We consider Thomson-like processes between dark matter dark photons and free electrons in the Milky Way. The result is a radio signal background that can be detectable with current or future radio telescope arrays. In particular, we computed sensitivity prospects for the Atacama Large Millimeter Array radio telescope and for the future Square Kilometer Array, concluding that unconstrained parameter space in a wide range of dark photon masses can be probed.

We investigate the cosmological first-order chiral phase transition of quantum chromodynamics (QCD), and for the first time calculate its parameters which can determine the gravitational wave spectrum. With the state-of-the-art calculation from the functional QCD method, we found that the large chemical potential of QCD phase transition results in very weak and fast first-order phase transitions at the temperature lower than $\mathcal{O}({10}^2)\mathrm{MeV}$. These results further suggest that the gravitational wave (GW) signals of NANOGrav are very unlikely sourced from the chiral phase transition of QCD.

We describe the chiral transition for the quark condensate and the melting of scalar and vector mesons in two-flavor holographic quantum chromodynamics (holographic QCD). This is done by extending the improved holographic soft wall models proposed in Ballon-Bayona et al. [Phys. Rev. D 104, 126029 (2021)] to finite temperature, by means of introducing an asymptotically anti–de Sitter black brane. We find that the chiral transition is second order in the chiral limit and a crossover for physical quark masses, as expected in two-flavor QCD. We investigate the melting of vector and scalar mesons in the deconfined plasma through the calculation of hadronic spectral functions. Fixing the model parameters by the meson spectrum at zero temperature, we find that the mesons melt at temperatures between 90 and 110 MeV and the chiral transition occurs around 129 MeV. We also provide a prediction for the hydrodynamic diffusion constant associated with a flavor current in the deconfined plasma.

It has long been established that certain higher-dimensional topological phases of matter support extended objects like quasistrings and quasimembranes in their bulk states. In this study, we investigate the physics of these topological systems using a phase-space approach in the semiclassical regime, incorporating tensorial coordinates related to the extended objects. Specifically, we explore the semiclassical currents associated to open quasistrings in topological phases in three spatial dimensions, considering the presence of both coordinate-space Kalb-Ramond fields and momentum-space tensor Berry connections. Our results show that the currents associated to the endpoints of open quasistrings on the system’s boundary exhibit a quantum Hall response. This formalism accurately reproduces the boundary response as predicted by an Abelian BF theory proving a novel way to study extended objects in topological matter.

We consider the scenario of a fluctuating spacetime due to a deformed commutation relation with a fluctuating deformation parameter or to a fluctuating metric tensor. By computing the resulting dynamics and averaging over these fluctuations, we find that a system experiences a decoherence in the momentum basis. We studied the predictions of the model for a free particle and an harmonic oscillator. Using experimental data taken from a mechanical oscillator prepared in quantum states of motion, we put a bound on the free parameters of the considered model. In addition, we comment on how these measurements can also provide bounds to other phenomenological quantum gravity models, such as the length scale for nonlocal dynamics.

It was previously shown that if an experimenter, Alice, puts a massive or charged body in a quantum spatial superposition, then the presence of a black hole (or more generally any Killing horizon) will eventually decohere the superposition. This decoherence was identified as resulting from the radiation of soft photons/gravitons through the horizon, thus suggesting that the global structure of the spacetime is essential for describing the decoherence. In this paper, we show that the decoherence can alternatively be described in terms of the local two-point function of the quantum field within Alice’s lab, without any direct reference to the horizon. From this point of view, the decoherence of Alice’s superposition in the presence of a black hole arises from the extremely low frequency Hawking quanta present in Alice’s lab. We explicitly calculate the decoherence occurring in Schwarzschild spacetime in the Unruh vacuum from the local viewpoint. We then use this viewpoint to elucidate (i) the differences in decoherence effects that would occur in Schwarzschild spacetime in the Boulware and Hartle-Hawking vacua; (ii) the difference in decoherence effects that would occur in Minkowski spacetime filled with a thermal bath as compared with Schwarzschild spacetime; (iii) the lack of decoherence in the spacetime of a static star even though the vacuum state outside the star is similar in many respects to the Boulware vacuum around a black hole; and (iv) the requirements on the degrees of freedom of a material body needed to produce a decoherence effect that mimics that of a black hole.

It has been argued that, while the individual terms in the generalized entropy, $S_{\mathrm{gen}}=A/4G_{\mathrm{N}}+S_{\mathrm{ext}}$, are ill-defined in the semiclassical limit, their sum is well-defined if one takes into account perturbative quantum gravitational effects. The first term diverges as $G_{\mathrm{N}}\to 0$, and the second diverges due to the infinite entanglement across the horizon which is characteristic of type III von Neumann algebras. It was recently shown that the von Neumann algebra of observables “gravitationally dressed” to the mass of a Schwarzschild-AdS black hole or the energy of an observer in de Sitter spacetime admit a well-defined trace. The algebras are type ${\mathrm{II}}_{\infty }$ (which does not admit a maximum entropy state) and type ${\mathrm{II}}_1$ (which admits a maximum entropy state) respectively, and the von Neumann entropy of “semiclassical” states was found to be (up to an additive constant) the generalized entropy. However, these arguments rely on the existence of a stationary “equilibrium (KMS) state” and do not apply to, for example, black holes formed from gravitational collapse, Kerr black holes, or black holes in asymptotically de Sitter spacetime. These spacetimes are stationary but not in thermal equilibrium. In this paper, we present a general framework for obtaining the algebra of “gravitationally dressed” observables for a linear, Klein-Gordon field on any spacetime with a (bifurcate) Killing horizon. We prove, assuming the existence of a stationary state—which is not necessarily KMS—and suitable asymptotic decay of solutions, a “structure theorem” that the algebra of “gravitationally dressed” observables always contains a type II factor of observables “localized” on the horizon. These assumptions have been rigorously proven in most cases of interest in this paper. Applying our general framework to the algebra of observables in the exterior of an asymptotically flat Kerr black hole where the fields are dressed to the black hole mass and angular momentum we find that the algebra is the product of a type ${\mathrm{II}}_{\infty }$ algebra on the horizon and a type ${\mathrm{I}}_{\infty }$ algebra at past null infinity. The full algebra is type ${\mathrm{II}}_{\infty }$, and the von Neumann entropy of semiclassical states is the generalized entropy. In the case of Schwarzschild-de Sitter, despite the fact that we must introduce an observer, the algebra of observables dressed to the perturbed areas of the black hole and cosmological horizons is the product of type ${\mathrm{II}}_{\infty }$ algebras on each horizon. The entropy of semiclassical states is given by the sum of the areas of the two horizons as well as the entropy of quantum fields in between the horizons. Our results suggest that in all cases where there exists another “boundary structure” (e.g., an asymptotic boundary or another Killing horizon) the algebra of observables is type ${\mathrm{II}}_{\infty }$ and in the absence of such structures (e.g. de Sitter spacetime) the algebra is type ${\mathrm{II}}_1$.

Many theoretical models were created to figure out the properties of magnetic reconnection process, among which the Sweet-Parker model is the most famous since it describes the magnetic reconnection in a concise way. However, the low reconnection rate expected by this model is generally not available in most astrophysical systems, which motivates people to seek fast reconnection models. Under the scheme of generalized magnetohydrodynamics for pair plasma, a fast magnetic reconnection model was established, in which the thermal electromotive force plays a key role to remarkably increase the reconnection rate. In this work, I would like to extend the discussions in my previous work about the general relativistic description of the Sweet-Parker model to the description of fast magnetic reconnection induced by thermal electromotive force. I will revisit the fast reconnection model briefly to initialize my discussions and show how the thermal electromotive force impacts the reconnection rate. Next, some basic setups will be exhibited before discussing specific examples about how the properties of fast magnetic reconnection are modified by gravitational effect or in observations. Results in this work consolidate my opinion reiterated in my previous work that properties of magnetic reconnection would never be modified by gravitational effect significantly if the magnetic reconnection process occurs in a local scale, while the modifications of properties could not be neglected when the process is detected by an observer who is moving with respect to the laboratory in the rest frame of which the magnetic reconnection occurs.

Recently, it was shown by Danielson et al. (DSW) that, for the massive or charged body in a quantum spatial separated superposition state, the presence of a black hole can decohere the superposition inevitably toward capturing the radiation of soft photons or gravitons [D. L. Danielson et al., Black holes decohere quantum superpositions, Int. J. Mod. Phys. D 31, 2241003 (2022); D. L. Danielson et al., Killing horizons decohere quantum superpositions, Phys. Rev. D 108, 025007 (2023); D. L. Danielson et al., Local description of decoherence of quantum superpositions by black holes and other bodies, arXiv:2407.02567]. In this work, we study the DSW decoherence effect for the static charged body in the Reissner-Nordström black holes. By calculating the decohering rate for this case, it is shown that the superposition is decohered by the low-frequency photons that propagate through the black hole horizon. For the extremal Reissner-Nordström black hole, the decoherence of quantum superposition is completely suppressed due to the black hole Meissner effect. It is also found that the decoherence effect caused by the Reissner-Nordström black hole is equivalent to that of an ordinary matter system with the same size and charge.

Gravitational waves have been shown to provide new constraints on gravitational theories beyond general relativity (GR), especially in the strong-field regime. Gravitational-wave signals from Galactic double white dwarfs, expected to be detected by the Laser Interferometer Space Antenna (LISA), also have the potential to place stringent bounds on certain theories that give rise to relatively large deviations from GR in less compact binaries, such as through scalar radiation. Nevertheless, the orbital evolution of close double white dwarf systems is also affected by various astrophysical effects, such as stellar rotation, tidal interactions, and magnetic interactions, which add complexity to the gravity tests. In this work, we employ the parametrized post-Einsteinian model to capture the leading beyond-GR effect on the signal and estimate the measurement uncertainties using the Fisher information matrix. We then study the systematic error caused by ignoring each astrophysical effect mentioned above on the parameter estimation. Our numerical results show that, to place bounds on the non-GR effects comparable to existing bounds from pulsar observations, tight priors on the mass of the binary and long observation time are required. At this level of sensitivity, we found that systematic errors from the astrophysical effects dominate statistical errors. The most significant effects investigated here are torques from tidal synchronization and magnetic unipolar induction for sufficiently large magnetic fields ($>{10}^7\mathrm{G}$). Meanwhile, even the weaker astrophysical effects from quadrupolar deformations are of a similar order of magnitude as the statistical uncertainty and, hence, cannot be ignored in the waveform model. We conclude that the astrophysical effects must be carefully accounted for in the parameter estimation to test gravity with Galactic double white dwarfs detected by LISA.

We investigate a complete family of spacetimes which represent black holes with (Kerr) rotation, Newman-Unti-Tamburino (NUT) twist, acceleration, as well as electric and magnetic charges. These are exact solutions of the Einstein-Maxwell equations with any cosmological constant, such that the (non-null) electromagnetic field is aligned with both the double-degenerate principal null directions (PNDs) of the Weyl tensor. In particular, we explicitly relate various coordinates and the corresponding physical parameters of such solutions, namely the original Plebański-Demiański (PD) form; the convenient Astorino (A) form, which was found recently and formally improved here (${\mathrm{A}}^{+}$); the Griffiths-Podolský form; and the Podolský-Vrátný form of the metric. It is demonstrated that, if properly mapped and physically interpreted, all these representations cover the complete class of type D black holes. Using the new A-parameters, the two main PD quartic metric functions are factorized into the product of quadratic expressions, enabling thus an explicit analysis. Moreover, we clarify the role of the twist parameter $\omega$, related to both the Kerr-like rotation and the NUT parameters $a$ and $l$, respectively. Special attention is paid to the elusive subclass of accelerating NUT black holes with $a=0$.

Fisher-matrix methods are widely used to predict how accurately parameters can be estimated. Being computationally efficient, this approach is prompted by the large number of signals simulated in forecast studies for future gravitational-wave detectors, for which adequate analysis tools and computational resources are still unavailable to the scientific community. However, approximating the full likelihood function with a Gaussian may lead to inaccuracies, which we investigate in this work. To assess the accuracy of the Fisher approximation, we compare the results of the Fisher code gwfish against real data from the Gravitational Wave Transient Catalogs provided by the Virgo/LIGO Bayesian analyses. Additionally, we present a sampling algorithm to include priors in gwfish, not only to ensure a fair comparison between gwfish results and the Virgo/LIGO posteriors but also to investigate the role of prior information and to assess the need to include it in standard Fisher analyses. We find that the impact of priors depends mostly on the level of signal-dependent degeneracy of the waveform parametrization, and priors are generally more important when the level of degeneracy is high. Our findings imply that Fisher-matrix methods are a valid tool for Einstein Telescope science-case studies.

Gravitational waves are now routinely detected from compact binary mergers, with binary neutron star mergers being of note for multimessenger astronomy as they have been observed to produce electromagnetic counterparts. Novel search pipelines for these mergers could increase the combined search sensitivity, and could improve the ability to detect real gravitational wave signals in the presence of glitches and nonstationary detector noise. Deep learning has found success in other areas of gravitational wave data analysis, but a sensitive deep learning-based search for binary neutron star mergers has proven elusive due to their long signal length. In this work, we present a deep learning pipeline for detecting binary neutron star mergers. By training a convolutional neural network to detect binary neutron star mergers in the signal-to-noise ratio time series, we concentrate signal power into a shorter and more consistent timescale than strain-based methods, while also being able to train our network to be robust against glitches. We compare our pipeline’s sensitivity to the three offline detection pipelines using injections in real gravitational wave data, and find that our pipeline has a comparable sensitivity to the current pipelines below the 1 per 2 months detection threshold. Furthermore, we find that our pipeline can increase the total number of binary neutron star detections by 12% at a false alarm rate of 1 per 2 months. The pipeline is also able to successfully detect the two binary neutron star mergers detected so far by the LIGO-Virgo-KAGRA collaboration, GW170817 and GW190425, despite the loud glitch present in GW170817.

Gravitational wave memory is said to arise when a gravitational wave burst produces changes in a physical system that persist even after that wave has passed. This paper analyzes gravitational wave bursts in plane wave spacetimes, deriving memory effects for timelike and null geodesics, massless scalar fields, and massless spinning particles whose motion is described by the spin Hall equations. We find that all such effects are characterized by four “memory tensors,” three of which are independent. We also show that memory effects for null geodesics can have strong longitudinal components, even in vacuum general relativity. When considering massless particles with spin, we solve the spin Hall equations analytically by showing that there exists a conservation law associated with each conformal Killing vector. For the scattering of fields by gravitational waves, we show that given any solution to the massless scalar field equation in flat spacetime, a weak-field solution in a plane wave spacetime can be generated just by applying an appropriate differential operator—an operator that is constructed from the aforementioned memory tensors. Memory effects for scalar fields are illustrated for both incoming plane waves and higher-order Gaussian beams. We also present a numerical comparison between the spin Hall equations and the full evolution of localized wave packets with angular momentum. Although we work in plane wave spacetimes, which are physically idealized, similar results are also expected to apply for sufficiently small systems affected by distantly generated gravitational waves. Using the Penrose limit, our results may also apply to ultrarelativistic systems at arbitrary locations in arbitrary (even nonradiating) spacetimes.

We propose an improved method to determine the sound horizon in a cosmological model-independent way by using the latest observations of baryon acoustic oscillation (BAO) measurements from DES, BOSS/eBOSS, and DESI surveys and gravitationally time-delay lensed quasars from H0LiCOW collaboration. Combined with the $6D_{\mathrm{\Delta }t}$ plus BAO measurements, we obtain a model-independent result of $r_d={139.5}_{-4.4}^{+5.2}\mathrm{Mpc}$, with the precision at the $\sim 3.7\%$ level, which is consistent with the Planck 2018 result within $\sim 1.7\sigma$ uncertainty. For the combination of four lenses $D_d$, the corresponding constraint result is $r_d={126.3}_{-10.5}^{+13.3}\mathrm{Mpc}$. Our method is independent of cosmological parameters such as the Hubble constant, dark energy, (and more importantly, cosmic curvature is not involved when using the $D_d$ measurements of the lenses, and also avoids the obstacle of mass-sheet degeneracy in gravitational lensing). Meanwhile, it does not need to consider the Eddington relation with concerning the transformation of distance. Finally, we forecast that there are about 50 or so high precision time-delay gravitational lensing systems capable of constraining the sound horizon to the 1% precision level.

Early dark energy, an additional component of dark energy active in the decade of redshift before recombination, has emerged as one of the most effective models at reducing the “$H_0$ tension” between direct measurement of the Hubble parameter $H_0$ in the late Universe and the $\mathrm{\Lambda }\mathrm{CDM}$ prediction when calibrated on Planck. However, it requires a slight increase in the dark matter density ${\omega }_{\mathrm{cdm}}$ and primordial tilt $n_s$ that worsens the “$S_8$ tension” between measurements of weak gravitational lensing at low redshifts and the Planck/$\mathrm{\Lambda }\mathrm{CDM}$ prediction. Using a phenomenological fluid model, we investigate whether the inclusion of a drag term between dark matter and early dark energy can compensate for the effect of the increase in power at small-scales, such that both $H_0$ and $S_8$ tensions are simultaneously alleviated. We find that this works if the drag term is dynamically relevant in the post-recombination universe. However, a drag term active before or just around the time at which the early dark energy contribution to the energy density is maximum is significantly constrained due to its impact on the matter perturbations before recombination, and the subsequent modifications to the cosmic microwave background power spectra.

We present a formalism for analyzing galaxy clustering on the light cone with the two-point correlation in the spherical Fourier-Bessel formalism, which is a natural choice to account for all wide-angle and relativistic (general relativity, GR) effects. We extend previous studies by including all projection and GR effects, developing an efficient numerical implementation that avoids the use of the Limber approximation, includes multibins correlations and a full nondiagonal covariance. Using this formalism, we investigate the impact of neglecting GR corrections, and in particular how much this could bias measurements of the non-Gaussianity parameter $f_{\mathrm{NL}}$. Our results show that not including relativistic projection terms can systematically and non-negligibly bias estimates of $f_{\mathrm{NL}}$. The exact results depend on survey specifications and galaxy population properties, but we stress that a bias will generally be present. Finally, we develop a novel prescription for cross-bin correlations that allow to search for a clean signal of relativistic corrections, and show that this requires the use of the 3D full-sky formalism.

Pulsar timing arrays (PTAs) have made tremendous progress and are now showing strong evidence for the gravitational-wave background (GWB). Further probing the origin and characteristics of the GWB will require more generalized analysis techniques. Bayesian methods are most often used but can be computationally expensive. On the other hand, frequentist methods, like the PTA optimal statistic (OS), are more computationally efficient and can produce results that are complementary to Bayesian methods, allowing for stronger statistical cases to be built from a confluence of different approaches. In this work we expand the capabilities of the OS through a technique we call the per-frequency optimal statistic (PFOS). The PFOS removes the underlying power-law assumption inherent in previous implementations of the OS, and allows one to estimate the GWB spectrum in a frequency-by-frequency manner. We have also adapted a recent generalization from the OS pipeline into the PFOS, making it capable of accurately characterizing the spectrum in the intermediate and strong GW signal regimes using only a small fraction of the necessary computational resources when compared with fully correlated Bayesian methods, while also empowering many new types of analyses not possible before. We find that even in the strong GW signal regime, where the GWB dominates over noise in all frequencies, the injected value of the signal lies within the 50th percentile of the PFOS uncertainty distribution in 41–45% of simulations, remaining $3\sigma$ consistent with unbiased estimation.

Due to the lack of a universally accepted model for determining the structure and composition of a hot hybrid neutron star (HHNS), continuous efforts are being made to improve the existing models. Thus, in this study, we set out to model HHNSs by utilizing a statistical model based on Thomas-Fermi approximation for baryonic matter, and the Polyakov-Nambu-Jona-Lasino model for quark matter to describe the baryon-quark phase transition using the Maxwell construction for neutrino-free matter and neutrino-trapped matter under the beta equilibrium conditions, while adopting the isothermal (isentropic) equation of state (EOS) of baryonic matter with the interactions MS96 and MS90, and the one of quark matter where the quark vector interactions are included. According to our research findings, besides the repulsive effects arising from the baryonic interactions in phase space, and the quark vector couplings, it is concluded that the temperature and neutrino trapping significantly affect the stiffness of the hot hybrid EOS. Within the isentropic picture, the EOS of neutrino-free matter turns out to be more sensitive to the value of the entropy per baryon than that of neutrino-trapped matter. Additionally, with an increase in entropy per baryon, the HHNS structures with larger gravitational (baryonic) masses are allowed, while the ones with smaller radii are excluded. Most importantly, our calculations show that the possibility of obtaining a stable mass structure of HHNSs cannot be excluded under the conditions of quark vector interaction and neutrino trapping.

The Laser Interferometer Space Antenna (LISA) will feature a prominent anisotropic astrophysical stochastic gravitational wave signal, arising from the tens of millions of unresolved mHz white dwarf binaries in the Milky Way: the Galactic foreground. While proper characterization of the Galactic foreground as a noise source will be crucial for every LISA science goal, it is extremely scientifically interesting in its own right, comprising—along with $\sim {10}^4$ resolvable white dwarf binaries—a complete sample of every mHz white dwarf binary in our Galaxy. We present a novel Bayesian analysis of the LISA Galactic foreground that directly treats its anisotropy via astrophysically motivated templates, allowing for a direct connection between the observed time modulation of the foreground amplitude and the underlying spatial distribution of the Milky Way. We validate the efficacy of this approach via simulated data and show that it is able to accurately recover the foreground spectrum in the presence of LISA instrumental noise.