Cosmological <i>N</i>-body simulations with suppressed variance

Angulo, Raúl E.; Pontzen, Andrew

doi:10.1093/mnrasl/slw098

Cited by 202 publications

(216 citation statements)

References 19 publications

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“…A more comprehensive exploration of how pairing can improve the accuracy of structure formation simulations, including on measurement correlations beyond the power spectrum, is presented in Ref. [46].…”

Section: Improving the Accuracy Of Structure Formation Simulationsmentioning

confidence: 99%

Inverted initial conditions: Exploring the growth of cosmic structure and voids

et al. 2016

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We introduce and explore "paired" cosmological simulations. A pair consists of an A and B simulation with initial conditions related by the inversion δ A ðx; t initial Þ ¼ −δ B ðx; t initial Þ (underdensities substituted for overdensities and vice versa). We argue that the technique is valuable for improving our understanding of cosmic structure formation. The A and B fields are by definition equally likely draws from ΛCDM initial conditions, and in the linear regime evolve identically up to the overall sign. As nonlinear evolution takes hold, a region that collapses to form a halo in simulation A will tend to expand to create a void in simulation B. Applications include (i) contrasting the growth of A-halos and B-voids to test excursion-set theories of structure formation, (ii) cross-correlating the density field of the A and B universes as a novel test for perturbation theory, and (iii) canceling error terms by averaging power spectra between the two boxes. Generalizations of the method to more elaborate field transformations are suggested.

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Section: Improving the Accuracy Of Structure Formation Simulationsmentioning

confidence: 99%

Inverted initial conditions: Exploring the growth of cosmic structure and voids

et al. 2016

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“…This becomes an important issue especially when the measurement is sensitive to large angular scales that fluctuate the most. We suppress this effect with a mode-cancellation technique that preserves Gaussianity in the initial density fields, unlike the method presented in Angulo & Pontzen (2016) that sacrificed this statistical property, but achieved a higher level of cancellation. Our approach has a significant advantage that becomes clear in the following use.…”

Section: Introductionmentioning

confidence: 99%

Cosmic shear covariance matrix in wCDM: Cosmology matters

Harnois-Déraps

Giblin

Joachimi

2019

A&A

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We present here the cosmo-SLICS, a new suite of simulations specially designed for the analysis of current and upcoming weak lensing data beyond the standard two-point cosmic shear. We sample the [Ω m , σ 8 , h, w 0 ] parameter space at 25 points organised in a Latin hyper-cube, spanning a range that contains most of the 2σ posterior distribution from ongoing lensing surveys. At each of these nodes we evolve a pair of N-body simulations in which the sampling variance is highly suppressed, and ray-trace the volumes 800 times to further increase the effective sky coverage. We extract a lensing covariance matrix from these pseudo-independent lightcones and show that it closely matches a brute-force construction based on an ensemble of 800 truly independent N-body runs. More precisely, a Fisher analysis reveals that both methods yield marginalized two-dimensional constraints that vary by less than 6% in area, a result that holds under different survey specifications and that matches to within 15% the area obtained from an analytical covariance calculation. Extending this comparison with our 25 wCDM models, we probe the cosmology dependence of the lensing covariance directly from numerical simulations, reproducing remarkably well the Fisher results from the analytical models at most cosmologies. We demonstrate that varying the cosmology at which the covariance matrix is evaluated in the first place might have an order of magnitude greater impact on the parameter constraints than varying the choice of covariance estimation technique. We present a test case in which we generate fast predictions for both the lensing signal and its associated variance with a flexible Gaussian process regression emulator, achieving an accuracy of a few percent on the former and 10% on the latter.

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“…A different approach has been recently proposed by Angulo & Pontzen (2016). In order to reduce the scatter between different realisations, i.e., the effects of cosmic variance, they suggest fixing the amplitudes of the Fourier harmonics.…”

Section: Introductionmentioning

confidence: 99%

Suppressing cosmic variance with paired-and-fixed cosmological simulations: average properties and covariances of dark matter clustering statistics

Klypin

Prada

Byun

2020

Monthly Notices of the Royal Astronomical Society

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ABSTRACT Making cosmological inferences from the observed galaxy clustering requires accurate predictions for the mean clustering statistics and their covariances. Those are affected by cosmic variance – the statistical noise due to the finite number of harmonics. The cosmic variance can be suppressed by fixing the amplitudes of the harmonics instead of drawing them from a Gaussian distribution predicted by the inflation models. Initial realizations also can be generated in pairs with 180○ flipped phases to further reduce the variance. Here, we compare the consequences of using paired-and-fixed versus Gaussian initial conditions on the average dark matter clustering and covariance matrices predicted from N-body simulations. As in previous studies, we find no measurable differences between paired-and-fixed and Gaussian simulations for the average density distribution function, power spectrum, and bispectrum. Yet, the covariances from paired-and-fixed simulations are suppressed in a complicated scale- and redshift-dependent way. The situation is particularly problematic on the scales of Baryon acoustic oscillations where the covariance matrix of the power spectrum is lower by only $\sim 20{{\ \rm per\ cent}}$ compared to the Gaussian realizations, implying that there is not much of a reduction of the cosmic variance. The non-trivial suppression, combined with the fact that paired-and-fixed covariances are noisier than from Gaussian simulations, suggests that there is no path towards obtaining accurate covariance matrices from paired-and-fixed simulations – result, that is theoretically expected and accepted in the field. Because the covariances are crucial for the observational estimates of galaxy clustering statistics and cosmological parameters, paired-and-fixed simulations, though useful for some applications, cannot be used for the production of mock galaxy catalogues.

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Cosmological N-body simulations with suppressed variance

Cited by 202 publications

References 19 publications

Inverted initial conditions: Exploring the growth of cosmic structure and voids

Inverted initial conditions: Exploring the growth of cosmic structure and voids

Cosmic shear covariance matrix in wCDM: Cosmology matters

Suppressing cosmic variance with paired-and-fixed cosmological simulations: average properties and covariances of dark matter clustering statistics

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