Given the lack of an absolute time parameter in general relativistic systems, quantum cosmology often describes the expansion of the universe in terms of relational changes between 2 degrees of freedom, such as matter and geometry. However, if clock degrees of freedom (self-)interact nontrivially, they in general have turning points where their momenta vanish. At and beyond a turning point, the evolution of other degrees of freedom is no longer described directly by changes of the clock parameter because it stops and then turns back, while time is moving forward. Previous attempts to describe quantum evolution relative to a clock with turning points have failed and led to frozen evolution in which degrees of freedom remain constant while the clock parameter, interpreted directly as a substitute for monotonic time, is being pushed beyond its turning point. Here, a new method previously used in oscillator systems is applied to a tractable cosmological model, given by an isotropic universe with spatial curvature and scalar matter. The recollapsing scale factor presents an example of a clock with a single turning point. The method succeeds in defining unitary and freeze-free evolution by unwinding the turning point of the clock, introducing an effective monotonic time parameter that is related to but not identical with the nonmonotonic clock degree of freedom. Characteristic new quantum features are found around the turning point, based on analytical and numerical calculations.

In this paper, we investigate the gravitational effects of a global monopole that couples nonminimally to gravity. Considering a coupling parameter of arbitrary strength, we have obtained an analytical solution for the field equations of the model in the asymptotic region outside the monopole’s core, thus generalizing previous results. Within the weak coupling regime, we have also analyzed how this matter-geometry interplay affects some physical properties of the monopole, such as its radius and the mass enclosed in its core, which are explicitly computed and expressed in terms of the parameter of the theory. Using the Hahari-Loustó toy model, we have also found that the modification of general relativity (GR) may render a positive sign to the monopole mass, which means a global monopole able to exert attractive gravitational force on matter particles surrounding it. Next, we have studied some properties of a hairy black hole carrying such a global monopole charge. In this vein, we verified that the deviation from GR may provide such a spacetime with both event and Cauchy horizons, thus emulating a Reissner-Nordström framework. Finally, we have performed a study of the geodesic motion of timelike particles around this black hole by setting the conditions for stable circular orbits. We also examined the gravitational bending experienced by photons traveling through this geometry, taking into account finite-distance corrections on the deflection angle. Additionally, we make an order-of-magnitude estimate for this extra contribution considering the light bending caused by Sagittarius A*.

Recently, Contreras et al. [Phys. Rev. D 103, 044020 (2021)] introduced a new type of black hole, called the hairy Kerr black hole (HKBH), which describes a Kerr BH surrounded by an axially symmetric fluid with conserved energy momentum tensor. In this paper, we compute the gravitational waves emitted from the extreme mass ratio inspirals around the HKBHs. We solve the Dudley-Finley equation, which describes the gravitational perturbations of the HKBH, and obtain the energy fluxes induced by a stellar-mass compact object moving on the equatorial, circular orbits. Using the adiabatic approximation, we evolved the radii of the circular orbits by taking into account the backreaction of gravitational radiation. Then we calculate the dephasing and mismatch of the extreme-mass-ratio inspiral (EMRI) waveforms from the HKBH and Kerr BH to assess the difference between them. The results demonstrate that the EMRI waveforms from the HKBH with deviation parameter larger than 0.001 and hair charge smaller than $1.5M$ can be discerned by LISA.

We study the flavor dependent $U(1{)}_{B_i-L_j}$ models, where an $i$th generation of quarks and $j(\ne i)$th generation of leptons are charged. By solving the anomaly free condition for the matter sector of the SM fermions and three generations of right-handed (RH) neutrinos, we find that the $j$th generation of RH neutrino is not necessarily charged under the $U(1{)}_{B_i-L_j}$ gauge symmetry with the charge $-1$ and the other (neither $i$th nor $j$th) generation of RH neutrino can also be. As a general solution for the anomaly cancellation conditions, the other two RN neutrinos than the charge $-1$ RH neutrino may have nonvanishing charge and be stable due to the gauge invariance, and hence it is a candidate for dark matter (DM) in our Universe. We apply this result to a $B_3-L_2$ model and consider a light thermal DM and a solution to the muon $g-2$ anomaly. We identify the parameter region to have the DM mass range from MeV to sub-GeV and simultaneously solve the muon $g-2$ anomaly. We also derive the constraints on the gauge kinetic mixing parameter by using the latest Borexino phase-II data.

An unidentified 3.55 keV x-ray line in stacked spectra of galaxies and clusters raises the interesting possibility that it originates from the decay of sterile neutrino dark matter. In this work, we explore mixed sterile neutrino dark matter models that combine cold dark matter and warmer sterile neutrino dark matter produced through lepton number-driven active-to-sterile neutrino transformation. We analyze the sensitivity of the sterile neutrino spectra on active-sterile mixing and on initial neutrino lepton numbers. We explore the parameter space of mixed sterile neutrino dark matter models, and assess their viability based on estimates of the number of subhalos formed as the host sites of satellite galaxies. We find constraints on the active-sterile mixing angle inferred from x-ray observations ($\lesssim {10}^{-9}$) and the fraction of dark matter consisting of 7.1 keV sterile neutrinos ($\lesssim 1$). Considering other probes of small-scale structure formation can make these constraints more stringent and future observations of the x-ray line can better constrain these models and their production mechanism.

Structure formation in the Universe has been well studied within the Eulerian and Lagrangian perturbation theories, where the latter performs substantially better in comparison with N-body simulations. Standing out is the celebrated Zel’dovich approximation for dust matter. In this work, we recall the description of gravitational noncollisional systems and extend both the Eulerian and Lagrangian approaches by including, possibly anisotropic, velocity dispersion. A simple case with plane symmetry is then studied with an exact, nonperturbative approach, and various approximations of the derived model are then compared numerically. A striking result is that linearized Lagrangian solutions outperform models based on Burgers’ equation in the multistream regime in comparison with the exact solution. These results are finally extended to a 3D case without symmetries, and master equations for the evolution of all parts of the perturbations are derived. The particular 3D case studied corresponds to a maximally anisotropic collapse, which involves an approximation based on the estimation of importance of the different levels of spatial derivatives of the local deformation field.

Nontrivial dark sector physics continues to be an interesting avenue in our quest to the nature of dark matter. In this paper, we study the cosmological signatures of mass-varying dark matter where its mass changes from zero to a nonzero value in the early Universe. We compute the changes in various observables, such as, the linear matter power spectrum and the cosmic microwave background anisotropy power spectrum. We explain the origin of the effects and point out a qualitative similarity between this model and a warm dark matter cosmology with no sudden mass transition. Finally, we do a simple analytical study to estimate the constraint on the parameters of this model from the Lyman-$\alpha$ forest data.

Over the past two decades, significant strides have been made in the study of dark matter (DM) admixed neutron stars and their associated properties. However, an intriguing facet regarding the effect of DM on magnetized neutron stars still remains unexplored. This study is carried out to analyze the properties of DM admixed magnetized neutron stars. The equation of state for the DM admixed neutron star is calculated using the relativistic mean-field model with the inclusion of a density-dependent magnetic field. Several macroscopic properties, such as mass, radius, particle fractions, tidal deformability, and the $f$-mode frequency, are calculated with different magnetic field strengths and DM configurations. The equation of state is softer with the presence of DM as well as for the parallel components of the pressure and vice versa for the perpendicular one. Other macroscopic properties, such as mass, radius, tidal deformability, etc., are also affected by both DM and magnetic fields. The change in the magnitude of different neutron star observables is proportional to the amount of DM percentage and the strength of the magnetic field. We observe that the change is seen mainly in the core part of the star without affecting the crustal properties.

Charged-current neutrino processes such as ${\nu }_e+n\rightleftharpoons p+e^{-}$ and ${\overline{\nu }}_e+p\rightleftharpoons n+e^{+}$ destroy the flavor coherence among the weak-interaction states of a single neutrino and thus damp its flavor oscillation. In a dense neutrino gas such as that inside a core-collapse supernova or the black hole accretion disk formed in a compact binary merger, however, these “collision” processes can trigger large flavor conversion in cooperation with the strong neutrino-neutrino refraction. We show that there exist two types of collisional flavor instability in a homogeneous and isotropic neutrino gas which are identified by the dependence of their real frequencies on the neutrino density $n_{\nu }$. The instability transitions from one type to the other and exhibits a resonancelike behavior in the region where the net electron lepton number of the neutrino gas is negligible. In the transition region, the flavor instability grows exponentially at a rate $\propto n_{\nu }^{1/2}$. We find that the neutrino gas in the black hole accretion disk is susceptible to the collision-induced flavor conversion where the neutrino densities are the highest. Further investigations are needed to confirm if the collisional flavor instability will indeed result in the production of large amounts of heavy-lepton flavor neutrinos in this environment which would have important ramifications in its subsequent evolution.

The axion dark matter echo technique, proposed in Arza and Sikivie [Phys. Rev. Lett. 123, 131804 (2019)], aims to search for axion dark matter by detecting the electromagnetic echo coming from stimulated decay of ambient axion dark matter interacting with a radio-microwave outgoing beam. In this work we consider deviations from the standard halo model (SHM) in the form of fine grained streams that are present in the solar system and use them as a target for the axion echo method, which demonstrates to be very sensitive to particular local axion dark matter phase space distributions. We show that the extremely small dispersion of the streams works in favor of the echo method performance, improving its sensitivity in the axion-photon coupling up to two orders of magnitude, with respect to the isotropic SHM. We also discuss the possibility of targeting high density dark matter hairs formed when fine grained streams focus by the inner Earth’s gravitational field.

We present a Mathematica package that takes any reductive gauge algebra and fully-reducible fermion representation, and outputs all semisimple gauge extensions under the condition that they have no additional fermions, and are free of local anomalies. These include all simple completions, also known as grand unified theories. We additionally provide a list of all semisimple completions for 5835 fermionic extensions of the one-generation Standard Model.

Lattice QCD in the strong coupling regime can be formulated in dual variables, which are integer-valued. It can be efficiently simulated for modest finite temperatures and finite densities via the worm algorithm, circumventing the finite density sign problem in this regime. However, the low temperature regime is more expensive to address. As the partition function is solely expressed in terms of integers, it can be cast as a combinatorial optimization problem that can be solved on a quantum annealer. We will first explain the setup of the system we want to study and then present its reformulation suitable for a quantum annealer, and in particular the D wave. As a proof of concept, we present first results obtained on D wave for gauge group U(1) and U(3), and outline the next steps towards gauge groups $\mathrm{SU}(3)$. We find that in addition, histogram reweighting greatly improves the accuracy of our observables when compared to analytic results.

The chiral magnetic wave (CMW) is a macroscopic quantum phenomenon that arises due to the mixing of the electric and chiral charge oscillations induced by the chiral anomaly. In this study we report the first quantum simulation (on classical hardware) of the real-time dynamics of CMWs in the Schwinger model. Our quench protocol is the following; at $t=0$ we suddenly place an electric dipole at the middle of our lattice. Due to chiral anomaly, this dipole excites the CMW that propagates towards the edges of the lattice. In the Schwinger model tuned to the conformal critical point (at $\theta =\pi$, $m/g\simeq 0.2$), we find a gapless linear CMW that propagates with the speed of light. For a massless Schwinger model ($\theta =0$, $m=0$), we find a gapped linear CMW, in accord with previous analytical analyses. For a massive Schwinger model (that is dual to strongly interacting bosonic theory), we enter the new regime of nonlinear CMWs, where we find a surprise. Specifically, for $m/g>1$, the frequency of electric charge oscillations becomes much smaller than the frequency of the oscillations of the chiral charge. For $m/g=4$, we find a solution corresponding to a nearly static electric dipole with fast oscillations of the chiral charge confined within. We call this solution a “thumper” and study its properties in detail. We speculate that thumpers might be relevant for CMWs in the strange quark sector of quark-gluon plasma and narrow gap semiconductors.

With the breakthrough in PeV gamma-ray astronomy brought about by the LHAASO experiment, the high-energy sky is becoming richer. Recently, the LHAASO Collaboration reported the observation of a gamma-ray diffuse emission with energy up to the PeV level from both the inner and outer Galactic plane. In these spectra, there is a bump that is hard to explain using the conventional cosmic-ray transport scenarios. Therefore, we introduce two extra components corresponding to unresolved sources with exponential-cutoff-power-law (ECPL) spectral shape—one with an index of 2.4 and cutoff energy of 20 TeV, and another with an index of 2.3 and cutoff energy of 2 PeV. With our constructed model, we simulate the Galactic diffuse neutrino flux and find that our results are in full agreement with the latest IceCube Galactic plane search. We estimate the Galactic neutrino contributes $\sim 9\%$ of astrophysical neutrinos at 20 TeV. In the high-energy regime, as expected, most of the neutrinos observed by IceCube should be from extragalactic environments.

Recently, Chandrasekaran, Penington, and Witten (CPW) have shown that the generalized entropy of the Schwarzschild black hole at the bifurcation surface equals the entropy of an extended von Neumann algebra of quantum observables in the black hole exterior, in semiclassical Einstein gravity. They also derive a version of the generalized second law. We generalize these results to a static black hole in an arbitrary diffeomorphism invariant theory of gravity. Thus, a version of the generalized second law for an arbitrary diffeomorphism invariant theory of gravity follows.

We consider the perturbative photon flows (PPFs, i.e., electromagnetic (EM) counterparts) generated by the EM resonance response to high-frequency gravitational waves (HFGWs) with additional polarization states in a rotating laboratory frame system. It is found that when the propagating direction of HFGWs and the symmetrical axis of the laboratory frame system are the same, the PPFs have the maximum value. In this case, using the rotation (the rotation of the azimuth $\varphi$) of the EM detection system, all six possible polarization states of HFGWs can be separated and displayed. For the current experimental conditions, it is quite prospective to detect the PPFs generated by the HFGWs predicted in the braneworld models, the primordial black hole theories, the interaction mechanism between astrophysical plasma and intense EM radiation, etc., due to the large amplitudes (or high spectral densities) and spectral characteristics of these HFGWs. Detecting the primordial HFGWs from inflation faces great challenges at present, but it is not impossible.

We present IMRPhenomXODE, a new phenomenological frequency-domain waveform approximant for gravitational wave (GW) signals from precessing binary black holes (BBHs) with generic spin configurations. We build upon the success of IMRPhenomXPHM [G. Pratten et al., Phys. Rev. D 103, 104056 (2021)], which is one of the most widely adopted waveform approximants in GW data analyses that include spin precession, and introduce two additional significant improvements. First, we employ an efficient technique to numerically solve the ${(\text{next-to})}^4$-leading-order post-Newtonian precession equations, which allows us to accurately determine the evolution of the orientation of the orbital angular momentum ${\widehat{\mathit{L}}}_{\mathrm{N}}$ even in cases with complicated precession dynamics, such as transitional precession. Second, we recalibrate the phase of GW modes in the frame coprecessing with ${\widehat{\mathit{L}}}_{\mathrm{N}}$ against SEOBNRv4PHM [S. Ossokine et al., Phys. Rev. D 102, 044055 (2020)] to capture effects due to precession such as variations in the spin components aligned with ${\widehat{\mathit{L}}}_{\mathrm{N}}$. By incorporating these new features, IMRPhenomXODE achieves matches with SEOBNRv4PHM that are better than 99% for most BBHs with mass ratios $q\ge 1/6$ and with arbitrary spin configurations. In contrast, the mismatch between IMRPhenomXPHM and SEOBNRv4PHM often exceeds 10% for a BBH with $q\lesssim 1/2$ and large in-plane or antialigned spin components. Our implementation is also computationally efficient, with waveform evaluation times that can even be shorter than those of IMRPhenomXPHM for BBH signals with long durations and hence high-frequency resolutions. The accuracy and efficiency of IMRPhenomXODE position it as a valuable tool for GW event searches, parameter estimation analyses, and the inference of underlying population properties.

Scalar-tensor theories are a natural alternative to general relativity, as they may provide an effective dark energy phenomenology on cosmological scales while passing local tests, but their black hole solutions are still poorly understood. Here, we generalize existing no-hair theorems for spherical black holes and specific theories in the scalar-tensor class. We show that shift symmetry prevents the appearance of scalar hairs in rotating (asymptotically flat, stationary, and axisymmetric) black holes for all theories in the Horndeski/beyond Horndeski/DHOST classes, but for those with a coupling between the scalar and the Gauss-Bonnet invariant. Our proof also applies to higher dimensions. We also compute the values of the scalar hair charges if shift symmetry and asymptotic flatness are violated by a time growth of the scalar field at infinity, under suitable regularity conditions at the event horizon.

We study quasiequilibrium sequences of binary neutron stars in the framework of Damour-Esposito-Farese-type scalar-tensor theory of gravity with a massive scalar field, paying particular attention to the case where neutron stars are already spontaneously scalarized at distant orbits, i.e., in the high-coupling constant case. Although scalar effects are largely quenched when the separation $a$ is $\gtrsim 3–6$ times of the Compton length scale that is defined by the scalar mass, we show that the interaction between the scalar fields of the two neutron stars generates a scalar cloud surrounding the binary at the price of orbital energy when $a\lesssim 3–6$ times of the Compton length scale. This enables us to constrain the scalar mass $m_{\varphi }$ from gravitational wave observations of binary neutron star mergers by inspecting the dephasing due to such phenomenon. In particular, the event GW170817 is suggestive of a constraint of $m_{\varphi }\gtrsim {10}^{-11}\mathrm{eV}$ and the coupling strength should be mild if the neutron stars in this system were spontaneously scalarized.

We consider gravitational collapse for the axially symmetric Einstein-Vlasov system. We investigate the weak cosmic censorship conjecture in the case of highly prolate initial data and we investigate the “only if” part of the Hoop conjecture. Shapiro and Teukolsky initiated a similar study in 1991 [Formation of Naked Singularities: The Violation of Cosmic Censorship, Phys. Rev. Lett. 66, 994 (1991)] where they found support that the weak cosmic censorship conjecture was violated for sufficiently prolate spheroidal initial data. More recently, independent studies of this problem have been carried out by Yoo et al. [3D simulation of spindle gravitational collapse of a collisionless particle system, Classical Quantum Gravity 34, 105010 (2017)] and by East [Cosmic Censorship Upheld in Spheroidal Collapse of Collisionless Matter, Phys. Rev. Lett. 122, 231103 (2019)]. A common feature in these works is that the initial data are dustlike. Dust can be considered as a singular case of matter described by the Einstein-Vlasov system. The original motivation by Shapiro and Teukolsky to study this problem is based on the Lin-Mestel-Shu instability for gravitational collapse of uniform spheroids in the case of dust in Newtonian gravity. We argue that the Lin-Mestel-Shu solution is not relevant for studying the weak cosmic censorship conjecture of the Einstein-Vlasov system and we argue that dustlike initial data is also not relevant. To investigate collapse of highly prolate spheroidal configurations for the Einstein-Vlasov system is nevertheless interesting in view of the Hoop conjecture. By choosing highly prolate initial data the weak cosmic censorship conjecture is seriously tested. We carry out such a study for initial data which are not dustlike. We find formation of an apparent horizon in all cases we consider, which provides support for the weak cosmic censorship conjecture. In our tests of the Hoop conjecture we compute the polar circumference ${\mathcal{C}}_{H,p}$ at the time when the apparent horizon forms and find that it is less than 12% above $4\pi M$, where $M$ is the irreducible mass of the apparent horizon, which agrees with the spirit of the Hoop conjecture.