This work investigates the possibility of achieving a conformally flat slicing of the Kerr spacetime. We consider a hypersurface of the form $t=F(r,\theta ,a)$, where $(t,r,\theta ,\varphi )$ are the Boyer-Lindquist coordinates; solve for a vanishing Cotton-York tensor of the induced metric order by order in the spin parameter $a$; and show that the procedure fails at the fifth order. We also prove that no coordinate change can induce a spatially flat recasting of the Kerr(-de Sitter) metric, beyond linear order in $a$, adopting a more general ansatz depending on $\varphi$.

First-order perturbations of homogeneous and hypersurface orthogonal locally rotationally symmetric class II cosmologies with a cosmological constant are considered in the framework of the $1+1+2$ covariant decomposition of spacetime. The perturbations, which are of perfect fluid type, include general scalar, vector, and tensor modes and extend some previous works in which vorticity perturbations were excluded. A harmonic decomposition is performed, and the field equations are then reduced to a set of eight evolution equations for eight harmonic coefficients, representing perturbations in density, shear, vorticity, and the Weyl tensor, in terms of which all other variables can be expressed algebraically. This system decouples into two subsystems, one for five and one for three coefficients. As previously known, vorticity perturbations cannot be generated to any order in a barotropic perfect fluid. Hence, the time development of existing first-order vorticity perturbations is seen to be completely determined by the background. However, an already existing vorticity will act as source terms in the evolution equations for the other quantities. In the high-frequency approximation, the four independent Weyl tensor harmonics evolve as gravitational waves on the anisotropic background in the same manner as in the case without vorticity, whereas vorticity gives a first-order disturbance of sonic waves.

Spin can have significant effects on the electromagnetic transients accompanying binary neutron star mergers. The measurement of spin can provide important information about binary formation channels. In the absence of a strong neutron star spin prior, the degeneracy of spin with other parameters leads to significant uncertainties in their estimation, in particular limiting the power of gravitational waves to place tight constraints on the nuclear equation of state. Thus detailed studies of highly spinning neutron star mergers are essential to understand all aspects of multimessenger observation of such events. We perform a systematic investigation of the impact of neutron star spin—considering dimensionless spin values up to $a_{\mathrm{NS}}=0.33$—on the merger of equal mass, quasicircular binary neutron stars using fully general-relativistic simulations. We find that the peak frequency of the post-merger gravitational wave signal is only weakly influenced by the neutron star spin, with cases where the spin is aligned (antialigned) with the orbital angular momentum giving slightly lower (higher) values compared to the irrotational case. We find that the one-arm instability arises in a number of cases, with some dependence on spin. Spin has a pronounced impact on the mass, velocity, and angular distribution of the dynamical ejecta, and the mass of the disk that remains outside the merger remnant. We discuss the implications of these findings on anticipated electromagnetic signals, and on constraints that have been placed on the equation of state based on multimessenger observations of GW170817.

The Noether charge associated to diffeomorphism invariance in teleparallel gravity is derived. It is shown that the latter yields the Arnovitt-Deser-Misner mass of an asymptotically flat spacetime. The black hole entropy is then investigated based on Wald’s prescription that relies on the Noether charge. It is shown that, like in general relativity, the surface gravity can be factored out from such a charge. Consequently, the similarity with the first law of thermodynamics implied by such an approach in general relativity does show up also in teleparallel gravity. It is found that, based on the expression of the first law of black hole mechanics, which is preserved in teleparallel gravity, entropy can thus be extracted from such a Noether charge. The resulting entropy can very naturally be expressed as a volume integral, though. As such, it is shown that the conformal issue that plagues the entropy-area law within general relativity does not arise in teleparallel gravity based on Wald’s approach. The physics behind these features is discussed.

The existence of the cosmological particle horizon as the maximum measurable length $l_{\max }$ in the Universe leads to a generalization of the quantum uncertainty principle (GUP) to the form $\mathrm{\Delta }x\mathrm{\Delta }p\ge \frac{\hslash }{2}\frac{1}{1-\alpha \mathrm{\Delta }{x}^2}$, where $\alpha \equiv l_{\max }^{-2}$. The implication of this GUP and the corresponding generalized commutation relation $[x,p]=i\hslash \frac{1}{1-\alpha x^2}$ on simple quantum mechanical systems has been discussed recently by one of the authors [Cosmological horizons, uncertainty principle and maximum length quantum mechanics, Phys. Rev. D 95, 103523 (2017).] and shown to have extremely small (beyond current measurements) effects on the energy spectra of these systems due to the extremely large scale of the current particle horizon. This may not be the case in the early Universe during the quantum generation of the inflationary primordial fluctuation spectrum. Here we estimate the effects of such a GUP on the primordial fluctuation spectrum and on the corresponding spectral index. In particular, motivated by the above GUP we generalize the field commutation (GFC) relation to $[\phi (\mathbf{k}),{\pi }_{\phi }({\mathbf{k}}^{\prime })]=i\delta (\mathbf{k}-{\mathbf{k}}^{\prime })\frac{1}{1-\mu {\phi }^2(\mathbf{k})}$, where $\mu \simeq {\alpha }^2\equiv l_{\max }^{-4}$ is a GFC parameter, $\phi$ denotes a scalar field, and ${\pi }_{\phi }$ denotes its canonical conjugate momentum. In the context of this GFC we use standard methods to obtain the primordial scalar perturbation spectrum and show that it is of the form $P_S(k)=P_S^{(0)}(k)(1+\frac{\overline{\mu }}{k})$, where $\overline{\mu }\equiv \mu V_{*}\simeq \sqrt{\alpha }=l_{\max }^{-1}$ (here $V_{*}\simeq l_{\max }^3$ is the volume corresponding to the maximum measurable scale $l_{\max }$) and $P_S^{(0)}(k)$ is the standard primordial spectrum obtained in the context of the Heisenberg uncertainty principle (HUP $\mu =0$). We show that the scalar spectral index predicted by the model, defined from $P_S(k)=A_S{k}^{n_s-1}$, is running and may be written as $n_s=1-\lambda -\frac{\overline{\mu }}{k}$ with $\lambda =6\epsilon -2\eta$ (where $\epsilon$ and $\eta$ are the slow-roll parameters). Using observational constraints on the scale dependence of the spectral index $n_s$, a cosmological constraint may be imposed on $\overline{\mu }$ as $\overline{\mu }=(0.9\pm 7.6)\times {10}^{-6}h/\mathrm{Mpc}$. Using this result we estimate the GUP parameter $\alpha \lesssim {10}^{-54}{\mathrm{m}}^{-2}$ at $1\sigma$ and $\alpha \lesssim {10}^{-52}{\mathrm{m}}^{-2}$ at $2\sigma$. The $2\sigma$ range of $\alpha$ corresponds to $l_{\max }\gtrsim {10}^{26}\mathrm{m}$, which is of the same order as the current particle horizon. Thus the assumption that a maximum measurable length could emerge as a result of the presence of the cosmological particle horizon remains a viable assumption at the $2\sigma$ level.

A network of cosmic strings would lead to gravitational waves which may be detected by pulsar timing or future interferometers. The details of the gravitational wave signal depend on the distribution of cosmic string loops, which are produced by intercommutations from the scaling network of long strings. We analyze the limits imposed by energy conservation, i.e., by the fact that the total amount of string flowing into loops cannot exceed the amount leaving the long strings. We show that some recent suggestions for the cosmic string loop production rate and distribution are ruled out by these limits. As a result, gravitational waves based on such suggestions, in particular “model 3” used in LIGO data analysis, are not to be expected.

The value of the Hubble constant inferred from Planck measurements of anisotropies in the cosmic-microwave background is at $4.4\sigma$ tension with direct astronomical measurements at low redshifts. Very recently, it has been conjectured that this discrepancy may be reconciled if a small fraction of the dark matter is described by three mutually orthogonal vector fields of the same mass. We study the thermal description of this model and use the observationally inferred primordial fractions of baryonic mass in ${}^4\mathrm{He}$ to constrain its phase space. We show that while the sterile vector fields may help to alleviate a little bit the existing tension in the measurements of the Hubble parameter, they cannot eliminate the discrepancy between low- and high-redshift observations.

The general kind noncanonical warm inflation is introduced and its total non-Gaussianity of perturbation on uniform energy density hypersurfaces is studied. The total non-Gaussianity contains two complementary parts: the three-point and the four-point correlations. The three-point correlation part denoted by $f_{NL}^{\mathrm{int}}$ comes from the three-point correlation of inflaton field, while the four-point correlation part, denoted by $f_{NL}^{\delta N}$, is the contribution due to the four-point correlation function of the inflaton field. The aforementioned two parts of non-Gaussianity in general noncanonical warm inflation are calculated and analyzed, respectively, and the comparisons and discussions of the two parts are finally carried out in this study.

In order to separate astrophysical gravitational-wave signals from instrumental noise, which often contains transient non-Gaussian artifacts, astronomers have traditionally relied on bootstrap methods such as time slides. Bootstrap methods sample with replacement, comparing single-observatory data to construct a background distribution, which is used to assign a false-alarm probability to candidate signals. While bootstrap methods have played an important role establishing the first gravitational-wave detections, there are limitations. First, as the number of detections increases, it makes increasingly less sense to treat single-observatory data as bootstrap-estimated noise, when we know that the data are filled with astrophysical signals, some resolved, some unresolved. Second, it has been known for a decade that background estimation from time slides eventually breaks down due to saturation effects, yielding incorrect estimates of significance. Third, the false alarm probability cannot be used to weight candidate significance, for example when performing population inference on a set of candidates. Given recent debate about marginally resolved gravitational-wave detection claims, the question of significance has practical consequences. We propose a Bayesian framework for calculating the odds that a signal is of astrophysical origin versus instrumental noise without bootstrap noise estimation. We show how the astrophysical odds can safely accommodate glitches. We argue that it is statistically optimal. We demonstrate the method with simulated noise and provide examples to build intuition about this new approach to significance.

The gravitational waveform of a merging stellar-mass binary is described at leading order by a quadrupolar mode. However, the complete waveform includes higher-order modes, which encode valuable information not accessible from the leading-order mode alone. Despite this, the majority of astrophysical inferences so far obtained with observations of gravitational waves employ only the leading order mode because calculations with higher-order modes are often computationally challenging. We show how to efficiently incorporate higher-order modes into astrophysical inference calculations with a technique known as importance sampling. There are two steps. First, we carry out Bayesian parameter estimation using a computationally cheap leading-order-mode waveform, which provides an initial estimate of binary parameters. Second, we weight the initial estimate using higher-order mode waveforms in order to fold in the extra information from the full waveform. We use mock data to demonstrate the effectiveness of this method. We apply the method to each binary black hole event in the first gravitational-wave transient catalog GWTC-1 to obtain posterior distributions and Bayesian evidence with higher-order modes. Performing Bayesian model selection on the events in GWTC-1, we find only a weak preference for waveforms with higher-order modes, indicating that the events observed to date are not sufficiently loud to resolve the higher-order modes. We discuss how this method can be generalized to a variety of other applications.

We show that current and imminent underground detectors are capable of precision astrometry of dark matter. First, we show that Galactic dark matter velocity distributions can be obtained from reconstructed tracks of dark matter scattering on multiple nuclei during transit; using the liquid scintillator neutrino detector $\mathrm{SNO}+$ as an example, we find that the dark matter velocity vector can be reconstructed event by event with such a small uncertainty that the precision of dark matter astrometry will be limited mainly by statistics. We then determine the number of dark matter events required to determine the dispersion speed, escape speed, and velocity anisotropies of the local dark matter halo and also find that with as few as $\mathcal{O}(10)$ events dark matter signals may be discriminated from potential backgrounds arising as power-law distributions. Finally, we discuss the prospects of dark matter astrometry at other liquid scintillator detectors, dark matter experiments, and the recently proposed MATHUSLA detector.

The HAWC Collaboration has discovered a $\gamma$-ray emission extended about 2 degrees around the Geminga and Monogem pulsar wind nebulae (PWNe) at $\gamma$-ray energies $E_{\gamma }>5\mathrm{TeV}$. We analyze, for the first time, almost 10 years of $\gamma$-ray data obtained with the Fermi Large Area Telescope at $E_{\gamma }>8\mathrm{GeV}$ in the direction of Geminga and Monogem. Since these two pulsars are close to the Galactic plane we run our analysis with ten different interstellar emission models (IEMs) to study the systematics due to the modeling of this component. We detect a $\gamma$-ray halo around Geminga with a significance in the range $7.8–11.8\sigma$ depending on the IEM considered. This measurement is compatible with $e^{+}$ and $e^{-}$ emitted by the PWN, which inverse Compton scatter (ICS) with photon fields located within a distance of about 100 pc from the pulsar, where the diffusion coefficient is estimated to be around $1.1\times {10}^{27}{\mathrm{cm}}^2/\mathrm{s}$ at 100 GeV. We include in our analysis the proper motion of the Geminga pulsar which is relevant for $\gamma$ rays produced for ICS in the Fermi-LAT energy range. We find that an efficiency of about 1% for the conversion of the spin-down energy of the pulsar into $e^{+}$ and $e^{-}$ is required to be consistent with $\gamma$-ray data from Fermi-LAT and HAWC. The inferred contribution of Geminga to the $e^{+}$ flux is at most 20% at the highest-energy AMS-02 data. Our results are compatible with the interpretation that the cumulative emission from Galactic pulsars explains the positron excess.

Two accretion columns have been argued to form over the surface of a newborn millisecond magnetar for an extremely high accretion rate $\gtrsim 1.8\times {10}^{-2}{M}_{\odot }{\mathrm{s}}^{-1}$ that may occur in the core collapse of a massive star. In this paper, we investigate the characteristics of these accretion columns and their gravitational wave (GW) radiation. For a typical millisecond magnetar (surface magnetic field strength $B\sim {10}^{15}\mathrm{G}$ and initial spin period $P\sim 1\mathrm{ms}$), we find (i) its accretion columns are cooled via neutrinos and can reach a height $\sim 1\mathrm{km}$ over the stellar surface; (ii) its column-induced characteristic GW strain is comparable to the sensitivities of the next-generation ground-based GW detectors within a horizon $\sim 1\mathrm{Mpc}$; (iii) the magnetar can survive only a few tens of seconds; (iv) during the survival timescale, the height of the accretion columns increases rapidly to the peak and subsequently decreases slowly; (v) the column mass, characteristic GW strain, and maximum GW luminosity have simultaneous peaks in a similar rise-fall evolution. In addition, we find that the magnetar’s spin evolution is dominated by the column accretion torque. A possible association with failed supernova is also discussed.

Observational constraints on gamma rays produced by the annihilation of weakly interacting massive particles around primordial black holes (PBHs) imply that these two classes of dark matter candidates cannot coexist. We show here that the successful detection of one or more PBHs by radio searches (with the Square Kilometer Array) and gravitational wave searches (with LIGO/Virgo and the upcoming Einstein Telescope) would set extraordinarily stringent constraints on virtually all weak-scale extensions of the standard model with stable relics, including those predicting a WIMP abundance much smaller than that of dark matter. Upcoming PBH searches have in particular the potential to rule out almost the entire parameter space of popular theories such as the minimal supersymmetric standard model and scalar singlet dark matter.

We propose a novel mechanism with two component dark matter models. The subdominant dark matter can thermalize the dominant one in galaxies, and leads to core density profiles. Using ultralight dark photons and GeV-TeV Dirac fermions as an example, we couple the two dark matter candidates with a $U(1)$ interaction. This mechanism differs from self-interacting dark matter, due to three effects: (1) higher occupation numbers, (2) forward-backward scattering cancellation, and (3) the multiple scatterings required for the heavy dark matter. Furthermore, the Bullet Cluster bound is evaded due to the reduced Bose enhancement factor. Unlike the fuzzy dark matter solution to the small structure problems which have tension with Lyman-$\alpha$, the ultralight dark photons with mass $\gtrsim {10}^{-21}\mathrm{eV}$ can have a core profile through interactions with $\psi$ and are not constrained by other astrophysical observations.

We show that the Hamiltonian of isotropic loop quantum cosmology is selected by physical criteria plus a choice implementing Occam’s razor. We also parametrize the freedom when this choice is relaxed and show boundedness of energy density for a broad class of cases. A criterion used is covariance under dilations, the continuous diffeomorphisms remaining in this context, which are not canonical but conformally canonical transformations. We propose how to implement such transformations in quantum theory. Removal of the infrared regulator makes the result independent of ordering ambiguities.

We present the first analytical computation of the (conservative) gravitational self-force correction to the periastron advance around a spinning black hole. Our result is accurate to the second order in the rotational parameter and through the 9.5 post-Newtonian level. It has been obtained as the circular limit of the correction to the gyroscope precession invariant along slightly eccentric equatorial orbits in the Kerr spacetime. The latter result is also new and we anticipate here the first few terms only of the corresponding post-Newtonian expansion.

In this paper we propose to use convolutional neural networks (CNNs) to improve the precision measurement of the Higgs boson-gluon effective coupling at lepton colliders. The CNN is employed to recognize the Higgs boson and a $Z$ boson associated production process, with the Higgs boson decaying to a gluon pair and the $Z$ boson decaying to a lepton pair at the center-of-mass energy 250 GeV and integrated luminosity $5{\mathrm{ab}}^{-1}$. By using CNNs, the uncertainty of the effective coupling measurement can be decreased from 1.94% to about 1.28% using the pythia data and from 1.82% to about 1.22% using the herwig data in the Monte Carlo simulation. Moreover, the performance of CNNs using different final state constituents shows that the energy distributions of the leading and subleading jets constituents play a major role in the identification and the optimal uncertainty of effective coupling using CNNs is reduced by about 35% compared to that using conventional method.

We discuss pure $q\overline{q}$ axial-vector mesons in the Tamm-Dancoff approximation of the Coulomb-gauge QCD model from North Carolina State University. While recent studies have put emphasis in configuration mixing with open meson-meson channels, we here concentrate on the simpler closed-channel problem and follow the $1^{+}$ mixing through a wide range of quark masses. We also examine their radial excitations and discuss with them the concept of insensitivity to chiral symmetry breaking.

We propose a simple grand unified theory (GUT) scenario in which supersymmetry (SUSY) is spontaneously broken in visible sector. Our model is based on the GUT model that has been proposed to solve almost all problems in conventional GUT scenarios. In the previous work, the problems can be solved by a natural assumption in a supersymmetric vacuum. In this paper, we consider an extension of the model (i.e., omitting one singlet field) and break SUSY spontaneously without a new sector. Our model does not have a hidden sector and predicts high-scale SUSY where sfermion masses are of order 100–1000 TeV and flavor violating processes are suppressed. In this scenario, we can see an explicit signature of GUT in the sfermion mass spectrum since the sfermion mass spectrum respects $SU(5)$ matter unification. In addition, we find a superheavy long lived charged lepton as a proof of our scenario, and it may be seen in the LHC.