We study the potential induced by imaginary self-dual 3-forms in compactifications of string theory and the cosmological evolution associated with it. The potential contains exponentials of the volume moduli of the compactification, and we demonstrate that the exponential form of the potential leads to a power law for the scale factor of the universe. This power law does not support accelerated expansion. We explain this result in terms of supersymmetry and comment on corrections to the potential that could lead to inflation or quintessence.

We continue the study of supersymmetric type IIB pp-wave solutions by Maldacena and Maoz (hep-th/0207284), who showed Ramond-Ramond five-forms can induce potential terms in the light cone string actions which are nonlinear sigma models with special holonomy target spaces. We show that nonvanishing Ramond-Ramond three-forms provide extra potential terms involving Killing vectors in the string action and identify the supersymmetry requirements. In particular, in solutions with $\mathrm{}(1,1)\mathrm{}$ worldsheet supersymmetry, the Killing vectors are required to be self-dual in Spin(7).

We extend the method of matrix partition to obtain explicitly the gauge field for noncommutative ADHM construction in some general cases. As an application of this method we apply it to the $U\left(2\right)\mathrm{}$ 2-instanton and get explicit results for the gauge fields in the coincident instanton limit. We also easily apply it to noncommutative ’t Hooft instantons.

We study the direct interaction between global and local monopoles. While in two previous papers the coupling between the two sectors was only “indirect” through the coupling to gravity, we here introduce a new term in the potential that couples the Goldstone field and the Higgs field directly. We investigate the influence of this term in curved space and compare it to the results obtained previously.

The presence of fields with negative mass-squared typically leads to some form of instability in standard field theories. The observation that, at least in the light-cone gauge, strings propagating in plane wave spacetimes can have world sheet scalars with such tachyonlike masses suggests that the supergravity background may itself be unstable. To address this issue, we perform a perturbative analysis around the type IIB vacuum plane wave, the solution which most obviously generates world sheet scalars with negative mass-squared. We argue that this background is perturbatively stable.

We compute the finite temperature correction to the induced fermion number for fermions coupled to a static SU(2) chiral background, using the derivative expansion technique. At zero temperature the induced fermion number is topological, being the winding number of the chiral background. At finite temperature, however, higher order terms in the derivative expansion give nontopological corrections to the winding number. We use this result to show that the standard cancellation of the fractional parts of the fermion number inside and outside the bag in an SU(2) x SU(2) hybrid chiral bag model of the nucleon does not occur at nonzero temperature.

We study the back-reaction effect of a massless minimally coupled scalar field at finite temperatures in the background of an Einstein universe. Substituting for the vacuum expectation value of the components of the energy-momentum tensor on the right hand side of the Einstein equation, we deduce a relationship between the radius of the universe and its temperature. This relationship exhibits a maximum temperature, below the Planck scale, at which the system changes its behavior drastically. The results are compared with the case of a conformally coupled field. An investigation into the values of the cosmological constant exhibits a remarkable difference between the conformally coupled case and the minimally coupled one.

The hypothesis that the Lorentz transformations may be modified at Planck scale energies is further explored. We present a general formalism for theories which preserve the relativity of inertial frames with a nonlinear action of the Lorentz transformations on momentum space. Several examples are discussed in which the speed of light varies with energy and elementary particles have a maximum momenta and/or energy. Energy and momentum conservation are suitably generalized and a proposal is made for how the new transformation laws apply to composite systems. We then use these results to explain the ultrahigh-energy cosmic ray anomaly and we find a form of the theory that explains the anomaly, and leads also to a maximum momentum and a speed of light that diverges with energy. We finally propose that the spatial coordinates be identified as the generators of translation in Minkowski spacetime. In some examples this leads to a commutative geometry, but with an energy dependent Planck constant.

The teleparallel gravity theory, treated physically as a gauge theory of translations, naturally represents a particular case of the most general gauge-theoretic model based on the general affine group of spacetime. On the other hand, geometrically, the Weitzenböck spacetime of distant parallelism is a particular case of the general metric-affine spacetime manifold. These physical and geometrical facts offer a new approach to teleparallelism. We present a systematic treatment of teleparallel gravity within the framework of the metric-affine theory. The symmetries, conservation laws and the field equations are consistently derived, and the physical consequences are discussed in detail. We demonstrate that the so-called teleparallel GR-equivalent model has a number of attractive features which distinguishes it among the general teleparallel theories, although it has a consistency problem when dealing with spinning matter sources.

We explore the physical consequences of a recently discovered class of exact solutions to the five dimensional Kaluza-Klein theory. We find a number of surprising features including the following: (1) In the presence of a Kaluza-Klein bubble, there are arbitrarily large black holes with topology $S^3.$ (2) In the presence of a black hole or a black string, there are expanding bubbles (with de Sitter geometry) which never reach null infinity. (3) A bubble can hold two black holes of arbitrary size in static equilibrium. In particular, two large black holes can be close together without merging to form a single black hole.

An exact expression for the quasinormal modes of scalar perturbation on a massless topological black hole in four and higher dimensions is presented. The massive scalar field is nonminimally coupled to the curvature, and the horizon geometry is assumed to have a negative constant curvature.

A typical stellar mass black hole with a lighter companion is shown to succumb to a chaotic precession of the orbital plane. The chaotic behavior is identified in the conservative system since there is no clear way to do so when dissipation is included and all binaries merge. The precession and the subsequent modulation of the gravitational radiation depend on the mass ratio, eccentricity, and spins. The smaller the mass of the companion, the more prominent the effect of the precession. The most important parameters are the spin magnitudes and misalignments. If the spins are small and nearly aligned with the orbital angular momentum, then there will be no chaotic precession, while increasing both the spin magnitudes and misalignments increases the erratic precession. A large eccentricity can be induced by large, misaligned spins but does not seem to be required for chaos. When dissipation due to gravitational radiation is included chaos is damped, but a further study is needed to determine if dissipation will erase all traces of chaos or if an imprint of irregularity survives.

In a static spacetime, the Killing time can be used to measure the time required for signals or objects to propagate between two of its orbits. By further restricting to spherically symmetric cases, one obtains a natural association between these orbits and timelike lines in Minkowski space. We prove a simple theorem to the effect that in any spacetime satisfying the weak energy condition the above signaling time is, in this sense, no faster than that for a corresponding signal in Minkowski space. The theorem uses a normalization of Killing time appropriate to an observer at infinity. We then begin an investigation of certain related but more local questions by studying particular families of spacetimes in detail. Here we are also interested in restrictions imposed by the dominant energy condition. Our examples suggest that signaling in spacetimes satisfying this stronger energy condition may be significantly slower than the fastest spacetimes satisfying only the weak energy condition.

We examine the possibility that thermal, rather than quantum, fluctuations are responsible for seeding the structure of our universe. We find that while the thermalization condition leads to nearly Gaussian statistics, a Harrisson-Zeldovich spectrum for the primordial fluctuations can only be achieved in very special circumstances. These depend on whether the universe gets hotter or colder in time, while the modes are leaving the horizon. In the latter case we find a no-go theorem which can only be avoided if the fundamental degrees of freedom are not particlelike, such as in string gases near the Hagedorn phase transition. The former case is less forbidding, and we suggest two potentially successful “warming universe” scenarios. One makes use of the Phoenix universe, the other of “phantom” matter.

We derive constraints on the time variation of the Higgs vacuum expectation value $〈\phi 〉$ through the effects on big bang nucleosynthesis (BBN) and the cosmic microwave background (CMB). In the former case, we include the (previously neglected) effect of the change in the deuteron binding energy, which alters both the ${}^4\mathrm{He}$ and deuterium abundances significantly. We find that the current BBN limits on the relative change in $〈\phi 〉$ are $-(0.6\mathrm{}-0.7)\times {10}^{-2}<\Delta 〈\phi 〉/〈\phi 〉<(1.5\mathrm{}-2.0)\times {10}^{-2},$ where the exact limits depend on the model we choose for the dependence of the deuteron binding energy on $〈\phi 〉.$ The limits from the current CMB data are much weaker.

Weakly interacting massive particles (WIMPs) are among the favored candidates for cold dark matter in the universe. The phenomenology of supersymmetric WIMPs has been quite developed during recent years. However, there are other possibilities which have not been discussed as much. One example is a right-handed massive neutrino, which has recently been proposed in the context of a version of the Zee model for massive neutrinos. This TeV-scale, leptonic WIMP (or LIMP, for short) may at first sight appear to be essentially undetectable. However, we point out that the radiatively induced annihilation rate into leptons and photons is bound to be substantial, and provides a conspicuous gamma-ray signature for annihilations in the galactic halo. This gives a window of opportunity for Air Čerenkov Telescopes with the ability to observe the galactic center, such as the HESS and CANGAROO arrays, and also for the GLAST space telescope. In addition, the contribution to the positron cosmic ray flux is in principle detectable, but this would require very strong local density enhancements in the dark matter halo distribution.

The observed baryon asymmetry of the Universe can be due to the $B-L$ violating decay of heavy right-handed (s)neutrinos. The amount of the asymmetry depends crucially on their number density. If the (s)neutrinos are generated thermally, in supersymmetric models there is limited parameter space leading to enough baryons. For this reason, several alternative mechanisms have been proposed. We discuss the nonperturbative production of sneutrino quanta by a direct coupling to the inflaton. This production dominates over the corresponding creation of neutrinos, and it can easily (i.e., even for a rather small inflaton-sneutrino coupling) lead to a sufficient baryon asymmetry. We then study the amplification of the minimal supersymmetric standard model (MSSM) degrees of freedom, via their coupling to the sneutrinos, during the rescattering phase which follows the nonperturbative production. This process, which mainly influences the (MSSM) D-flat directions, is very efficient as long as the sneutrino quanta are in the relativistic regime. The rapid amplification of the light degrees of freedom may potentially lead to a gravitino problem. We estimate the gravitino production by means of a perturbative calculation, discussing the regime in which we expect it to be reliable.

Motivated by recent developments in particle physics and cosmology, there has been growing interest in a unified description of dark matter and dark energy scenarios. In this paper we explore observational constraints from age estimates of high-z objects on cosmological models dominated by an exotic fluid with an equation of state $p=-A/{\rho }^{\alpha }$ (the so-called generalized Chaplygin gas) which has the interesting feature of interpolating between nonrelativistic matter and negative-pressure dark energy regimes. As a general result we find that, if the age estimates of these objects are correct, they impose very restrictive limits on some of these scenarios.

The back reaction of structure formation influences the cosmological evolution equation for the homogenous and isotropic average metric. In a cold dark matter universe this effect leads only to small corrections unless a substantial fraction of matter is located in regions where strong gravitational fields evolve in time. A“cosmic virial theorem” states that the sum of gravitational and matter pressures vanishes and, therefore relates the average kinetic energy to a suitable average of the Newtonian potential. In the presence of a scalar “cosmon” field mediating quintessence, however, cosmology could be modified if local cosmon fluctuations grow large. We speculate that this may trigger the accelerated expansion of the universe after the formation of structure.

We estimate the growth of matter perturbations in a class of recently proposed dark-energy models based on the (loop-corrected) gravidilaton string effective action, and characterized by a global attractor epoch in which dark-matter and dark-energy density scale with the same effective equation of state. Unlike most dark-energy models, we find that the accelerated phase might start even at redshifts as high as $z\approx 5$ (thus relaxing the coincidence problem), while still producing at present an acceptable level of matter fluctuations. We also show that such an early acceleration is not in conflict with the recently discovered supernova SN 1997ff at $z\approx \mathrm{1.7.}$ The comparison of the predicted value of ${\sigma }_8$ with the observational data provides interesting constraints on the fundamental parameters of the given model of dilaton–dark matter interactions.