  We provide a formula for extracting the value of the rms of the linear matter
fluctuations on a scale R directly from redshift surveys data. It allows to
constrain the real-space amplitude of sigma_R without requiring any modeling of
the nature and power spectrum of the matter distribution. Furthermore, the
formalism is completely insensitive to the character of the bias function,
namely its eventual scale or non-linear dependence. By contrasting measurements
of sigma_R with predictions from linear perturbation theory, one can test for
eventual departures from the standard description of gravity on large
cosmological scales.
  The proposed estimator exploits the information contained in the 1-point
moments and 2-point correlators of the matter and galaxy density fields, and it
can be applied on cosmic scales where linear and semi-linear perturbative
approximations of the evolution of matter overdensities offer a satisfactory
description of the full underlying theory. We implement the test with N-body
simulations to quantify potential systematics and successfully show that we are
able to recover the present day value of sigma_8 `hidden' in the simulation. We
also design a consistency check to gauge the soundness of the results inferred
when the formalism is applied to real (as opposed to simulated) data. We expect
that this approach will provide a sensitive probe of the clustering of matter
when applied to future large redshift survey such as BigBOSS and EUCLID.
