Cosmologists are still asking the same questions that the first stargazers posed as they surveyed the heavens. Where did the universe come from？ What,if anything,preceded it？ How did the universe arrive at its present state,and what will be its future？ Although theorists have long speculated on the ori-gin of the cosmos,until recently they had no way to probe the universe's earlier moments to test their hypotheses. In recent years,however,researchers have identified a method for observing the universe as it was in the very first fraction of a second after the big bang. This method involves looking for traces of gravitational waves in the cosmic microwave background,the cooled radiation that has permeated the universe for nearly 15 billion years.