This is the final paper in a series of four in which we incorporate backreaction among the homogeneous and between the homogeneous and inhomogeneous degrees of freedom in quantum cosmological perturbation theory using space adiabatic methods. Here, we consider the gauge-invariant scalar (Mukhanov-Sasaki) and tensor (primordial gravitational wave) perturbations of general relativity coupled to a scalar field which arise from a careful constraint analysis of this system up to second order in the perturbations. The simultaneous quantization of the homogeneous and inhomogeneous degrees of freedom requires the use of a suitable constructive perturbation scheme for which we employ space adiabatic perturbation theory, a rigorous extension of the standard Born-Oppenheimer theory. We are confronted with several challenges arising for theories with an infinite number of degrees of freedom. We are able to compute the backreaction effects up to second order in the adiabatic parameter and find modifications as compared to earlier derivations of the effective quantum dynamics of the homogeneous sector.

This is the third paper in a series of four in which we incorporate backreaction among the homogeneous as well as between the homogeneous and inhomogeneous degrees of freedom in quantum cosmological perturbation theory using space adiabatic methods. Here, we consider a gauge-fixed version of cosmological perturbation theory which uses Gaussian dust as a material reference system. The observable matter content is a real-valued scalar field. We explore the sector of that theory which is purely homogeneous and isotropic with respect to the geometry degrees of freedom but which contains inhomogeneous perturbations up to second order of the scalar field. We explore the quantum field theoretical challenges of the space adiabatic framework in a cosmological model of the early universe which is technically still relatively simple. We compute the quantum backreaction effects from the inhomogeneous matter modes on the homogeneous geometry up to second order in the adiabatic parameter. It turns out that these contributions are not negligible.

In this second paper in a series of four, we continue with the program of incorporating backreaction among the homogeneous and between the homogeneous and inhomogeneous degrees of freedom in quantum cosmological perturbation theory. The purpose of the present paper is to illustrate the formalism of space adiabatic perturbation theory for two simple quantum mechanical models, and to prove that backreaction indeed leads to additional correction terms in effective Hamiltonians that one would otherwise neglect in a crude Born-Oppenheimer approximation. The first model consists of a harmonic oscillator coupled to an anharmonic oscillator. The second model describes the coupling between a scalar matter field and gravity restricted to the purely homogeneous and isotropic sector. These results have potential phenomenological consequences for quantum cosmological models.

Quantum cosmology, including quantum cosmological inhomogeneities, is a promising framework for describing the very early universe in which all degrees of freedom are being considered as dynamical and quantum. However, many previous discussions on this subject consider the quantum cosmological perturbations as test fields on a curved spacetime with effective quantum modifications and thus neglect certain interactions of the subsystems, namely the backreaction of the inhomogeneous quantum fields on the cosmological background. In a series of four papers, of which this is the first, we aim at improving on the treatment of quantum effects that arise due to backreactions between matter and geometry as well as between the cosmological perturbations and the homogeneous degrees of freedom. We employ the technique of space adiabatic perturbation theory in the form developed by Panati, Spohn, and Teufel which relies on the familiar Born-Oppenheimer approximation. We extend the scheme to quantum field theory in the cosmological perturbative setting and show that this leads to presently neglected correction terms in the quantum Friedmann equations. In this first article, we provide a detailed introduction to the iterative scheme and examine the generic challenges encountered in its application to perturbative quantum cosmology. Our results will allow for a direct comparison to cosmological observations.

Electric fields in QED are known to discharge due to Schwinger pair production of charged particles. Corresponding electric fields in non-Abelian theory are known to discharge due to the production of gluons. Yet electric flux tubes in QCD ought to be stable to the production of charged gluons as they confine quarks. We resolve this conundrum by finding electric field configurations in pure non-Abelian gauge theory in which the Schwinger process is absent and the electric field is protected against quantum dissipation. We comment on the implications for QCD flux tubes.

We present a method to study the semiclassical gravitational collapse of a radially symmetric scalar quantum field in a coherent initial state. The formalism utilizes a Fock space basis in the initial metric, is unitary and time reversal invariant up to numerical precision. It maintains exact compatibility of the metric with the expectation values of the energy momentum tensor in the scalar field coherent state throughout the entire time evolution. We find a simple criterion for the smallness of discretization effects, which is violated when a horizon forms. As a first example, we study the collapse of a specific state in the angular momentum $l=0$ approximation. Outside the simulated volume, it produces a Schwarzschild metric with $r_s\sim 3.5{\ell }_p$. We see behavior that is compatible with the onset of horizon formation both in the semiclassical and corresponding classical cases in a regime where we see no evidence for large discretization artifacts. In our example setting, we see that quantum effects accelerate the possible horizon formation and move it radially outward. We find that this effect is robust against variations of the radial resolution, the time step, the volume, the initial position and shape of the inmoving state, the vacuum subtraction, the discretization of the time evolution operator, and the integration scheme of the metric. We briefly discuss potential improvements of the method and the possibility of applying it to black hole evaporation. We also briefly touch on the extension of our formalism to higher angular momenta but leave the details and numerics for a forthcoming publication.

The general parametrization of spherically symmetric and asymptotically flat black hole spacetimes in arbitrary metric theories of gravity was suggested in Rezzolla and Zhidenko [Phys. Rev. D 90, 084009 (2014)]. The parametrization is based on the continued fraction expansion in terms of the compact radial coordinate and has superior convergence and a strict hierarchy of parameters. It is known that some observable quantities, related to particle motion around the black hole, such as the eikonal-quasinormal modes, radius of the shadow, frequency at the innermost stable circular orbit, and others, depend mostly on only a few of the lowest coefficients of the parametrization. Here we continue this approach by studying the dominant (low-lying) quasinormal modes for such generally parametrized black holes. We show that, due to the hierarchy of parameters, the dominant quasinormal frequencies are also well determined by only the first few coefficients of the expansion for the so-called moderate black hole geometries. The latter are characterized by a relatively slow change of the metric functions in the radiation zone near the black hole. The nonmoderate metrics, which change strongly between the event horizon and the innermost stable circular orbit, are usually characterized by echoes or by the distinctive (from the Einstein case) quasinormal ringing which does not match the current observational data. Therefore, the compact description of a black hole spacetime in terms of the truncated general parametrization is an effective formalism for testing strong gravity and imposing constraints on allowed black hole geometries.

We investigate physical implications of a gravitational analog of axion electrodynamics with a parity-violating gravitoelectromagnetic theta term. This is related to the Nieh-Yan topological invariant in gravity with torsion, in contrast to the well-studied gravitational Chern-Simons term quadratic in curvature, coupled via a dynamical axionlike scalar field. Axion gravitodynamics is the corresponding linearized theory. We find that potentially observable effects are over 80 orders of magnitude stronger than for its Chern-Simons counterpart and could be in reach for detection by experiments in the near future. For a near-Earth scenario, we derive corrections to the Lense-Thirring effect and compare them to data from satellite-based experiments (Gravity Probe B). For gravitational waves, we find modified dispersion relations, derive the corresponding polarization-dependent modified group and phase velocities, and compare them to data from neutron star mergers (GW170817) to derive even stronger bounds.

We perform general relativistic, magnetohydrodynamic simulations of merging binary neutron stars incorporating neutrino transport and magnetic fields. Our new radiative transport module for neutrinos adopts a general relativistic, truncated-moment formalism. The binaries consist of two identical, irrotational stars modeled by the Skyrme Lyon (SLy) nuclear equation of state. They are initially in quasicircular orbit and threaded with a poloidal magnetic field that extends from the stellar interior into the exterior, as in typical pulsars. We insert neutrino processes shortly after the merger and focus on the role of neutrinos in launching a jet following the collapse of the hypermassive neutron star (HMNS) remnant to a spinning black hole (BH). We treat two microphysical versions: one (a “warm-up”) evolving a single neutrino species and considering only charged-current processes and the other evolving three species $({\nu }_e,{\overline{\nu }}_e,{\nu }_{\mathrm{x}})$ and related processes. We trace the evolution until the system reaches a quasiequilibrium state after BH formation. We find that the $\mathrm{BH}+\text{disk}$ remnant eventually launches an incipient jet. The electromagnetic Poynting luminosity is $\sim {10}^{53}\mathrm{erg}{\mathrm{s}}^{-1}$, consistent with that of typical short gamma-ray bursts. The effect of neutrino cooling shortens the lifetime of the HMNS and lowers the amplitude of the major peak of the gravitational wave power spectrum somewhat. After BH formation, neutrinos help clear out the matter near the BH poles, resulting in lower baryon-loaded surrounding debris. The neutrino luminosity resides in the range $\sim {10}^{52-53}\mathrm{erg}{\mathrm{s}}^{-1}$ once quasiequilibrium is achieved. Comparing with the neutrino-free models, we observe that the inclusion of neutrinos yields similar ejecta masses and is inefficient in carrying off additional angular momentum.

Numerical studies of the dynamics of gravitational systems, e.g., black hole-neutron star systems, require physical and constraint-satisfying initial data. In this article, we present the newly developed pseudospectral code elliptica, an infrastructure for construction of initial data for various binary and single gravitational systems of all kinds. The elliptic equations under consideration are solved on a single spatial hypersurface of the spacetime manifold. Using coordinate maps, the hypersurface is covered by patches whose boundaries can adapt to the surface of the compact objects. To solve elliptic equations with arbitrary boundary condition, elliptica deploys a Schur complement domain decomposition method with a direct solver. In this version, we use cubed sphere coordinate maps and the fields are expanded using Chebyshev polynomials of the first kind. Here, we explain the building blocks of elliptica and the initial data construction algorithm for a black hole-neutron star binary system. We perform convergence tests and evolve the data to validate our results. Within our framework, the neutron star can reach spin values close to breakup with arbitrary direction, while the black hole can have arbitrary spin with dimensionless spin magnitude $\sim 0.8$.

We study the inflationary implications of a novel parity-violating gravity model, which modifies the teleparallel equivalent of general relativity by introducing the Nieh-Yan term coupled to an axionlike field. The parity-violating Nieh-Yan term results in the velocity birefringence of gravitational waves (GWs) and triggers the tachyonic instability only for one of the two circular polarization states. We consider that the inflaton is identified as the coupled axionlike field with a wiggly potential characterized by steep cliffs connected by smooth plateaus. During inflation, the temporary fast roll of the axion on the clifflike region leads to the significant enhancement of the tensor perturbations in one polarization state with the wave numbers that exit the horizon around this period. In this setup, the resulting energy spectrum for GWs presents a sizable localized bump involving the contribution of only one polarization state. This chiral GW background is detectable by GW detectors like LISA and Taiji, and its chirality can be determined by correlating two detectors, which provides an opportunity to probe the inflation and test the gravity model.

We prove the universality of predictions for linear perturbations from the entire class of models of inflation driven by a pseudoscalar field coupled to an $\mathrm{SU}(N)$ gauge boson, where SU(2) subgroups in the $\mathrm{SU}(N)$ crossed with the background spatial SO(3) spontaneously break into a single SO(3). The effect of which SU(2) subgroup in $\mathrm{SU}(N)$ acquires a vacuum expectation value through spontaneous symmetry breaking can be quantified by a single parameter $\lambda$, which always appears in combination with the gauge coupling constant $g$. In the linear perturbations, as well as the background system, the same dynamics and predictions as in the chromo-natural inflation hold for its $\mathrm{SU}(N)$ extension by replacing $g\to g\lambda$. The latter class of models thereby draws the same prediction curve on the $n_s–r$ plane as the former at the tree level as long as $g\lambda$ stays constant during inflation. We briefly discuss possible transitions from one value of $\lambda$ to another during inflation and the observational prospects.

This paper examines the general relativistic model of a geometrically thick configuration of an accretion disk around an electrically charged black hole in an accelerated motion, as described by the $C$-metric family. We aim to study effects of the spacetime background on the magnetized version of the thick disk model via the sequences of figures of equilibrium. While maintaining the assumption of non-self-gravitating (test) fluid, we newly explore the influence of the strength of the large-scale magnetic field with field lines organised over the length scale of the black-hole horizon. We systematically analyze the dependence on a very broad parameter space of the adopted scenario. We demonstrate that the $C$ metric can, in principle, be distinguished from Kerr black-hole metric by resolving specific (albeit rather fine) features of the torus, such as the location of its center, inner, and outer rims, and the overall shape. The analytical setup can serve as a test bed for numerical simulations.

In order to extract the precise physical information encoded in the gravitational and electromagnetic signals from powerful neutron-star merger events, we need to include as much of the relevant physics as possible in our numerical simulations. This presents a severe challenge, given that many of the involved parameters are poorly constrained. In this paper we focus on the role of nuclear reactions. Combining a theoretical discussion with an analysis connecting to state-of-the-art simulations, we outline multiple arguments that lead to a reactive system being described in terms of a bulk viscosity. The results demonstrate that in order to properly account for nuclear reactions, future simulations must be able to handle different regimes where rather different assumptions/approximations are appropriate. We also touch upon the link to models based on the large-eddy-strategy required to capture turbulence.

The increasing observational pressure on the standard cosmological model motivates analyses going beyond the paradigm of the collisionless cold dark matter (DM). Since the only clear evidence for the existence of DM is based on gravitational interactions, it seems particularly fitting to study them in this sector where extensions to the standard model can be naturally introduced. A promising avenue can be obtained using modifications of the space-time metric coupled to DM and induced by the presence of a new ultralight scalar field $\varphi$. The $\varphi$ field can contribute to the DM density and can couple all the matter species universally, including additional heavy DM particles. We present a simple two-component DM model employing derivative conformal interactions between the two DM species. This can simultaneously (1) guarantee the necessary symmetries to stabilize the dark species, (2) predict a subdominant thermal relic density of the heavy DM component, and (3) alleviate small-scale structure tensions of the cold DM scenario due to the possible cointeractions in the dark sector. The scenario is highly predictive with future observational prospects ranging from the Large Hadron Collider to gravitational-wave searches and can be generalized to more rich and realistic dark sector models.

The existence of quark matter inside the cores of heavy neutron stars is a possibility which can be probed with modern astrophysical observations. We use a vector and axial-vector meson extended quark-meson model to describe quark matter in the core of neutron stars. We discover that an additional parameter constraint is necessary in the quark model to ensure chiral restoration at high densities. By investigating hybrid star sequences with various parameter sets, we show that low sigma meson masses are needed to fulfill the upper radius constraints and that the maximum mass of stable hybrid stars is only slightly dependent on the parameters of the crossover-type phase transition. Using this observation and results from recent astrophysical measurements, a constraint of $2.5<g_V<4.3$ is set for the constituent quark–vector meson coupling. The effect of a nonzero bag constant is also investigated, and we observe that its effect is small for values adopted in previous works.

Using data from the HAWC gamma-ray telescope, we have studied a sample of 37 millisecond pulsars (MSPs), selected for their spindown power and proximity. From among these MSP, we have identified four which favor the presence of very high-energy gamma-ray emission at a level of $(2\mathrm{\Delta }\ln \mathcal{L}{)}^{1/2}\ge 2.5$. Adopting a correlation between the spindown power and gamma-ray luminosity of each pulsar, we performed a stacked likelihood analysis of these 37 MSPs, finding that the data supports the conclusion that these sources emit very high-energy gamma-rays at a level of $(2\mathrm{\Delta }\ln \mathcal{L}{)}^{1/2}=4.24$. Among sets of randomly selected sky locations within HAWC’s field-of-view, less than 1% of such realizations yielded such high statistical significance. Our analysis suggests that MSPs produce very high-energy gamma-ray emission with a similar efficiency to that observed from the Geminga TeV-halo, ${\eta }_{\mathrm{MSP}}=(0.39-1.08)\times {\eta }_{\text{Geminga}}$. This conclusion poses a significant challenge for pulsar interpretations of the Galactic Center gamma-ray excess, as it suggests that any population of MSPs potentially capable of producing the GeV excess would also produce TeV-scale emission in excess of that observed by HESS from this region. Future observations by CTA will be able to substantially clarify this situation.

In this paper, we study the $B_s\to D_s$ transition form factors by using the light-cone sum rules within the framework of heavy quark effective field theory. We adopt a chiral current correlation function to do the calculation, the resultant transition form factors $f_{+}^{B_s\to D_s}(q^2)$ and $f_0^{B_s\to D_s}(q^2)$ are dominated by the contribution of $D_s$-meson leading-twist distribution amplitude, while the contributions from less certain $D_s$-meson twist-3 distribution amplitudes are greatly suppressed. At the largest recoil point, we obtain $f_{+,0}^{B_s\to D_s}(0)=0.533_{-0.094}^{+0.112}$. By further extrapolating the transition form factors into all the physically allowable $q^2$ region with the help of the $z$-series parametrization approach, we calculate the branching fractions $\mathcal{B}(B_s\to D_s{\ell }^{\prime }{\nu }_{{\ell }^{\prime }})$ with $({\ell }^{\prime }=e,\mu )$ and $\mathcal{B}(B_s\to D_s\tau {\nu }_{\tau })$, which gives $\mathcal{R}(D_s)=0.334\pm 0.017$.

Despite their inextricable quantum mechanical nature, events at a high energy particle collider experiment typically have very few unambiguous quantum signatures, due the type of data and the manner in which they are collected. We present a general analysis of one feature of quantum mechanics, interference between two orthogonal states on Hilbert space, projected onto the basis of states that span a collider experiment observable space. Identification of quantum interference can be considered as a binary discrimination between a pure state and a mixed state, and we introduce several statistical measures that quantify the amplitude of interference in a pure state with respect to a mixed state that exhibits no interference. Two explicit examples from particle physics are provided to demonstrate features of the general formalism.

We consider the propagation of a neutrino or an antineutrino in a medium composed of fermions ($f$) and scalars ($\varphi$) interacting via a Yukawa-type coupling of the form $\overline{f}\nu \varphi$, for neutrino energies at which the processes like $\nu +\varphi \leftrightarrow f$ or $\nu +\overline{f}\leftrightarrow \overline{\varphi }$, and the corresponding ones for the antineutrino, are kinematically accessible. The relevant energy values are around $|m_{\varphi }^2-m_f^2|/2m_{\varphi }$ or $|m_{\varphi }^2-m_f^2|/2m_f$, where $m_{\varphi }$ and $m_f$ are the masses of $\varphi$ and $f$, respectively. We refer to either one of these regions as a resonance energy range. Near these points, the one-loop formula for the neutrino self-energy has a singularity. From a technical point of view, that feature is indicative that the self-energy acquires an imaginary part, which is associated with damping effects and cannot be neglected, while the integral formula for the real part must be evaluated using the principal value of the integral. We carry out the calculations explicitly for some cases that allow us to give analytic results. Writing the dispersion relation in the form $\omega =\kappa +V_{\mathrm{eff}}-i\gamma /2$, we give the explicit formulas for $V_{\mathrm{eff}}$ and $\gamma$ for the cases considered. When the neutrino energy is either much larger or much smaller than the resonance energy, $V_{\mathrm{eff}}$ reduces to the effective potential that has been already determined in the literature in the high or low momentum regime, respectively. The virtue of the formula we give for $V_{\mathrm{eff}}$ is that it is valid also in the resonance energy range, which is outside the two limits mentioned. As a guide to possible applications we give the relevant formulas for $V_{\mathrm{eff}}$ and $\gamma$, and consider the solution to the oscillation equations including the damping term, in a simple two-generation case.