Observations that we are highly unlikely to be vacuum fluctuations suggest that our universe is decaying at a rate faster than the asymptotic volume growth rate, in order that there not be too many observers produced by vacuum fluctuations to make our observations highly atypical. An asymptotic linear e-folding time of roughly 16 Gyr (deduced from current measurements of cosmic acceleration) would then imply that our universe is more likely than not to decay within a time that is less than 19 Gyr in the future.

It is shown that the generating function for tree graphs in the “in-in” formalism may be calculated by solving the classical equations of motion subject to certain constraints. This theorem is illustrated by application to the evolution of a single inflaton field in a Robertson-Walker background.

We investigate electron-positron pair production in pulse-shaped electric background fields using a non-Markovian quantum kinetic equation. We identify a pulse length range for subcritical fields still in the nonperturbative regime where the number of produced pairs significantly exceeds that of a naive expectation based on the Schwinger formula. From a conceptual viewpoint, we find a remarkable quantitative agreement between the (real time) quantum kinetic approach and the (imaginary time) effective action approach.

We study correlations between center vortices and the low-lying eigenmodes of the Dirac operator, in both the overlap and asqtad formulations. In particular we address a puzzle raised some years ago by Gattnar et al. [Nucl. Phys. B716, 105 (2005)], who noted that the low-lying Dirac eigenmodes required for chiral symmetry breaking do not appear to be present in center-projected configurations. We show that the low-lying modes are in fact present in the staggered (asqtad) formulation, but not in the overlap and “chirally improved” formulations, and suggest a reason for this difference. We also confirm and extend the results of Kovalenko et al. [Phys. Lett. B 648, 383 (2007)], showing that there is a correlation between center vortex locations, and the scalar density of low-lying Dirac eigenmodes derived from unprojected configurations. This correlation is strongest at points which are associated, in the vortex picture, with nonvanishing topological charge density, such as vortex intersection and writhing points. We present supporting evidence that the lowest Dirac eigenmodes, in both asqtad and overlap formulations, have their largest concentrations in pointlike regions, rather than on submanifolds of higher dimensionality.

We investigate chiral properties of local (nonderivative) fields of baryons consisting of three quarks with flavor $SU(3)$ symmetry. We construct explicitly independent local three-quark fields belonging to definite Lorentz and flavor representations. Chiral symmetry is spontaneously broken and therefore the baryon fields can have different chiral representations. Because of the color antisymmetric and spatially symmetric structure of the local three-quark fields, the allowed chiral representations are strongly correlated with the Lorentz group representations. We discuss some implications of the allowed chiral symmetry representations on physical quantities such as the axial coupling constants.

We extend previous results from the preceding paper on factorization in high energy nucleus-nucleus collisions by computing the inclusive multigluon spectrum to next-to-leading order. The factorization formula is strictly valid for multigluon emission in a slice of rapidity of width $\Delta Y\le {\alpha }_s^{-1}$. Our results shows that often neglected disconnected graphs dominate the inclusive multigluon spectrum, and are crucial in order to achieve factorization for this quantity. These results provide a dynamical framework for the Glasma flux tube picture of the striking “ridge”-like correlation seen in heavy ion collisions.

We derive a high energy factorization theorem for inclusive gluon production in $\mathrm{A}+\mathrm{A}$ collisions. Our factorized formula resums (i) all-order leading logarithms $(g^2\ln (1/x_{1,2}){)}^n$ of the incoming parton momentum fractions, and (ii) all contributions $(g{\rho }_{1,2}{)}^n$ that are enhanced when the color charge densities in the two nuclei are of order of the inverse coupling—${\rho }_{1,2}\sim g^{-1}$. The resummed inclusive gluon spectrum can be expressed as a convolution of gauge invariant distributions $W[{\rho }_{1,2}]$ from each of the nuclei with the leading order gluon number operator. These distributions are shown to satisfy the JIMWLK equation describing the evolution of nuclear wave functions with rapidity. As a by-product, we demonstrate that the JIMWLK Hamiltonian can be derived entirely in terms of retarded light-cone Green’s functions without any ambiguities in their pole prescriptions. We comment on the implications of our results for understanding the Glasma produced at early times in $A+A$ collisions at collider energies.

We address the possible scenario that the Large Hadron Collider (LHC) discovers only a Higgs boson after $10{\mathrm{fb}}^{-1}$ of operation, and attempt to identify this Higgs boson as that of the standard model, the minimal universal extra dimension model, the littlest Higgs model with $T$-parity (LHT), or the minimal supersymmetric standard model (MSSM), using only the measurement of the product of gluon-fusion production cross section and the di-photon branching ratio. In the minimal universal extra dimension model, by decoupling any new physics sufficiently to evade the discovery reach at the LHC, the deviation of the signal from the SM is not statistically significant. However, in LHT and MSSM, it is possible to have a significant deviation in the signal that is consistent with this “lone Higgs scenario,” and, in the case of a very large suppression, we can distinguish MSSM and LHT before the discovery of any new resonances. Starting with the lone Higgs scenario and the deviation in this measurement from the standard model prediction (whether or not statistically significant), we offer tests that may discriminate the models and search strategies of discovering new physics signatures with increasing integrated luminosity.

We present a stringy mechanism to generate Dirac neutrino masses by $D$-instantons in an experimentally relevant mass scale without fine-tuning. Within type IIA string theory with intersecting $D6$-branes, we spell out specific conditions for the emergence of such couplings and provide a class of supersymmetric local SU(5) grand unified models, based on the $Z_2\times Z_2^{\prime }$ orientifold compactification, where perturbatively absent Dirac neutrino masses can be generated by $D2$-brane instantons in the experimentally observed mass regime, while Majorana masses remain absent, thus providing an intriguing mechanism for the origin of small neutrino masses due to nonperturbative stringy effects.

We discuss an analysis presented by Berkovits in Ref [1] in the context of classical mechanics and perform its extension to the case of pure spinor string in a general background.

We present two generic classes of supersymmetric solutions of $N=2$, $d=4$ supergravity coupled to non-Abelian vector supermultiplets with a gauge group that includes an $SU(2)$ factor. The first class consists of embeddings of the ’t Hooft-Polyakov monopole and in the considered model is a globally regular, asymptotically flat spacetime. The other class of solutions consists of regular non-Abelian extreme black holes. There is a covariant attractor at the horizon of these non-Abelian black holes.

We study the question of stability of the ground state of a scalar theory which is a generalization of the ${\varphi }^3$ theory and has some similarity to gravity with a cosmological constant. We show that the ground state of the theory at zero temperature becomes unstable above a certain critical temperature, which is evaluated in closed form at high temperature.

We elaborate the problem of energy-momentum in general relativity by energy-momentum prescriptions theory. In this regard, we calculate Møller, Landau-Lifshitz, Papapetrou, Einstein, Bergmann, Tolman, and Weinberg’s energy-momentum complexes in static and nonstatic cosmic string space-times. We obtain strong coincidences between the results. These coincidences can be considered an extension of Virbhadra’s viewpoint that different energy-momentum prescriptions may provide some basis to define a unique quantity. In addition, our results disagree with Lessner’s belief about Møller’s prescription and support the Virbhadra’s conclusion about the power of Einstein’s prescription.

The Dirac equation for the electron around a five-dimensional rotating black hole with two different angular momenta is separated into purely radial and purely angular equations. The general solution is expressed as a superposition of solutions derived from these two decoupled ordinary differential equations. By separating variables for the massive Klein-Gordon equation in the same spacetime background, I derive a simple and elegant form for the Stäckel-Killing tensor, which can be easily written as the square of a rank-three Killing-Yano tensor. I have also explicitly constructed a symmetry operator that commutes with the scalar Laplacian by using the Stäckel-Killing tensor, and the one with the Dirac operator by the Killing-Yano tensor admitted by the five-dimensional Myers-Perry metric, respectively.

We consider the question of whether consistency arguments based on measurement theory show that the gravitational field must be quantized. Motivated by the argument of Eppley and Hannah, we apply a DeWitt-type measurement analysis to a coupled system that consists of a gravitational wave interacting with a mass cube. We also review the arguments of Eppley and Hannah and of DeWitt, and investigate a second model in which a gravitational wave interacts with a quantized scalar field. We argue that one cannot conclude from the existing gedanken experiments that gravity has to be quantized. Despite the many physical arguments which speak in favor of a quantum theory of gravity, it appears that the justification for such a theory must be based on empirical tests and does not follow from logical arguments alone.

We introduce new black hole solutions to the Einstein-Yang-Mills-Born-Infeld (EYMBI), Einstein-Yang-Mills-Born-Infeld-Gauss-Bonnet (EYMBIGB), and Einstein-Yang-Mills-Born-Infeld-Gauss-Bonnet-Lovelock (EYMBIGBL) gravities in higher dimensions $N\ge 5$ to investigate the roles of Born-Infeld parameter $\beta$. It is shown that these solutions in the limits of $\beta \to 0$ and $\beta \to \infty$ represent pure gravity and gravity coupled with Yang-Mills fields, respectively. For $0<\beta <\infty$ it yields a variety of black holes, supporting even regular ones at $r=0$.

We discuss different kinds of Killing horizons possible in static, spherically symmetric configurations and recently classified as “usual,” “naked,” and “truly naked” ones depending on the near-horizon behavior of transverse tidal forces acting on an extended body. We obtain the necessary conditions for the metric to be extensible beyond a horizon in terms of an arbitrary radial coordinate and show that all truly naked horizons, as well as many of those previously characterized as naked and even usual ones, do not admit an extension and therefore must be considered as singularities. Some examples are given, showing which kinds of matter are able to create specific space-times with different kinds of horizons, including truly naked ones. Among them are fluids with negative pressure and scalar fields with a particular behavior of the potential. We also discuss horizons and singularities in Kantowski-Sachs spherically symmetric cosmologies and present horizon regularity conditions in terms of an arbitrary time coordinate and proper (synchronous) time. It turns out that horizons of orders 2 and higher occur in infinite proper times in the past or future, but one-way communication with regions beyond such horizons is still possible.

In a recent paper [Vasileios Paschalidis, Phys. Rev. D 78, 024002 (2008).], the first coauthor presented a new parabolic extension (PADM) of the standard $3+1$ Arnowitt, Deser, Misner (ADM) formulation of the equations of general relativity. By parabolizing first-order ADM in a certain way, the PADM formulation turns it into a well-posed system which resembles the structure of mixed hyperbolic–second-order parabolic partial differential equations. The surface of constraints of PADM becomes a local attractor for all solutions and all possible well-posed gauge conditions. This paper describes a numerical implementation of PADM and studies its accuracy and stability in a series of standard numerical tests. Numerical properties of PADM are compared with those of standard ADM and its hyperbolic Kidder, Scheel, Teukolsky (KST) extension. The PADM scheme is numerically stable, convergent, and second-order accurate. The new formulation has better control of the constraint-violating modes than ADM and KST.

We examine Bousso’s covariant entropy bound conjecture in the context of radiation filled, spatially flat, Friedmann-Robertson-Walker models. The bound is violated near the big bang. However, the hope has been that quantum gravity effects would intervene and protect it. Loop quantum cosmology provides a near ideal setting for investigating this issue. For, on the one hand, quantum geometry effects resolve the singularity and, on the other hand, the wave function is sharply peaked at a quantum corrected but smooth geometry, which can supply the structure needed to test the bound. We find that the bound is respected. We suggest that the bound need not be an essential ingredient for a quantum gravity theory but may emerge from it under suitable circumstances.

I calculate the first corrections to the dynamical preexponential factor of the bubble nucleation rate for a relativistic first-order phase transition in an expanding cosmological background by estimating the effects of the Hubble expansion rate on the critical bubbles of Langer’s statistical theory of metastability. I also comment on possible applications and problems that arise when one considers the field theoretical extensions of these results (the Coleman–De Luccia and Hawking-Moss instantons and decay rates).