Newtonian kinematical backreaction in cosmological $N$-body simulations with Delaunay Tesselation: "zero test" and scale dependence

Kazimierczak, Tomasz A.

doi:10.48550/arxiv.1601.00110

Cited by 2 publications

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“…But it is on the order of Q ∼ u 2 λ 2 /r 4 and so is even smaller than the second term for large V . We believe this is what Kazimierczak (2016) has measured in N-body simulations.…”

Section: Appendix A: Acceleration In Terms Of Surface Averagessupporting

confidence: 76%

Why there is no Newtonian backreaction

Kaiser¹

2017

Monthly Notices of the Royal Astronomical Society

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In the conventional framework for cosmological dynamics the scale factor a(t) is assumed to obey the 'background' Friedmann equation for a perfectly homogeneous universe while particles move according to equations of motions driven by the gravity of the density fluctuations. It has recently been suggested that the emergence of structure modifies the evolution of a(t) viaNewtonian (or 'kinematic') backreaction and that this may avoid the need for dark energy. Here we point out that the conventional system of equations is exact in Newtonian gravity and there is no approximation in the use of the homogeneous universe equation for a(t). The recently proposed modification of Racz et al. (2017) does not reduce to Newtonian gravity in the limit of low velocities. We discuss the relation of this to the 'generalised Friedmann equation' of Buchert and Ehlers. These are quite different things; their formula describes individual regions and is obtained under the restrictive assumption that the matter behaves like a pressure-free fluid whereas our result is exact for collisionless dynamics and is an auxiliary relation appearing in the structure equations. We use the symmetry of the general velocity autocorrelation function to show how Buchert's Q tends very rapidly to zero for large volume and that this does not simply arise 'by construction' through the adoption of periodic boundary conditions as has been claimed. We conclude that, to the extent that Newtonian gravity accurately describes the low-z universe, there is no backreaction of structure on a(t) and that the need for dark energy cannot be avoided in this way.

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“…But it is on the order of Q ∼ u 2 λ 2 /r 4 and so is even smaller than the second term for large V . We believe this is what Kazimierczak (2016) has measured in N-body simulations.…”

Section: Appendix A: Acceleration In Terms Of Surface Averagessupporting

confidence: 76%

Why there is no Newtonian backreaction

Kaiser¹

2017

Monthly Notices of the Royal Astronomical Society

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“…applying post-Newtonian corrections) suggest that the mean evolution is well approximated by the Friedmannian evolution [6][7][8]. On the other hand, studies that try to implement backreaction-type effects into the Newtonian Cosmology and N -body codes find large effects [9,10] -such an approach has sparked recently a debate, see ref. [11] and the rebuttal response in ref.…”

Section: Introductionmentioning

confidence: 99%

Cosmological backreaction within the Szekeres model and emergence of spatial curvature

Bolejko

2017

J. Cosmol. Astropart. Phys.

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This paper discusses the phenomenon of backreaction within the Szekeres model.Cosmological backreaction describes how the mean global evolution of the Universe deviates from the Friedmannian evolution. The analysis is based on models of a single cosmological environment and the global ensemble of the Szekeres models (of the Swiss-Cheese-type and Styrofoam-type). The obtained results show that non-linear growth of cosmic structures is associated with the growth of the spatial curvature Ω R (in the FLRW limit Ω R → Ω k ). If averaged over global scales the result depends on the assumed global model of the Universe. Within the Swiss-Cheese model, which does have a fixed background, the volume average follows the evolution of the background, and the global spatial curvature averages out to zero (the background model is the ΛCDM model, which is spatially flat). In the Styrofoamtype model, which does not have a fixed background, the mean evolution deviates from the spatially flat ΛCDM model, and the mean spatial curvature evolves from Ω R = 0 at the CMB to Ω R ∼ 0.1 at z = 0. If the Styrofoam-type model correctly captures evolutionary features of the real Universe then one should expect that in our Universe, the spatial curvature should build up (local growth of cosmic structures) and its mean global average should deviate from zero (backreaction). As a result, this paper predicts that the low-redshift Universe should not be spatially flat (i.e. Ω k = 0, even if in the early Universe Ω k = 0) and therefore when analysing low-z cosmological data one should keep Ω k as a free parameter and independent from the CMB constraints.

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Newtonian kinematical backreaction in cosmological $N$-body simulations with Delaunay Tesselation: "zero test" and scale dependence

Cited by 2 publications

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Why there is no Newtonian backreaction

Why there is no Newtonian backreaction

Cosmological backreaction within the Szekeres model and emergence of spatial curvature

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