Robert Kirshner of Harvard is one of the world's most distinguished astrophysicists. So to kick off his Big Think interview, we asked the hardest-hitting astrophysics question in our arsenal: what's all that stuff up there in the night sky, anyway? Kirshner's answer begins with the Moon and rockets out to the farthest galaxies, providing a full-fledged guided tour of our universe—which, thanks to his work, we now know is expanding faster and faster.Popular science shows may have made it sound inevitable by now, but Kirshner's discovery that we inhabit an "accelerating cosmos" overturned decades of scientific assumption, and before it had been published, even prompted his own mother to ask: "You think you're smarter than Einstein?" (In a moment of sheepishness he answered, "No, Mom.") Yet Kirshner, along with one of his intrepid graduate students, ultimately stuck to his guns, and the rest is astrophysics history.