This paper deals with a cosmological model in which the universe is filled with tachyon dark energy in order to describe current and future accelerating expansion. We obtain that the simplest condition for the regime of phantom energy to occur in this scenario is that the scalar field be Wick rotated to imaginary values which correspond to an axionic field classically. By introducing analytical expressions for the scale factor or the Hubble parameter that satisfy all constraint equations of the used models we show that such models describe universes which may develop a big rip singularity in the finite future. It is argued that, contrary to a recent claim, the entropy for a universe filled with dark energy is definite positive even on the phantom regime where the universe would instead acquire a negative temperature. It is also seen that, whichever the fate of the tachyonic accelerating universe, it will be stable to any fluctuations of the scalar field, and that since the considered models have all an imaginary sound speed, any overdense regions will undergo an accelerated collapse leading rapidly to formation of giant black holes. Finally the conjecture is advanced that these black holes may be the supermassive black holes that most galaxies harbor at their center.

The mass shift induced by one-loop quantum fluctuations on self-dual ANO vortices is computed using heat kernel/generalized zeta function regularization methods.

The constraints on the neutral gauge boson couplings, $ZZ\gamma$ and $Z\gamma \gamma$, are investigated at linear $e^{+}{e}^{-}$ collider energies through the $Z\gamma$ production with longitudinal and transverse polarization states of the final $Z$ boson. Because of high energy of linear electron-positron beams, initial state radiation (ISR) considerably changes the production cross section. We obtain an increase in the cross section by a factor of 2–3 due to ISR for transverse polarization and by a factor of 10–100 for longitudinal polarization states depending on the energy. We find the 95% C.L. limits on the CP conserving form factors $h_3^Z$, $h_4^Z$, $h_3^{\gamma }$, and $h_4^{\gamma }$ with integrated luminosity $500{\mathrm{f}\mathrm{b}}^{-1}$ and $\sqrt{s}=0.5,1,1.5\mathrm{T}\mathrm{e}\mathrm{V}$ energies. It is shown that the longitudinal polarization of the $Z$ boson, together with ISR, can improve sensitivities by factors 2–3.

We calculate the probability that an $e^{-}-e^{+}$ pair is created at rest from vacuum at the focus of two equal and oppositely directed pulsed laser beams. The effects of the finite duration and of the temporal behavior of the resulting laser pulse are taken into account perturbatively and the production probability is compared with the corresponding quantity calculated in the presence of an infinite laser beam with a constant pulse shape. By inserting theoretically achievable numerical values of the pulse parameters it is shown that the induced correction to the production probability cannot be neglected.

We study the scalar and pseudoscalar condensates and the ${\eta }^{\prime }$ meson correlators of the two-flavor massive Schwinger model in the $\theta \ne 0$ vacuum. Exploiting our new method which was developed to investigate topological effects in the previous work, we find that $\langle \overline{\psi }{\gamma }_5\psi {\rangle }^{\theta }\ne 0$ and there exists a long-range correlation of the ${\eta }^{\prime }$ meson. This phenomenon is well described by the clustering property. We also find that even in $\theta =0$ case the cancellation of the long-range correlation is nontrivial and requires accurate contributions from higher topological sectors. Our results imply that the fluctuation of the disconnected diagram originates from the pseudoscalar condensate in each topological sector.

We describe an extension of special relativity characterized by three invariant scales, the speed of light $c$, a mass $\kappa$, and a length $R$. This is defined by a nonlinear extension of the Poincaré algebra $\mathcal{A}$, which we describe here. For $R\to \infty$, $\mathcal{A}$ becomes the Snyder presentation of the $\kappa$-Poincaré algebra, while for $\kappa \to \infty$ it becomes the phase space algebra of a particle in de Sitter spacetime. We conjecture that the algebra is relevant for the low energy behavior of quantum gravity, with $\kappa$ taken to be the Planck mass, for the case of a nonzero cosmological constant $\Lambda =R^{-2}$. We study the modifications of particle motion which follow if the algebra is taken to define the Poisson structure of the phase space of a relativistic particle.

We consider Maxwell-Chern-Simons models involving different nonminimal coupling terms to a non relativistic massive scalar and further coupled to an external uniform background charge. We study how these models can be constrained to support static radially symmetric vortex configurations saturating the lower bound for the energy. Models involving Zeeman-type coupling support such vortices provided the potential has a “symmetry breaking” form and a relation between parameters holds. In models where minimal coupling is supplemented by magnetic and electric field dependent coupling terms, non trivial vortex configurations minimizing the energy occur only when a nonlinear potential is introduced. The corresponding vortices are studied numerically.

To investigate how chaos affects gravitational waves, we study the gravitational waves from a spinning test particle moving around a Kerr black hole, which is a typical chaotic system. To compare the result with those in a nonchaotic dynamical system, we also analyze a spinless test particle, whose orbit can be complicated in the Kerr back ground although the system is integrable. We estimate the emitted gravitational waves by the multipole expansion of a gravitational field. We find a striking difference in the energy spectra of the gravitational waves. The spectrum for a chaotic orbit of a spinning particle contains various frequencies, while some characteristic frequencies appear in the case of a spinless particle.

Backgrounds depending on time and on angular variable, namely, polarized and unpolarized $S^1\times S^2$ Gowdy models, are generated as the sector inside the horizons of the manifold corresponding to axisymmetric solutions. As is known, an analytical continuation of ordinary $D$-branes, $iD$-branes allow one to find $S$-brane solutions. Simple models have been constructed by means of analytic continuation of the Schwarzschild and the Kerr metrics. The possibility of studying the $i$-Gowdy models obtained here is outlined with an eye toward seeing if they could represent some kind of generalized $S$-branes depending not only on time but also on an angular variable.

The Hamiltonian treatment for the collapse of thin shells for a family of Lanczos-Lovelock theories is studied. This formalism allows us to carry out a concise analysis of these theories. It is found that the black holes solution can be created by collapsing a thin shell. Naked singularities cannot be formed by this mechanism. Among the different Lanczos-Lovelock theories, the Chern-Simons theory corresponds to an exceptional case, because naked singularities can emerge from the collapse of a thin shell. This kind of theory does not possess a gravitational self-interaction analogous to the Newtonian case.

We investigate here models that suggest that the vacuum energy decays into cold dark matter (CDM) and show that the density fluctuation spectrum obtained from the cosmic microwave background (CMB) data together with large galaxy surveys (e.g., the Sloan Digital Sky Survey), puts strong limits on the rate of decay of the vacuum energy. CDM produced by a decaying vacuum energy would dilute the density fluctuation spectrum, created in the primordial universe and observed with large galaxy surveys at low redshifts. Our results indicate that the decay rate of the vacuum energy into CDM is extremely small.

High-energy modifications to general relativity introduce changes to the perturbations generated during inflation, and the latest high-precision cosmological data can be used to place constraints on such modified inflation models. Recently it was shown that Randall-Sundrum–type braneworld inflation leads to tighter constraints on quadratic and quartic potentials than in general relativity. We investigate how this changes with a Gauss-Bonnet correction term, which can be motivated by string theory. Randall-Sundrum models preserve the standard consistency relation between the tensor spectral index and the tensor-to-scalar ratio. The Gauss-Bonnet term breaks this relation, and also modifies the dynamics and perturbation amplitudes at high energies. We find that the Gauss-Bonnet term tends to soften the Randall-Sundrum constraints. The observational compatibility of the quadratic potential is strongly improved. For a broad range of energy scales, the quartic potential is rescued from marginal rejection. Steep inflation driven by an exponential potential is excluded in the Randall-Sundrum case, but the Gauss-Bonnet term leads to marginal compatibility for sufficient e-folds.

We study the cooling behavior of an isolated strange-quark star, using an equation of state derived from perturbative QCD up to second order in a strong coupling constant, and we compare it with that of a neutron star. After an initial rapid cooling, a quark star may undergo the QCD phase transition to become a neutron star. We propose several signatures for such a scenario: a large amount of energy can be released due to latent heat, a long duration $\gamma$-ray source, and a second neutrino burst after a supernova explosion.

The acceleration of the expansion of the Universe, ascribed to a dark energy, is one of the most intriguing discoveries in science. In addition to precise, systematics controlled data, clear, robust interpretation of the observations is required to reveal the nature of dark energy. Even for the simplest question: is the data consistent with the cosmological constant? there are important subtleties in the reconstruction of the dark energy properties. We discuss the roles of analysis both in terms of the Hubble expansion rate or dark energy density ${\rho }_{DE}(z)$ and in terms of the dark energy equation of state $w(z)$, arguing that each has its carefully defined place. Fitting the density is best for learning about the density, but using it to probe the equation of state can lead to instability and bias.

We study the effects of improvement on the locality of square-rooted staggered Dirac operators in lattice QCD simulations. We find the localization lengths of the improved operators (fat7tad and asqtad) to be very similar to that of the one-link operator studied by Bunk et al. being at least the Compton wavelength of the lightest particle in the theory, even in the continuum limit. We conclude that improvement has no effect. We discuss the implications of this result for the locality of the $n^{\mathrm{t}\mathrm{h}}$-rooted fermion determinant used to reduce the number of sea quark flavors, and for possible staggered valence quark formulations.

We study the mass spectrum of baryons with two and three charmed quarks. For double charm baryons the spin splitting is found to be smaller than standard quark-model potential predictions. This splitting is not influenced either by the particular form of the confining potential or by the regularization taken for the contact term of the spin-spin potential. We consistently predict the spectra for triply charmed baryons.

The coupling of the electromagnetic field directly with gravitational gauge fields leads to new physical effects that can be tested using astronomical data. Here we consider a particular case for closer scrutiny, a specific nonminimal coupling of torsion to electromagnetism, which enters into a metric-affine geometry of space-time. We show that under the assumption of this nonminimal coupling, space-time is birefringent in the presence of such a gravitational field. This leads to the depolarization of light emitted from extended astrophysical sources. We use polarimetric data of the magnetic white dwarf RE J0317-853 to set, for the very first time, constraints on the essential coupling constant for this effect, giving $k^2\lesssim (19\mathrm{m}{)}^2$.

We reinvestigate the infrared behavior of the pressure in the $g{\varphi }^3$ scalar theory in six dimensions. This problem was first studied by Almeida and Frenkel and more recently by Carrington et al., certifying that their results under certain approximations. We employ an alternative technique, instead of the approximation methods necessary to truncate the Schwinger-Dyson equations, often considered to calculate the pressure nonperturbatively. A daisy-type sum, implemented through the modified self-consistent resummation (MSCR), is enough to take care of the infrared divergences ensuring the finiteness of the pressure.

We have constructed a quantum field theory in a finite box, with periodic boundary conditions, using the hypothesis that particles living in a finite box are created and/or annihilated by the creation and/or annihilation operators, respectively, of a quantum harmonic oscillator on a circle. An expression for the effective coupling constant is obtained, showing explicitly its dependence on the dimension of the box.

We consider the consistent evaluation of perturbative (divergent) Green functions associated with fermionic tensor densities and the derivation of symmetry relations for them. We show that, in spite of current algebra methods being not applicable, it is possible to derive symmetry properties analogous to the Ward identities of vector and axial-vector densities. The proposed method, which is applicable to any previously chosen order of perturbative calculation, gives the same results as those of current algebra when such a tool is applicable. By using a very general calculational strategy, concerning the manipulations and calculations involving divergent Feynman integrals, we evaluate the purely fermionic two-point functions containing tensor vertices and derive their symmetry properties. The present investigation is the first step in the study and characterization of possible anomalies involving fermionic tensor densities, particularly in purely fermionic three-point functions.