In a binary, when the orbital plane of the companion star is almost edge on along the line-of-sight direction, this would produce an observable self-gravitational lensing effect, which would slightly increase the overall optical intensity of the binary. However, the probability of getting one observable self-lensing binary (SLB) is very low. There are only five observed SLBs so far and all of them are eclipsing binaries. In this article, we theoretically show that the neutron star–white dwarf (WD) binary PSR J1910-5959A could be an observable noneclipsing SLB. It might be the first binary showing both periodic optical amplification and Shapiro time delay of radio signals, which is useful to verify our understanding about gravitational lensing in relativistic binaries. Moreover, we show that the observed peak amplification limit of the PSR J1910-5959A can help constrain the radius of the WD, which is a crucial parameter to examine the mass-radius and temperature-radius relationship for helium WDs.

The gravitational waves emitted (some time) after two black holes merge are well described by the theory of linear perturbations on a spacetime characterized by the mass and spin of the remnant. However, in the very early stages right after merger, both the mass and spin are changing. In this work we explore, in a setup based on Vaidya’s spacetime, the dynamical consequences of a change of mass in the spacetime due to the accretion of null matter (for example, gravitational waves). We show that accretion imprints time-dependent frequencies and amplitude to a ringdown waveform, and we show how to model accurately this effect in certain regimes. We also comment on the direct emission of gravitational waves due to perturbations in the infalling matter, which is of relevance for black holes embedded in astrophysical environments.

A number of modified gravity theories [e.g., $f(R)$-gravity] lead to a Yukawa-like metric in the weak field limit which can be described by two Yukawa parameters, i.e., the strength $\kappa$ and the length scale $\lambda$. The S-stars, orbiting around the supermassive black hole in the Galactic Center, are unique probes to test these gravity theories in a relatively strong gravitational field. The Newtonian Yukawa gravity potential, or a simple approximation to the Yukawa metric, was usually adopted in previous studies when using the orbital motion of S-stars to constrain such modified gravity theories, which may be not sufficiently accurate considering recent and future high resolution observations. In this paper, we first derive the post-Newtonian (PN) Yukawa motion equation at the 2PN order, and then investigate the high order effects on the orbital motions by comparison with those from the Newtonian Yukawa gravity potential. We further obtain constraints on $\kappa$ by using the observations on the orbital motions of several S-stars (i.e., S2, S38, and S55). Our results show that the current observations of these stars are compatible with general relativity, and $\kappa$ can be constrained to $|\kappa |<0.01$ with 95% confidence if $\lambda \in (100,250)\mathrm{A}\mathrm{.}\mathrm{U}\mathrm{.}$ We also estimate the possible improvements (about an order of magnitude or more) to the constraints by future higher resolution observations and the inclusion of closer S-stars, such as S4716.

The geometries with $\mathrm{SL}(2,\mathbb{R})$ and some axial U(1) isometries are called “near-horizon extremal geometries” and are found usually, but not necessarily, in the near-horizon limit of the extremal black holes. We present a new member of this family of solutions in five-dimensional Einstein-Hilbert gravity that has only one nonzero angular momentum. In contrast with the single-rotating Myers-Perry extremal black hole and its near-horizon geometry in five dimensions, this solution may have a nonvanishing and finite entropy. Although there is a uniqueness theorem that prohibits the existence of such single-rotating near-horizon geometries in five-dimensional general relativity, this solution has a curvature singularity at one of the poles, which breaks the smoothness conditions in the theorem.

We investigate the charged scalar field propagating in a ($2+1$) charged Bañados-Teitelboim-Zanelli black hole. The conditions for stability are studied unveiling, for each black hole geometry, the existence of a critical scalar charge as of the evolution is unstable. The existence of growing profiles is substantiated by the deep in the effective potential that intensifies as the scalar charge increases. The phenomenon happens in every black hole geometry even for small geometry charge. In the small scalar charge regime, the field evolution is stable and in such we calculate the quasinormal modes.

A quantum analysis of the vacuum Bianchi IX model is performed, focusing in particular on the chaotic nature of the system. The framework constructed here is general enough for the results to apply in the context of any theory of quantum gravity, since it includes only minimal approximations that make it possible to encode the information of all quantum degrees of freedom in the fluctuations of the usual anisotropy parameters. These fluctuations are described as canonical variables that extend the classical phase space. In this way, standard methods for dynamical systems can be applied to study the chaos of the model. Two specific methods are applied that are suitable for time-reparametrization invariant systems. First, a generalized version of the Misner-Chitre variables is constructed, which provides an isomorphism between the quantum Bianchi IX dynamics and the geodesic flow on a suitable Riemannian manifold, extending, in this way, the usual billiard picture. Second, the fractal dimension of the boundary between points with different outcomes in the space of initial data is numerically analyzed. While the quantum system remains chaotic, the main conclusion is that its strength is considerably diminished by quantum effects as compared to its classical counterpart.

We study the precession caused by electromagnetic radiation on a magnetic dipole located far from the source. As we show, this entails a net rotation of the dipole in the plane orthogonal to the direction of wave propagation, providing an electromagnetic analogue of gyroscopic gravitational memory. Like its gravitational cousin, the precession rate falls off with the square of the distance to the source, and is related to electric-magnetic duality and optical helicity on the celestial sphere. We use a multipolar expansion to compute the memory effect due to localized sources such as moving point charges, and highlight its occurrence in setups that break parity symmetry.

Principal component analysis (PCA) is an efficient tool to optimize multiparameter tests of general relativity (GR), wherein one looks for simultaneous deviations in multiple post-Newtonian phasing coefficients. This is accomplished by introducing non-GR deformation parameters in the phase evolution of the gravitational-wave templates used in the analysis. A PCA is performed to construct the “best-measured” linear combinations of the deformation parameters. This helps to set stringent limits on deviations from GR and to more readily detect possible beyond-GR physics. In this paper, we study the effectiveness of this method with the proposed next-generation gravitational-wave detectors, Cosmic Explorer (CE) and Einstein Telescope (ET). For compact binaries at a luminosity distance of 500 Mpc and the detector-frame total mass in the range $20–200M_{\odot }$, CE can measure the most dominant linear combination with a $1\text{−}\sigma$ uncertainty $\sim 0.1\%$ and the next two subdominant linear combinations with a $1\text{−}\sigma$ uncertainty of $\le 10\%$. For a specific range of masses, constraints from ET are better by a factor of a few than CE. This improvement is because of the improved low frequency sensitivity of ET compared to CE (between 1–7 Hz). In addition, we explain the sensitivity of the PCA parameters to the different post-Newtonian deformation parameters and discuss their variation with total mass. We also discuss a criterion for quantifying the number of most dominant linear combinations that capture the information in the signal up to a threshold.

The population of black holes inferred from the detection of gravitational waves by the LIGO-Virgo-KAGRA Collaboration has revealed interesting features in the properties of black holes in the Universe. We analyze the GWTC-3 dataset assuming the detected black holes in each event had an either astrophysical or primordial origin. In particular, we consider astrophysical black holes described by the fiducial $\mathrm{P}\mathrm{O}\mathrm{W}\mathrm{E}\mathrm{R}\mathrm{L}\mathrm{A}\mathrm{W}+\mathrm{P}\mathrm{E}\mathrm{A}\mathrm{K}$ distribution and primordial black holes whose mass function obeys a broken power law. These primordial black holes can be generated by vacuum bubbles that nucleate during inflation. We find that astrophysical black holes dominate the events with mass less than $\sim 30M_{\odot }$, whereas primordial black holes are responsible for the massive end, and also for the peak at $\sim 30M_{\odot }$ in the mass distribution. More than half of the observed events could come from primordial black hole mergers. We also discuss the implications on the primordial black hole formation mechanism and the underlying inflationary model.

We investigate the thermodynamic properties of the Hayward regular black hole using both Euclidean path integral and Hamiltonian methods, in asymptotically anti–de Sitter, Minkowski, and de Sitter spacetimes. With the inclusion of matter fields which act as a source for the regular black hole geometry, an effective temperature emerges that differs from the conventional definition related to the Killing surface gravity. We posit that this temperature is the appropriate choice for studying thermodynamic phenomena, by demonstrating consistency between the Euclidean and Hamiltonian formulations in the appropriate limits. We examine the thermodynamic properties and phase structure of the Hayward black hole in the canonical ensemble and show that, counter to some earlier indications, standard mean-field theory critical behavior is observed when the cosmological constant is treated as a thermodynamic pressure. We note the absence of a Hawking-Page transition, and conjecture that quantum gravity corrections which are suitably strong to regulate the Schwarzschild singularity generically prevent the transition from occurring. We also show that the Smarr relation remains linear in all cases, despite the absence of a linearity proof for nonlinear electrodynamic theories with nonsymmetry inheriting fields.

Weak gravitational lensing of the cosmic microwave background (CMB) carries imprints of the physics operating at redshifts much lower than that of recombination and serves as an important probe of cosmological structure formation, dark matter physics, and the mass of neutrinos. Reconstruction of the CMB lensing deflection field through use of quadratic estimators has proven successful with existing data, but is known to be suboptimal on small angular scales ($\ell >3000$) for experiments with low-noise levels. Future experiments will provide better observations in this regime, but these techniques will remain statistically limited by their approximations. We show that correlations between fluctuations of the large-scale temperature gradient power of the CMB sourced by $\ell <2000$ and fluctuations of the local small-scale temperature power reveal a lensing signal that is prominent in even the real-space pixel statistics across a CMB temperature map. We present the development of the small-correlated-against-large estimator (SCALE), a novel estimator for the CMB lensing spectrum that offers promising complementary analysis alongside other reconstruction techniques in this regime. The SCALE method computes correlations between both the large-/small-scale temperature gradient power in harmonic space, and it is able to quantitatively recover unbiased statistics of the CMB lensing field without the need for map-level reconstruction. SCALE can outperform quadratic estimator signal-to-noise by a factor of up to 1.5 in current and upcoming experiments for CMB lensing power spectra $C_{6000<L<8000}^{\varphi \varphi }$.

We study how well field-level inference with a perturbative forward model can constrain cosmological parameters compared to conventional analyses. We exploit the fact that in perturbation theory the field-level posterior can be computed analytically in the limit of small noise. In the idealized case where the only relevant parameter for the nonlinear evolution is the nonlinear scale, we argue that information content in this posterior is the same as in the $n$-point correlation functions computed at the same perturbative order. In the real universe other parameters can be important, and there are possibly enhanced effects due to nonlinear interactions of long- and short-wavelength fluctuations that can either degrade the signal or increase covariance matrices. We identify several different parameters that control these enhancements and show that for some shapes of the linear power spectrum they can be large. This leads to degradation of constraints in the standard analyses, even though the effects are not dramatic for a $\mathrm{\Lambda }\mathrm{CDM}$-like cosmology. The aforementioned long-short couplings do not affect the field-level inference which remains optimal. Finally, we show how in these examples calculation of the perturbative posterior motivates new estimators that are easier to implement in practice than the full forward modeling but lead to nearly optimal constraints on cosmological parameters. These results generalize to any perturbative forward model, including galaxies in redshift space.

We investigate a mechanism of primordial black hole (PBH) formation that avoids any dependence on specific inflationary features or exotic physics. In this scenario, the required large curvature fluctuations leading to PBH formation are generated after inflation by the quantum fluctuations of a light stochastic spectator field during inflation, when this field transiently dominates the energy density. We calculate the dynamics of such a spectator field during and after inflation, the distribution of induced curvature perturbations and their non-Gaussian tails leading to the copious production of PBHs. For a plateaulike potential, this scenario produces an extended PBH mass distribution with a peak at the solar-mass scale when one takes into account the effects of the thermal history. What is remarkable in this scenario is the absence of parameter fine-tuning. Instead, it invokes an anthropic selection over all the realizations of PBH abundances predicted by the field stochasticity. This scenario offers a novel perspective for the formation of PBHs with minimal ingredients and without the need of fine-tuning. It is amenable to observational tests, notably with the gravitational-wave observations of black hole mergers and of a background at nanohertz frequency, as recently observed by pulsar timing arrays.

The postinflationary Universe can pass through a long epoch of effective matter-dominated expansion. This era may allow for both the parametric amplification of initial fluctuations and the gravitational collapse of inflaton perturbations. We perform first-of-their-kind high-resolution simulations that span the resonant phase and subsequent gravitational collapse of the inflaton field by segueing from a full Klein-Gordon treatment of resonance to a computationally efficient Schrödinger-Poisson description that accurately captures the gravitational dynamics when most quanta are nonrelativistic. We consider a representative example in which resonance generates $\mathcal{O}({10}^{-1})$ overdensities and gravitational collapse follows promptly as resonance ends. We observe the formation of solitonic cores inside inflaton halos and complex gravitational dynamics on scales of ${10}^{-27}\mathrm{m}$, greatly extending the possible scope of nonlinear postinflationary gravitational dynamics.

A recent report has identified a central compact object (CCO) within the supernova remnant HESS J1731-347, with a mass and radius of $M={0.77}_{-0.17}^{+0.20}{M}_{\odot }$ and $R={10.4}_{-0.78}^{+0.86}\mathrm{km}$, respectively. To investigate this light compact star, a density-dependent relativistic mean-field (DDRMF) model, specifically the DDVT model, has been employed. The DDVT model incorporates tensor couplings of vector mesons, which can successfully describe the properties of finite nuclei, such as charge radius, binding energy, and spin-orbit splitting. The introduction of tensor coupling reduces the influence of scalar mesons and generates a softer equation of state (EOS) in the outer core of the neutron star. Moreover, it has been found that the crust segment plays a crucial role in reproducing the mass-radius relation of HESS J1731-347, indicating a preference for a soft crust EOS. By manipulating the coupling strength of the isovector meson in the DDVT parameter set, a reasonable hadronic EOS has been obtained, satisfying the constraints from the gravitational-wave signal GW170817, the simultaneous mass-radius measurements from the NICER collaboration, and the properties of finite nuclei. Notably, the mass-radius relations derived from this hadronic EOS also accurately describe the observables of HESS J1731-347. Therefore, based on our estimation, the CCO in HESS J1731-347 may represent the lightest known neutron star, within the estimations on the uncertainty from the carbon atmosphere model and the inhomogeneous temperatures on the stellar surface of the neutron star.

Ultrasensitive cryogenic calorimeters have become a favored technology with widespread application where eV-scale energy resolutions are needed. In this article, we characterize the performance of an x-ray magnetic microcalorimeter (MMC) using a ${}^{55}\mathrm{Fe}$ source. Employing an optimum filter-based amplitude estimation and energy reconstruction, we demonstrate that a full-width half-maximum (FWHM) resolution of $\mathrm{\Delta }{E}_{\mathrm{FWHM}}=(1.25\pm 0.17(\mathrm{stat}{)}_{-0.07}^{+0.05}(\mathrm{syst}))\mathrm{eV}$ can be achieved, leading to an unprecedented energy resolving power $E/\mathrm{\Delta }{E}_{\mathrm{FWHM}}\sim 4700$ among existing energy-dispersive detectors for soft and tender x-rays. We also derive the best possible resolution and discuss limiting factors affecting the measurement. The analysis pipeline for the MMC data developed in this paper is furthermore an important step for the realization of the proposed superfluid helium-based experiment DELight, which will search for direct interaction of dark matter particles with masses below $100\mathrm{MeV}/c^2$.

A wormhole might have two new kinds of marginally unstable photon spheres. They could be formed either by the merger of the photon sphere and throat or by the coincidence of the photon sphere, antiphoton sphere, and throat. Both of them are unique for an object with throat. We investigate the strong deflection gravitational lensing near these marginally unstable photon spheres of a static and spherically symmetric wormhole. Its deflection angle in the strong deflection limit diverges in a manner of a power law with a specific exponent, rather than by the well-known logarithmic law for the photon sphere. We analytically obtain its observables, and apply them to a wormhole in the beyond Horndeski theory.

The paradigm-changing possibility of collective neutrino-antineutrino oscillations was recently advanced in analogy to collective flavor oscillations. However, the amplitude for the backward scattering process ${\nu }_{{\mathbf{p}}_1}{\overline{\nu }}_{{\mathbf{p}}_2}\to {\nu }_{{\mathbf{p}}_2}{\overline{\nu }}_{{\mathbf{p}}_1}$ is helicity suppressed and vanishes for massless neutrinos, implying that there is no off-diagonal refractive index between $\nu$ and $\overline{\nu }$ of a single flavor of massless neutrinos. For a nonvanishing mass, collective helicity oscillations are possible, representing de facto $\nu -\overline{\nu }$ oscillations in the Majorana case. However, such phenomena are suppressed by the smallness of neutrino masses as discussed in the previous literature.

We present the design, control system, and noise analysis of a 6-axis seismometer comprising a mass suspended by a single fused silica fiber. We utilize custom-made, compact Michelson interferometers for the readout of the mass motion relative to the table and successfully overcome the sensitivity of existing commercial seismometers by over an order of magnitude in the angular degrees of freedom. We develop the sensor for gravitational-wave observatories, such as LIGO, Virgo, and KAGRA, to help them observe intermediate-mass black holes, increase their duty cycle, and improve localization of sources. Our control system and its achieved sensitivity makes the sensor suitable for other fundamental physics experiments, such as tests of semiclassical gravity, searches for bosonic dark matter, and studies of the Casimir force.

Arm locking is one of the key technologies to suppress the laser phase noise in spaced-based gravitational waves observatories. Since arm locking was proposed, the phase margin criterion was always used as the fundamental design strategy for the controller development. In this paper, we find that this empirical method from engineering actually cannot guarantee the arm locking stability. Therefore, most of the advanced arm locking controllers reported so far may have stability problems. After comprehensive analysis of the single arm locking’s transient responses, strict analytical stability criterions are summarized for the first time. These criterions are then generalized to dual arm locking, modified-dual arm locking, and common arm locking, and special considerations for the design of arm locking controllers in different architectures are also discussed. It is found that proportional integral controllers can easily meet our stability criterions in most of the arm locking systems. Using a simple high gain proportional integral controller, it is possible to suppress the laser phase noise by 5 orders of magnitude within the science band. Our stability criterions can also be used in other feedback systems, where several modules with different delays are connected in parallel.