The interaction of a kink and a monochromatic plane wave in one dimensional scalar field theories is studied. It is shown that in a large class of models the radiation pressure exerted on the kink is negative, i.e. the kink is pulled towards the source of the radiation. This effect has been observed by numerical simulations in the ${\varphi }^4$ model, and it is explained by a perturbative calculation assuming that the amplitude of the incoming wave is small. Quite importantly the effect is shown to be robust against small perturbations of the ${\varphi }^4$ model. In the sine-Gordon (SG) model the time-averaged radiation pressure acting on the kink turns out to be zero. The results of the perturbative computations in the SG model are shown to be in full agreement with an analytical solution corresponding to the superposition of a SG kink with a cnoidal wave. It is also demonstrated that the acceleration of the kink satisfies Newton’s law.

We investigate the nonlinear evolution of black hole ringdown in the framework of higher-order metric perturbation theory. By solving the initial-value problem of a simplified nonlinear field model analytically as well as numerically, we find that (i) second-order quasinormal modes (QNMs) are indeed excited at frequencies different from those of first-order QNMs, as predicted recently. We also find serendipitously that (ii) the evolution is dominated by a new type of power-law tail at late times. This “second-order power-law tail” decays more slowly than any late-time tails known in the first order (i.e., linear) perturbation theory, and is generated at the wave front of the first-order perturbation by an essentially nonlinear mechanism. These nonlinear components should be particularly significant for binary black hole coalescences, and could open a new precision science in gravitational-wave studies.

Pulsars in close binary systems with white dwarfs or other neutron stars make ideal laboratories for testing the predictions of gravitational radiation and self-gravitational effects. We report new timing measurements of the pulsar–white-dwarf binary PSR J1141–6545. The orbit is found to be decaying at a rate of $1.04\pm 0.06$ times the general relativistic prediction and the Shapiro delay is consistent with the orbital inclination angle derived from scintillation measurements. The system provides a unique testbed for tensor-scalar theories of gravity. Our measurements place stringent constraints in the theory space, with a limit of ${\alpha }_0^2<2.1\times {10}^{-5}$ for weakly nonlinear coupling and an asymptotic limit of ${\alpha }_0^2<3.4\times {10}^{-6}$ for strongly nonlinear coupling (where ${\alpha }_0$ is the linear coupling strength of matter to an underlying scalar field), which is nearly 3 times smaller than the Cassini bound (${\alpha }_0^2\approx {10}^{-5}$).

We present results obtained by scattering a scalar field off the curved background of a coalescing binary black hole system. A massless scalar field is evolved on a set of fixed backgrounds, each provided by a spatial hypersurface generated numerically during a binary black hole merger. We show that the scalar field scattered from the merger region exhibits quasinormal ringing once a common apparent horizon surrounds the two black holes. This occurs earlier than the onset of the perturbative regime as measured by the start of the quasinormal ringing in the gravitational waveforms. We also use the scalar quasinormal frequencies to associate a mass and a spin with each hypersurface, and observe the compatibility of this measure with the horizon mass and spin computed from the dynamical horizon framework.

We present a new model of $D$-term dominated chaotic inflation in supergravity. The $F$-flat direction present in this model is lifted by the dominant $D$-term, which leads to chaotic inflation and subsequent reheating. No cosmic string is formed after inflation because the $U(1)$ gauge symmetry is broken during inflation. The leptogenesis scenario via the inflaton decay in our $D$-term chaotic inflation scenario is also discussed.

In the context of $f(R)$ theories of gravity, we study the evolution of scalar cosmological perturbations in the metric formalism. Using a completely general procedure, we find the exact fourth-order differential equation for the matter density perturbations in the longitudinal gauge. In the case of sub-Hubble modes, the expression reduces to a second-order equation which is compared with the standard (quasistatic) equation used in the literature. We show that for general $f(R)$ functions the quasistatic approximation is not justified. However, for those functions adequately describing the present phase of accelerated expansion and satisfying local gravity tests, it provides a correct description for the evolution of perturbations.

This paper provides an analytical description of the transport of ultrahigh energy cosmic rays in an inhomogeneously magnetized intergalactic medium. The latter is modeled as a collection of magnetized scattering centers, such as radio cocoons, magnetized galactic winds, clusters or magnetized filaments of large scale structure, with negligible magnetic fields in between. Magnetic deflection is no longer a continuous process, it is rather dominated by scattering events. We study the interaction between high-energy cosmic rays and the scattering agents. We then compute the optical depth of the Universe to cosmic ray scattering and discuss the phenomenological consequences for various source scenarios. For typical parameters of the scattering centers, the optical depth is greater than unity at $5\times {10}^{19}\mathrm{eV}$, but the total angular deflection is smaller than unity. One important consequence of this scenario is the possibility that the last scattering center encountered by a cosmic ray be mistaken with the source of this cosmic ray. In particular, we suggest that part of the correlation recently reported by the Pierre Auger Observatory may be affected by such delusion: this experiment may be observing in part the last scattering surface of ultrahigh energy cosmic rays rather than their source population. Since the optical depth falls rapidly with increasing energy, one should probe the arrival directions of the highest energy events beyond ${10}^{20}\mathrm{eV}$ on an event by event basis to circumvent this effect.

We reanalyze the production of seed magnetic fields during inflation in $(R/m^2{)}^n{F}_{\mu \nu }{F}^{\mu \nu }$ and $I{F}_{\mu \nu }{F}^{\mu \nu }$ models, where $n$ is a positive integer, $R$ the Ricci scalar, $m$ a mass parameter, and $I\propto {\eta }^{\alpha }$ a power-law function of the conformal time $\eta$, with $\alpha$ a positive real number. If $m$ is the electron mass, the produced fields are uninterestingly small for all $n$. Taking $m$ as a free parameter, we find that, for $n\ge 2$, the produced magnetic fields can be sufficiently strong in order to seed the dynamo mechanism and then to explain galactic magnetism. For $\alpha \gtrsim 2$, there is always a window in the parameters defining inflation such that the generated magnetic fields are astrophysically interesting. Moreover, if inflation is (almost) de Sitter and the produced fields almost scale invariant ($\alpha \simeq 4$), their intensity can be strong enough to directly explain the presence of microgauss galactic magnetic fields.

We provide what we believe is the minimal three family $\mathcal{N}=1$ SUSY and conformal Pati-Salam model from type IIB superstring theory. This $Z_3$ orbifolded $\mathrm{AdS}\otimes S^5$ model has long lived protons and has potential phenomenological consequences for LHC (Large Hadron Collider).

Motivated by the vast string landscape, we consider the shear viscosity to entropy density ratio in conformal field theories dual to Einstein gravity with curvature square corrections. After field redefinitions these theories reduce to Gauss-Bonnet gravity, which has special properties that allow us to compute the shear viscosity nonperturbatively in the Gauss-Bonnet coupling. By tuning of the coupling, the value of the shear viscosity to entropy density ratio can be adjusted to any positive value from infinity down to zero, thus violating the conjectured viscosity bound. At linear order in the coupling, we also check consistency of four different methods to calculate the shear viscosity, and we find that all of them agree. We search for possible pathologies associated with this class of theories violating the viscosity bound.

Gravitational wave production from bubble collisions was calculated in the early 1990s using numerical simulations. In this paper, we present an alternative analytic estimate, relying on a different treatment of stochasticity. In our approach, we provide a model for the bubble velocity power spectrum, suitable for both detonations and deflagrations. From this, we derive the anisotropic stress and analytically solve the gravitational wave equation. We provide analytical formulas for the peak frequency and the shape of the spectrum which we compare with numerical estimates. In contrast to the previous analysis, we do not work in the envelope approximation. This paper focuses on a particular source of gravitational waves from phase transitions. In a companion article, we will add together the different sources of gravitational wave signals from phase transitions: bubble collisions, turbulence and magnetic fields and discuss the prospects for probing the electroweak phase transition at LISA.

We study the effect of primordial non-Gaussianity on large-scale structure, focusing upon the most massive virialized objects. Using analytic arguments and $N$-body simulations, we calculate the mass function and clustering of dark matter halos across a range of redshifts and levels of non-Gaussianity. We propose a simple fitting function for the mass function valid across the entire range of our simulations. We find pronounced effects of non-Gaussianity on the clustering of dark matter halos, leading to strongly scale-dependent bias. This suggests that the large-scale clustering of rare objects may provide a sensitive probe of primordial non-Gaussianity. We very roughly estimate that upcoming surveys can constrain non-Gaussianity at the level of $|f_{\mathrm{NL}}|\lesssim 10$, which is competitive with forecasted constraints from the microwave background.

We extend a recent numerical calculation of the pion electromagnetic form factor $F_{\pi }(Q^2)$ in holographic QCD to study two important issues regarding the behavior of fields in the bulk. First, we show that using a chiral symmetry-breaking field formally satisfying the boundary conditions of the “soft-wall” model changes numerical results very little from the earlier calculation that ignores these constraints. Second, we use a background field that interpolates between “hard-wall” and soft-wall models to obtain an improved model that reproduces the desirable phenomenological features of both. In all cases, $F_{\pi }$ for large $Q^2$ is shallower than data, an effect that can be cured by relaxing the fit to one of the static observables, particularly the decay constant $f_{\pi }$.

The single $t$-quark production in proton-proton collisions can proceed through three distinct processes: $tq$, $t\overline{b}$, $tW$ productions. With the running of the Large Hadron Collider (LHC) at CERN, it has good potential to measure each single $t$-quark production mode. The topcolor-assisted technicolor (TC2) model, one of the promising dynamical theories, predicts some new particles at the several hundred GeV scale: three top-pion bosons (${\Pi }_t^{\pm }$, ${\Pi }_t^0$) and one top-Higgs boson ($h_t$). These particles are regarded as the typical feature of the TC2 model and can contribute to some processes. In this paper, we systematically study the contribution of the TC2 model to the single $t$-quark production at the Hadron colliders, especially at the LHC. The TC2 model can contribute to the cross section of the single $t$-quark production in two different ways. First, the existence of the top pions and top Higgs can modify the $Wtb$ coupling via their loop contributions, and such modification can cause the correction to the cross sections of all three production modes. Our study shows that this kind of correction is negative and very small in all cases. Thus it is difficult to observe such a correction even at the LHC. On the other hand, there exist the tree-level flavor-changing (FC) couplings in the TC2 model which can also contribute to the cross sections of the $tq$ and $t\overline{b}$ production processes. The resonant effect can greatly enhance the cross sections of the $tq$ and $t\overline{b}$ productions. The first evidence of the single $t$-quark production has been reported by the D0 collaboration and the measured cross section for the single $t$-quark production of $\sigma (p\overline{p}\to tb+X,tqb+X)$ is compatible at the 10% level with the standard model prediction. Because the light top pion can make a great contribution to the $t\overline{b}$ production, the top-pion mass should be very large in order to make the predicted cross section in the TC2 model be consistent with the Tevatron experiments. More detailed information about the top-pion mass and the FC couplings in the TC2 model should be obtained with the running of the LHC.

We study discretization effects in a mixed-action lattice theory with domain-wall valence quarks and Asqtad-improved staggered sea quarks. At the level of the chiral effective Lagrangian, discretization effects in the mixed-action theory give rise to two new parameters as compared to the lowest order Lagrangian for rooted-staggered fermions—the residual quark mass $m_{\mathrm{res}}$ and the mixed valence-sea meson mass splitting ${\Delta }_{\mathrm{mix}}$. We find that $m_{\mathrm{res}}$, which parametrizes explicit chiral symmetry breaking in the mixed-action theory, is approximately one-quarter the size of our lightest valence quark mass on our coarser lattice spacing and of comparable size to that of simulations by the RBC and UKQCD Collaborations. We also find that the size of ${\Delta }_{\mathrm{mix}}$ is comparable to the size of the smallest of the staggered meson taste splittings measured by the MILC Collaboration. Because lattice artifacts are different in the valence and sea sectors of the mixed-action theory, they give rise to unitarity-violating effects that disappear in the continuum limit, some of which should be described by mixed-action chiral perturbation theory ($\mathrm{MA}\chi \mathrm{PT}$). Such effects are expected to be mild for many quantities of interest but are expected to be significant in the case of the isovector scalar ($a_0$) correlator. Specifically, once the parameters $m_{\mathrm{res}}$, ${\Delta }_{\mathrm{mix}}$, and two others that can be determined from the light pseudoscalar meson spectrum are known, the two-particle intermediate state “bubble” contribution to the scalar correlator is completely predicted within $\mathrm{MA}\chi \mathrm{PT}$. We find that the behavior of the scalar meson correlator is quantitatively consistent with the $\mathrm{MA}\chi \mathrm{PT}$ prediction; this supports the claim that $\mathrm{MA}\chi \mathrm{PT}$ describes the dominant unitarity-violating effects in the mixed-action theory and can therefore be used to remove lattice artifacts and recover physical quantities.

We discuss a possible interpretation of the scalar $B_{s0}^{*}(5725)$ and axial $B_{s1}(5778)$ bottom-strange mesons as hadronic molecules—bound states of $BK$ and $B^{*}K$ mesons, respectively. Using a phenomenological Lagrangian approach we analyze the strong $B_{s0}^{*}\to B_s{\pi }^0$, $B_{s1}\to B_s^{*}{\pi }^0$ and the radiative $B_{s0}^{*}\to B_s^{*}\gamma$, $B_{s1}\to B_s\gamma$, $B_{s1}\to B_s^{*}\gamma$, $B_{s1}\to B_{s0}^{*}\gamma$ decays. We give predictions for the decay properties: effective couplings and decay widths.

We derive the cross section for the diffractive gluon production in high energy onium-nucleus collisions that includes the low-$x$ evolution effects in the rapidity interval between the onium and the produced gluon and in the rapidity interval between the gluon and the target nucleus. We analyze our result in two limiting cases: when the onium size is much smaller than the saturation scale and when its size is much larger than the saturation scale. In the later case the gluon multiplicity is very small in the quasiclassical case and increases when the low-$x$ evolution effects in onium become significant. We discuss the implications of our result for the Relativistic Heavy Ion Collider, Large Hadron Collider, and Electron Ion Collider phenomenology.

In the current work we investigate the propagation of electromagnetic waves in the field of gravitational waves. Starting with the simple case of an electromagnetic wave traveling in the field of a plane monochromatic gravitational wave, we introduce the concept of the surfing effect and analyze its physical consequences. We then generalize these results to an arbitrary gravitational wave field. We show that, due to the transverse nature of gravitational waves, the surfing effect leads to significant observable consequences only if the velocity of gravitational waves deviates from the speed of light. This fact can help to place an upper limit on the deviation of gravitational wave velocity from the speed of light. The microarcsecond resolution promised by the upcoming precision interferometry experiments allow one to place stringent upper limits on $ϵ=(v_{\mathrm{gw}}-c)/c$ as a function of the energy density parameter for gravitational waves ${\Omega }_{\mathrm{gw}}$. For ${\Omega }_{\mathrm{gw}}\approx {10}^{-10}$ this limit amounts to $ϵ\lesssim 2·{10}^{-2}$.

We study the effective gravitational theory on a brane in a six-dimensional Einstein-Maxwell model of flux compactification, regularizing a conical defect as a codimension-one brane. We employ the gradient expansion technique valid at low energies. A lowest order analysis showed that standard four-dimensional Einstein gravity is reproduced on the brane. We extend this study to include second order corrections in the effective equations, and show that the correction term is given by a quadratic energy-momentum tensor. Taking the thin-brane limit where the regularized brane shrinks to the pole, we find that the second order metric diverges logarithmically on the brane, giving rise to divergences in the brane effective action. Away from the branes, the effective action is, however, well defined.

Nonlocal quantum corrections to gravity have been recently proposed as a possible solution to the cosmological fine-tuning problems. We study the dynamics of a class of nonlocal actions defined by the function of the inverse d’Alembertian of the Ricci scalar. Power-law and exponential functions are considered in detail, but we also show a method to reconstruct a nonlocal correction that generates a given background expansion. We find that even the simplest terms can, while involving only Planck scale constants, drive the late time acceleration without changing early cosmology. This leads to a sudden future singularity which, however, may be avoided by regularizing the d’Alembertian. We also consider the Einstein frame versions of these models.