gevolution: a cosmological N-body code based on General Relativity

We present a new N-body code, gevolution, for the evolution of large scale structure in the Universe. Our code is based on a weak field expansion of General Relativity and calculates all six metric degrees of freedom in Poisson gauge. N-body particles are evolved by solving the geodesic equation which we write in terms of a canonical momentum such that it remains valid also for relativistic particles. We validate the code by considering the Schwarzschild solution and, in the Newtonian limit, by comparing with the Newtonian N-body codes Gadget-2 and RAMSES. We then proceed with a simulation of large scale structure in a Universe with massive neutrinos where we study the gravitational slip induced by the neutrino shear stress. The code can be extended to include different kinds of dark energy or modified gravity models and going beyond the usually adopted quasi-static approximation. Our code is publicly available.Comment: 28 pages + appendix, 10 figures. v2: revised and extended version accepted by JCAP; code available at https://github.com/gevolution-cod

Unified dark energy and dark matter from a scalar field different from quintessence

We explore unification of dark matter and dark energy in a theory containing a scalar field of non-Lagrangian type, obtained by direct insertion of a kinetic term into the energy-momentum tensor. This scalar is different from quintessence, having an equation of state between -1 and 0 and a zero sound speed in its rest frame. We solve the equations of motion for an exponential potential via a rewriting as an autonomous system, and demonstrate the observational viability of the scenario, for sufficiently small exponential potential parameter \lambda, by comparison to a compilation of kinematical cosmological data.Comment: 10 pages RevTeX4 with 5 figures incorporate

Constraining the dark fluid

Cosmological observations are normally fit under the assumption that the dark sector can be decomposed into dark matter and dark energy components. However, as long as the probes remain purely gravitational, there is no unique decomposition and observations can only constrain a single dark fluid; this is known as the dark degeneracy. We use observations to directly constrain this dark fluid in a model-independent way, demonstrating in particular that the data cannot be fit by a dark fluid with a single constant equation of state. Parameterizing the dark fluid equation of state by a variety of polynomials in the scale factor $a$, we use current kinematical data to constrain the parameters. While the simplest interpretation of the dark fluid remains that it is comprised of separate dark matter and cosmological constant contributions, our results cover other model types including unified dark energy/matter scenarios.Comment: 5 pages, 5 figures incorporated. Updated to new observational data including SHOES determination of H0; new citations adde

Section 4(f): Analyzing Differing Interpretations and Examining Proposals for Reform

Section 4(f): Analyzing Differing Interpretations and Examining Proposals for Reform, focuses on the ongoing debate over the scope, meaning and application of Section 4(f) of the Department of Transportation Act of 1966. Section 4(f) affects the construction and development of transportation-related projects that require the use of public parks, wildlife refuges or historic sites. Since its enactment, it has been a source of controversy and debate within the transportation and environmental law communities. Recent congressional and executive branch actions aimed at reforming the law have garnered increased attention. This article analyzes the current state of play concerning section 4(f) and proposed changes to it

Spatiotemporal variability of lightning activity in Europe and the relation to the North Atlantic Oscillation teleconnection pattern

Comprehensive lightning statistics are presented for a large, contiguous domain covering several European countries such as France, Germany, Austria, and Switzerland. Spatiotemporal variability of convective activity is investigated based on a 14-year time series (2001–2014) of lightning data. Based on the binary variable thunderstorm day, the mean spatial patterns of lightning activity and regional peculiarities regarding seasonality are discussed. Diurnal cycles are compared among several regions and evaluated with respect to major seasonal changes. Further analyses are performed regarding interannual variability and the impact of teleconnection patterns on convection. Mean convective activity across central Europe is characterized by a strong northwest-to-southeast gradient with pronounced secondary features superimposed. The zone of maximum values of thunderstorm days propagates southwestward along the southern Alpine range from April to July. Diurnal cycles vary substantially between both different months and regions, particularly regarding the incidence of nighttime lightning. The North Atlantic Oscillation (NAO) is shown to have a significant impact on convective activity in several regions, which is primarily caused by variations of the large-scale lifting pattern in both NAO phases. This dynamical effect is partly compensated for by thermodynamical modifications of the pre-convective environment. The results point to a crucial role of large-scale flow in steering the spatiotemporal patterns of convective activity