodesic of a large Kerr black hole, with the geodesic slowly evolving due to radiative backreaction. Up to initial conditions, geodesics are typically parameterized in two ways: using the integrals of motion energy $E$, axial angular momentum $L_z$, and Carter constant $Q$; or, using orbit geometry parameters semilatus rectum $p$, eccentricity $e$, and (cosine of) inclination $x_I\equiv \cos I$. The community has long known how to compute orbit integrals as functions of the orbit geometry parameters, i.e., as functions expressing $E(p,e,x_I)$, and likewise for $L_z$ and $Q$. Mappings in the other direction—functions $p(E,L_z,Q)$, and likewise for $e$ and $x_I$—have not yet been developed in general. In this note, we develop generic mappings from ($E,L_z,Q$) to ($p,e,x_I$). The mappings are particularly simple for equatorial orbits ($Q=0$, $x_I=\pm 1$), and can be evaluated efficiently for generic cases. These results make it possible to more accurately compute adiabatic inspirals by eliminating the need to use a Jacobian which becomes singular as inspiral approaches the last stable orbit.

The quintet of Dirac $4\times 4$ matrices suggests that the fundamental dimension of the internal (spin) space is $n=5$, instead of the conventional dimension $n=4$. Then instead of the conventional $4\times 4$ tetrads (vierbein), gravity is described in terms of the 5-bein (fünfbein or five legs). We discuss the properties of the spacetime geometry induced from this 5-leg Dirac spinor theory, where the spin connection contains $10\times 4=40$ elements instead of 24 elements in the tetrad formulation of the general relativity theory.

The ringdown (RD) phase of gravitational waves is of prime interest for testing general relativity (GR). The modeling of the linear quasinormal modes (QNMs) within the Kerr spectrum—or with agnostic parametrized deviations to that GR spectrum—has become ordinary; however, specific attention has recently emerged to calibrate the effects of nonlinear perturbations for the predominant quadrupolar $l=2$, $m=2$ mode. In this paper, we test the performance of a few nonlinear toy models and of the nonlinear inspiral-merger-ringdown (IMR) model IMRPhenomD for faithfully representing the RD regime and we compare them with the results obtained using linear solutions as sums of QNM tones. Using several quasicircular, nonprecessing numerical waveforms, we fit the dominant $l=2$, $m=2$ mode of the strain, and we assess the results in terms of both the Bayes factor and the inferred posterior distributions for the mass and spin of the final black hole. We find that the nonlinear models can be comparable or preferred over the linear QNM-only solutions when the analysis is performed from the peak of the strain, especially at high signal-to-noise ratios consistent with third-generation observatories. Since the calibration of the tones’ relative amplitudes and phases in high-overtone models to the progenitor parameters is still missing, or even not achievable, we consider the use of nonlinear models to be more pertinent for performing confident tests of general relativity based on the RD regime starting from early times.

In symmetric teleparallel geometry the curvature and torsion tensors are assumed to vanish identically, while the dynamics of gravity is encoded by nonmetricity. Here the spatially homogeneous and isotropic connections that can accompany the flat Friedmann-Lemaître-Robertson-Walker metric come in three sets. As the trivial set has received much attention, we focus on the two alternative sets, which introduce an extra degree of freedom into the equations. Working in the context of symmetric teleparallel scalar-tensor gravity with generic nonminimal coupling and potential, we show that the extra free function in the connection cannot play the role of dark matter nor dark energy, but it drastically alters the scalar field behavior. We determine the restrictions on the model functions which permit the standard cosmological scenario of successive radiation, dust matter, and scalar potential domination eras to be stable. However, the alternative connections also introduce a rather general possibility of the system meeting a singularity in finite time.

We analyze the spectrum of gravitational waves generated by the induced spectrum of tensor fluctuation during warm natural inflation. In our previous work, it has been demonstrated that an epoch of warm natural inflation can lead to cosmologically relevant dark matter production in the form of primordial black holes. Here, we show that models that solve the dark-matter production also produce a contribution to the cosmic gravitational wave background that satisfies current constraints from pulsar timing and big bang nucleosynthesis. More importantly, this gravitational wave background may be observable in the next generation of space-based and ground-based gravitational wave interferometers.

We derive constraints on the injection of free-streaming dark radiation after big bang nucleosynthesis by considering the decay of a massive hidden sector particle into dark radiation. Such a scenario has the potential to alleviate the Hubble tension by introducing a new energy component to the evolution of the early Universe. We employ observations of the cosmic microwave background (CMB) from Planck 2018 and South Pole Telescope 2018, measurements of the primordial deuterium abundance, $\mathrm{Pantheon}+$ type Ia supernovae data, and baryon acoustic oscillation measurements from BOSS DR12 to constrain these decay scenarios. Prerecombination decays are primarily restricted by observations of the CMB via their impact on the effective number of relativistic species. On the other hand, long-lived decay scenarios in which the massive particle lifetime extends past recombination tend to decrease the late-time matter density inferred from the CMB and are thus subject to constraints from $\mathrm{Pantheon}+$ and baryon acoustic oscillations. We find that, when marginalizing over lifetimes of ${\tau }_Y=[{10}^{-12.08},{10}^{-1.49}]\mathrm{Gyr}$, the decaying particle is limited at $2\sigma$ to only contribute a maximum of 3% of the energy density of the Universe. With limits on these decays being so stringent, neither short-lived nor long-lived scenarios are successful at substantially mitigating the Hubble tension.

In this paper, we study a possible early universe source for the recent observation of a stochastic gravitational wave background at the NANOGrav pulsar timing array. The source is a tachyonic instability in a dark gauge field induced by an axionlike particle (ALP), a known source for gravitational waves. We find that relative to the previous analysis with the NANOGrav 12.5-year dataset, the current 15-year dataset favors parameter space with a relatively larger axion mass and decay constant. This favored parameter space is heavily constrained by $\mathrm{\Delta }{N}_{\mathrm{eff}}$ and overproduction of ALP dark matter. While there are potential mechanisms for avoiding the second problem, evading the $\mathrm{\Delta }{N}_{\mathrm{eff}}$ constraint remains highly challenging. In particular, we find that the gravitational wave magnitude is significantly suppressed with respect to the gauge boson dark radiation, which implies that successfully explaining the NANOGrav observation requires a large additional dark radiation, violating the cosmological constraints. Satisfying the $\mathrm{\Delta }{N}_{\mathrm{eff}}$ constraint will limit the potential contribution from this mechanism to the observed signal to at most a percent level.

In cosmological $N$-body simulations of warm dark matter, thermal velocities of dark-matter particles are sometimes taken into account by adding random initial velocities to the particles of simulation. However, a particle in the $N$-body system represents a huge collection of dark-matter particles, whose average thermal velocity is very close to zero. We consider justification of the procedure of adding thermal velocities in $N$-body simulations and build a simple model of their influence on the power spectrum. Our model captures the physical effect of suppression of the power spectrum at small wave numbers and also explains its artificial enhancement at large wave numbers, observed in numerical simulations with added thermal velocities. The cause of this enhancement is the disturbance of the growth rate of the density profile introduced when adding random initial thermal velocities. Specifically, the model predicts a turnover in the behavior of the simulated power spectrum at a certain wave number $k_{*}$, beyond which it grows as $P(k)\propto k^2$. Our treatment is generalized to a system consisting of several matter components with different thermal velocity dispersion. We also estimate the effects of discreteness related to the bulk velocity field and establish the conditions under which these effects dominate over those of thermal velocities.