A unified model is developed within the context of the braneworld paradigm, where a single scalar field can act as both the inflaton field in the very early universe and also as strong, self-interacting dark matter in the post-inflationary universe. Reheating proceeds due to the overproduction and subsequent evaporation of primordial black holes. Observational constraints, most notably from gravitational waves, are satisfied if the probability of PBH formation is sufficiently high.

We formulate the general requirements, set by classical electrodynamics, on the sources of extremely high-energy cosmic rays (EHECRs). It is shown that the parameters of EHECR accelerators are strongly limited not only by the particle confinement in large-scale magnetic fields or by the difference in electric potentials (generalized Hillas criterion) but also by the synchrotron radiation, the electro-bremsstrahlung, or the curvature radiation of accelerated particles. Optimization of these requirements in terms of an accelerator’s size and magnetic field strength results in the ultimate lower limit to the overall source energy budget, which scales as the fifth power of attainable particle energy. Hard γ rays accompanying generation of EHECRs can be used to probe potential acceleration sites. We apply the results to several populations of astrophysical objects—potential EHECR sources—and discuss their ability to accelerate protons to ${10}^{20}\hspace{0.5em}\mathrm{eV}$ and beyond. The possibility of gain from ultrarelativistic bulk flows is addressed, with active galactic nuclei and gamma-ray bursts being the examples.

We consider the $N=(1,1)\mathrm{}$ super Yang-Mills (SYM) theory that is obtained by dimensionally reducing SYM theory in 2+1 dimensions to 1+1 dimensions and discuss soft supersymmetry breaking. We discuss the numerical simulation of this theory using supersymmetric discrete light-cone quantization when either the boson or the fermion has a large mass. We compare our result to the pure adjoint fermion theory and pure adjoint boson discrete light-cone quantization calculations of Klebanov, Demeterfi, Bhanot and Kutasov. With a large boson mass we find that it is necessary to add additional operators to the theory to obtain sensible results. When a large fermion mass is added to the theory we find that it is not necessary to add operators to obtain a sensible theory. The theory of the adjoint boson is a theory that has stringy bound states similar to the full SYM theory. We also discuss another theory of adjoint bosons with a spectrum similar to that obtained by Klebanov, Demeterfi, and Bhanot.

The big bang of the Friedmann-Robertson-Walker- (FRW-)brane universe is studied. In contrast with the spacelike initial singularity of the usual FRW universe, the initial singularity of the FRW-brane universe is pointlike from the viewpoint of causality including gravitational waves propagating in the bulk. The existence of null singularities (“seam singularities”) is also shown in the flat and open FRW-brane universe models.

We show that generically the initial singularity is isotropic in spatially homogeneous cosmological models in the brane-world scenario. We then argue that it is plausible that the initial singularity is isotropic in typical brane world cosmological models. Therefore, brane cosmology naturally gives rise to a set of initial data that provides the conditions for inflation to subsequently take place, thereby solving the initial conditions problem and leading to a self-consistent and viable cosmology.

We propose to use T-odd momentum correlations as physical observables to study T violation in $\overrightarrow{B}{K}^{*}{l}^{+}{l}^{-}(l=e,\mu )$ decays. We show that these observables are zero in the standard model but sizable at the level of $10\%$ in the supersymmetric model. These large T violating effects are measurable at the B factories in KEK and SLAC and future hadron colliders. We also point out that the T violating effects are insensitive to nonperturbative QCD contributions.

We report on evidence that confinement is related to the dual superconductivity of the vacuum in full QCD, as in quenched QCD. The vacuum is a dual superconductor in the confining phase, while the $U\left(1\right)\mathrm{}$ magnetic symmetry is realized a la Wigner in the deconfined phase.

An expansion scheme is developed for Feynman diagrams describing the production of one massive and one massless particle near the threshold. As an example application, we compute the correlators of heavy-light quark currents, $\overline{b}{\gamma }_{\mu }u,$ $\overline{b}{\gamma }_{5}u,$ through $\mathcal{O}\left({\alpha }_s^2\right).\mathrm{}$

We conjecture that the end point of bulk closed string tachyon decay at any nonzero coupling is the annihilation of space-time by Witten’s bubble of nothing, resulting in a topological phase of the theory. In support of this we present a variety of situations in which there is a correspondence between the existence of perturbative tachyons in one regime and the semiclassical annihilation of space-time. Our discussion will include many recently investigated scenarios in string theory including Scherk-Schwarz compactifications, Melvin magnetic backgrounds, and noncompact orbifolds. We use this conjecture to investigate a possible web of dualities relating the eleven-dimensional Fabinger-Horava background with nonsupersymmetric string theories. Along the way we point out where our conjecture resolves some of the puzzles associated with bulk closed string tachyon condensation.

In this paper we consider the influence of trans-Planckian physics on the CMBR anisotropies produced by inflation. We consider a simple toy model that allows for analytic calculations and argue on general grounds, based on ambiguities in the choice of vacuum, that effects are expected with a magnitude of the order of $H/\Lambda ,$ where H is the Hubble constant during inflation and Λ the scale for new physics, e.g., the Planck scale.

We reanalyze the conditions under which we have a primordial thermal population of axions. We extend a previous study and take into account processes involving gluons and quarks. We conclude that if the Peccei-Quinn scale satisfies $F_a<1.2\mathrm{}\times {10}^{12}\hspace{0.5em}\mathrm{GeV}$ there is thermal axion production. In this case, a period in the early universe exists where axions would interact with the QCD plasma and we point out that nonthermal axions produced before the end of this period would thermalize.

We reconsider the virial theorem in the presence of a positive cosmological constant Λ. Assuming steady state, we derive an inequality of the form $\rho >~A(\Lambda /8\mathrm{}\pi G_N)$ for the mean density ρ of the astrophysical object. The parameter A depends only on the shape of the object. With a minimum at $A_{\mathrm{sphere}}=2,$ its value can increase by several orders of magnitude as the shape of the object deviates from a spherically symmetric one. This indicates that flattened matter distributions such as, e.g., clusters or superclusters, with low density, cannot be in gravitational equilibrium.

In the high density low temperature limit, quantum chromodynamics (QCD) exhibits a transition to a phase characterized by color superconductivity and by energy gaps in the fermion spectra. Under specific circumstances the gap parameter has a crystalline pattern, breaking translational and rotational invariance. The corresponding phase is the the crystalline color superconductive phase [or Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) phase]. In the effective theory the fermions couple to the phonon arising from the breaking of rotation and translation invariance. We compute for the single-plane-wave condensate ansatz the parameters of the low energy effective Lagrangian describing the motion of the free phonon in the high density medium and derive the phonon dispersion law.

It is shown in first order perturbation theory that anharmonic oscillators in noncommutative space behave smoothly in the commutative limit just as harmonic oscillators do. The noncommutativity provides a method for converting a problem in degenerate perturbation theory to a nondegenerate problem.

We investigate spherically symmetric spacetimes which contain a perfect fluid obeying the polytropic equation of state and admit a kinematic self-similar vector of the second kind which is neither parallel nor orthogonal to the fluid flow. We assume two kinds of polytropic equations of state and show in general relativity that such spacetimes must be a vacuum, which is in contrast with the result in the Newtonian case.

We show that at one loop the magnetic mass vanishes at finite temperature in QED in any dimension. In three-dimensional QED, even the zero temperature part can be regularized to zero. We calculate the two-loop contributions to the magnetic mass in three-dimensional QED with a Chern-Simons term and show that it vanishes. We give a simple proof which shows that the magnetic mass vanishes to all orders at finite temperature in this theory. This proof also holds for QED in any dimension.

Starting from Ooguri’s construction for $\mathrm{BF}$ theory in three (and four) dimensions, we show how to construct a well defined theory with an infinite number of degrees of freedom. The spin network states that are kept invariant by the evolution operators of the theory are exact solutions of the Hamiltonian constraint of quantum gravity proposed by Thiemann. The resulting theory is the first example of a well defined, finite, consistent, spin-foam-based theory in a situation with an infinite number of degrees of freedom. Since it solves the quantum constraints of general relativity it is also a candidate for a theory of quantum gravity. It is likely, however, that the solutions constructed correspond to a spurious sector of solutions of the constraints. The richness of the resulting theory makes it an interesting example to be analyzed by forthcoming techniques that construct the semiclassical limit of spin network quantum gravity.

Does the measurement of a quantum system necessarily break Lorentz invariance? We present a simple model of a detector that measures the spacetime localization of a relativistic particle in a Lorentz invariant manner. The detector does not select a preferred Lorentz frame as a Newton-Wigner measurement would do. The result indicates that there exists a Lorentz invariant notion of quantum measurement and sheds light on the issue of the localization of a relativistic particle. The framework considered is that of single-particle mechanics as opposed to field theory. The result may be taken as support for the interpretation postulate of the spacetime-states formulation of single-particle quantum theory.

The thermal emission of strongly magnetized neutron-star atmospheres is thought to be highly polarized. However, because of the different orientations of the magnetic field over the surface of the neutron star (NS), it is commonly assumed that the net observed polarization will be significantly reduced as the polarization from different regions will cancel each other. We show that the birefringence of the magnetized QED vacuum decouples the polarization modes in the magnetosphere; therefore, the direction of the polarization follows the direction of the magnetic field up to a large distance from the stellar surface. At this distance, the rays that leave the surface and are destined for our detectors pass through only a small solid angle; consequently, the polarization direction of the emission originating in different regions will tend to align together. The net observed polarization of the thermal radiation of NSs should therefore be very large. Measurement of this polarization will be the first direct evidence of the birefringence of the magnetized vacuum due to QED and a direct probe of behavior of the vacuum at magnetic fields of order of and above the critical QED field of $4.4\times {10}^{13}\hspace{0.5em}\mathrm{G}.$ The large observable polarization will also help us learn more about the atmospheric properties of NSs.

Just as noncommutative gauge theories arise from quantizing open strings in a large magnetic field, non-Abelian two-form gauge theories may conceivably be constructed by quantizing open membranes in a large three-form magnetic background. We make some observations that arise in following this strategy, with an emphasis on the relation to the quantization of volume-preserving diffeomorphisms (VPD). In particular, we construct consistent non-Abelian interactions of a two-form in 3+1 dimensions, based on gauge invariance under VPD.