Friedmann universe with dust and scalar field

We study a spatially flat Friedmann model containing a pressureless perfect fluid (dust) and a scalar field with an unbounded from below potential of the form V(\fii)=W_0 - V_0\sinh(\sqrt{3/2}\kappa\fii), where the parameters $W_0$ and $V_0$ are arbitrary and $\kappa=\sqrt{8\pi G_N}=M_p^{-1}$. The model is integrable and all exact solutions describe the recollapsing universe. The behavior of the model near both initial and final points of evolution is analyzed. The model is consistent with the observational parameters. We single out the exact solution with the present-day values of acceleration parameter $q_0=0.5$ and dark matter density parameter $\Omega_{\rho 0}=0.3$ describing the evolution within the time approximately equal to $2H_0^{-1}$.Comment: 11 pages, 10 figure

Particle creation from the vacuum by an exponentially decreasing electric field

We analyze the creation of fermions and bosons from the vacuum by the exponentially decreasing in time electric field in detail. In our calculations we use QED and follow in main the consideration of particle creation effect in a homogeneous electric field. To this end we find complete sets of exact solutions of the $d$-dimensional Dirac equation in the exponentially decreasing electric field and use them to calculate all the characteristics of the effect, in particular, the total number of created particles and the probability of a vacuum to remain a vacuum. It should be noted that the latter quantities were derived in the case under consideration for the first time. All possible asymptotic regimes are discussed in detail. In addition, switching on and switching off effects are studied.Comment: We add some references and minor comments. Version accepted for publication in Physica Scripta as a Invited Commen

Integration of D-dimensional 2-factor spaces cosmological models by reducing to the generalized Emden-Fowler equation

The D-dimensional cosmological model on the manifold $M = R \times M_{1} \times M_{2}$ describing the evolution of 2 Einsteinian factor spaces, $M_1$ and $M_2$, in the presence of multicomponent perfect fluid source is considered. The barotropic equation of state for mass-energy densities and the pressures of the components is assumed in each space. When the number of the non Ricci-flat factor spaces and the number of the perfect fluid components are both equal to 2, the Einstein equations for the model are reduced to the generalized Emden-Fowler (second-order ordinary differential) equation, which has been recently investigated by Zaitsev and Polyanin within discrete-group analysis. Using the integrable classes of this equation one generates the integrable cosmological models. The corresponding metrics are presented. The method is demonstrated for the special model with Ricci-flat spaces $M_1,M_2$ and the 2-component perfect fluid source.Comment: LaTeX file, no figure

Gravity wave intensity and momentum fluxes in the mesosphere over Shigaraki, Japan (35°N, 136°E) during 1986-1997

International audienceAveraged seasonal variations of wind perturbation intensities and vertical flux of horizontal momentum produced by internal gravity waves (IGWs) with periods 0.2-1 h and 1-6 h are studied at the altitudes 65-80 km using the MU radar measurement data from the middle and upper atmosphere during 1986-1997 at Shigaraki, Japan (35° N, 136° E). IGW intensity has maxima in winter and summer, winter values having substantial interannual variations. Mean wave momentum flux is directed to the west in winter and to the east in summer, opposite to the mean wind in the middle atmosphere. Major IGW momentum fluxes come to the mesosphere over Shigaraki from the Pacific direction in winter and continental Asia in summer

Toda chains with type A_m Lie algebra for multidimensional m-component perfect fluid cosmology

We consider a D-dimensional cosmological model describing an evolution of Ricci-flat factor spaces, M_1,...M_n (n > 2), in the presence of an m-component perfect fluid source (n > m > 1). We find characteristic vectors, related to the matter constants in the barotropic equations of state for fluid components of all factor spaces. We show that, in the case where we can interpret these vectors as the root vectors of a Lie algebra of Cartan type A_m=sl(m+1,C), the model reduces to the classical open m-body Toda chain. Using an elegant technique by Anderson (J. Math. Phys. 37 (1996) 1349) for solving this system, we integrate the Einstein equations for the model and present the metric in a Kasner-like form.Comment: LaTeX, 2 ps figure

Violation of vacuum stability by inverse square electric fields

In the framework of QED with a strong background, we study particle creation (the Schwinger effect) by a time-dependent inverse square electric field. To this end corresponding exact in- and out-solutions of the Dirac and Klein-Gordon equations are found. We calculate the vacuum-to-vacuum probability and differential and total mean numbers of pairs created from the vacuum. For electric fields varying slowly in time, we present detailed calculations of the Schwinger effect and discuss possible asymptotic regimes. The obtained results are consistent with universal estimates of the particle creation effect by electric fields in the locally constant field approximation. Differential and total quantities corresponding to asymmetrical configurations are also discussed in detail. Finally, the inverse square electric field is used to imitate switching on and off processes. Then the case under consideration is compared with the one where an exponential electric field is used to imitate switching on and off processes.Comment: 30 pages, 9 figures. Published versio

Schwinger mechanism of magnon-antimagnon pair production on magnetic field inhomogeneities and the bosonic Klein effect

Effective field theory of low-energy exitations-magnons that describes antiferromagnets is mapped into scalar electrodynamics of a charged scalar field interacting with an external electromagnetic potential. In the presence of a constant inhomogeneous external magnetic field the latter problem is technically reduced to the problem of charged-particle creation from the vacuum by an electric potential step (x-step). Magnetic moment plays here the role of the electric charge, and magnons and antimagnons differ from each other in the sign of the magnetic moment. In the framework of such a consideration, it is important to take into account the vacuum instability (the Schwinger effect) under the magnon-antimagnon production on magnetic field inhomogeneities (an analog of pair creation from the vacuum by electric-like fields). We demonstrate how to use the strong field QED with x-steps developed by the authors (SPG and DMG) to study the magnon-antimagnon pair production on magnetic field inhomogeneities. Characteristics of the vacuum instability obtained for some magnetic steps that allows exact solving the Klein-Gordon equation are presented. In particular, we consider examples of magnetic steps with very sharp field derivatives that correspond to a regularization of the Klein step. In the case of smooth-gradient steps, we describe an universal behavior of the flux density of created magnon pairs. We also note that since the low-energy magnons are bosons with small effective mass, then for the first time maybe the opportunity will arise to observe the Schwinger effect in the case of the Bose statistics, in particular, the bosonic Klein effect in laboratory conditions. Moreover, it turns out that in the case of the Bose statistics appears a new mechanism for amplifying the effect of the pair creation, which we call statistically-assisted Schwinger effect.Comment: 26 page

Schwinger pair creation in multilayer graphene

The low energy effective field model for the multilayer graphene (at ABC stacking) in external Electric field is considered. The Schwinger pair creation rate and the vacuum persistence probability are calculated using the semi - classical approach.Comment: Latex, 5 pages, accepted for publication in JETP let

Empirical estimation of peak pressure level from sound exposure level. Part II: Offshore impact pile driving noise

Numerical models of underwater sound propagation predict the energy of impulsive signals and its decay with range with a better accuracy than the peak pressure. A semi-empirical formula is suggested to predict the peak pressure of man-made impulsive signals based on numerical predictions of their energy. The approach discussed by Galindo-Romero, Lippert, and Gavrilov [J. Acoust. Soc. Am. 138, in press (2015)] for airgun signals is modified to predict the peak pressure from offshore pile driving, which accounts for impact and pile parameters. It is shown that using the modified empirical formula provides more accurate predictions of the peak pressure than direct numerical simulations of the signal waveform