Earlier, I pointed out that a bubble could be hard to identify. Indeed, I think this is especially true of bubbles in the stock market. Central banks or government officials are unlikely to have an informational advantage over market participants. If a central bank were able to identify bubbles in the stock market, wouldn't market participants be able to do so as well?
If so, then a bubble would be unlikely to develop, because market participants would know that prices were getting out of line with fundamentals.