New York’s Whitney Museum of American Art has made official what had been rumored for several days, that it will build a new branch at a site in Greenwich Village that was abandoned last week by the Dia Foundation. “Meanwhile, the Museum of Modern Art, which you would think would leave well enough alone after raising $825 million to overhaul and endow its Midtown mega-complex, is also talking about another addition. This churning has to do with three trends that are forcing museums to make extraordinary, difficult bets on the future: spiraling construction costs, dizzy real-estate prices and an art market that’s gone bonkers.”