David Gross takes a look ahead at the looming concern about pension plans.    Author’s note: This article focuses on defined benefit pension plans in the private sector. To learn more about the pension challenges facing municipalities and states, check out this article by Mary Williams Walsh at The New York Times and the Federal Reserve’s analysis from December 2019.  Stock markets plummet by 20 to 30 percent. Long-term interest rates drop. Future economic growth and asset returns are…