The S(igma) 8 tension at the onset of Stage-IV surveys

In 1970, Allan Sandage famously described Cosmology as "A search for two numbers". In the half-century since that description of the field was penned, as Stage III cosmic surveys come to an end and Stage-IV surveys begin taking data, the field finds itself having measured the six parameters of the concordance ΛCDM model at nearly 1% precision. However, different experiments now report different values for two of these parameters -- namely the Hubble Constant and the variance of dark matter density fluctuations, S(igma) 8 -- at varying levels of significance. This new "search for two numbers" is now a significant science driver for stage-IV cosmic surveys. I will present an overview of the current status of measurements of S(igma) 8 and why we expect the current picture will soon change significantly. I will specifically argue for the importance of galaxy clustering surveys as an intermediate arbiter between large-scale measurements from the CMB and small-scale measurements from cosmic shear surveys. The incredible precision of next-generation surveys also begets highly accurate modelling of summary statistics. I will describe the challenges associated with deriving unbiased cosmological information from new surveys and show how two seemingly disparate modelling approaches -- perturbation theory and simulations -- can be fruitfully combined to pave the way for model-building in the stage-IV era.