The motion of compact binaries is influenced by the spin of their components starting at the 1.5 post-Newtonian (PN) order. On the other hand, in the large mass ratio limit, the spin of the lighter object appears in the equations of motion at first order in the mass ratio, coinciding with the leading gravitational self-force. Frame and gauge choices make it challenging to compare between the two limits, especially for generic spin configurations. We derive novel closed formulas for the gauge-invariant actions and frequencies for the motion of spinning test particles near Kerr black holes. We use this to express the Hamiltonian perturbatively in terms of action variables up to 3PN and compare it with the 1.5 PN action-angle Hamiltonian at finite mass ratios. This allows us to match the actions across both systems, providing a new gauge-invariant dictionary for interpolation between the two limits.

We explore the possibility that quadratic gravity, as a renormalizable theory, describes the interior of quantum black holes. We find new exact power-law solutions to pure quadratic gravity under spherical symmetry, which are complex valued. The resulting solutions, dubbed powerballs, are horizonless compact objects that become Schwarzschild-like a small distance (of the order of the Planck length) outside the would-be Schwarzschild horizon. We present a description of the global eternal geometry, whose right and left exteriors are Lorentzian and Euclidean Schwarzschild-like regions, respectively, while the complex interior is a form of spiraling spacetime. We compute the total on-shell action integral as a saddle point to a gravitational path integral and discuss the Lorentzian and Euclidean interpretations thereof.

In the context of $f(R)$ gravity, as well as other extended theories of gravity, the correct counting of globally well-defined dynamical modes has recently drawn a vivid interest. In this communication we present a consistent approach shedding light on such issues for both so-called degenerate and nondegenerate $f(R)$ models embedded in maximally symmetric backgrounds. We find that the linearized spectrum of degenerate models on these backgrounds is empty, lacking both the graviton and scalaron modes which appear in generic nondegenerate models. Our work generalizes previous results in the literature applicable only to the specific (degenerate) model $f(R)=\alpha R^2$; in fact, we find that the same pathologies discovered therein emerge for all choices of $f(R)$ belonging to the wide class of degenerate models.

A finitely supertranslated Schwarzschild black hole possesses nontrivial super-Lorentz charges compared with the standard one. This may impact the quasinormal modes of the black hole. Since the Einstein’s equations are generally covariant, the quasinormal modes of a supertranslated black hole can be obtained by supertranslating the familiar results for a standard black hole. It turns out that the supertranslated quasinormal modes can be obtained by simply shifting the retarded time of the standard modes by an angle-dependent function parametrizing the supertranslation. Therefore, the supertranslated quasinormal modes oscillate at the same frequencies and decay at the same rates as the corresponding standard ones. The supertranslated metric is time translation invariant, but does not explicitly respect spherical symmetries, although it is implicitly rotationally symmetric. So the supertranslated perturbations can still be written as linear combinations of the eigenfunctions of the generalized angular momentum operators for the underlying rotational symmetry. With a suitably defined asymptotic parity transformation, any perturbation can be decomposed into the even and odd parity parts. Then, one may conclude that the isospectrality still holds. To detect such supertranslated quasinormal modes, one has to place multiple gravitational wave interferometers around the supertranslated black hole, and measure the differences in the time shifts between interferometers. Gravitational lensing may also be helpful in the same spirit.

A long-standing problem in physics is why observed masses are always positive. While energy conditions in quantum field theory can partly answer this problem, in this paper we find evidence that classical general relativity abhors negative masses, without the need for quantum theory or energy conditions. This is done by considering many different models of negative-mass “stars” and showing they are dynamically unstable. A fortiori, we show that any barotropic negative-mass star must be dynamically unstable.

We study the robustness of parametrized tests of general relativity (GR) with gravitational waves, described within the parametrized post-Einsteinian (ppE) framework, due to waveform inaccuracy. In particular, we determine the properties of the signal—signal-to-noise ratio (SNR) and source parameters—such that we are led to falsely identify a ppE deviation in the post-Newtonian (PN) inspiral phase at $-1\mathrm{PN}$, 1PN, or 2PN order, due to neglecting spin precession or higher models in the recovery model. To characterize the statistical significance of the biases, we compute the Bayes factor between the non-GR and GR models, and the fitting factor of the ppE model. For highly precessing, edge-on signals, we find that mismodeling the signal leads to a significant systematic bias in the recovery of the ppE parameters, even at an SNR of 30. However, these biased inferences are characterized by a significant loss of SNR and a weak preference for the non-GR model (over the GR model). At a higher SNR, the biased inferences display a strong preference for the non-GR model (over the GR model) and a significant loss of SNR. For edge-on signals containing asymmetric masses, at an SNR of 30, we find that excluding higher modes does not impact the ppE tests as much as excluding spin precession. Our analysis, therefore, identifies the spin-precessing and mass-asymmetric systems for which parametrized tests of GR are robust. With a toy model and using the linear signal approximation, we illustrate these regimes of bias and characterize them by obtaining bounds on the ratio of systematic to statistical error and the effective cycles incurred due to mismodeling. As a by-product of our analysis, we explicitly connect different measures and techniques commonly used to estimate systematic errors—linear-signal approximation, Laplace approximation, fitting factor, effective cycles, and Bayes factor—that are generally applicable to all studies of systematic uncertainties in gravitational wave parameter estimation.

Within the context of a axion-Chern-Simons running-vacuum-model (RVM) cosmology, one expects an early matter dominated (eMD) reheating period after RVM inflation driven by the axion field. Treating thus in this work Chern-Simons RVM cosmology as an effective $f(R)$ gravity theory characterized by logarithmic corrections of the spacetime curvature, we study the gravitational-wave (GW) signal induced by the nearly scale invariant inflationary adiabatic curvature perturbations during the transition from the eMD era driven by the axion to the late radiation-dominated era. Remarkably, by accounting for the extra GW scalaron polarization present within $f(R)$ gravity theories, we find regions in the parameter space of the theory where one is met with a distinctive induced GW signal with a universal $f^6$ high-frequency scaling compared to the $f^7$ scaling present in general relativity (GR). In particular, for axion masses $m_a$ higher than 1 GeV and axion gauge couplings $f_a$ above ${10}^{-3}$ reduced Planck masses, one can produce induced GW spectra within the sensitivity bands of future GW observatories such as the Einstein Telescope (ET), the Laser Interferometer Space Antenna (LISA), the Big Bang Observer (BBO) and the Square Kilometer Arrays (SKA).

Primordial black holes (PBHs) are considered viable candidates for dark matter and the seeds of supermassive black holes (SMBHs), with their fruitful physical influences providing significant insights into the conditions of the early Universe. Cosmic microwave background (CMB) $\mu$ distortion tightly constrains the abundance of PBHs in the mass range of ${10}^4–{10}^{11}{M}_{\odot }$ recently, limiting their potential to serve as seeds for the SMBHs observed. Given that $\mu$ distortion directly constrains the primordial power spectrum, it is crucial to employ more precise methods in computing PBH abundance to strengthen the reliability of these constraints. By a Press-Schechter (PS) type method utilizing the compaction function, we find that the abundance of PBHs could be higher than previously estimated constraints from $\mu$ distortion observations. Furthermore, our analysis shows that variations in the shape of the power spectrum have a negligible impact on our conclusions within the mass ranges under consideration. This conclusion provides us a perspective for further research on the constraints of PBH by $\mu$ distortion.

We discuss an implementation of a deep learning framework to gain insight into dark matter (DM) structure formation. We investigate the contribution of velocity and density field information to the construction of the halo mass function (HMF) in cosmological $N$-body simulations. We train a convolutional neural network (CNN) on the initial snapshot of a DM-only simulation to predict the halo mass that individual particles fall into at $z=0$, in the halo mass range of $10.5<\log (M/M_{\odot })<14$. We show that for the standard $\mathrm{\Lambda }\mathrm{CDM}$ cosmology with amplitude of initial perturbations $A_s=2\times {10}^{-9}$, the initial velocity and density fields have equivalent information, as expected in the linear regime, and manifest the power of our CNN to diagnose the redundant information. To investigate the nonlinear effects, we increase the initial power spectrum. In the linear regime, this is equivalent to decreasing the initial redshift. The CNN model trained on the simulation snapshots with large $A_s$ shows a considerable improvement in the HMF prediction when adding the velocity field information. Using our CNN map without further physical assumptions, we precisely evaluate when these nonlinear effects become vital. Eventually, for the simulation with $A_s=8\times {10}^{-8}$, the model trained with only density information shows at least an 80% increase in the mean squared error relative to the model with both velocity and density information. Our work shows the interpretability and ability of CNNs to read higher-order information from simple images, making them an excellent tool for cosmological studies.

Neutrino quantum kinetics is a rapidly evolving field in computational astrophysics, with a primary focus on collective neutrino oscillations in core-collapse supernovae and postmerger phases of binary neutron star mergers. In recent years, there has been considerable debate concerning resolution dependence in numerical simulations. In this paper, we conduct a comprehensive resolution study in both angular- and spatial directions by using two independent schemes of quantum kinetic neutrino transport: finite volume and pseudospectral methods. We complement our discussion by linear stability analysis including inhomogeneous modes. Our result suggests that decreasing spatial resolutions underestimates the growth of flavor instability, and then leads to wrong asymptotic states of flavor conversions, which potentially has a critical impact on astrophysical consequences. We further delve into numerical results of low resolution simulations, that reveals the underlying mechanism responsible for numerical artifacts caused by insufficient resolutions. This study settles the debate on requirements of resolutions and serves as a guideline for numerical modeling of quantum kinetic neutrino transport.

Future space-based laser interferometric detectors, such as the Laser Interferometer Space Antenna (LISA), will be able to detect gravitational waves (GWs) generated during the inspiral phase of stellar-mass binary black holes (SmBBHs). These detections contain a wealth of important information concerning astrophysical formation channels and fundamental physics constraints. However, the detection and characterization of GWs from SmBBHs poses a formidable data analysis challenge, arising from the large number of wave cycles that make the search extremely sensitive to mismatches in signal and template parameters in a likelihood-based approach. This makes the search for the maximum of the likelihood function over the signal parameter space an extremely difficult task, with grid-based deterministic global optimization methods becoming computationally infeasible. We present a data analysis method that addresses this problem using both algorithmic innovations and hardware acceleration driven by graphics processing units. The method follows a hierarchical approach in which a semicoherent $\mathcal{F}$-statistic is computed with different numbers of frequency domain partitions at different stages, with multiple particle swarm optimization (PSO) runs used in each stage for global optimization. An important step in the method is the judicious partitioning of the parameter space at each stage to improve the convergence probability of PSO and avoid premature convergence to noise-induced secondary maxima in the semicoherent $\mathcal{F}$-statistic. The hierarchy of stages confines the semicoherent searches to progressively smaller parameter ranges, with the final stage performing a search for the global maximum of the fully coherent $\mathcal{F}$-statistic. We test our method on 2.5 yr f a single LISA time delay interferometry combination and find that for an injected SmBBH signal with a signal-to-noise ratio between $\approx 11$ and $\approx 14$, the method can estimate (i) the chirp mass with a relative error of $\lesssim 0.01\%$, (ii) the time of coalescence within $\approx 100\mathrm{s}$, (iii) the sky location within $\approx 0.2{\mathrm{deg}}^2$, and (iv) orbital eccentricity at a fiducial signal frequency of 10 mHz with a relative error of $\lesssim 1\%$.

Neutron stars provide ideal astrophysical laboratories for probing new physics beyond the Standard Model. If axions exist, then photons can develop linear polarization during photon-axion conversion in the magnetic field of a neutron star. We find that the plasma in the neutron star magnetosphere could dramatically enhance the polarization through the resonant conversion effect. With the polarization measurements from PSR $\mathrm{B}0656+14$, 4U $0142+61$, and the benchmark polarization measurement in the midinfrared band, we demonstrate that optical and infrared polarization from neutron stars can provide strong constraints on the axion-photon coupling over a broad axion mass range ${10}^{-11}\lesssim m_a\lesssim {10}^{-3}\mathrm{eV}$.

Recently Usov’s mechanism of pair creation on the surface of compact astrophysical objects has been revisited with a conclusion that the pair creation rate was previously underestimated in the literature by nearly two orders of magnitude. Here we consider an alternative hypothesis of pair creation due to a perturbation of the surface of a compact object. Radial perturbation is induced in hydrodynamic velocity resulting in a microscopic displacement of the negatively charged component with respect to the positively charged one. The result depends on the ratio between the spatial scale of the perturbation $\lambda$ and the mean free path $l$. When $\lambda \sim l$ the perturbation energy is converted into a burst of electron-positron pairs that are created in collisionless plasma oscillations at the surface; after energy excess is dissipated, the electrosphere returns to its electrostatic configuration. When instead $\lambda \gg l$, the perturbation is thermalized, its energy is transformed into heat, and pairs are created continuously by the heated electrosphere. We discuss the relevant astrophysical scenarios.

Multiple pulsar timing arrays (PTAs) have recently reported evidence for nHz stochastic gravitational wave background (SGWB), stimulating intensive discussions about its physical origin. In principle, the sources may be either supermassive black hole binaries (SMBHBs) or processes in the early Universe. One key difference between the two lies in the statistics of the SGWB frequency power spectrum. In particular, the often assumed Gaussian random SGWB does not accurately describe the distribution of the collective SMBHB emission. This work presents a semianalytical framework for calculating the non-Gaussian statistics of SGWB power expected from SMBHBs. We find that (a) wave interference between individual SMBHBs with indistinguishable observed frequencies and (b) the Poisson fluctuation of the source numbers, together shape the non-Gaussian statistics. Implementing the non-Gaussian statistics developed in this work, we investigate the sensitivity of current and future PTA datasets in distinguishing the origin of the SGWB through non-Gaussian information. Additionally, we find an interesting approximation of the non-Gaussian statistics, which has implications for accurately and practically treating non-Gaussianity in PTA Bayesian analyses.

We revisit and update the axion-induced pair production process in a nuclear electric field mediated by the axion-electron coupling, $a+{}_Z{}^{A}X\to {}_Z{}^{A}X+e^{+}+e^{-}$. This process emerges as one of the most efficient channels for detecting axions with energies above a few MeV in large underground detectors. It is particularly relevant for detecting axions produced in nuclear reactions, such as the $p+d\to {}^3\mathrm{He}+a(5.5\mathrm{MeV})$ reaction in the solar pp-chain, and for axions originating in supernovae. Despite recent interest in detecting high-energy axions, the pair production process has received limited attention, even in scenarios where it is the dominant detection channel. This study fills this gap by demonstrating that pair production is a highly effective detection mechanism for high-energy axions. We apply our results to axions from supernovae and the solar 5.5 MeV line, recasting the current bounds of Borexino and comparing the detection capabilities of the JUNO and Hyper-Kamiokande detectors.

The merging black hole binaries detected by the LIGO-Virgo-KAGRA (LVK) gravitational-wave observatories may help us shed light on how such binaries form. In addition, these detections can help us probe the hypothesized primordial black holes, a candidate for the observed abundance of dark matter. In this work, we study the black-hole mass distribution obtained from the LVK binary black hole merger events. In particular, we study the primary mass $m_1$-distribution of the observed black hole binaries, and also the secondary to primary mass ratio $q=m_2/m_1$ distribution. We obtain those distributions by first associating a skewed normal distribution to each event detected with a signal to noise ratio $(\mathrm{SNR})>8$ and then summing all such distributions. We also sample black hole binaries from two separate populations of merging binaries to which we associate a redshift from the redshift distribution. One of these is a stellar-origin population that follows a mass-distribution similar to the zero-age mass function of stars. The second population of black holes follows a Gaussian mass-distribution. Such a distribution could approximate a population of black hole binaries formed from earlier black hole mergers in dense stellar environments, or binaries of primordial black holes among other astrophysical processes. For those populations, we evaluate the number of detectable events and fit their combination to the LVK observations. In our work, we assume that stellar-origin binary black holes follow a similar mass-distribution to that of the initial mass-function of stars. We simulate a wide range of stellar-origin black-hole mass distributions. In agreement with the binary black hole merger rates-analysis of the LVK collaboration, we find that studying the observed LVK events can be fitted better by the combination of such a stellar-origin mass distribution and a Gaussian distribution, than by the stellar-origin mass distribution alone. As we demonstrate with some simple examples, our methodology allows for rapid testing of potential theoretical models for the binary black hole mergers to the observed events by LVK.

Due to its excellent optical properties, such as low absorption and scattering, amorphous ${\text{Ta}}_2{\mathrm{O}}_5$ is commonly used as an optical coating material, often in combination with ${\mathrm{SiO}}_2$ layers to produce a highly reflective stack. However, the high mechanical loss of ${\text{Ta}}_2{\mathrm{O}}_5$ limits the thermal-noise performance of such coatings when used in precision measurement applications. Doping with ${\mathrm{TiO}}_2$ has previously been shown to slightly reduce the mechanical loss, but it is still very high compared to many other materials, particularly at low temperatures. In this paper, we present a detailed study of different heat treatment temperatures and of Ti concentrations of up to nominally 75%. We show a significant mechanical-loss reduction for the mixture with the highest Ti cation content, which crystallized after heat treatment at 500°C. The resulting loss is much lower than that of pure ${\mathrm{TiO}}_2$ or that of ${\text{Ta}}_2{\mathrm{O}}_5$ after crystallization, making further studies highly interesting, in particular investigations of scattering which may pose a major drawback for optical applications.

We derive self-consistent constraint conditions for collision terms in quantum kinetic theory using the Wigner function formalism. We present specific solutions for these collision terms that align with the constraints. We develop quantum kinetic theory at relaxation-time approximation. With this framework, we discuss spin polarization and electric charge separation effects in relativistic heavy-ion collisions.

Based on the nonextensive statistical mechanics and the gluon polarization tensor obtained from kinetic theory, we derive the longitudinal and transverse gluon self-energies for the quark-gluon plasma. The electric permittivity $ϵ$ and the magnetic permeability ${\mu }_M$ are evaluated from the gluon self-energies through which the real part of the square of the refraction index $\mathrm{Re}{n}^2$ and the Depine-Lakhtakia index $n_{\mathrm{DL}}$ are investigated. The real part of $ϵ$ displays a frequency pole ${\omega }_d=p$, which is just the position of the frequency inflexion of the imaginary part of $ϵ$. The nonextensive parameter $q$ significantly affects the real and imaginary parts of $ϵ$ in the spacelike region $\omega <p$, while the frequency pole ${\omega }_d=p$ remains unchanged as $q$ increases. The magnetic permeability, $\mathrm{Re}{n}^2$ and the Depine-Lakhtakia index $n_{\mathrm{DL}}$ diverge at frequency ${\omega }_m$. As $q$ increases, the pole frequency ${\omega }_m$ shifts to large frequency region. The Depine-Lakhtakia index $n_{\mathrm{DL}}$ becomes negative in a quite large frequency region $\omega \in [{\omega }_c,{\omega }_m]$. When $q$ increases, the frequency range for $n_{\mathrm{DL}}<0$ becomes wider. Nevertheless, there are no propagating modes for the negative refraction. In addition, as momentum $p$ increases, the electric permittivity, the magnetic permeability, $\mathrm{Re}{n}^2$ and $n_{\mathrm{DL}}$ are sensitive to the change of $p$, which indicates the importance of the spatial dispersion in the electromagnetic responses of the quark-gluon plasma.