CARPool Covariance: Fast, unbiased covariance estimation for large-scale
  structure observables
  The covariance matrix $\boldsymbol{\Sigma}$ of non-linear clustering
statistics that are measured in current and upcoming surveys is of fundamental
interest for comparing cosmological theory and data and a crucial ingredient
for the likelihood approximations underlying widely used parameter inference
and forecasting methods. The extreme number of simulations needed to estimate
$\boldsymbol{\Sigma}$ to sufficient accuracy poses a severe challenge.
Approximating $\boldsymbol{\Sigma}$ using inexpensive but biased surrogates
introduces model error with respect to full simulations, especially in the
non-linear regime of structure growth. To address this problem we develop a
matrix generalization of Convergence Acceleration by Regression and Pooling
(CARPool) to combine a small number of simulations with fast surrogates and
obtain low-noise estimates of $\boldsymbol{\Sigma}$ that are unbiased by
construction. Our numerical examples use CARPool to combine GADGET-III $N$-body
simulations with fast surrogates computed using COmoving Lagrangian
Acceleration (COLA). Even at the challenging redshift $z=0.5$, we find variance
reductions of at least $\mathcal{O}(10^1)$ and up to $\mathcal{O}(10^4)$ for
the elements of the matter power spectrum covariance matrix on scales
$8.9\times 10^{-3}<k_\mathrm{max} <1.0$ $h {\rm Mpc^{-1}}$. We demonstrate
comparable performance for the covariance of the matter bispectrum, the matter
correlation function and probability density function of the matter density
field. We compare eigenvalues, likelihoods, and Fisher matrices computed using
the CARPool covariance estimate with the standard sample covariance estimators
and generally find considerable improvement except in cases where $\Sigma$ is
severely ill-conditioned.
