What Can We Learn by Combining the Skew Spectrum and the Power Spectrum?
  Clustering of the large scale structure provides complementary information to
the measurements of the cosmic microwave background anisotropies through power
spectrum and bispectrum of density perturbations. Extracting the bispectrum
information, however, is more challenging than it is from the power spectrum
due to the complex models and the computational cost to measure the signal and
its covariance. To overcome these problems, we adopt a proxy statistic, skew
spectrum which is a cross-spectrum of the density field and its quadratic
field. By applying a large smoothing filter to the density field, we show the
theory fits the simulations very well. With the spectra and their full
covariance estimated from $N$-body simulations as our "mock" Universe, we
perform a global fits for the cosmological parameters. The results show that
adding skew spectrum to power spectrum the $1\sigma$ marginalized errors for
parameters $ b_1^2A_s, n_s$ and $f_{\rm NL}^{\rm loc}$ are reduced by $31\%,
22\%, 44\%$, respectively. This is the answer to the question posed in the
title and indicates that the skew spectrum will be a fast and effective method
to access complementary information to that enclosed in the power spectrum
measurements, especially for the forthcoming generation of wide-field galaxy
surveys.
