Slow growth may do more than just signal trouble; it may cause it. Economics isn’t rocket science, but the U.S. economy is a little like a rocket. If it has enough thrust, it can escape the tug of economic gravity. Not enough and it just might go into a tailspin. Economists at the Federal Reserve and elsewhere are studying whether today’s slow growth is a precursor to an outright recession—and if so, why. It’s widely accepted that a slowly growing economy is more likely to tip into recession, for the obvious reason that it’s already too close to the line; any shock can knock it into negative territory. And today’s slow growth is at least in part a symptom of underlying problems such as consumer indebtedness, high energy prices, and the jitters induced by …..