The Simons Observatory: science goals and forecasts. Issue 2 (27th February 2019) - Ade, Peter; Aguirre, James; Ahmed, Zeeshan; Aiola, Simone; Ali, Aamir; Alonso, David; Alvarez, Marcelo A.; Arnold, Kam; Ashton, Peter; Austermann, Jason; Awan, Humna; Baccigalupi, Carlo; et al.