One use of it has been to suggest that a large axion decay constant, naively ruled out due to its prediction of an overabundance of dark matter, is allowed due to a highly tuned initial angle. Hoyle's argument for the existence of a particular resonance of carbon-12 is often considered a use of this principle. In cosmology, it is the subject of a book by Barrow and Tipler. Its name was coined by Brandon Carter, and one prominent use is Stephen Weinberg's 1987 prediction of a small cosmological constant on the basis of galaxy formation. Its weak form is a tautological statement that the universe must be compatible with our knowledge that carbon-based life exists. For 10 points, what is this controversial principle which in one strong form relies on an ensemble of universes in which we live 
One use of it has been to suggest that a large axion decay constant, naively ruled out due to its prediction of an overabundance of dark matter, is allowed due to a highly tuned initial angle. Hoyle's argument for the existence of a particular resonance of carbon-12 is often considered a use of this principle. In cosmology, it is the subject of a book by Barrow and Tipler. Its name was coined by Brandon Carter, and one prominent use is Stephen Weinberg's 1987 prediction of a small cosmological constant on the basis of galaxy formation. Its weak form is a tautological statement that the universe must be compatible with our knowledge that carbon-based life exists. For 10 points, what is this controversial principle which in one strong form relies on an ensemble of universes in which we live in a rare habitable region?
One use of it has been to suggest that a large axion decay constant, naively ruled out due to its prediction of an overabundance of dark matter, is allowed due to a highly tuned initial angle.
the anthropic principle