The chief real constraint is energy. Professionals outside of economics are thinking about the economic consequences of energy resources on growth, and therefore on the current credit-based economic system, which presupposes constant growth, as Michael Hudson has demonstrated in his work. The following links question this presumption on physical grounds.
Gail the Actuary is an expert on energy and her work is often posted at the the Oil Drum. She blogs at Our Finite World. Tom Murphy is an associate professor of physics at the University of California, San Diego. His articles were reposted at the Oil Drum.
Gail the Actuary: How Limited Oil Supply can lead to a Continuing Financial Crisis
Prof. Tom Murphy: Galactic-scale energy: Part 1: [Appreciating the genuine limits to indefinite growth]