We present and study new nontopological soliton solutions in the $U(1)$ gauged nonlinear $O(3)$ sigma-model with a symmetry breaking potential in $3+1$ dimensional flat space-time. The configurations are endowed with an electric and magnetic field and also carry a nonvanishing angular momentum density. We discuss properties of these solitons and investigate the domains of their existence.

An unsolved problem of particle physics is a discrepancy between the measured value of the muon anomalous magnetic moment and the theoretical prediction based on standard quantum electrodynamics. In this paper we show that if spacetime possesses a fundamental length scale, the ensuing modifications to the photon propagator can account for the discrepancy if the scale is chosen to be ${10}^{-22}\mathrm{m}$; the corresponding energy being about 30 TeV. The possibility that spacetime possesses a graininess on a fine enough scale has a long history. One class of theories that develops this idea is doubly special relativity, and we choose this as a model for our calculation. We note that the derived length scale is many orders of magnitude larger than the Planck length but comparable to that of some higher dimensional gravitational theories. It is also within the scope of experimental confirmation in the next generation of colliders.

An exact solution to the vacuum Einstein equations is presented whose structure is based on the Hopf fibration. The solution employs a geodesic null vector field that defines a twisting congruence and appears in the metric in Kerr–Schild form. This solution is of Petrov type D and involves two parameters. Remarkably, the resulting spacetime is regular, with no curvature singularities. Both the Kretschmann scalar and the Chern–Pontryagin scalar are nonzero and remain finite throughout the spacetime. In addition, the Newman–Penrose Weyl scalar ${\mathrm{\Psi }}_2$ possesses both nonzero real and imaginary parts, reflecting the topologically nontrivial nature of the gravitational field. The spacetime also admits two Killing vector fields and a Killing–Yano tensor, which induces an associated Killing tensor, revealing its hidden symmetry. The derivation is simple and self-contained, offering a transparent and geometrically guided approach to finding new exact solutions in general relativity.

We explore scalarized Einstein-Gauss-Bonnet (EGB) theories within the context of the parametrized post-Newtonian (PPN) formalism, which serves as a robust framework for examining modifications to general relativity that exhibit scalarization. This approach enables us to impose a variety of constraints on the parameter space, particularly focusing on the PPN parameters $\gamma$ and $\beta$. Our analysis reveals several significant bounds for each scalarized model, utilizing data from the Cassini mission, lunar laser ranging, and the forthcoming BepiColombo mission, in conjunction with geodetic very long baseline interferometry measurements around the Sun. In particular, we conducted a combined analysis of the MESSENGER mission’s precision measurements of Mercury’s perihelion precession $\dot{\omega }$ alongside the Cassini constraints on $\gamma$, yielding intriguing limits for the Ricci-EGB model. Furthermore, we investigate the impact of the new post-Newtonian potential introduced by the Gauss-Bonnet term at fourth order, which necessitates an expanded formulation of the PPN parameters. This highlights that the parameter $\beta$ exhibits different effects when analyzing the Ricci-EGB model. Additional constraints are derived from this framework by estimating the $\gamma$ PPN parameter using data from strong-lensing galaxy systems.

We construct static and stationary versions of a black bounce geometry using as inspiration a line element that arises in loop quantum gravity (LQG) scenarios. Analyzing the line element from the framework of general relativity, we trace its origin to nonlinear electrodynamics with electric and magnetic charges and check the energy conditions (null energy condition, weak energy condition, and strong energy condition). By extending the geometry using the Simpson-Visser procedure, we construct a black hole–wormhole bounce structure, influenced by LQG parameters. We analyze the horizon structure to constrain parameters for black holes and wormholes, and examine curvature and new sources, including a phantom-type scalar field, to ensure spacetime regularity and adherence to energy conditions. Thermodynamic properties are also studied, revealing the existence of remnants and phase transitions. Additionally, we derive a rotating black bounce solution, verifying its regularity, and putting forward that the spherical bouncing surface turns into an ellipsoid with no ring singularity. Finally, we find that increasing the LQG parameter leads to smaller ergospheres and reduced shadows, with potential implications for observational astrophysics and quantum gravitational signatures.

We study the impact of quantum corrections to gravitational waveforms on the gravitational wave memory effect. In certain quantum gravity theories and semiclassical frameworks, black holes (or other exotic compact objects) exhibit reflective properties that cause quasinormal modes of a binary merger waveform to partially reflect off the horizon. If these reflections reach the detector, the measured gravitational wave signal may show echolike features following the initial ringdown phase. Detecting such echoes, or their indirect signatures, would offer compelling evidence for the quantum nature of black holes. Given that direct detection of echoes requires finely tuned waveform templates, exploring alternative imprints of this phenomenon is crucial. In this work, we pursue this goal by calculating corrections to the null memory arising from echolike features, formulated in terms of the Newman-Penrose scalar ${\mathrm{\Psi }}_0$. We demonstrate that the morphology of the resulting features is model independent rendering them conceptually much easier to detect in real interferometer data than the raw echo. The corresponding signal-to-noise ratio of echo-induced features appearing in the gravitational wave memory is estimated subsequently. We further compute the physical fluxes associated to the echo at both the black hole horizon and null infinity and identify novel distinguishing features of the underlying reflectivity models in measurement data.

We derive the model of the Schwarzschild black hole immersed into a dark matter halo with a relativistic Hernquist profile, the Schwarzschild-Hernquist black hole, and obtain its tidal Love numbers and quasinormal modes. We thoroughly compare our odd and even parity perturbation equations with the literature and point out that two distinct choices of matter perturbations lead to distinct spectra. We establish that the quasinormal modes admit qualitatively distinct scaling laws in terms of dark matter densities for nonrelativistic and relativistic halos. We develop a stable numerical scheme for computing tidal Love numbers based on asymptotic series expansions. We further comment upon the existence of matter configurations obeying the dominant energy condition that leads to multiple light rings.

Many theories of physical interest, which admit a Hamiltonian description, exhibit symmetries under a particular class of nonstrictly conformal transformation, known as dynamical similarities. The presence of such symmetries allows a reduction process to be carried out, eliminating a single degree of freedom from the system, which we associate with an overall scale. This process of “contact reduction” leads to theories of a frictional nature, in which the physically observable quantities form an autonomous subsystem, that evolves in a predictable manner. We demonstrate that this procedure has a natural generalization to theories of higher order; detailed examples are provided, and physical implications discussed.

We obtain the equations of motion for compact binary systems (black holes or neutron stars) in an alternative theory of gravity known as Einstein-Æther theory, which supplements the standard spacetime metric with a timelike four-vector (the Æther field) that is constrained to have unit norm. The equations make use of solutions obtained in Paper I for the gravitational and Æther field potentials within the near zone of the system, evaluated to 2.5 post-Newtonian (PN) order ($O(v/c{)}^5$ beyond Newtonian gravity), sufficient to obtain the effects of gravitational radiation reaction to the same order as the quadrupole approximation of general relativity. Those potentials were derived by applying the post-Minkowskian method to the field equations of the theory. Using a modified geodesic equation that is a consequence of the effects of the interaction between the Æther field and the internal strong-gravity fields of the compact bodies, we obtain explicit equations of motion in terms of the positions and velocities of the bodies, focusing on the radiation-reaction terms that contribute at 1.5PN and 2.5PN orders. We obtain the rate of energy loss by the system, including the effects of dipole gravitational radiation (conventionally denoted $-1\mathrm{PN}$ order) and the analogue of quadrupole radiation (denoted 0PN order). We find significant disagreements with published results, based on calculating the energy flux in the far zone using a “Noether current” construction.

Eccentric black-hole binaries are among the most awaited sources of gravitational waves, yet their dynamics lacks a consistent framework that provides a detailed and physically robust evolutionary description due to gauge issues. We present a new set of nonorbit-averaged equations, free from radiation-reaction gauge parameters, that accurately describe the evolution of orbital elements for eccentric, nonspinning black-hole binaries. We derive these equations by mapping the Keplerian orbital elements to a new set of characteristic parameters using energy and angular momentum definitions combined with near-identity transformations. The resulting framework is valid for arbitrary eccentricities, including parabolic and hyperbolic limits. Using this framework, we demonstrate effects of the nonadiabatic emission of gravitational waves—characteristic of eccentric binaries—on the orbital parameters. Furthermore, we assess the regime of validity of the widely used orbit-averaged equations first derived by Peters in 1964. Importantly, their breakdown becomes evident at the first pericenter passage, implying that the validity of the orbit-averaged approximation cannot be inferred solely from binary initial conditions. The formalism we introduce, accurate up to 2.5 post-Newtonian order, aims to provide a robust tool for making reliable astrophysical predictions and accurately interpreting current and future gravitational wave data, paving the way for deeper insights into the dynamics of eccentric black hole binaries.

Previously, we attempted to overspin Kerr-Taub-NUT (Kerr-Taub-Newman-Unti-Tamburino) black holes with test fields satisfying the null energy condition. We derived that a nongeneric overspinning could be achieved with strict restrictions on the test field modes and the initial spacetime parameters. Here, we incorporate the effect of the second order variations by evaluating the Sorce-Wald condition for Kerr-Taub-NUT black holes. We calculate the second variation of the area for the subcollection of Kerr-Taub-NUT black holes for which overspinning is possible due to first order perturbations. We carry out an analysis to second order, avoiding the common order of magnitude problems where the second order variations appear in the form of a product with the square of an additional small parameter. We derive that the contribution of the second order variations fix the final state of the spacetime parameters driving the Kerr-Taub-NUT black hole away from extremality. The event horizon is preserved in accord with the cosmic censorship conjecture and the laws of black hole dynamics.

A broad class of modified gravities can result in a modified friction effect in the propagation of gravitational waves (GWs). This effect changes the amplitude-damping rate of GWs during their propagation in the cosmological distance and thus modifies the standard luminosity distance of GWs in general relativity. Therefore, one can constrain this modified friction by measuring both the luminosity distance and redshift of the GW sources. In this paper, we investigate the prospects of constraining such modified friction effects by using the precessing binary black holes with ground-based GW detectors. For this purpose, we consider 20 precessing events detected by the GW detector network consisting of two LIGO detectors and two third-generation GW detectors (the Einstein Telescope and the Cosmic Explorer). The redshift information of these events is obtained by identifying their possible host galaxies in the GLADE+ galaxy catalog. We show that the precession in the binary system can significantly improve the precision of the luminosity distance and thus lead to a tighter constraint on the modified friction. By assuming narrow priors on cosmological parameters that are consistent with the uncertainties of Planck 2018 results, our analysis shows that the modified friction effect, characterized by two parameters $({\mathrm{\Xi }}_0,n)$, can be constrained to be ${\mathrm{\Xi }}_0=1.002_{-0.004}^{+0.004}$ and $n=3.257_{-2.192}^{+2.595}$, in which the result of ${\mathrm{\Xi }}_0$ is about 2 orders of magnitude better than current results from an analysis with GWTC-3. Our result sets the stage for future research with third-generation GW detectors, offering new insights into gravitational parameter modifications. It also contributes to the understanding of the properties and applications of binary black hole systems with precession.

We study families of solutions depending only on one coordinate for the theories of $N=2$, five-dimensional supergravity coupled to vector multiplets. Four families of solutions, valid for all space-time signatures, are obtained. The explicit metrics of the solutions depend on the Jordan normal forms of constrained matrices. Examples with three charges are given for the so-called STU model.

Recent observations from the James Webb Space Telescope revealed a surprisingly large number of galaxies at high redshift, challenging the standard Lambda cold dark matter cosmology with a power-law primordial power spectrum. Previous studies alleviated this tension with a blue-tilted primordial power spectrum [$P(k)\propto k^{m_s}$ with $m_s>1$ at small scales $>1{\mathrm{cMpc}}^{-1}$]. In this study, we examine whether the blue-tilted model can boost dark matter substructures especially at low redshift, thereby addressing other potential challenges to the standard cosmology. First, substructures in the standard cosmological model may not be sufficient to explain the anomalous flux ratio problem observed in strong gravitational lensing. Second, the number of observed nearby satellite galaxies could be higher than the theoretical predictions of the standard cosmology, after completeness correction and tidal stripping by baryonic disks. To study the impact of a blue-tilted primordial power spectrum on substructures, we perform high-resolution cosmological zoom-in dark matter-only simulations of Milky Way host mass halos, evolving to redshift $z=0$. At $z=0$, we find that the blue-tilted subhalo mass functions can be enhanced by more than a factor of 2 for subhalo masses $M_{\mathrm{sub}}\lesssim {10}^{10}{M}_{\odot }$, whereas the subhalo $V_{\max }$ functions can be enhanced by a factor of 4 for maximum circular velocities $V_{\max }\lesssim 30\mathrm{km}/\mathrm{s}$. The blue-tilted scaled cumulative substructure fraction can be an order of magnitude higher at $\sim 10\%$ of the virial radius. The blue-tilted subhalos also have higher central densities, since the blue-tilted subhalos reach the same $V_{\max }$ at a smaller distance $R_{\max }$ from the center. We have also verified these findings with higher-resolution simulations.

Ultralight (or fuzzy) dark matter (ULDM) is an alternative to cold dark matter. A key feature of ULDM is the presence of solitonic cores at the centers of collapsed halos. These would potentially increase the drag experienced by supermassive black hole (SMBH) binaries, changing their merger dynamics and the resulting gravitational wave background. We perform detailed simulations of high-mass SMBH binaries in the soliton of a massive halo. We find more rapid decay than previous simulations and semianalytic approximations. We confirm expectations that the drag depends strongly on the ULDM particle mass, finding masses greater than ${10}^{-21}\mathrm{eV}$ could potentially alleviate the final parsec problem and that ULDM may even suppress gravitational wave production at lower frequencies in the pulsar timing band.

A strong first-order phase transition in a dark sector may produce all or part of the low-frequency gravitational wave signal recently reported by the NANOGrav Collaboration and other pulsar timing arrays. Here we point out, with a simple toy model, that even if the amplitude of the gravitational wave background from the dark phase transition is insufficient to match the NANOGrav signal, a modified expansion rate at early times may considerably enhance the gravitational wave signal. In particular, a faster-than-standard expansion rate, triggered, for instance, by the presence of one or more additional sources of energy density redshifting with higher powers of temperatures than radiation, boosts upper limits on the gravitational wave signal from first-order cosmological phase transitions, enlarging the slate of possible dark sector scenarios matching the NANOGrav signal.

The latest baryon acoustic oscillation (BAO) measurements from the Dark Energy Spectroscopic Instrument (DESI) Collaboration, when combined with Planck satellite cosmic microwave background (CMB) data and type Ia supernovae, suggest a preference for dynamical dark energy (DDE) at a significance level ranging from $2.8\sigma$ to $4.2\sigma$. In this work, I test whether, and to what extent, this preference is supported by CMB experiments other than Planck. I analyze the latest Atacama Cosmology Telescope (ACT) and South Pole Telescope (SPT) temperature, polarization, and lensing spectra at small scales, eventually combining them with Planck or Wilkinson microwave anisotropy probe (WMAP) 9-yr observations at large angular scales. My analysis shows that ACT and WMAP data, when combined with DESI BAO and $\mathrm{Pantheon}+$ supernovae, yield independent constraints with a precision comparable to Planck. Notably, in this case, the cosmological constant value is recovered within 2 standard deviations. A preference for DDE reappears when $\mathrm{Pantheon}+$ is replaced with distance moduli measurements from the Dark Energy Survey supernova program (DESy5). However, it remains less pronounced compared to the Planck-based results. When considering SPT data, no clear preference for DDE is found in combinations involving $\mathrm{Pantheon}+$ supernovae, and the preference is significantly weaker in combinations involving DESy5. Overall, CMB experiments other than Planck generally weaken the evidence for DDE. I argue that the subsets of Planck data that strengthen the shift toward DDE are the temperature and E-mode polarization anisotropy measurements at large angular scales $\ell \lesssim 30$.

We study the effects of the oscillating axion field present in our environment on the Casimir pressure between two metallic plates. We take into account the finite conductivity of the boundary plates and model the interactions between matter and photons in the Schwinger-Keldysh formalism. This allows us to take into account dissipation in the quantum field description of this open quantum system and retrieve the Lifschitz results for the Casimir interaction between two metallic plates. We then compute the leading correction to the Lifschitz theory in inverse powers of the axion suppression scale and show that the Casimir pressure receives oscillating corrections depending on the product of the axion mass and the distance between the plates. This contribution is repulsive at large distance compared to the axion Compton wavelength as a consequence of the breaking of parity invariance by the axion dark matter background.

Leptophilic sub-MeV spin-zero dark matter (DM) decays into photons via one-loop processes, a scenario that has been in part overlooked in current literature. In this work, we provide updated and comprehensive upper limits on scalar, pseudoscalar, and axionlike DM-electron couplings based on the latest NPIPE cosmic microwave background data from Planck. Our bounds on the couplings are not only competitive with astrophysical and terrestrial experiments, but outperform them in certain regions of parameter space. Notably, we present the most stringent limits to date on scalar DM with masses around a few keV and pseudoscalar DM with masses between 100 eV and a few keV. Additionally, we explore, for the first time, the impact of implementing a cosmology-consistent treatment of energy deposition into the cosmic medium.