The farther we get from the last economic collapse, the closer we get to the next economic collapse. But which bubble is next to pop? Student loans? Desert real estate? Today, we turn our untrained eye to the backbone of the U.S. stock market.

We're not talking about some fly-by-night, overhyped, boiler room penny stocks here. We're talking about the S&P 500, the big boys, the best that corporate America has to offer. A staple of even the most conservative portfolios. As terrified investors flee emerging markets, the S&P becomes an ever more popular destination of choice, for your retirement money. The S&P 500 index has been an outstanding investment since the end of the recession.

Put it all in perspective for us, Bloomberg:

The benchmark gauge for U.S. equities has risen 14 percent relative to income over the past 12 months to 16 times earnings, according to data compiled by Bloomberg. Valuations last climbed this fast in the final year of the 1990s technology bubble, just before the index began a 49 percent tumble. The rally that started in March 2009 has now outlasted the average gain since 1946, the data show.

A replay of the year just before the collapse of the tech bubble, and a rally that is, historically speaking, ripe for its end. What could go wrong?

Other bubble candidates today: central banking, Chinese mortgage lending, and the "bubble" metaphor itself.

[Bloomberg. Image via]