It is well known that helical magnetic fields undergo a so-called inverse cascade by which their correlation length grows due to the conservation of magnetic helicity in classical ideal magnetohydrodynamics (MHD). At high energies above approximately 10 MeV, however, classical MHD is necessarily extended to chiral MHD and then the conserved quantity is $⟨\mathcal{H}⟩+2⟨{\mu }_5⟩/\lambda$ with $⟨\mathcal{H}⟩$ being the mean magnetic helicity and $⟨{\mu }_5⟩$ being the mean chiral chemical potential of charged fermions. Here, $\lambda$ is a (phenomenological) chiral feedback parameter. In this paper, we study the evolution of the chiral MHD system with the initial condition of nonzero $⟨\mathcal{H}⟩$ and vanishing ${\mu }_5$. We present analytic derivations for the time evolution of $⟨\mathcal{H}⟩$ and $⟨{\mu }_5⟩$ that we compare to a series of laminar and turbulent three-dimensional direct numerical simulations. We find that the late-time evolution of $⟨\mathcal{H}⟩$ depends on the magnetic and kinetic Reynolds numbers ${\mathrm{Re}}_{\mathrm{M}}$ and ${\mathrm{Re}}_{\mathrm{K}}$. For a high ${\mathrm{Re}}_{\mathrm{M}}$ and ${\mathrm{Re}}_{\mathrm{K}}$ where turbulence occurs, $⟨\mathcal{H}⟩$ eventually evolves in the same way as in classical ideal MHD where the inverse correlation length of the helical magnetic field scales with time $t$ as $k_{\mathrm{p}}\propto t^{-2/3}$. For a low Reynolds numbers where the velocity field is negligible, the scaling is changed to $k_{\mathrm{p}}\propto t^{-1/2}\ln (t/t_{\log })$. After being rapidly generated, $⟨{\mu }_5⟩$ always decays together with $k_{\mathrm{p}}$, i.e., $⟨{\mu }_5⟩\approx k_{\mathrm{p}}$, with a time evolution that depends on whether the system is in the limit of low or high Reynolds numbers.

We study the prospect of using TianQin to detect stellar-mass binary black holes (SBBHs). We estimate the expected detection number as well as the precision of parameter estimation on SBBH inspirals, using five different population models. We note TianQin can possibly detect a few SBBH inspirals with signal to noise ratios greater than 12; lowering the threshold and combining multiple detectors can both boost the detection number. The source parameters can be recovered with good precision for most events above the detection threshold. For example, the precision of the merger time most likely occurs near 1 s, making it possible to guide the detection of the ground-based detectors, the precision of the eccentricity $e_0$ most likely occurs near ${10}^{-4}$, making it possible to distinguish the formation channels, and the precision of the mass parameter is better than ${10}^{-6}$ in general and most likely occurs near ${10}^{-7}$. We note, in particular, that for a typical merger event, the error volume is likely to be small enough to contain only the host galaxy, which could greatly help in the study of gravitational wave cosmology and relevant studies through the multimessenger observation.

With a reinterpretation of recent results, the GRACE Follow-On mission can be applied to gravitational wave astronomy. Existing GRACE Follow-On data constrain the stochastic gravitational wave background to ${\mathrm{\Omega }}_{\mathrm{GW}}<3.3\times {10}^7$ at 100 mHz. With a dedicated analysis, GRACE Follow-On may be able to detect the inspiral of local neutron star binaries, inspiral of subgalactic stellar-mass black hole binaries, or mergers of intermediate-mass black hole binaries within the Milky Way.

We revisit the chiral anomaly in the quantum kinetic theory in the Wigner function formalism under the background field approximation. Our results show that the chiral anomaly is actually from the Dirac sea or the vacuum contribution in the un-normal-ordered Wigner function. We also demonstrate that this contribution modifies the chiral kinetic equation for antiparticles.

Nonstandard interactions (NSI) of neutrinos with matter mediated by a scalar field would induce medium-dependent neutrino masses which can modify oscillation probabilities. Generating observable effects requires an ultralight scalar mediator. We derive general expressions for the scalar NSI using techniques of quantum field theory at finite density and temperature, including the long-range force effects, and discuss various limiting cases applicable to the neutrino propagation in different media, such as the Earth, Sun, supernovae and early Universe. We also analyze various terrestrial and space-based experimental constraints, as well as astrophysical and cosmological constraints on these NSI parameters, applicable to either Dirac or Majorana neutrinos. By combining all these constraints, we show that observable scalar NSI effects, although precluded in terrestrial experiments, are still possible in future solar and supernovae neutrino data, and in cosmological observations such as cosmic microwave background and big bang nucleosynthesis data.

Using the general formalism presented in [Phys. Rev. D 94, 013008 (2016); Phys. Rev. D 100, 034511 (2019)], we study the finite-volume effects for the $\mathbf{2}+\mathcal{J}\to \mathbf{2}$ matrix element of an external current coupled to a two-particle state of identical scalars with perturbative interactions. Working in a finite cubic volume with periodicity $L$, we derive a $1/L$ expansion of the matrix element through $\mathcal{O}(1/L^5)$ and find that it is governed by two universal current-dependent parameters, the scalar charge and the threshold two-particle form factor. We confirm the result through a numerical study of the general formalism and additionally through an independent perturbative calculation. We further demonstrate a consistency with the Feynman-Hellmann theorem, which can be used to relate the $1/L$ expansions of the ground-state energy and matrix element. The latter gives a simple insight into why the leading volume corrections to the matrix element have the same scaling as those in the energy, $1/L^3$, in contradiction to Phys. Rev. D 91, 074509 (2015), which found a $1/L^2$ contribution to the matrix element. We show here that such a term arises at intermediate stages in the perturbative calculation, but cancels in the final result.

We study three-body decays of ${\mathrm{\Lambda }}_c^{*}(2765)\to {\mathrm{\Lambda }}_c^{+}{\pi }^{+}{\pi }^{-}$ by using effective Lagrangians in a nonrelativistic framework. We consider the sequential decays through ${\mathrm{\Sigma }}_c(2455)\pi$ and ${\mathrm{\Sigma }}_c^{*}(2520)\pi$ in intermediate states which are dominant contributions. The coupling constants in the effective Lagrangians are computed in the quark model. We demonstrate that the ratio $R=\mathrm{\Gamma }({\mathrm{\Lambda }}_c^{*}\to {\mathrm{\Sigma }}_c^{*}(2520)\pi )/\mathrm{\Gamma }({\mathrm{\Lambda }}_c^{*}\to {\mathrm{\Sigma }}_c(2455)\pi )$ and angular correlations are sensitive to the spin and parity of ${\mathrm{\Lambda }}_c^{*}(2765)$. Thus, the measurement of these observables in experimental facilities such as Belle and LHCb can provide useful constraints to determine the spin and parity of ${\mathrm{\Lambda }}_c^{*}(2765)$.

We discuss the general properties of the $B$-meson decay form factors $F(q^2,q^{\prime 2})$, describing $B$-meson decays induced by two currents, e.g., $B$ decays into four leptons in the final state. We study the analytic properties of these complicated objects and identify those regions of $q^2$ and $q^{\prime 2}$, where perturbative QCD can be applied for obtaining predictions for the physical form factors.

We study and reveal the relation between the electroweak sphaleron energy and the gravitational wave signals from a first-order electroweak phase transition. For the first time, we find that a larger sphaleron energy corresponds to a stronger gravitational wave signal for a sufficiently strong phase transition. We also present the correlation between the sphaleron and the properties of the gravitational waves, such as the signal-to-noise ratio, peak frequency, and amplitude, with the intention that traces of the sphaleron can be obtained with detailed measurement and studies of the stochastic gravitational wave at future space-based detectors. This study paves the way for a more practical revelation of the sphaleron which is otherwise unprobable at high energy colliders.

The adiabatic regularization method was originally proposed by Parker and Fulling to renormalize the energy-momentum tensor of scalar fields in expanding universes. It can be extended to renormalize the electric current induced by quantized scalar fields in a time-varying electric background. This can be done in a way consistent with gravity if the vector potential is considered as a variable of adiabatic order one. Assuming this, we further extend the method to deal with Dirac fields in four spacetime dimensions. This requires a self-consistent ansatz for the adiabatic expansion, in presence of a prescribed time-dependent electric field, which is different from the conventional expansion used for scalar fields. Our proposal is consistent, in the massless limit, with the conformal anomaly. We also provide evidence that our proposed adiabatic expansion for the fermionic modes parallels the Schwinger-DeWitt adiabatic expansion of the two-point function. We give the renormalized expression of the electric current and analyze, using numerical and analytical tools, the pair production induced by a Sauter-type electric pulse. We also analyze the scaling properties of the current for a large field strength.

In $CP$-violating conformal field theories in four dimensions, the Pontryagin density can appear in the Weyl anomaly. The Pontryagin density in the Weyl anomaly is consistent, but it has a peculiar feature that the parent three-point function of the energy-momentum tensor can violate $CP$ only (semi)locally. In this paper, we study the supersymmetric completion of the Pontryagin density in the Weyl anomaly, where the central charge $c$ effectively becomes a complex number. The supersymmetry suggests that it accompanies the graviphoton $\theta$ term associated with the R-symmetry gauging in the Weyl anomaly. It also accompanies new $CP$-violating terms in the R-current anomaly. While there are no conclusive perturbative examples of $CP$-violating super Weyl anomaly, we construct explicit supersymmetric dilaton effective action which generates these anomalies.

It has been argued that the symmetries of gravity at null infinity should include a $\mathrm{Diff}(S^2)$ factor associated with diffeomorphisms on the celestial sphere. However, the standard phase space of gravity does not support the action of such transformations. Building on earlier work by Laddha and one of the authors, we present an extension of the phase space of gravity at null infinity on which $\mathrm{Diff}(S^2)$ acts canonically. The Poisson brackets of supertranslation and $\mathrm{Diff}(S^2)$ charges reproduce the generalized BMS algebra introduced in [Campiglia and Laddha Phys. Rev. D 90, 124028 (2014)].

We consider the observation of stellar-mass black holes binaries with the Laser Interferometer Space Antenna (LISA). Preliminary results based on Fisher information matrix analyses have suggested that gravitational waves from those sources could be very sensitive to possible deviations from the theory of general relativity and from the strong equivalence principle during the low-frequency binary inspiral. We perform a full Markov Chain Monte Carlo Bayesian analysis to quantify the sensitivity of these signals to two phenomenological modifications of general relativity, namely a putative gravitational dipole emission and a nonzero mass for the graviton, properly accounting for the detector’s response. Moreover, we consider a scenario where those sources could be observed also with Earth-based detectors, which should measure the coalescence time with precision better than 1 ms. This constraint on the coalescence time further improves the bounds that we can set on those phenomenological deviations from general relativity. We show that tests of dipole radiation and the graviton’s mass should improve respectively by seven and half an order(s) of magnitude over current bounds. Finally, we discuss under which conditions one may claim the detection of a modification to general relativity.

We derive the equations of motion of a test particle in the equatorial plane around a static and spherically symmetric wormhole influenced by a radiation field including the general relativistic Poynting-Robertson effect. From the analysis of this dynamical system, we develop a diagnostic to distinguish a black hole from a wormhole, which can be timely supported by several and different observational data. This procedure is based on the possibility of having some wormhole metrics, which smoothly connect to the Schwarzschild metric in a small transition surface layer very close to the black hole event horizon. To detect such a metric change, we analyze the emission proprieties from the critical hypersurface (stable region where radiation and gravitational fields balance) together with those from an accretion disk in the Schwarzschild spacetime toward a distant observer. Indeed, if the observational data are well fitted within such a model, it immediately implies the existence of a black hole; while in the case of strong departures from such a description it means that a wormhole could be present. Finally, we discuss our results and draw conclusions.

In this article, we use the dynamical system approach to study the linear dynamics of a Randall-Sundrum braneworld model with a tachyon scalar field confined to the brane. We recognize that the form of the tachyon potential plays a significant role in the evolution of the Universe. For the case of an inverse square potential we find that one of our new variables, $\lambda$, is constant. We obtain critical points of the system in this situation and investigate their stability using the linear perturbation method. Then we turn to a Gaussian potential in which $\lambda$ is not constant. Using the idea of instantaneous critical points we study the behavior of the Universe and its possible fates. One of the interesting results of this article is that our Universe will probably experience another phase transition from acceleration to deceleration in the future.

Dark energy models can be seen as dynamical systems. In this paper, we show that multifield models with a curved field space give rise to new critical points and we analyze their stability. These are new accelerating solutions in late-time cosmology which exist even for steep potentials. This opens up the possibility to realize quintessence models even when quantum corrections spoil the flatness of the underlying potential. These nonlinear sigma models arise naturally in supergravity and string models where their multifield dynamics can help to avoid swampland bounds.

We build upon previous investigation of the one-dimensional conformal symmetry of the Friedman-Lemaître-Robertson-Walker (FLRW) cosmology of a free scalar field and make it explicit through a reformulation of the theory at the classical level in terms of a manifestly $\mathrm{SL}(2,\mathbb{R})$-invariant action principle. The new tool is a canonical transformation of the cosmological phase space to write it in terms of a spinor, i.e., a pair of complex variables that transform under the fundamental representation of $\mathrm{SU}(1,1)\sim \mathrm{SL}(2,\mathbb{R})$. The resulting FLRW Hamiltonian constraint is simply quadratic in the spinor and FLRW cosmology is written as a Schrödinger-like action principle. Conformal transformations can then be written as proper-time dependent $\mathrm{SL}(2,\mathbb{R})$ transformations. We conclude with possible generalizations of FLRW to arbitrary quadratic Hamiltonian and discuss the interpretation of the spinor as a gravitationally-dressed matter field or matter-dressed geometry observable.

The global contribution of unresolved gamma-ray point sources to the extragalactic gamma-ray background has been recently measured down to gamma-ray fluxes lower than those reached with standard source detection techniques, and by employing the statistical properties of the observed gamma-ray counts. We investigate and exploit the complementarity of the information brought by the one-point statistics of photon counts (using more than 10 years of Fermi-Large Area Telescope (LAT) data) and by the recent measurement of the angular power spectrum of the unresolved gamma-ray background (based on 8 years of Fermi-LAT data). We determine, under the assumption that the source-count distribution of the brightest unresolved objects is dominated by blazars, their gamma-ray luminosity function and spectral energy distribution down to fluxes almost two orders of magnitude smaller than the threshold for detecting resolved sources. The different approaches provide consistent predictions for the gamma-ray luminosity function of blazars, and they show a significant complementarity.

With the first detections of binary neutron star mergers by gravitational-wave detectors, it proves timely to consider how the internal structure of neutron stars affects the way in which they can be asymmetrically deformed. Such deformations may leave measurable imprints on gravitational-wave signals and can be sourced through tidal interactions or the formation of mountains. We detail the formalism that describes fully relativistic neutron star models with elastic crusts undergoing static perturbations. This formalism primes the problem for studies into a variety of mechanisms that can deform a neutron star. We present results for a barotropic equation of state and a realistic model for the elastic crust, which enables us to compute relevant quantities such as the tidal deformability parameter. We find that the inclusion of an elastic crust provides a very small correction to the tidal deformability. The results allow us to demonstrate when and where the crust starts to fail during a binary inspiral, and we find that the majority of the crust will remain intact up until merger.

The LIGO observatories detect gravitational waves through monitoring changes in the detectors’ length down to below ${10}^{-19}\mathrm{m}/\sqrt{\mathrm{Hz}}$ variations—a small fraction of the size of the atoms that make up the detector. To achieve this sensitivity, the detector and its environment need to be closely monitored. Beyond the gravitational-wave data stream, LIGO continuously records hundreds of thousands of channels of environmental and instrumental data in order to monitor for possibly minuscule variations that contribute to the detector noise. A particularly challenging issue is the appearance in the gravitational wave signal of brief, loud noise artifacts called “glitches,” which are environmental or instrumental in origin but can mimic true gravitational waves and therefore hinder sensitivity. Currently, they are primarily identified by analysis of the gravitational-wave data stream, and auxiliary data channels often provide corroborating evidence. Here we present a machine learning approach that can identify glitches by considering all environmental and detector data channels, a task that has not previously been pursued due to its scale and the number of degrees of freedom within gravitational-wave detectors. The presented method is capable of reducing the gravitational-wave detector network’s false alarm rate and improving the LIGO instruments, consequently enhancing detection confidence.