Recently, there has been quite a lot of interest in static, spherical wormhole spacetimes and the question of their stability with respect to time-dependent perturbations. The consideration of linearized perturbations usually leads to a master wave equation with effective potential, which can then be analyzed using standard tools from quantum mechanics. However, in the wormhole case, particular care must be taken with the gauge conditions when formulating the master equation. A poor coordinate choice, based for example on fixing the areal radial coordinate, may lead to singularities at the throat, which complicate the stability analysis, or might even lead to erroneous conclusions regarding the stability of the underlying wormhole configuration. In this work, we present a general method for deriving a gauge-invariant wave system of linearized perturbation equations in the spherically symmetric case, assuming that the matter supporting the wormhole is a phantom scalar field—that is, a self-interacting scalar field whose kinetic energy has the reversed sign. We show how this system can be decoupled and reduced to a single master wave equation with a regular potential, with no intermediate steps involving singularities at the throat. Two applications of our formalism are given. First, we rederive the master equation for the linearly perturbed Ellis-Bronnikov wormhole using our new, singularity-free method. As a second application, we derive the master equation describing the linear perturbations of a certain anti–de Sitter wormhole, provide a detailed analysis of the spectral properties of the underlying operator, and prove that, as in the Ellis-Bronnikov case, this wormhole is linearly unstable and possesses a single unstable mode. In the final part of the paper, we consider a wormhole with de Sitter–type ends, whose spacetime presents horizons and admits a nonstatic extension beyond them; for this system, we derive partial results of linear instability.

We study the response function of Unruh-deWitt detectors placed in a flat spacetime inside a thin matter shell. We show that the response function distinguishes between the local and global (Minkowski) inertial frames and picks up the presence of the shell even when the detector is switched on for a finite time interval within which a light signal cannot travel to the shell and back as required by a classical measurement. We also analyze how the response of the detector depends on its location within the shell.

We calculate the cross section for $e^{+}{e}^{-}$ annihilation into $D^{*0}{\overline{D}}^0+\gamma$ at center-of-mass energies near the $D^{*0}{\overline{D}}^{*0}$ threshold under the assumption that $X(3872)$ is a weakly bound charm-meson molecule. The Dalitz plot has a ${\overline{D}}^{*0}$ resonance band in the squared invariant mass $t$ of ${\overline{D}}^0\gamma$. In the limit as the decay width of the $D^{*0}$ goes to 0, the Dalitz plot also has a narrow band in the squared invariant mass $u$ of $D^{*0}{\overline{D}}^0$ from a charm-meson triangle singularity. At the physical value of the $D^{*0}$ width, the narrow band reduces to a shoulder. Thus, the triangle singularity cannot be observed directly as a peak in a differential cross section as a function of $u$. It may however be observed indirectly as a local minimum in the $t$ distribution for events with $u$ below the triangle singularity. The minimum is produced by the Schmid cancellation between triangle loop diagrams and a tree diagram. The observation of this minimum would support the identification of $X(3872)$ as a weakly bound charm-meson molecule.

We present the conformal freeze-in (COFI) scenario for dark matter production. At high energies, the dark sector is described by a gauge theory flowing toward a Banks-Zaks fixed point, coupled to the Standard Model via a nonrenormalizable portal interaction. In the early Universe, a nonthermal freeze-in process transfers energy from the standard model plasma to the dark sector. During the freeze-in, the dark sector is described by a strongly coupled conformal field theory. As the Universe cools, cosmological phase transitions in the Standard Model sector, either electroweak or QCD, induce conformal symmetry breaking and confinement in the dark sector. One of the resulting dark bound states is stable on the cosmological time scales and plays the role of dark matter. With the Higgs portal, the COFI scenario provides a viable dark matter candidate with mass in a phenomenologically interesting sub-MeV range. With the quark portal, a dark matter candidate with mass around 1 keV is consistent with observations. Conformal bootstrap may put a nontrivial constraint on model building in this case.

Nonstandard interactions (NSIs), possible subleading effects originating from new physics beyond the Standard Model, may affect the propagation of neutrinos and eventually contribute to measurements of neutrino oscillations. Besides this, $\mu -\tau$ reflection symmetry, naturally predicted by non-Abelian discrete flavor symmetries, has been very successful in explaining the observed leptonic mixing patterns. In this work, we study the combined effect of both. We present an $S_4$ flavor model with $\mu -\tau$ reflection symmetry realized in both neutrino masses and NSIs. Under this formalism, we perform a detailed study for the upcoming neutrino experiments DUNE and T2HK. Our simulation results show that under the $\mu -\tau$ reflection symmetry, NSI parameters are further constrained and the mass ordering sensitivity is less affected by the presence of NSIs.

We evaluate the gluon and quark contributions to the spin of the proton using an ensemble of gauge configurations generated at physical pion mass. We compute all valence and sea quark contributions to high accuracy. We perform a nonperturbative renormalization for both quark and gluon matrix elements. We find that the contribution of the up, down, strange, and charm quarks to the proton intrinsic spin is $\frac{1}{2}{\sum }_{q=u,d,s,c}\mathrm{\Delta }{\mathrm{\Sigma }}^{q^{+}}=0.191(15)$ and to the total spin ${\sum }_{q=u,d,s,c}{J}^{q^{+}}=0.285(45)(10)$. The gluon contribution to the spin is $J^g=0.187(46)(10)$ yielding $J=J^q+J^g=0.473(71)(14)$ confirming the spin sum. The momentum fraction carried by quarks in the proton is found to be 0.618(60) and by gluons 0.427(92), the sum of which gives 1.045(118) confirming the momentum sum rule. All scale and scheme dependent quantities are given in the $\overline{\mathrm{MS}}$ scheme at 2 GeV.

We investigate the characteristics of $\sigma$, $f_0(980)$, and $a_0(980)$ with the formalism of chiral unitary approach. With the dynamical generation of them, we make a further study of their properties by evaluating the couplings, the compositeness, the wave functions, and the radii. We also research their properties in the single channel interactions, where the $a_0(980)$ cannot be reproduced in the $K\overline{K}$ interactions alone with isospin $I=1$ since the potential is too weak. In our results, the states of $\sigma$ and $f_0(980)$ can be dynamically reproduced stably with varying cutoffs both in the coupled channel and the single channel cases. We find that the $\pi \eta$ components is much important in the coupled channel interactions to dynamically reproduce the $a_0(980)$ state, which means that $a_0(980)$ state cannot be a pure $K\overline{K}$ molecular state. We obtain their radii as: $|\sqrt{⟨r^2⟩}{|}_{f_0(980)}=1.80\pm 0.35\mathrm{fm}$, $|\sqrt{⟨r^2⟩}{|}_{\sigma }=0.68\pm 0.05\mathrm{fm}$ and $|\sqrt{⟨r^2⟩}{|}_{a_0(980)}=0.94\pm 0.09\mathrm{fm}$. Based on our investigation results, we conclude that the $f_0(980)$ state is mainly a $K\overline{K}$ bound state, the $\sigma$ state a resonance of $\pi \pi$ and the $a_0(980)$ state a loose $K\overline{K}$ bound state with the significant compositeness of $\pi \eta$. From the results of the compositeness, they are not pure molecular states and have something nonmolecular components, especially for the $\sigma$ state.

An effective Lagrangian approach, based on an extended Einstein-Aether (EA) model, is proposed. This model is presented as an alternative to GR and mimetic gravity models in order to deal with the big bang cosmological singularity issue. In particular, working on non flat Friedmann–Lemaitre–Robertson–Walker (FLRW) space-time, a generalized Friedmann equation is derived, and it is shown that, for a suitable choice of the action within the extended EA theory, a regular bounce solution is present, generalizing the bounce solution obtained in quantum loop cosmology (QLC) in the flat case. Furthermore, perturbation theory of the extended model is investigated, and the main perturbation quantities are evaluated and the conditions to avoid physical (superluminarity) and mathematical instabilities are discussed. Finally, the static spherically symmetric (SSS) case is also investigated. It is found that, with the same action proposed for the FLRW space-time, a Schwarschild like solution with a correction is obtained.

The recent detections of the gravitational wave (GW) event GW170817 and its electromagnetic counterpart GRB 170817A produced by a binary neutron star (NS) merger is a new milestone of multimessenger astronomy. The time interval between these two signals has attracted widespread attention from physicists. In the braneworld scenario, GWs could propagate through the bulk while electromagnetic waves (EMWs) are bounded on the brane, i.e., our Universe. Therefore, the trajectories of GWs and EMWs may follow different pathes. If GWs and EMWs are originated simultaneously from the same source on the brane, they are expected to arrive at the observer successively. Consequently, the time delay between GW170817 and GRB 170817A may carry the information of the extra dimension. In this paper, we try to investigate the phenomenon in the context of a five-dimensional dS (${\mathrm{dS}}_5$) spacetime. We first study two special Universe models, i.e., de Sitter and Einstein-de Sitter models, and calculate the gravitation horizon radius in each case. For the real Universe, we then consider the $\mathrm{\Lambda }\mathrm{CDM}$ model. Our results show that for the de Sitter model of the Universe, the ${\mathrm{dS}}_5$ radius could not contribute to the time delay. With the data of the observation, we constrain the ${\mathrm{dS}}_5$ radius to $\ell \gtrsim 7.5\times {10}^2\mathrm{Tpc}$ for the Einstein-de Sitter model and $\ell \gtrsim 2.4\times {10}^3\mathrm{Tpc}$ for the $\mathrm{\Lambda }\mathrm{CDM}$ model. After considering the uncertainty in the source redshift and the time-lags given by different astrophysical processes of the binary NS merger, we find that our constraints are not sensitive to the redshift in the range of (0.005, 0.01) and the time-lag in the range of ($-100\mathrm{s}$, 1.734 s).

The detection of axion fields can unfold intriguing facets of our Universe in several astrophysical and cosmological scenarios. In four dimensions, such a field owes its origin to the completely antisymmetric Kalb-Ramond field strength tensor. Its invisibility in the Solar System–based tests compels one to look for its signatures in the strong-field regime. The recent observation of the shadow of the supermassive black hole in the galaxy M87 ushers in a new opportunity to test for the footprints of axions in the near-horizon regions of black holes, where the gravity is expected to be strong. In this paper, we explore the impact of axions on the black hole shadow and compare the result with the available image of M87*. Our analysis indicates that an axion which violates the energy condition seems to be favored by observations. The implications are discussed.

Using a dynamical system approach we study the cosmological phase space of the generalized hybrid metric-Palatini gravity theory, characterized by the function $f(R,\mathcal{R})$, where $R$ is the metric scalar curvature and $\mathcal{R}$ the Palatini scalar curvature of the spacetime. We formulate the propagation equations of the suitable dimensionless variables that describe FLRW universes as an autonomous system. The fixed points are obtained for four different forms of the function $f(R,\mathcal{R})$, and the behavior of the cosmic scale factor $a(t)$ is computed. We show that due to the structure of the system, no global attractors can be present and also that two different classes of solutions for the scale factor $a(t)$ exist. Numerical integrations of the dynamical system equations are performed with initial conditions consistent with the observations of the cosmological parameters of the present state of the Universe. In addition, using a redefinition of the dynamic variables, we are able to compute interesting solutions for static universes.

The Kerr geometry is believed to represent the exterior spacetime of astrophysical black holes. We here reanalyze the geometry of Kerr-like metrics (Kerr, Kerr-Newman, Kerr–de Sitter, and Kerr–anti-de Sitter), paying particular attention to the region near the singular set. We find that, although the Kretschmann scalar vanishes at the singular set along a given direction, a certain combination of curvature invariants diverges regardless of the direction of approach. We also find that the two-dimensional geometry induced by the spacetime metric on the orbits of the isometry group also possesses a singularity regardless of the direction of approach. Likewise, the two-dimensional geometry in the directions orthogonal to the isometry orbits is $C^2$-divergent, but extends continuously at the singular set as a cone with opening angle $4\pi$. We conclude by showing that tidal forces lead to infinite stresses on neighboring geodesics that approach the singular set, destroying any such observers in finite proper time. Those geodesics that come in from infinity and do not hit the singular set but approach it are found to need tremendous energy to get close to the singular set, experiencing an acceleration transversal to the equatorial plane which grows without bound when the minimal distance of approach goes to zero. While establishing these results, we also present an alternative description of some other known properties, as well as introducing toroidal coordinates that provide a hands-on description of the double covering for the geometry near the singular set.

We develop an approach to calculate the self-force on a charged particle held in place in a curved spacetime, in which the particle is attached to a massless string and the force is measured by the string’s tension. The calculation is based on the Weyl class of static and axially symmetric spacetimes, and the presence of the string is manifested by a conical singularity; the tension is proportional to the angular deficit. A remarkable and appealing aspect of this approach is that the calculation of the self-force requires no renormalization of the particle’s electric field. This is in contract with traditional methods, which incorporate a careful and elaborate subtraction of the singular part of the field. We implement the approach in a number of different situations. First, we examine the case of an electric charge in Schwarzschild spacetime, and recover the classic Smith-Will force in addition to a purely gravitational contribution to the self-force. Second, we turn to the case of electric and magnetic dipoles in Schwarzschild spacetime, and correct expressions for the self-force previously obtained in the literature. Third, we replace the electric charge by a scalar charge, and recover Wiseman’s no-force result, which we generalize to a scalar dipole. And fourth, we calculate the force exerted on extended bodies such as Schwarzschild black holes and Janis-Newman-Winicour objects, which describe scalarized naked singularities.

Torsion is a non-Riemannian geometrical extension of general relativity that allows including the spin of matter and the twisting of spacetime. Cosmological models with torsion have been considered in the literature to solve problems of either the very early (high redshift $z$) or the present-day Universe. This paper focuses on distinguishable observational signatures of torsion that could not be otherwise explained with a scalar field in pseudo-Riemannian geometry. We show that when torsion is present, the cosmic duality relation between the angular diameter distance, $D_{\mathrm{A}}$, and the luminosity distance, $D_{\mathrm{L}}$, is broken. We show how the deviation described by the parameter $\eta =D_{\mathrm{L}}/[D_{\mathrm{A}}(1+z{)}^2]-1$ is linked to torsion and how different forms of torsion lead to special-case parametrizations of $\eta$, including ${\eta }_{0}z$, ${\eta }_{0}z/(1+z)$, and ${\eta }_0\ln (1+z)$. We also show that the effects of torsion could be visible in low-redshift data, inducing biases in supernovae-based $H_0$ measurements. We also show that torsion can impact the Clarkson-Bassett-Lu (CBL) function $\mathcal{C}(z)=1+H^2(D{D}^{\prime \prime }-D^{\prime 2})+H{H}^{\prime }D{D}^{\prime }$, where $D$ is the transverse comoving distance. If $D$ is inferred from the luminosity distance, then, in general, nonzero torsion models, $\mathcal{C}(z)\ne 0$. For pseudo-Riemannian geometry, the Friedmann-Lemaître-Robertson-Walker metric has $\mathcal{C}(z)\equiv 0$; thus, the measurement of the Clarkson-Bassett-Lu function could provide another diagnostic of torsion.

We consider dark matter represented by the light scalar field whose coupling to the ordinary matter is extremely suppressed. We assume that this interaction can be described as the coupling of the square of the field to the energy-momentum tensor. We study the effect of this interaction on the evolution of dark matter scalar, as well as bounds on the model parameters that come from the variation of fundamental constants at big bang nucleosynthesis.

Cosmological models describing the nongravitational interaction between dark matter and dark energy are based on some phenomenological choices of the interaction rates between dark matter and dark energy. There is no such guiding rule to select such rates of interaction. In the present work we show that various phenomenological models of the interaction rates might have a strong field theoretical ground. We explicitly derive several well known interaction functions between dark matter and dark energy under some special conditions and finally constrain them using the latest cosmic microwave background observations from final Planck legacy release together with baryon acoustic oscillations distance measurements. Our analyses report that one of the interacting functions is able to alleviate the $H_0$ tension. We also perform Bayesian evidence analyses for all the models with reference to the $\mathrm{\Lambda }\mathrm{CDM}$ model. From the Bayesian evidence analyses, although the reference scenario is preferred over the interacting scenarios, however, we found that two interacting models are close to the reference $\mathrm{\Lambda }\mathrm{CDM}$ model.

Dark matter can form bound states upon the emission of quanta of energy equal to the binding energy. The rate of this process is large for strongly interacting dark matter, and further enhanced by long-distance effects. The resulting monochromatic and diffuse $\gamma$ rays can be tested in indirect-detection experiments. If dark matter has electroweak charge, indirect signals include multiple observable photon lines for masses in the TeV range. Otherwise, if it couples only via a dark photon portal, diffuse spectra from dwarf galaxies and cosmic microwave background reionization set powerful limits for masses below a tera-electron volt. This mechanism provides a powerful means of probing asymmetric dark matter today.

Dark matter could have a small electromagnetic charge, provided the charge-to-mass ratio is much less than that of electrons or protons. This candidate, commonly known as millicharged dark matter (mDM), would form a plasma and interact with the interstellar medium and electromagnetic fields within galaxies. In general, understanding the dynamics of mDM requires consideration of collective plasma effects. It has been proposed that mDM can be accelerated in supernova remnants, forming a dark cosmic ray population that would leave distinct experimental signatures. In this work, we study a microphysical model where mDM is shocked by a supernova remnant and isotropized in the frame of the expanding fluid. We find that for $|q_{\chi }/m_{\chi }|\gtrsim {10}^{-13}e/\mathrm{MeV}$, the isotropization length for electromagnetic plasma instabilities is much shorter than the size of the supernova remnant. This is a necessary, though not sufficient, first step for formation of a Fermi-accelerated mDM component, and determining the size of this component requires further study. We discuss additional implications of mDM interactions in supernova remnants.