Some critics worry stocks' big climb isn't on solid footing

— -- There's no shortage of reasons why skeptics think stocks will peter out. The market's resilience, though, keeps proving them wrong.

More than six months after the market hit rock bottom, stocks continue to fix the damage left by the worst bear market in recent memory. The Standard & Poor's 500 is now 54% above its March low, and all three major U.S. indexes are at 2009 highs. Investors got a clear view of the market's persistence Thursday: Despite rampant worries that the market has moved up too much too fast, stocks capped a five-session run that created at least $100 billion in market value each day, as measured by the Wilshire 5000 index.

The rally continues to cost its critics in opportunities lost. But each day stocks creep higher, investors' expectations for what companies and the economy must deliver to justify the rally increase, too. Even bulls say there will need to be tangible signs the rally is being held up by real signs of recovery, including:

•Improving demand growth. There are just a few weeks left in the third quarter, and companies had better be seeing an increase in revenue, says Karl Mills of money management firm Jurika Mills and Keifer. "We need to see a case for growth," he says.

•Strengthening global economy. While U.S. companies will participate in the rebound, this recovery hinges on an upswing by foreign economies, says Doug Sandler of Riverfront Investment. There are signs that is happening: The iShares MSCI Emerging Markets exchange traded fund, which mirrors the performance of stocks in countries including China and Brazil, is up 83% since March 9.

•Justification of valuation. Companies' earnings will need to expand to keep stock prices compelling despite the run-up, says Charles Crane of Douglass Winthrop Advisors. Stocks in the S&P 500 are trading for 26 times operating earnings the previous four quarters, S&P says. That's up from 18 in September 2008 and 17 in September 2007.