A number of open problems hinder our present ability to extract scientific information from data that will be gathered by the near-future gravitational-wave mission LISA. Many of these relate to the modeling, detection, and characterization of signals from binary inspirals with an extreme component-mass ratio of $\lesssim {10}^{-4}$. In this paper, we draw attention to the issue of systematic error in parameter estimation due to the use of fast but approximate waveform models; this is found to be relevant for extreme-mass-ratio inspirals even in the case of waveforms with $\gtrsim 90\%$ overlap accuracy and moderate ($\gtrsim 30$) signal-to-noise ratios. A scheme that uses Gaussian processes to interpolate and marginalize over waveform error is adapted and investigated as a possible precursor solution to this problem. Several new methodological results are obtained, and the viability of the technique is successfully demonstrated on a three-parameter example in the setting of the LISA Data Challenge.

We consider the Einstein-scalar-Gauss-Bonnet theory and assume that, at regimes of large curvature, the Ricci scalar may be ignored compared to the quadratic Gauss-Bonnet term. We then look for static, spherically symmetric, regular black-hole solutions with a nontrivial scalar field. Despite the use of a general form of the spacetime line element, no black-hole solutions are found. In contrast, solutions that resemble irregular particlelike solutions or completely regular gravitational solutions with a finite energy-momentum tensor do emerge. In addition, in the presence of a cosmological constant, solutions with a horizon also emerge, however, the latter corresponds to a cosmological rather than to a black-hole horizon. It is found that, whereas the Ricci term works towards the formation of the positively curved topology of a black-hole horizon, the Gauss-Bonnet term exerts a repulsive force that hinders the formation of the black hole. Therefore, a pure scalar-Gauss-Bonnet theory cannot sustain any black-hole solutions. However, it could give rise to interesting cosmological or particlelike solutions where the Ricci scalar plays a less fundamental role.

We discuss the optical properties of the solar gravitational lens (SGL). We estimate the power of the EM field received by an imaging telescope. Studying the behavior of the EM field at the photometric detector, we develop expressions that describe the received power from a point source as well as from an extended resolved source. We model the source as a disk with uniform surface brightness and study the contribution of blur to a particular image pixel. To describe this process, we develop expressions describing the power received from the directly imaged region of the exoplanet, from the rest of the exoplanet and also the power for off-image pointing. We study the SGL’s amplification and its angular resolution in the case of observing an extended source with a modest size telescope. The results can be applied to direct imaging of exoplanets using the SGL.

Detailed observations of phenomena involving black holes, be it via gravitational waves or more traditional electromagnetic means, can probe the strong field regime of the gravitational interaction. The prediction of features in such observations requires detailed knowledge of the black hole spacetime, both within and outside of general relativity. We present here a new numerical code that can be used to obtain stationary solutions that describe black hole spacetimes in a wide class of modified theories of gravity. The code makes use of a relaxed Newton-Raphson method to solve the discretized field equations with a Newton’s polynomial finite difference scheme. We test and validate this code by considering static and spherically symmetric black holes both in general relativity and in scalar-Gauss-Bonnet gravity with a linear (linear scalar-Gauss-Bonnet) and an exponential (Einstein-dilaton-Gauss-Bonnet) coupling. As a by-product of the latter, we find that analytic solutions obtained in the small coupling approximation are in excellent agreement with our fully nonlinear solutions when using a linear coupling. As expected, differences arise when using an exponential coupling. We then use these numerical solutions to construct a fitted analytical model, which we then use to calculate physical observables such as the innermost stable circular orbit and photon sphere and compare them to the numerical results. This code lays the foundation for more detailed calculations of black hole observables that can be compared with data in the future.

We investigate the escape of photons from the vicinity of the horizon to infinity in the Kerr-Newman black hole spacetime. We assume that a light source is at rest in a locally nonrotating frame on the equatorial plane and photons are emitted isotropically. Then, we evaluate the escape probability of the emitted photons. The main result of this paper is the following. If the black hole is extremal with the nondimensional spin parameter $a_{*}>1/2$, however close to the horizon the light source would be, the escape probability remains nonzero. The near-horizon limit value of the escape probability is a monotonically increasing function of $a_{*}$ and takes a maximum $\sim 29.1\%$ at $a_{*}=1$, i.e., for the extremal Kerr case. On the other hand, if the black hole is extremal with $0\le a_{*}\le 1/2$ or if the black hole is subextremal, the near-horizon limit value is zero.

In this paper, we argue and show numerically that the threshold to form primordial black holes from an initial spherically symmetric perturbation is, to an excellent approximation, universal, whenever given in terms of the compaction function averaged over a sphere of radius $r_m$, where $r_m$ is the scale on which the compaction function is maximum. This can be understood as the requirement that, for a black hole to form, each shell of the averaged compaction function should have an amplitude exceeding the so-called Harada-Yoo-Kohri limit. For a radiation dominated universe we argued, supported by the numerical simulations, that this limit is ${\delta }_c=0.40$, which is slightly below the one quoted in the literature. Additionally, we show that the profile dependence of the threshold for the compaction function is only sensitive to its curvature at the maximum. We use these results to provide an analytic formula for the threshold amplitude of the compaction function at its maximum in terms of the normalized compaction function curvature at $r_m$.

In the context of a special class of tensor-multiscalar theories of gravity for which the target-space metric admits an isometry under which the theory is invariant, we present rotating vacuum solutions, namely, with no matter fields. These objects behave like nontopological solitons, whose primary stability is due to the conserved charge arising from the global symmetry. We consider theories with two and three scalar fields, different Gauss curvatures, and quartic interaction coefficients. As it occurs for boson stars, their angular momentum is quantized.

We conduct a thorough investigation into the possibility that residing in an overdense region of the Universe may induce bias in measurements of the large-scale structure. We compute the conditional correlation function and angular power spectrum of density and lensing fluctuations while holding the local spherically averaged density fixed and show that for Gaussian fields this has no effect on the angular power at $l>0$. We identify a range of scales where a perturbative approach allows analytic progress to be made, and we compute leading-order conditional power spectra using an Edgeworth expansion and second-order perturbation theory. We find no evidence for any significant bias to cosmological power spectra from our local density contrast. We show that when smoothed over a large region around the observer, conditioning on the local density typically affects density power spectra by less than a percent at cosmological distances, below cosmic variance. We find that while typical corrections to the lensing angular power spectrum can be at the 10% level on the largest angular scales and for source redshifts $z_s\lesssim 0.1$, for the typical redshifts targeted by upcoming wide imaging surveys the corrections are subpercent and negligible, in contrast to previous claims in the literature. Using an estimate of the local spherically averaged density from a composite galaxy redshift catalogue we find that the corrections from conditioning on our own local density are below cosmic variance and subdominant to other nonlinear effects. We discuss the potential implications of our results for cosmology and point out that a measurement of the local density contrast may be used as a consistency test of cosmological models.

The expansion history of the Universe reconstructed from a combination of recent data indicates a preference for a changing dark energy (DE) density. Moreover, the DE density appears to be increasing with cosmic time, with its equation of state being below $-1$ on average, and possibly crossing the so-called phantom divide. Scalar-tensor theories, in which the scalar field mediates a force between matter particles, offer a natural framework in which the effective DE equation of state can be less than $-1$ and cross the phantom barrier. We consider the generalized Brans-Dicke (GBD) class of scalar-tensor theories and reconstruct their Lagrangian given the effective DE density extracted from recent data. Then, given the reconstructed Lagrangian, we solve for the linear perturbations and investigate the characteristic signatures of these reconstructed GBD in the cosmological observables, such as the cosmic microwave background (CMB) anisotropy, the galaxy number counts, and their cross-correlations. In particular, we demonstrate that the integrated Sachs-Wolfe effect probed by the cross-correlation of CMB with the matter distribution can rule out scalar-tensor theories as the explanation of the observed DE dynamics independently from the laboratory and Solar System fifth force constraints.

We explore higher-dimensional generalizations of the Runge-Kutta-Wentzel-Kramers-Brillouin method for integrating coupled systems of first-order ordinary differential equations with highly oscillatory solutions. Such methods could improve the performance and adaptability of the codes which are used to compute numerical solutions to the Einstein-Boltzmann equations. We test Magnus expansion-based methods on the Einstein-Boltzmann equations for a simple universe model dominated by photons with a small amount of cold dark matter. The Magnus expansion methods achieve an increase in run speed of about 50% compared to a standard Runge-Kutta integration method. A comparison of approximate solutions derived from the Magnus expansion and the Wentzel-Kramers-Brillouin (WKB) method implies the two are distinct mathematical approaches. Simple Magnus expansion solutions show inferior long range accuracy compared to WKB. However, we also demonstrate how one can improve on the standard Magnus approach to obtain a new “Jordan-Magnus” method. This has a WKB-like performance on simple two-dimensional systems, although its higher-dimensional generalization remains elusive.

Recently it has been noticed that the Fermi-LAT data of gamma-rays from some galactic pulsars and supernova remnants reveal spectral modulations that might be explained by the conversion of photons to ALPs (axionlike particles) induced by the conventional ALP coupling to photon in the presence of galactic magnetic fields. However the corresponding ALP mass and coupling are in a severe tension with the observational constraints from CAST, SN1987A, and other gamma-ray observations. Motivated by this, we examine an alternative possibility that those spectral modulations are explained by other type of ALP coupling involving both the ordinary photon and a massless dark photon, when nonzero background dark photon gauge fields are assumed. We find that our scheme results in oscillations among the photon, ALP, and dark photon, which can explain the gamma-ray spectral modulations of galactic pulsars or supernova remnants, while satisfying the known observational constraints.

For direct detection of sub-MeV dark matter, a promising strategy is to search for individual phonon excitations in a crystal. We perform an analytic calculation of the rate for light dark matter ($\mathrm{keV}<m_{\mathrm{DM}}<\mathrm{MeV}$) to produce two acoustic phonons through scattering in cubic crystals such as GaAs, Ge, Si, and diamond. The multiphonon rate is always smaller than the rate to produce a single optical phonon, whenever the latter is kinematically accessible. In Si and diamond, there is a dark matter mass range for which multiphonon production can be the most promising process, depending on the experimental threshold.

Recently, quantum simulators have been constructed to investigate experimentally the most prominent theoretical four-point many-body system described by the Hubbard model. By varying the coupling strength of the four-point interaction in relation to the kinetic term, it is possible to analyze the phase structure of the model. This intriguing fact leads us to ask the question as to whether similar Hamiltonians with four-point interactions can also be studied as a function of their four-point coupling strength. In this paper, we reexamine the Nambu–Jona-Lasinio model, regarding it generally beyond the context of quantum chromodynamics. Essentially, it is a model in which particle-antiparticle pairing leads to a Bardeen-Cooper-Schrieffer-like condensate, with the result that chiral symmetry is broken dynamically in the strong-coupling regime, where $G{\mathrm{\Lambda }}^2$ is larger than a critical value, i.e., $G{\mathrm{\Lambda }}^2>G_c{\mathrm{\Lambda }}^2$. To study the behavior of the system, it is necessary to move from this regime to a hypothetical regime of weak coupling, altering the coupling strength of the interaction arbitrarily. In order to do this, the gap equation must be regarded as complex and its Riemann surface structure must be known. We do this and obtain a continuous quantum phase transition characterized by the development of a complex order parameter (the dynamically generated mass) from the second sheet of the Riemann surface associated with the gap equation, as we move into the weak-coupling regime. The power-law behavior of the order parameter in the vicinity of the phase transition point is demonstrated to be independent of the choice of the regularization scheme with the critical exponent as $\beta \approx 0.55$. At the same time, the isovector pseudoscalar modes retain their feature as Goldstone modes and still have zero mass, while the isoscalar scalar meson follows the behavior of the order parameter and gains a width. Energetically, this mode is not favored over the normal, uncondensed mode but would have to be accessed through an excitation process.

In this paper, limits are set on bileptons masses and couplings in the context of a 331 model, as well as generic, non-331 models predicting bileptons. The following measurable processes are studied: $pp\to {\ell }^{+}{\ell }^{+}{\ell }^{-}{\ell }^{-}X$, $pp\to {\ell }^{+}{\ell }^{-}\nu \nu X$ and $pp\to {\ell }^{+}{\ell }^{-}X$. Experimental limits on singly charged bileptons masses and couplings within a 331 model are also obtained for the first time. With the results, an over 20-year-old experimental limit on vector bileptons is increased by 60%. The computed limits are now the most stringent ones for these particles.

We propose a $U(1{)}^{\prime }$ model inspired by $E_6$, which has an isospin-violation dark matter. With extra an two pairs of vectorlike quarks, we can assign the proper $U(1{)}^{\prime }$ charges for the first two-generation quark doublets and explain why the first two-generation quarks are lighter than the third generation. By choosing a proper linear combination of two extra $U(1)$ gauge symmetries in $E_6$, it is natural to realize the ratio $f_n/f_p=-0.7$ so as to maximally relax the constraints from the xenon based direct detection experiments. We study the sensitivities of the dark matter direct and indirect detection experiments and identify the parameter spaces that can give the observed relic density. We also study the sensitivities of the future colliders with a center mass energy $\sqrt{s}=33/50/100\mathrm{TeV}$ and compare the different detection methods. We show that in some parameter spaces, the future colliders can give much stronger limits.

We study the two-dimensional lattice multicomponent Abelian-Higgs model, which is a lattice compact U(1) gauge theory coupled with an $N$-component complex scalar field, characterized by a global $\mathrm{SU}(N)$ symmetry. In agreement with the Mermin-Wagner theorem, the model has only a disordered phase at finite temperature, and a critical behavior is observed only in the zero-temperature limit. The universal features are investigated by numerical analyses of the finite-size scaling behavior in the zero-temperature limit. The results show that the renormalization-group flow of the 2D lattice $N$-component Abelian-Higgs model is asymptotically controlled by the infinite gauge-coupling fixed point, associated with the universality class of the 2D $C{P}^{N-1}$ field theory.

We report nucleon mass, isovector vector and axial-vector charges, and tensor and scalar couplings, calculated using two recent $2+1$-flavor dynamical domain-wall fermion lattice-QCD ensembles generated jointly by the RIKEN-BNL-Columbia and UKQCD Collaborations. These ensembles were generated with Iwasaki $\times$ dislocation-suppressing-determinant-ratio gauge action at an inverse lattice spacing of 1.378(7) GeV and pion mass values of 249.4(3) and 172.3(3) MeV. The nucleon mass extrapolates to a value $m_N=0.950(5)\mathrm{GeV}$ at the physical point. The isovector vector charge renormalizes to unity in the chiral limit, narrowly constraining excited-state contamination in the calculation. The ratio of the isovector axial-vector to the vector charges shows a deficit of about 10%. The tensor coupling no longer depends on mass and extrapolates to 1.04(5) in $\overline{\mathrm{MS}}2\mathrm{GeV}$ renormalization at the physical point, in a good agreement with the value obtained at the lightest mass in our previous calculations and other calculations that followed. The scalar charge, though noisier, does not show mass dependence and is in agreement with other calculations.

A new model-independent parametrization is proposed for the hadronic form factors in the semileptonic $\overline{B}\to D\ell {\overline{\nu }}_{\ell }$ decay. By a combined consideration of the recent experimental and lattice QCD data, we determine precisely the Cabibbo-Kobayashi-Maskawa matrix element $|V_{cb}|=41.01(75)\times {10}^{-3}$ and the ratio ${\mathcal{R}}_D=\frac{\mathcal{B}\mathcal{R}(\overline{B}\to D\tau {\overline{\nu }}_{\tau })}{\mathcal{B}\mathcal{R}(\overline{B}\to D\ell {\overline{\nu }}_{\ell })}=0.301(5)$. The coefficients in this parametrization, related to phase shifts by sumrulelike dispersion relations and hence called phase moments, encode important scattering information of the $\overline{B}\overline{D}$ interactions which are poorly known so far. Thus, we give strong hints about the existence of at least one bound and one virtual $\overline{B}\overline{D}$ $S$-wave $0^{+}$ states, subject to uncertainties produced by potentially sizable inelastic effects. This formalism is also applicable for any other semileptonic processes induced by the weak $b\to c$ transition.

We determine the strange electromagnetic form factors of the nucleon within the lattice formulation of quantum chromodynamics using simulations that include light, strange and charm quarks in the sea all tuned to their physical mass values. We employ state-of-the-art techniques to accurately extract the form factors for values of the momentum transfer square up to $0.8{\mathrm{GeV}}^2$. We find that both the electric and magnetic form factors are statistically nonzero. We obtain for the strange magnetic moment ${\mu }^s=-0.017(4){(}_0^{+1}){(}_0^{+5})$, the strange magnetic radius $⟨r_M^2{⟩}^s=-0.015(9){(}_0^{+3}){(}_0^{+4}){\mathrm{fm}}^2$, and the strange charge radius $⟨r_E^2{⟩}^s=-0.0048(6){(}_{-8}^0){(}_0^{+12}){\mathrm{fm}}^2$, where the first error is statistical, and the second and third systematic.