Einstein’s theory of general relativity predicts that the only stationary configuration of an isolated black hole is the Kerr spacetime, which has a unique multipolar structure and a spherical shape when nonspinning. This is in striking contrast to the case of other self-gravitating objects, which instead can in principle have arbitrary deformations even in the static case. Here, we develop a general perturbative framework to construct stationary stars with small axisymmetric deformations and study explicitly compact stars with an intrinsic quadrupole moment. The latter can be sustained, for instance, by crust stresses or strong magnetic fields. While our framework is general, we focus on quadrupolar deformations of neutron stars induced by an anisotropic crust, which continuously connect to spherical neutron stars in the isotropic limit. Deformed neutron stars might provide a more accurate description for isolated and binary compact objects and can be used to improve constraints on the neutron-star equation of state through gravitational-wave detections and through the observation of low-mass x-ray binaries. We argue that, for values of the dimensionless intrinsic quadrupole moment of few percent or higher (which can be sustained by an elastic crust with ordinary parameters), the effect of the deformation is stronger than that of tidal interactions in coalescing neutron-star binaries, and might also significantly affect the electromagnetic signal from accreting neutron stars. Current observational bounds on the post-Newtonian coefficients in the gravitational waveform signal from GW170817 do not exclude that the neutron stars in the binary had some significant intrinsic deformation.

We construct the field of an arbitrarily accelerating and rotating point mass which specializes to the Kerr solution when the acceleration vanishes and specializes to Kinnersley’s arbitrarily accelerating point mass when the rotation vanishes.

Anisotropies generically dominate the earliest stages of expansion of a homogeneous universe. They are particularly relevant in bouncing models, since shears grow in the contracting phase of the cosmos, making the isotropic situation unstable. This paper extends the study of cosmological perturbations in loop quantum cosmology (LQC) to anisotropic Bianchi I models that contain a bounce followed by a phase of slow-roll inflation. We show that, although the shear tensor dilutes and the universe isotropizes soon after the bounce, cosmic perturbations retain memory of this short anisotropic phase. We develop the formalism needed to describe perturbations in anisotropic, effective LQC, and apply it to make predictions for the cosmic microwave background (CMB), while respecting current observational constraints. We show that the anisotropic bounce induces: (i) anisotropic features in all angular correlation functions in the CMB, and in particular a quadrupolar modulation that can account for a similar feature observed in the temperature map by the Planck satellite, and (ii) quantum entanglement between scalar and tensor modes, that manifests itself in temperature-polarization (T-B and E-B) correlations in the CMB.

A spinning neutron star (NS) that is asymmetric with respect to its spin axis can emit continuous gravitational wave (GW) signals. The spin frequencies and their distribution of radio millisecond pulsars (MSPs) and accreting MSPs provide some evidences of GW radiation, and MSPs are ideal probes detecting high frequency GW signals. It is generally thought that MSPs originate from the recycled process, in which the NS accretes the material and angular momentum from the donor star. The accreted matter would be confined at the polar cap zone by an equatorial belt of compressed magnetic field fixed in the deep crust of the NS, and yields “magnetic mountain.” Based on an assumption that the spin-down rates of three transitional MSPs including PSR $\mathrm{J}1023+0038$ are the combinational contribution of the accretion torque, the propeller torque, and the GW radiation torque, in this work we attempt to constrain the ellipticities of MSPs with observed spin-down rates. Assuming some canonical parameters of NSs, the ellipticities of three transitional MSPs and ten redbacks are estimated to be $\epsilon =(0.9-23.4)\times {10}^{-9}$. The electrical resistivities of three transitional MSPs are also derived to be in the range $\eta =(1.2-15.3)\times {10}^{-31}\mathrm{s}$, which display an ideal power law relation with the accretion rate. The characteristic strains ($h_{\mathrm{c}}=(0.6-2.5)\times {10}^{-27}$) of GW signals emitting by these sources are obviously beyond the sensitivity scope of the aLIGO. We expect that the third-generation GW detectors like the Einstein Telescope can seize the GW signals from these sources in the future.

Axion helioscopes search for solar axions and axionlike particles via inverse Primakoff conversion in strong laboratory magnets pointed at the Sun. Anticipating the detection of solar axions, we determine the potential for the planned next-generation helioscope, the International Axion Observatory (IAXO), to measure or constrain the solar magnetic field. To do this we consider a previously neglected component of the solar axion flux at sub-keV energies arising from the conversion of longitudinal plasmons. This flux is sensitively dependent to the magnetic field profile of the Sun, with lower energies corresponding to axions converting into photons at larger solar radii. If the detector technology eventually installed in IAXO has an energy resolution better than 200 eV, then solar axions could become an even more powerful messenger than neutrinos of the magnetic field in the core of the Sun. For energy resolutions better than 10 eV, IAXO could access the inner 70% of the Sun and begin to constrain the field at the tachocline: the boundary between the radiative and convective zones. The longitudinal plasmon flux from a toroidal magnetic field also has an additional 2% geometric modulation effect which could be used to measure the angular dependence of the magnetic field.

We investigate the implications of the general frame approach for conformal Bjorken flow beyond the earlier studies. We show that the power series solution at late times is not unique and is accompanied by an exact solution of the form $1/\tau$, which becomes unphysical if taken on shell. In contrast to the Müller-Israel-Stewart formalism, a matching between $\mathcal{N}=4\mathrm{SYM}$ results and the hydro expansion is only possible up to the first order, which gives rise to $\eta /s=1/4\pi$. Matching the results to the next order gives rise to causality/stability-violating values. Furthermore, we show that the pressure anisotropy in the general frame cannot capture the hydrodynamization, and we introduce an alternative measure to find the attractor. Using slow-roll expansion, we find an analytical approximation form for the attractor. We also show that the early-time behavior of attractors is related to stability and causality conditions. The attractor solutions outside the stable and causal regime give rise to reheating and negative longitudinal pressures in early times, in contrast to the stable and causal ones. We also comment on the violation of the second law of thermodynamics by the off-shell parameters. We show that for the stable and causal choice of parameters, the off-shell canonical entropy of the attractors, which is not a physical quantity, has a negative divergence in early times before tending to its on-shell limit. On the other hand, the unstable and acausal attractors have non-negative entropy divergence. We speculate that the violation of the second law by stable and causal off-shell parameters is required for stability of the first-order hydrodynamics. We investigate the analytical structure of the Borel-transformed series and find the proper relation between the poles and nonhydro modes.

We consider the possibility of probing the left-right symmetric model (LRSM) via the cosmic microwave background (CMB). We adopt the minimal LRSM with Higgs doublets, also known as the doublet left-right model (DLRM), where all fermions including the neutrinos acquire masses only via their couplings to the Higgs bidoublet. Because of the Dirac nature of light neutrinos, there exist additional relativistic degrees of freedom which can thermalize in the early Universe by virtue of their gauge interactions corresponding to the right sector. We constrain the model from Planck 2018 bound on the effective relativistic degrees of freedom and also estimate the prospects for planned CMB stage IV experiments to constrain the model further. We find that the $W_R$ boson mass below 4.06 TeV can be ruled out from the Planck 2018 bound at $2\sigma$ C.L. in the exact left-right symmetric limit which is equally competitive as the LHC bounds from dijet resonance searches. On the other hand, the Planck 2018 bound at $1\sigma$ C.L. can rule out a much larger parameter space out of reach of present direct search experiments, even in the presence of additional relativistic degrees of freedom around the TeV corner. We also study the consequence of these constraints on dark matter in the DLRM by considering a right-handed real fermion quintuplet to be the dominant dark matter component in the Universe.

We present the first study of $N_f=2+1+1+1$ lattice QCD with domain-wall quarks. The ($\mathbf{b}$, $\mathbf{c}$, $\mathbf{s}$) quarks are physical, while the ($\mathbf{u}$, $\mathbf{d}$) quarks are heavier than their physical masses, with the pion mass $\sim 700\mathrm{MeV}$. The gauge ensemble is generated by hybrid Monte Carlo simulation with the Wilson gauge action for the gluons, and the optimal domain-wall fermion action for the quarks. Using point-to-point quark propagators, we measure the time-correlation functions of quark-antiquark meson interpolators with quark contents $\overline{\mathbf{b}}\mathbf{b}$, $\overline{\mathbf{b}}\mathbf{c}$, $\overline{\mathbf{b}}\mathbf{s}$, and $\overline{\mathbf{c}}\mathbf{c}$, and obtain the masses of the low-lying mesons. They are in good agreement with the experimental values, plus some predictions which have not been observed in experiments. Moreover, we also determine the masses of ($\mathbf{b}$, $\mathbf{c}$, $\mathbf{s}$) quarks.

Measurements of a permanent neutron electric dipole moment (EDM) potentially probe beyond-the-Standard Model (BSM) sources of $CP$-violation. At low energy the $CP$-violating BSM interactions are parametrized by flavor-conserving $CP$-violating operators of dimension higher than four. QCD calculations of the nucleon matrix elements of these operators are required to fully reconstruct the sources and magnitudes of the different $CP$-violating contributions to the nucleon EDM. Herein we study the quark-chromoelectric dipole moment (qCEDM) operator and the three-gluon Weinberg operator. The nonperturbative determination, using lattice QCD, of the nucleon matrix elements of these $CP$-violating operators is hampered by their short-distance behavior. Under renormalization these operators mix with lower-dimensional operators, which induces power divergences in the lattice spacing, as the continuum limit is approached. We study the short-distance behavior of the qCEDM and the Weinberg operators using the gradient flow. We perform a short flow time expansion and determine, in perturbation theory, the expansion coefficients of the linearly divergent terms stemming from the mixing with the pseudoscalar density and the topological charge, confirming the expectations of the operator product expansion. We introduce a new method to perform calculations at nonzero flow-time for arbitrary values of the external momenta. This method allows us to work in four dimensions for most of the calculations described in this paper, avoiding the complications associated with defining ${\gamma }_5$ in generic $d$ dimensions. We show that leading contributions in the external momenta can be reproduced by defining ${\gamma }_5$ using the t Hooft-Veltman-Breitenlohner-Maison scheme.

To study the $CP$-violation using the $K_0-{\overline{K}}_0$ oscillation, we need the kaon bag parameter which represents QCD corrections in the leading Feynman diagrams. The lattice QCD provides us with the only way to evaluate the kaon bag parameter directly from the first principles of QCD. However, a calculation of relevant four quark operators with theoretically sound Wilson-type lattice quarks had to carry a numerically big burden of extra renormalizations and resolution of extra mixings due to the explicit chiral violation. Recently, the small flow-time expansion ($\text{SF}t\text{X}$) method was proposed as a general method based on the gradient flow to correctly calculate any renormalized observables on the lattice, irrespective of the explicit violations of related symmetries on the lattice. To apply the $\text{SF}t\text{X}$ method, we need matching coefficients, which relate finite operators at small flow times in the gradient flow scheme to renormalized observables in conventional renormalization schemes. In this paper, we calculate the matching coefficients for four quark operators and quark bilinear operators, relevant to the kaon bag parameter.

In a qualitative study, the low-energy properties of the $\mathrm{SO}(6)$-symmetric quark-meson-diquark model as an effective model for two-color quantum chromodynamics are investigated within the functional renormalization group (FRG) approach. In particular, we compute the infrared scaling behavior of fluctuation-induced higher-derivative couplings of the linear quark-meson-diquark model and map the resulting renormalized effective action onto its nonlinear counterpart. The higher-derivative couplings of the nonlinear model, which we identify as the low-energy couplings of the quark-meson-diquark model, are therefore entirely determined by the FRG flow of their linear equivalents. This grants full access to their scaling behavior and provides insights into conceptual aspects of purely bosonic effective models, as they are treated within the FRG. In this way, the presented work is understood as an immediate extension of our recent advances in the $\mathrm{SO}(4)$-symmetric quark-meson model beyond common FRG approximations.

Inspired by the newly observed ${\mathrm{\Xi }}_c^0$ states by the LHCb Collaboration, we investigate the OKubo-Zweig-Iizuka-allowed two-body strong decays of the $\lambda$-mode $1P$ wave ${\mathrm{\Xi }}_c^{\prime }$ states within the chiral quark model. Our results indicate that: (i) The newly observed states ${\mathrm{\Xi }}_c(2923{)}^0$ and ${\mathrm{\Xi }}_c(2939{)}^0$ are good candidates of the $\lambda$-mode $1P$ wave ${\mathrm{\Xi }}_c^{\prime }$ states with the spin-parity $J^P=3/2^{-}$, namely ${|}^4{P}_{\lambda }3/2^{-}⟩$ and ${|}^2{P}_{\lambda }3/2^{-}⟩$, respectively. (ii) The other newly observed state ${\mathrm{\Xi }}_c(2965{)}^0$ mostly corresponds to the $\lambda$-mode $1P$-wave ${\mathrm{\Xi }}_c^{\prime }$ state with the spin-parity $J^P=5/2^{-}$, namely ${|}^4{P}_{\lambda }5/2^{-}⟩$. (iii) For the two $\lambda$-mode $J^P=1/2^{-}$ mixed states, the $|P_{\lambda }1/2^{-}{⟩}_1$ is a narrow state with a width of $\mathrm{\Gamma }\sim 15\mathrm{MeV}$ and mainly decays into ${\mathrm{\Xi }}_c^{\prime }\pi$, while the $|P_{\lambda }1/2^{-}{⟩}_2$ state has a width of $\mathrm{\Gamma }\sim 52\mathrm{MeV}$ and dominantly decays into ${\mathrm{\Xi }}_c\pi$ and ${\mathrm{\Lambda }}_{c}K$ channels. If the broad structure around 2880 MeV observed at LHCb arises from the new ${\mathrm{\Xi }}_c^0$ state, this state is very likely to be the $|P_{\lambda }1/2^{-}{⟩}_2$ state.

We investigate the idea of detecting pure neutrino-hydrogen interactions in a multinuclear target using the transverse kinematic imbalance technique [Lu et al., Phys. Rev. D 92, 051302 (2015)] in a high-pressure time projection chamber (HPTPC). With full solid-angle acceptance, MeV-level proton tracking threshold, state-of-the-art tracking resolution, and an $\mathcal{O}(100{\mathrm{m}}^3)$ gas volume at 10 bar, an HPTPC could provide an opportunity to realize this technique. We propose the use of hydrogen-rich gases in the TPC to achieve high detection purity with a large hydrogen mass. With the projected neutrino beam exposure at the DUNE experiment, neutrino-hydrogen events of the order of ${10}^4$ per year with purity above 90% could be achieved with such an HPTPC using methane gas. In this paper, we present a systematic study of the event rate and purity for a variety of argon-alkane mixtures, and examine these gas candidates for the TPC tracking-related properties.

We investigate two measures of quantum correlations and entanglement, namely the violation of the Bell-Mermin-Klyshko (BMK) inequalities and the quantum discord, for Dirac fermions in the cosmological de Sitter background of dimension four. The BMK violation is focused on the vacuum for two- and four-mode squeezed states and the maximum violation is demonstrated. For the quantum discord, we investigate a maximally entangled in-vacuum state. Qualitative similarities as well as differences of our results with that of different coordinations of de Sitter in the context of scalar and fermionic field theory are discussed.

Tensor-multiscalar theories (TMST) are among the most natural generalizations of Einstein’s theory; they are mathematically self-consistent and free from pathologies. They pass through all the known observations but contrary to standard scalar-tensor theories, TMST offer extremely rich spectrum of solutions and allow for large deviations from general relativity. One of the most interesting objects in these theories are the topological neutron stars. The main goal in the present paper is to study their radial stability since many branches of solutions can exist even for a fixed value of the topological charge. It turns out that only one of these branches is stable for each value of the topological charge while all the rest are unstable. The stable branch is exactly the one that spans from small to large neutron star masses having as well moderate values of the radii in agreement with the observations. As expected, it becomes unstable at the maximum of the mass. The frequencies of the radial modes are examined and it turns out that in most cases the mode frequencies are larger than the general relativistic ones and they increase with the increase of the topological charge. All this shows that the topological neutron stars are viable astrophysical objects that should be explored further in order to determine their observational manifestations.

It is now known that, apart from black holes, some naked singularities can also cast shadows which provide their possible observable signatures. We examine the relevant question here as to how to distinguish then these entities from each other, in terms of further physical signatures. In earlier literature, it has been pointed out that the Schwarzschild black holes always admit timelike bound orbits having positive perihelion precession. It has been also shown, while some naked singularities [e.g., Joshi-Malafarina-Narayan spacetime (JMN)] with a photon sphere can cast a shadow, it could also admit positive perihelion precession for such orbits, thereby mimicking a Schwarzschild black hole. This indicates that nonspinning compact objects with photon spheres may always admit positive perihelion precession of timelike bound orbits around them. In this paper, we construct a static, nonrotating spacetime configuration which has a central naked singularity but no photon sphere, and it can give both shadow and a negative perihelion precession. In the present case, the shadow is cast by a spherically symmetric thin shell of matter, not by a photon sphere. Our results imply that, whereas the presence of a shadow and a positive perihelion precession implies either a Schwarzschild black hole or a naked singularity, the presence of a shadow and a negative perihelion precession simultaneously may imply a naked singularity only. We discuss our results in the context of stellar motions (motion of the “S” stars) around the Sgr-A* galactic center.

We study horizon shells and soldering freedom for extreme black holes and how supertranslation-like Bondi-Metzner-Sachs (BMS) symmetries appear as soldering transformations. Further, for a null shell placed infinitesimally close to the horizon of an extreme Reissner–Nordström (RN) black hole, we show superrotation-like symmetries also arise as soldering freedom. Next, considering the interaction of impulsive gravitational waves supported at the horizon shell with test particles, we study how the “memory” (or the imprints) of BMS-like symmetries gets encoded in the geodesics (test particles) crossing the shell. Our study shows, timelike test particles get displaced from their initial plane when they cross the horizon shell. For a null geodesic congruence crossing the horizon shell, the optical tensors corresponding to the congruence suffer jumps. In both cases, the changes are induced by BMS parameters that constitute the gravity wave and matter degrees of freedom of the shell.

Using nonlinear, fully relativistic, simulations we investigate the dynamics and gravitational-wave signature associated with instabilities in neutron star postmerger remnants. For simplified models of the remnant we establish the presence of an instability in stars with moderate $T/|W|$, the ratio between the kinetic and the gravitational potential energies. Detailed analysis of the density oscillation pattern reveals a local instability in the inner region of the more realistic differential rotation profile. We apply Rayleigh’s inflection theorem and Fjørtoft’s theorem to analyze the stability criteria concluding that this inner local instability originates from a shear instability close to the peak of the angular velocity profile, and that it later evolves into a fast-rotating $m=2$ oscillation pattern. We discuss the importance of the presence of a corotation point in the fluid, its connection with the shear instability, and comparisons to the Rossby wave and Papaloizou Pringle instabilities considered in the wider literature.

In recent work minimal theories allowing the variation of the cosmological constant, $\mathrm{\Lambda }$, by means of a balancing torsion, have been proposed. It was found that such theories contain parity violating homogeneous and isotropic solutions, due to a torsion structure called the Cartan spiral staircase. Their dynamics are controlled by Euler and Pontryagin quasitopological terms in the action. Here we show that such theories predict a dramatically different picture for gravitational wave fluctuations in the parity violating branch. If the dynamics are ruled solely by the Euler-type term, then linear tensor mode perturbations are entirely undetermined, hinting at a new type of gauge invariance. The Pontryagin term not only permits for phenomenologically sounder background solutions (as found in previous literature), but for realistic propagation of gravitational wave modes. These have the general property that the right and left handed gravitational waves propagate with different speeds. More generally they imply modified dispersion relations for the graviton, with both parity violating and non-violating deformations, including an effective mass for both gravitational wave polarizations. We discuss the observational constraints and predictions of these theories.

Using a continued fraction ansatz we obtain an analytic approximation for a spherically symmetric black hole solution to Einsteinian quartic gravity (EQG), the next simplest generalized quasitopological gravity (GQTG) after Einsteinian cubic gravity (ECG). This approximate solution is valid everywhere outside of the horizon and we use it to investigate the orbit of massive test bodies near a black hole, specifically computing the innermost stable circular orbit. Using Shapiro time delay we calculate the constraints on the EQG coupling parameter. Finally we compute the shadow of an EQG black hole and figure out it to be larger than its Einsteinian counterpart in general relativity for the same value of the mass. By applying our results to Sagittarius A* (Sgr A*) at the center of Milky Way we find, similar to ECG black holes, that departures from general relativity are small but distinguishable for EQG black holes.