We consider scalar-Gauss-Bonnet and modified Gauss-Bonnet gravities and reconstruct these theories from the universe expansion history. In particular, we are able to construct versions of those theories (with and without ordinary matter), in which the matter-dominated era makes a transition to the cosmic acceleration epoch. In several of the cases under consideration, matter dominance and the deceleration-acceleration transition occur in the presence of matter only. The late-time acceleration epoch is described asymptotically by de Sitter space but may also correspond to an exact $\Lambda \mathrm{CDM}$ cosmology, having in both cases an effective equation of state parameter $w$ close to $-1$. The one-loop effective action of modified Gauss-Bonnet gravity on the de Sitter background is evaluated and it is used to derive stability criteria for the ensuing de Sitter universe.

Field theories with Lorentz (or diffeomorphism invariant) action can exhibit superluminal behavior through the breaking of local Lorentz invariance. Quantum induced superluminal velocities are well-known examples of this effect. The issue of the causal behavior of such propagation is somewhat controversial in the literature and we intend to clarify it. We provide a careful analysis of the meaning of causality in classical relativistic field theories and stress the role played by the Cauchy problem and the notion of chronology. We show that, in general, superluminal behavior threatens causality only if one assumes that a prior chronology in spacetime exists. In the case where superluminal propagation occurs, however, there are at least two nonconformally related metrics in spacetime and thus two available notions of chronology. These two chronologies are on equal footing, and it would thus be misleading to choose ab initio one of them to define causality. Rather, we provide a formulation of causality in which no prior chronology is assumed. We argue that this is the only way to deal with the issue of causality in the case where some degrees of freedom propagate faster than others. In that framework, then, it is shown that superluminal propagation is not necessarily noncausal, the final answer depending on the existence of an initial data formulation. This also depends on global properties of spacetime that we discuss in detail. As an illustration of these conceptual issues, we consider two field theories, namely, $k$-essence scalar fields and bimetric theories of gravity, and we derive the conditions imposed by causality. We discuss various applications such as the dark energy problem, modified-Newtonian-dynamics-like theories of gravity, and varying speed of light theories.

The nonexistence of asymptotically flat, neutral black holes and asymptotically flat, charged black holes in the Maxwell electrodynamics, with a nontrivial scalar field has been proved for a large class of scalar-tensor theories. The no-scalar-hair theorems, however, do not apply in the case of nonlinear electrodynamics. In the present work numerical solutions describing charged black holes coupled to Born-Infeld type nonlinear electrodynamics in scalar-tensor theories of gravity with massless scalar field are found. The causal structure and properties of the solutions are studied, and a comparison between these solutions and the corresponding solutions in the general relativity is made. The presence of the scalar field leads to a much more simple causal structure. The present class of black holes has a single, nondegenerate horizon, i.e., its causal structure resembles that of the Schwarzschild black hole.

Starting from a Hamiltonian description of the photon within the set of Bargmann-Wigner equations we derive new semiclassical equations of motion for the photon propagating in a static gravitational field. These equations which are obtained in the representation diagonalizing the Hamiltonian at the order $\hslash$, present the first order corrections to the geometrical optics. The photon Hamiltonian shows a new kind of helicity-torsion coupling. However, even for a torsionless space-time, photons do not follow the usual null geodesic as a consequence of an anomalous velocity term. This term is responsible for the gravitational birefringence phenomenon: photons with distinct helicity follow different geodesics in a static gravitational field.

The topology of the Universe can leave an imprint on the cosmic microwave background (CMB) radiation. Clues to the shape of our Universe can be found by searching the CMB for matching circles of temperature patterns. A full sky search of the CMB, mapped extremely accurately by NASA’s WMAP satellite, returned no detection of such matching circles and placed a lower bound on the size of the Universe at 24 Gpc. This lower bound can be extended by optimally filtering the WMAP power spectrum. More stringent bounds can be placed on specific candidate topologies by using a combination statistic. We use optimal filtering and the combination statistic to rule out the suggestion that we live in a Poincaré dodecahedral space.

We discuss the definition of quantum probability in the context of “timeless” general-relativistic quantum mechanics. In particular, we study the probability of sequences of events, or multievent probability. In conventional quantum mechanics this can be obtained by means of the “wave function collapse” algorithm. We first point out certain difficulties of some natural definitions of multievent probability, including the conditional probability widely considered in the literature. We then observe that multievent probability can be reduced to single-event probability, by taking into account the quantum nature of the measuring apparatus. In fact, by exploiting the von-Neumann freedom of moving the quantum/classical boundary, one can always trade a sequence of noncommuting quantum measurements at different times, with an ensemble of simultaneous commuting measurements on the joint system+apparatus system. This observation permits a formulation of quantum theory based only on single-event probability, where the results of the wave function collapse algorithm can nevertheless be recovered. The discussion also bears on the nature of the quantum collapse.

We propose a novel but natural definition of conserved quantities for gravity models of quadratic and higher order in curvature. Based on the spatial asymptotics of curvature rather than of metric, it avoids the more egregious problems—such as zero-energy “theorems” and failure in flat backgrounds—in this fourth-derivative realm. In $D>4$, the present expression indeed correctly discriminates between second-derivative Gauss-Bonnet and generic, fourth-derivative actions.

The intracavity topologies of laser gravitational-wave detectors proposed several years ago are the promising way to obtain sensitivity of these devices significantly better than the Standard Quantum Limit (SQL). In essence, the intracavity detector is a two-stage device where the end mirrors displacement created by the gravitational wave is transferred to the displacement of an additional local mirror by means of the optical rigidity. The local mirror positions have to be monitored by an additional local meter. It is evident that the local meter precision defines the sensitivity of the detector. To overcome the SQL, the quantum variational measurement can be used in the local meter. In this method a frequency-dependent correlation between the meter backaction noise and measurement noise is introduced, which allows us to eliminate the backaction noise component from the meter output signal. This correlation is created by means of an additional filter cavity. In this article the sensitivity limitations of this scheme imposed by the optical losses both in the local meter itself and in the filter cavity are estimated. It is shown that the main sensitivity limitation stems from the filter cavity losses. In order to overcome it, it is necessary to increase the filter cavity length. In a preliminary prototype experiment, an approximate 10 m long filter cavity can be used to obtain sensitivity approximately 2–3 times better than the SQL. For future Quantum Non-Demolition (QND) gravitational-wave detectors with sensitivity about 10 times better than the SQL, the filter cavity length should be within kilometer range.

There has been a certain interest in some recent works in the derivation of Noether charges for Hopf algebra space-time symmetries. Such analyses relied rather heavily on delicate manipulations of the fields of noncommuting coordinates whose charges were under study. Here we derive the same charges in a “coordinate-independent” symplectic-geometry type of approach and find results that are consistent with the ones of Agostini, Amelino-Camelia, Arzano, Marciano, and Tacchi [hep- th/0607221].

It has been recently suggested that the vacuum magnetic dichroism observed by the PVLAS experiment could be explained by pair production of a new light, $m_{\epsilon }\simeq 0.1\mathrm{eV}$, millicharged, $q_{\epsilon }\simeq 3\times {10}^{-6}e$, fermions ($\epsilon$). In addition, it has been pointed out that millicharged particles with $q_{\epsilon }\gtrsim {10}^{-9}e$ appear naturally in models based on the string theory. We show that low energy reactor neutrino experiments provide a sensitive probe of millicharged particles. Considering, as an example, recent results of the TEXONO experiment searching for the neutrino magnetic moment, a new upper bound $q_{\epsilon }\lesssim {10}^{-5}e$ for the mass region $m_{\epsilon }<1\mathrm{keV}$ is derived. These results enhance motivations for a more sensitive search for such particles in near future experiments. Furthermore, a direct experimental limit on the electric charge of the electron antineutrino $q_{{\overline{\nu }}_e}<3.7\times {10}^{-12}e$ is obtained.

In a variety of models of neutrino masses and mixings the lighter top squark decays into competing $R$-parity violating and $R$-parity conserving channels. Using PYTHIA we have estimated in a model independent way the minimum value of $P\equiv \mathrm{BR}({\tilde{t}}_1\to c{\tilde{\chi }}_1^0)\times \mathrm{BR}({\tilde{t}}_1\to l_i^{+}b)$, where $l_i=e^{+}$ and ${\mu }^{+}$, corresponding to an observable signal involving the final state $1l+\mathrm{\text{jets}}+{\mathrm{E̸}}_T$ (carried by the neutrinos from the ${\tilde{\chi }}_1^0$ decay) at Tevatron Run II. For the kinematical cuts designed in this paper $P$ depends on $m_{{\tilde{t}}_1}$ only. We then compute $P$ for representative choices of the model parameters constrained by the oscillation data and find that over a significant region of the allowed parameter space $P$ is indeed larger than $P_{\min }$. This signal is complementary to the $\mathrm{\text{dilepton}}+\mathrm{\text{dijet}}$ signal studied in several earlier experimental and phenomenological analyses and may be observed even if $\mathrm{BR}({\tilde{t}}_1\to l_i^{+}b)$ is an order of magnitude smaller than $\mathrm{BR}({\tilde{t}}_1\to c{\tilde{\chi }}_1^0)$. The invariant mass distribution of the hardest lepton and the hardest jet may determine $m_{{\tilde{t}}_1}$ and reveal the lepton number violating nature of the underlying interaction. The invariant mass distribution of the two lowest energy jets may determine $m_{{\tilde{\chi }}_1^0}$.

In this paper we compute the one loop chiral-logarithmic corrections to all $\mathcal{O}(p^4)$ counterterms in the three site Higgsless model. The calculation is performed using the background field method for both the chiral and gauge fields, and using Landau gauge for the quantum fluctuations of the gauge fields. The results agree with our previous calculations of the chiral-logarithmic corrections to the $S$ and $T$ parameters in ’t Hooft-Feynman gauge. The work reported here includes a complete evaluation of all one loop divergences in an $SU(2)\times U(1)$ nonlinear sigma model, corresponding to an electroweak effective Lagrangian in the absence of custodial symmetry.

We argue that the collinear factorization of the fragmentation functions in high energy hadron and nuclei collisions breaks down at transverse momenta $k_T\lesssim Q_s/g$ due to high parton densities in the colliding hadrons and/or nuclei. We calculate, at next-to-leading order in projectile parton density and to all orders in target parton density, the double-inclusive cross-section for production of a pair of gluons in the scalar $J^{PC}=0^{++}$ channel. Using the low energy theorems of QCD we find the inclusive cross-section for $\pi$-meson production.

The charmed $B$ decays, $B\to DP$, $D^{*}P$, and $DV$, are reanalyzed using the latest experimental data, where $P$ and $V$ denote the pseudoscalar meson and vector meson, respectively. We perform global fits under the assumption of flavor SU(3) symmetry. The size of the decay amplitudes and the strong phases between the topologically distinct amplitudes are studied. Predictions of the related $B_s$ decay rates are made based upon the fitted results. We also note a serious SU(3) symmetry breaking or inconsistency in the $DV$ sector.

We use an $s$-channel picture of hard hadronic collisions to investigate the parton distribution function for quarks at small momentum fraction $x$, which corresponds to very high energy scattering. We study the renormalized quark distribution at one loop in this approach. In the high-energy picture, the quark distribution function is expressed in terms of a Wilson-line correlator that represents the cross section for a color dipole to scatter from the proton. We model this Wilson-line correlator in a saturation model. We relate this representation of the quark distribution function to the corresponding representation of the structure function $F_T(x,Q^2)$ for deeply inelastic scattering.

We present a phenomenological study of the single-transverse spin asymmetry in azimuthal correlations of two jets produced nearly “back-to-back” in $pp$ collisions at RHIC. We properly take into account the initial- and final-state interactions of partons that can generate this asymmetry in QCD hard-scattering. Using distribution functions fitted to the existing single-spin data, we make predictions for various weighted single-spin asymmetries in dijet correlations that are now readily testable at RHIC.

Decay form factors of $B$ to light pseudoscalar ($P$) and vector ($V$) mesons are analyzed with the momentum cutoff near the end point. The cutoff is caused by possible Cherenkov gluon radiation when an energetic light parton from the weak vertex travels through the “brown muck” of light degrees of freedom. The end-point singularities and the double countings are naturally absent. The soft-overlap contributions where the partonic momenta configuration is highly asymmetric are very suppressed in this framework. A simple calculation gives a plausible results for $B\to P$, $V$ form factors.

We study whether the broad peak $X(1576)$ observed by BES Collaboration arises from the final state interaction effect of $\rho (1450\hspace{0.17em}1700)$ decays. The interference effect could produce an enhancement around 1540 MeV in the $K^{+}{K}^{-}$ spectrum with typical interference phases. However, the branching ratio $B[J/\psi \to {\pi }^0\rho (1450\hspace{0.17em}1700)]·B[\rho (1450\hspace{0.17em}1700)\to K^{+}{K}^{-}]$ from the final state interaction effect is far less than the experimental data.

We investigate the phenomenology of a wino lightest superparticle as obtained in anomaly mediated supersymmetry breaking and some string models. The Wilkinson Microwave Anisotropy Probe constraint on the dark matter relic density implies a wino lightest superparticle mass of 2.0–2.3 TeV. We find a viable signature for such a heavy wino at CLIC, operating at its highest center of mass energy of 5 TeV. One also expects a viable monochromatic $\gamma$-ray signal from its pair-annihilation at the galactic center at least for cuspy dark matter halo profiles.

This note derives the analogue of the Mukhanov-Sasaki variables both for scalar and tensor perturbations in the $1+3$ covariant formalism. The possibility of generalizing them to nonflat Friedmann-Lemaître universes is discussed.