Some of the brightest minds in finance are sounding the alarm about a stock market bubble.
They aren't warning of an imminent crash, but their comments should remind investors that the current bull market -- over five years long -- can't last forever.
1. Nobel Prize-winning economist Robert Shiller: Valuations at "worrisome" levels.
"The United States stock market looks very expensive right now," Robert Shiller wrote in a recent column for The New York Times.
Shiller, a Yale University professor who is often cited as one of the most influential people in economics and finance in the world, created a metric that compares stock prices with corporate profits. The metric recently climbed above 25. That level has only been surpassed three times since 1881: 1929, 1999 and 2007.
Steep market tumbles followed each instance, including the bursting of the dotcom bubble in the early 2000s. The Nasdaq still hasn't fully recovered from that meltdown.
The Yale professor sounds bewildered by the lofty valuations for the stock market, which has nearly tripled since the March 2009 bear market lows.
But none of this means it's time to sell everything. Shiller notes that his gauge is a "very imprecise timing indicator" and said the market could "remain at these valuations for years."
2. Hedge fund king Carl Icahn believes there's a bubble.
"We can no longer simply depend on the Federal Reserve to keep filling the bunch bowl," the hedge fund billionaire wrote on Tumblr last week, referring to the numerous measures the Fed has taken to stimulate the U.S. economy.
Icahn described a "dangerous financial situation" that includes challenges tied to monetary policy, unemployment and income inequality.
He also said recent comments from Fed chief Janet Yellen at the International Monetary Fund "suggest, and I agree, that we are in an asset bubble."
Still, Icahn isn't calling for an imminent crash by any means. He acknowledged a bubble might not burst for "the next one, five, ten or 20 years."
It's also important to recall that Icahn currently owns billions of dollars worth of stocks. During the second quarter he even raised his stake in eBay (Tech30) and added a new investment in , Gannett (. He still thinks there's value out there. )
3. Ex-Treasury secretary Robert Rubin: Low 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.
These financial luminaries (Feldstein served as chief economic adviser to President Ronald Reagan) didn't explicitly say whether a bubble already exists or if the Fed needs to hike rates now to prevent one.
However, they did advise the central bank to consider the possibility that the "excesses" caused by extremely low interest rates could "create financial crises."
Rubin and Feldstein pointed to record high stock prices, "dramatically" lower spreads on low-quality junk bonds and surging volumes of high-risk leveraged loans as alarming signs.
If hedge funds are holding assets that suddenly pop in a bubble, there's a risk of "contagion and snowballing effect" when they all hit the exits at the same time, the duo wrote.
Rubin should know about this threat. He was in charge of Treasury in 1998 when collapsing hedge fund Long-Term Capital Management imperiled the whole system. Ultimately Wall Street was forced to come to the rescue with a $3.6 billion industry-funded bailout.