  Multiple tracers of the cosmic density field, with different bias, number and
luminosity evolution, can be used to measure the large-scale properties of the
Universe. We show how an optimal combination of tracers can be used to detect
general-relativistic effects in the observed density of sources. We forecast
for the detectability of these effects, as well as measurements of primordial
non-Gaussianity and large-scale lensing magnification with current and upcoming
large-scale structure experiments. In particular we quantify the significance
of these detections in the short term with experiments such as the Dark Energy
Survey (DES), and in the long term with the Large Synoptic Survey Telescope
(LSST) and the Square Kilometre Array (SKA). We review the main observational
challenges that must be overcome to carry out these measurements.
