Living with ghosts in Horava-Lifshitz gravity

We consider the branch of the projectable Horava-Lifshitz model which exhibits ghost instabilities in the low energy limit. It turns out that, due to the Lorentz violating structure of the model and to the presence of a finite strong coupling scale, the vacuum decay rate into photons is tiny in a wide range of phenomenologically acceptable parameters. The strong coupling scale, understood as a cutoff on ghosts' spatial momenta, can be raised up to $\Lambda \sim 10$ TeV. At lower momenta, the projectable Horava-Lifshitz gravity is equivalent to General Relativity supplemented by a fluid with a small positive sound speed squared ($10^{-42}\lesssim$) $c^2_s \lesssim 10^{-20}$, that could be a promising candidate for the Dark Matter. Despite these advantages, the unavoidable presence of the strong coupling obscures the implementation of the original Horava's proposal on quantum gravity. Apart from the Horava-Lifshitz model, conclusions of the present work hold also for the mimetic matter scenario, where the analogue of the projectability condition is achieved by a non-invertible conformal transformation of the metric.Comment: 33 pages, 1 figure. The proof of an equivalence between the IR limit of the projectable Horava-Lifshitz gravity and the mimetic matter scenario is given in Appendix A. Version accepted for publication in JHE

Evaluation of the greenhouse gas balance in the Eucalyptus globulus sector in Portugal

Este estudio evalúa el balance de gas invernadero en el sector del E. globulus en Portugal. Las eliminaciones y las emisiones de dióxido de carbono (CO2) y el metano (CH4) fueron calculadas a lo largo en todo el sector forestal, incluyendo el ecosistema forestal, el tratamiento industrial de madera y las etapas de empleo y la disposición final de productos forestales. El balance de gas invernadero fue calculado restándose la eliminación de carbono neto a las emisiones de carbono fósil y sumándose las emisiones de carbono como el CH4. El balance global de gas invernadero fue una eliminación neta de carbono que varió entre 401 y 1033 Gg Ceq yr-1, respectivamente con el cambio de reservas y los accesos de flujo atmosférico. La diferencia entre las dos estimaciones es equivalente al carbón exportado en productos de madera y forestales. Tanto productos forestales como forestales eran sumideros de carbono, como sus reservas han estado aumentando. Aproximadamente el 94 % del cambio total de reservas de carbono en el sector era debido al bosque, mientras que el papel era el producto forestal con la contribución principal al aumento de reservas de carbono. Emisiones de carbono de fósil consideradas para el 13 % de las emisiones totales de carbono en el sector y disminuido el retiro neto de carbono por 18 y el 34 %, respectivamente con el flujo atmosférico y los cambio de reservas. El carbón emitido como CH4 tuvo la importancia menor y, por consiguiente, era responsable de una disminución en el retiro neto de carbón de sólo 4 y el 8 %, respectivamente con el flujo atmosférico y los accesos de cambio de reservas.____________________________________This study evaluates the greenhouse gas balance in the E. globulus sector in Portugal. Removals and emissions of carbon dioxide (CO2) and methane (CH4) were calculated along the whole forest sector, including the forest ecosystem, the industrial processing of wood and the stages of use and final disposal of forest products. The greenhouse gas balance was calculated by subtracting to the net carbon removal, the fossil carbon emissions and the additional emissions of carbon as CH4. Two different approaches were applied for estimating the net carbon removal in the sector: the stock-change and the atmospheric-flow approach. The global greenhouse gas balance was a net removal of carbon that varied between 401 and 1033 Gg Ceq yr-1, respectively with the stock-change and the atmospheric-flow approaches. The difference between the two estimates is equivalent to the carbon exported in wood and forest products. Both forest and forest products were carbon sinks, as their stocks have been increasing. About 94% of the total change in carbon stocks in the sector was due to forest, whereas paper was the forest product with the major contribution to the increase of carbon stocks. Fossil carbon emissions accounted for 13% of the total carbon emissions in the sector and decreased the net carbon removal by 18 and 34%, respectively with the atmospheric-flow and the stock-change approaches. Carbon emitted as CH4 was of minor importance and, consequently, was responsible for a decrease in the net carbon removal of only 4 and 8%, respectively with the atmospheric-flow and the stock-change approaches

The inflationary bispectrum with curved field-space

We compute the covariant three-point function near horizon-crossing for a system of slowly-rolling scalar fields during an inflationary epoch, allowing for an arbitrary field-space metric. We show explicitly how to compute its subsequent evolution using a covariantized version of the separate universe or "delta-N" expansion, which must be augmented by terms measuring curvature of the field-space manifold, and give the nonlinear gauge transformation to the comoving curvature perturbation. Nonlinearities induced by the field-space curvature terms are a new and potentially significant source of non-Gaussianity. We show how inflationary models with non-minimal coupling to the spacetime Ricci scalar can be accommodated within this framework. This yields a simple toolkit allowing the bispectrum to be computed in models with non-negligible field-space curvature.Comment: 22 pages, plus appendix and reference

Large non-Gaussianities in the Effective Field Theory Approach to Single-Field Inflation: the Bispectrum

The methods of effective field theory are used to study generic theories of inflation with a single inflaton field and to perform a general analysis of the associated non-Gaussianities. We investigate the amplitudes and shapes of the various generic three-point correlators, the bispectra, which may be generated by different classes of single-field inflationary models. Besides the well-known results for the DBI-like models and the ghost inflationary theories, we point out that curvature-related interactions may give rise to large non-Gaussianities in the form of bispectra characterized by a flat shape which, quite interestingly, is independently produced by several interaction terms. In a subsequent work, we will perform a similar general analysis for the non-Gaussianities generated by the generic four-point correlator, the trispectrum.Comment: Version matching the one published in JCAP, 2 typos fixed, references added. 30 pages, 20 figure

Large non-Gaussianity from two-component hybrid inflation

We study the generation of non-Gaussianity in models of hybrid inflation with two inflaton fields, (2-brid inflation). We analyse the region in the parameter and the initial condition space where a large non-Gaussianity may be generated during slow-roll inflation which is generally characterised by a large f_NL, tau_NL and a small g_NL. For certain parameter values we can satisfy tau_NL>>f_NL^2. The bispectrum is of the local type but may have a significant scale dependence. We show that the loop corrections to the power spectrum and bispectrum are suppressed during inflation, if one assume that the fields follow a classical background trajectory. We also include the effect of the waterfall field, which can lead to a significant change in the observables after the waterfall field is destabilised, depending on the couplings between the waterfall and inflaton fields.Comment: 16 pages, 6 figures; v2: comments and references added, typos corrected, matches published versio

Large slow-roll corrections to the bispectrum of noncanonical inflation

Nongaussian statistics are a powerful discriminant between inflationary models, particularly those with noncanonical kinetic terms. Focusing on theories where the Lagrangian is an arbitrary Lorentz-invariant function of a scalar field and its first derivatives, we review and extend the calculation of the observable three-point function. We compute the "next-order" slow-roll corrections to the bispectrum in closed form, and obtain quantitative estimates of their magnitude in DBI and power-law k-inflation. In the DBI case our results enable us to estimate corrections from the shape of the potential and the warp factor: these can be of order several tens of percent. We track the possible sources of large logarithms which can spoil ordinary perturbation theory, and use them to obtain a general formula for the scale dependence of the bispectrum. Our result satisfies the next-order version of Maldacena's consistency condition and an equivalent consistency condition for the scale dependence. We identify a new bispectrum shape available at next-order, which is similar to a shape encountered in Galileon models. If fNL is sufficiently large this shape may be independently detectable.Comment: v1: 37 pages, plus tables, figures and appendices. v2: supersedes version published in JCAP; some clarifications and more detailed comparison with earlier literature. All results unchanged. v3:improvements to some plots; text unchange

Combined local and equilateral non-Gaussianities from multifield DBI inflation

We study multifield aspects of Dirac-Born-Infeld (DBI) inflation. More specifically, we consider an inflationary phase driven by the radial motion of a D-brane in a conical throat and determine how the D-brane fluctuations in the angular directions can be converted into curvature perturbations when the tachyonic instability arises at the end of inflation. The simultaneous presence of multiple fields and non-standard kinetic terms gives both local and equilateral shapes for non-Gaussianities in the bispectrum. We also study the trispectrum, pointing out that it acquires a particular momentum dependent component whose amplitude is given by $f_{NL}^{loc} f_{NL}^{eq}$. We show that this relation is valid in every multifield DBI model, in particular for any brane trajectory, and thus constitutes an interesting observational signature of such scenarios.Comment: 38 pages, 11 figures. Typos corrected; references added. This version matches the one in press by JCA

Primordial Trispectrum from Entropy Perturbations in Multifield DBI Model

We investigate the primordial trispectra of the general multifield DBI inflationary model. In contrast with the single field model, the entropic modes can source the curvature perturbations on the super horizon scales, so we calculate the contributions from the interaction of four entropic modes mediating one adiabatic mode to the trispectra, at the large transfer limit ($T_{RS}\gg1$). We obtained the general form of the 4-point correlation functions, plotted the shape diagrams in two specific momenta configurations, "equilateral configuration" and "specialized configuration". Our figures showed that we can easily distinguish the two different momenta configurations.Comment: 17pages, 7 figures, version to appear in JCA

Local non-Gaussianity from rapidly varying sound speeds

We study the effect of non-trivial sound speeds on local-type non-Gaussianity during multiple-field inflation. To this end, we consider a model of multiple-field DBI and use the deltaN formalism to track the super-horizon evolution of perturbations. By adopting a sum separable Hubble parameter we derive analytic expressions for the relevant quantities in the two-field case, valid beyond slow variation. We find that non-trivial sound speeds can, in principle, curve the trajectory in such a way that significant local-type non-Gaussianity is produced. Deviations from slow variation, such as rapidly varying sound speeds, enhance this effect. To illustrate our results we consider two-field inflation in the tip regions of two warped throats and find large local-type non-Gaussianity produced towards the end of the inflationary process.Comment: 30 pages, 7 figures; typos corrected, references added, accepted for publication in JCA