Economist, economist on the wall, is there a market bubble after all?

Some of the brightest minds in finance are sounding the alarm about a stock market bubble, CNNMoney reported.

 

They aren't warning of an imminent crash, but their comments are a reminder that the current bull market -- over five years long -- can't last forever.

 

Economist Robert Shiller says valuations are at "worrisome" levels. "The United States stock market looks very expensive right now," he wrote in a recent column for The New York Times.

 

Carl Icahn believes we are riding the bubble. "We can no longer simply depend on the Federal Reserve to keep filling the punch bowl," he wrote on Tumblr.

 

Ex-Treasury secretary Robert Rubin believes low interest rates could spark another financial crisis. "The risk of excesses and the consequent instability have increased substantially," Rubin and Harvard professor Martin Feldstein wrote in an Op-Ed in The Wall Street Journal last week.