This is something that I have been watching for...with the Federal Reserve beginning to pull back QE (Quantitative Easing) and actually raising interest rates above zero the economy may not be able to sustain any real expansion for much longer. For most of the past few years it seems like most of the economic growth has come from abnormally low borrowing costs and share buybacks from companies, which are artificially inflating stock market prices.
Eventually this "financial heroin" has to run out, and when it does the stock market should begin to come down in earnest, possibly touching off a major recession like we had in 2008. After the initial downdraft the Fed will likely react with "accommodative" monetary policies again, which should be good for gold/silver and possibly bitcoin as well.