An increase in mortgage defaults among prime borrowers in 2009 is likely to accelerate this year, slowing the real estate recovery even as Americans become more optimistic about the economy, said Robert Shiller and Karl Case, the economists who created the S&P/Case-Shiller Home Price Index. “There will be continuing foreclosures, and not just subprime, it will be prime mortgages,” Shiller, a professor at Yale University, said in an interview. “This is creating a huge shadow inventory of homes that are still owned, but they’re going to be on the market in the next year or so.”