Extra polarization states of cosmological gravitational waves in alternative theories of gravity

Cosmological Gravitational Waves (GWs) are usually associated with the transverse-traceless part of the metric perturbations in the context of the theory of cosmological perturbations. These modes are just the usual polarizations `+' and `x' which appear in the general relativity theory. However, in the majority of the alternative theories of gravity, GWs can present more than these two polarization states. In this context, the Newman-Penrose formalism is particularly suitable for evaluating the number of non-null GW modes. In the present work we intend to take into account these extra polarization states for cosmological GWs in alternative theories of gravity. As an application, we derive the dynamical equations for cosmological GWs for two specific theories, namely, a general scalar-tensor theory which presents four polarization states and a massive bimetric theory which is in the most general case with six polarization states for GWs. The mathematical tool presented here is quite general, so it can be used to study cosmological perturbations in all metric theories of gravity.Comment: 26 pages, 1 figure. Accepted for publication in Classical and Quantum Gravity

Preheating in an Expanding Universe: Analytic Results for the Massless Case

Analytic results are presented for preheating in both flat and open models of chaotic inflation, for the case of massless inflaton decay into further inflaton quanta. It is demonstrated that preheating in both these cases closely resembles that in Minkowski spacetime. Furthermore, quantitative differences between preheating in spatially-flat and open models of inflation remain of order $10^{-2}$ for the chaotic inflation initial conditions considered here.Comment: 15pp, revtex. No figures. Very minor revisions; forthcoming in Phys Rev

Modeling scale-dependent bias on the baryonic acoustic scale with the statistics of peaks of Gaussian random fields

Models of galaxy and halo clustering commonly assume that the tracers can be treated as a continuous field locally biased with respect to the underlying mass distribution. In the peak model pioneered by BBKS, one considers instead density maxima of the initial, Gaussian mass density field as an approximation to the formation site of virialized objects. In this paper, the peak model is extended in two ways to improve its predictive accuracy. Firstly, we derive the two-point correlation function of initial density peaks up to second order and demonstrate that a peak-background split approach can be applied to obtain the k-independent and k-dependent peak bias factors at all orders. Secondly, we explore the gravitational evolution of the peak correlation function within the Zel'dovich approximation. We show that the local (Lagrangian) bias approach emerges as a special case of the peak model, in which all bias parameters are scale-independent and there is no statistical velocity bias. We apply our formulae to study how the Lagrangian peak biasing, the diffusion due to large scale flows and the mode-coupling due to nonlocal interactions affect the scale dependence of bias from small separations up to the baryon acoustic oscillation (BAO) scale. For 2-sigma density peaks collapsing at z=0.3, our model predicts a ~ 5% residual scale-dependent bias around the acoustic scale that arises mostly from first-order Lagrangian peak biasing (as opposed to second-order gravity mode-coupling). We also search for a scale dependence of bias in the large scale auto-correlation of massive halos extracted from a very large N-body simulation provided by the MICE collaboration. For halos with mass M>10^{14}Msun/h, our measurements demonstrate a scale-dependent bias across the BAO feature which is very well reproduced by a prediction based on the peak model.Comment: (v1): 23 pages text, 8 figures + appendix (v2): typos fixed, references added, accepted for publication in PR

Microstructural evolution during liquid phase sintering: Part II. Microstructural coarsening

During liquid phase sintering, microstructural coarsening takes place. One mechanism by which this occurs is Ostwald ripening. Alternatively, particle coalescence also leads to a concomitant reduction in the solid particle surface area per unit volume. In isolated structures in which particle-particle contacts are made, the rate of coarsening by coalescence is limited by the time between particle contacts, for this is long compared to the time to fuse two particles together. In skeletal structures the coalescence time limits coarsening by coalescence since this is long in comparison to the time between contacts. Expressions for the rate of particle coarsening are developed for the different mechanisms and different particle morphologies. The results of these calculations are combined with the microstructure maps developed in Part I of this paper to refine these maps so that they predict both the morphology developed and the dominant mechanism of coarsening in liquid phase sintered systems. © 1977 The Metallurgical Society of American Institute of Mining, Metallurgical and Petroleum Engineers, Inc., and American Society for Metals