A bias expansion relates the density of selected tracers to the matter distribution using allowed combinations of density contrast, tidal effects, spatial derivatives and stochastic contributions. For example, illustrates a local part. The coefficients encode tracer-formation physics. Including further allowed operators makes the expansion systematically improvable on sufficiently large scales.[ref]
Large-Scale Galaxy Bias by Vincent Desjacques and others. Abstract and perturbative-bias derivation: gravitational observables and finite bias coefficients
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