President Barack Obama made a confession Thursday at Carnegie Mellon University. “I am a science geek. I am a nerd,” he said in a speech before more than 700 people. “And I don’t make any apologies for it. It’s cool stuff.” The assertion was widely applauded at an event that marked the capstone of the White House Frontiers Conference, held at CMU and the University of Pittsburgh. And Mr. Obama spent much of Thursday afternoon demonstrating his geek credentials — though before he flew back to the White House, he voiced concern that scientific progress also presented social challenges.