Measuring bias from unbiased observable
  Since Kaiser introduced galaxies as a biased tracer of the underlying total
mass field, the linear galaxies bias, b(z) appears ubiquitously both in
theoretical calculations and in observational measurements related to galaxy
surveys. However, the generic approaches to the galaxy density is a non-local
and stochastic function of the underlying dark matter density and it becomes
difficult to make the analytic form of b(z). Due to this fact, b(z) is known as
a nuisance parameter and the effort has been made to measure bias free
observable quantities. We provide the exact and analytic function of b(z) which
also can be measured from galaxy surveys using the redshift space distortions
parameters, more accurately unbiased observable \beta \sigma_{\rm{gal}} = f
\sigma_8. We also introduce approximate solutions for b(z) for different
gravity theories. One can generalize these approximate solutions to be exact
when one solves the exact evolutions for the dark matter density fluctuation of
given gravity theories. These analytic solutions for b(z) make it advantage
instead of nuisance.
