Galaxy bias ($b$)

Galaxy bias describes the statistical relation between fluctuations in galaxy counts and the underlying matter density contrast. In the simplest large-scale limit, ${\delta }_g=b_1{\delta }_m$, so the corresponding deterministic power spectrum is $b_1^2{P}_m$. Different selections can have different $b_1$ because they sample different formation environments. More generally, the bias expansion includes nonlinear, tidal and stochastic terms, so a single multiplicative constant is an approximation.^[ref]