Ben Bernanke, the former Federal Reserve chairman and one of the world’s most closely watched economic thinkers, is joining one of the most successful hedge funds as an advisor, Citadel LLC said in a statement on Thursday. Bernanke, who retired from the U.S. central bank in 2014 after eight years at its helm, will advise the $25 billion hedge fund’s teams plus its investors on developments in monetary policy, financial markets and the global economy. “Citadel is a dynamic firm with tremendously talented people and a rigorous approach to research and investing.