Are we in a tech bubble? Fortune’s 40 Under 40 sound off by Robert Hackett @FortuneMagazine September 29, 2015, 8:12 AM EDT E-mail Tweet Facebook Linkedin Share icons Get ready to bust out your flask of unicorn tears. So-called “unicorns”—startups valued at $1 billion or more—are breeding like bunnies these days. Hot and bothered, venture-backed bunnies. In fact, Fortune counts some 140 of these hopeful, hopped-up monsters on our own “Unicorn List,” up from 80 when the magazine first wrote a cautionary cover story on “The Age of Unicorns” at the beginning of the year. That spike in investor spending raises an uneasy, VC-queasy question: Are we in a tech bubble? The brains here at Fortune HQ ponder this question often, to be sure. It certainly seems as though something funky is going on in the private markets. But don’t just take our word for it. Fortune opened the floor to another set of influential insiders: our new 40 Under 40. Here, then, the top young-guns of our generation weigh in: Bubble, or not?