Safely smoothing spacetime: backreaction in relativistic cosmological simulations

Adamek, Julian; Clarkson, Chris; Daverio, David; Durrer, Ruth; Kunz, M.

doi:10.1088/1361-6382/aaeca5

Cited by 42 publications

(52 citation statements)

References 70 publications

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“…Our numerical experiment provides conclusive evidence that the relativistic evolution of inhomogeneities, once consistently combined with the kinematics of light propagation on the inhomogeneous spacetime geometry, does not lead to an unexpectedly large bias of the distance-redshift correlation. This corroborates the conclusions of [1][2][3][4][5][6]28]. On the other hand, inhomogeneities introduce a significant non-Gaussian scatter that can give a large standard error on the mean when only a small sample of sources is available.…”

Section: Discussionsupporting

confidence: 87%

Bias and scatter in the Hubble diagram from cosmological large-scale structure

et al. 2019

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An important part of cosmological model fitting relies on correlating distance indicators of objects (for example type Ia supernovae) with their redshift, often illustrated on a Hubble diagram. Comparing the observed correlation with a homogeneous model is one of the key pieces of evidence for dark energy. The presence of cosmic structures introduces a bias and scatter, mainly due to gravitational lensing and peculiar velocities, but also due to smaller non-linear relativistic contributions which are more difficult to account for. For the first time we perform ray tracing onto halos in a relativistic N-body simulation. Our simulation is the largest that takes into account all leading-order corrections from general relativity in the evolution of structure, and we present a novel methodology for working out the non-linear projection of that structure onto the observer's past light cone. We show that the mean of the bias in the Hubble diagram is indeed as small as expected from perturbation theory. However, the distribution of sources is significantly skewed with a very long tail of highly magnified objects and we illustrate that the bias of cosmological parameters strongly depends on the function of distance which we consider.

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Section: Discussionsupporting

confidence: 87%

Bias and scatter in the Hubble diagram from cosmological large-scale structure

et al. 2019

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“…For some of the largest structures in our Universe that have been observationally well-characterized, we find that nonlinear effects can approach the percent level even in harmonic and Newtonian gauges, while linear theory is entirely unable describe the synchronous gauge metric. These findings coincide with expectations: it is well-known that the amplitude of metric perturbations can vary significantly between gauges or "slicing conditions" [15], as noted in other approximate and analytic treatments [16,17] as well. For density perturbations of cosmologically common amplitudes in synchronous gauge, the metric amplitude scales roughly with the density contrast δ ρ , and can therefore become quite large, while in harmonic slicing and a quasi-Newtonian gauge, metric perturbations remain small.…”

Section: Introductionsupporting

confidence: 89%

Limited accuracy of linearized gravity

et al. 2019

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Standard cosmological models rely on an approximate treatment of gravity, utilizing solutions of the linearized Einstein equations as well as physical approximations. In an era of precision cosmology, we should ask: are these approximate predictions sufficiently accurate for comparison to observations, and can we draw meaningful conclusions about properties of our Universe from them? In this work we examine the accuracy of linearized gravity in the presence of collisionless matter and a cosmological constant utilizing fully general relativistic simulations. We observe the gauge-dependence of corrections to linear theory, and note the amplitude of these corrections. For perturbations whose amplitudes are in line with expectations from the standard ΛCDM model, we find that the full, general relativistic metric is well-described by linear theory in Newtonian and harmonic gauges, while the metric in comoving-synchronous gauge is not. For the largest observed structures in our Universe, our results suggest that corrections to linear gravitational theory can reach or surpass the percent-level. Contents

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“…However, as pointed out many times before (e.g. [44,53,62,65,66,[80][81][82]), a possible issue with such a procedure is that observations are mainly made on the light cone. Therefore, spatially averaged quantities (including backreaction terms) are only relevant if they can be related to observables in a sensible manner.…”

Section: Average Light Propagationmentioning

confidence: 99%

Another look at redshift drift and the backreaction conjecture

Koksbang

2019

J. Cosmol. Astropart. Phys.

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Earlier studies have conjectured that redshift drift is described by spatially averaged quantities and thus becomes positive if the average expansion of the Universe accelerates. This conclusion is reevaluated here by considering exact light propagation in a simple toy-model with average accelerated expansion. The toy-model and light propagation setup is explicitly designed for concordance between spatial averages and averages along light rays. While it is verified that redshift-distance relations are well described by average quantities in this setup, it is found that the redshift drift is not. Specifically, the redshift drift is negative despite the on-average late-time accelerated expansion of the model. This result implies that measuring redshift drift signals at low redshifts gives the potential for directly falsifying the backreaction conjecture. However, the results are based on a toy-model so it is in principle possible that the result is an artifact and that redshift drift is in reality well described by spatially averaged quantities. The result therefore highlights the importance of developing exact solutions to the Einstein equations which exhibit average accelerated expansion without local expansion so that the relation between spatial averages and observations can be firmly established.

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Safely smoothing spacetime: backreaction in relativistic cosmological simulations

Cited by 42 publications

References 70 publications

Bias and scatter in the Hubble diagram from cosmological large-scale structure

Bias and scatter in the Hubble diagram from cosmological large-scale structure

Limited accuracy of linearized gravity

Another look at redshift drift and the backreaction conjecture

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