66 research outputs found
The large-scale general-relativistic correction for Newtonian mocks
We clarify the subtle issue of finding the correct mapping of Newtonian
simulations to light-cone observables at very large distance scales. A faithful
general-relativistic interpretation specifies a gauge, i.e. a chart that
relates the simulation data to points of the space-time manifold. It has
already been pointed out that the implicit gauge choice of Newtonian
simulations is indeed different from the Poisson gauge that is commonly adopted
for relativistic calculations, the difference being most significant at large
scales. It is therefore inconsistent, for example, to predict weak-lensing
observables from simulations unless this gauge issue is properly accounted for.
Using perturbation theory as well as fully relativistic N-body simulations we
quantify the systematic error introduced this way, and we discuss several
solutions that would render the calculations relativistically self-consistent.Comment: 10 pages, 5 figures; v2: minor revision with additional content,
matches accepted manuscrip
Dissipative fields in de Sitter and black hole spacetimes: Quantum entanglement due to pair production and dissipation
For free fields, pair creation in expanding universes is associated with the
building up of correlations that lead to nonseparable states, i.e., quantum
mechanically entangled ones. For dissipative fields, i.e., fields coupled to an
environment, there is a competition between the squeezing of the state and the
coupling to the external bath. We compute the final coherence level for
dissipative fields that propagate in a two-dimensional de Sitter space, and we
characterize the domain in parameter space where the state remains
nonseparable. We then apply our analysis to (analogue) Hawking radiation by
exploiting the close relationship between Lorentz violating theories
propagating in de Sitter and black hole metrics. We establish the robustness of
the spectrum and find that the entanglement among Hawking pairs is generally
much stronger than that among pairs of quanta with opposite momenta.Comment: Final version published in prd, 22 page
N-body methods for relativistic cosmology
We present a framework for general relativistic N-body simulations in the
regime of weak gravitational fields. In this approach, Einstein's equations are
expanded in terms of metric perturbations about a Friedmann-Lema\^itre
background, which are assumed to remain small. The metric perturbations
themselves are only kept to linear order, but we keep their first spatial
derivatives to second order and treat their second spatial derivatives as well
as sources of stress-energy fully non-perturbatively. The evolution of matter
is modelled by an N-body ensemble which can consist of free-streaming
nonrelativistic (e.g. cold dark matter) or relativistic particle species (e.g.
cosmic neutrinos), but the framework is fully general and also allows for other
sources of stress-energy, in particular additional relativistic sources like
modified-gravity models or topological defects. We compare our method with the
traditional Newtonian approach and argue that relativistic methods are
conceptually more robust and flexible, at the cost of a moderate increase of
numerical difficulty. However, for a LambdaCDM cosmology, where nonrelativistic
matter is the only source of perturbations, the relativistic corrections are
expected to be small. We quantify this statement by extracting post-Newtonian
estimates from Newtonian N-body simulations.Comment: 30 pages, 3 figures. Invited contribution to a Classical and Quantum
Gravity focus issue on "Relativistic Effects in Cosmology", edited by Kazuya
Koyam
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
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