Suppressing cosmic variance with pairedandfixed cosmological simulations: average properties and covariances of dark matter clustering statistics
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 pairedandfixed versus Gaussian initial conditions on the average dark matter clustering and covariance matrices predicted from Nbody simulations. As in previous studies, we find no measurable differences between pairedandfixed and Gaussian simulations for the average density distribution function, power spectrum, and bispectrum. Yet, the covariances from pairedandfixed simulations are suppressed in a complicated scale and redshiftdependent 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 nontrivial suppression, combined with the fact that pairedandfixed covariances are noisier than from Gaussian simulations, suggests that there is no path towards obtaining accurate covariance matrices from pairedandfixed 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, pairedandfixed simulations, though useful for some applications, cannot be used for the production of mock galaxy catalogues.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 August 2020
 DOI:
 10.1093/mnras/staa734
 arXiv:
 arXiv:1903.08518
 Bibcode:
 2020MNRAS.496.3862K
 Keywords:

 Dark matter;
 methods: numerical;
 largescale structure of Universe;
 dark matter;
 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 Submitted to MNRAS