  We present a new, model-independent measurement of the clustering amplitude
of galaxies and the growth of cosmic large-scale structures from the Baryon
Oscillation Spectroscopic Survey (BOSS) 12th data release (DR12). This is
achieved by generalising harmonic-space power spectra for galaxy clustering to
measure separately the magnitudes of the density and the redshift-space
distortion terms, respectively related to the clustering amplitude of
structures, $b\sigma_8(z)$, and their growth, $f\sigma_8(z)$. We adopt a
tomographic approach with 15 redshift bins in $z\in[0.15,0.67]$. We restrict
our analysis to strictly linear scales, implementing a redshift-dependent
maximum multipole for each bin. The measurements do not appear to suffer from
systematic effects and show excellent agreement with the theoretical
predictions from the Planck cosmic microwave background analysis assuming a
$\Lambda$CDM cosmology. Our results also agree with previous analyses by the
BOSS collaboration. Furthermore, our method provides the community with a new
tool for data analyses of the cosmic large-scale structure complementary to
state-of-the-art approaches in configuration or Fourier space. Amongst its
merits, we list: it being more agnostic with respect to the underlying
cosmological model; its roots in a well-defined and gauge-invariant observable;
the possibility to account naturally for wide-angle effects and even
relativistic corrections on ultra-large scales; and the capability to perform
an almost arbitrarily fine redshift binning with little computational effort.
These aspects are all the more relevant for the oncoming generation of
cosmological experiments such as Euclid, the Dark Energy Spectroscopic
Instrument (DESI), the Legacy Survey of Space and Time (LSST), and the SKA
Project.
