A different take on the same thing: the best models we have of the risk-free returns of equity markets are unbiased random walks, or martingales. Whatever value

A different take on the same thing: the best models we have of the risk-free returns of equity markets are unbiased random walks, or martingales. Whatever value the index is at now is the best guess at what value it will be in the future, at least when absolute polynomial loss is the error function.