It has long been suggested that certain conditions prevailing in the universe encourage the development of life. These include, among others, the gravitational constant, which allows for the formation of galaxies and stars; the fine-tuned expansion rate of the universe; and the nuclear reaction rate in stars, which is responsible for creating the rich abundance of elements found throughout the universe. The notion describing this life-accommodating universe is called the anthropic cosmological principle.

Astronomers know that the abundance of carbon in today's universe was produced largely in the interiors of red giant stars, and without just the right amount of carbon, carbon-based life that's us would be impossible.

The process that creates carbon is called the "triple-alpha process." This is a nuclear reaction in which three helium atoms fuse to form carbon (helium nuclei are known as alpha particles). The force that acts to fuse the constituents of these atoms protons and neutrons is the strong nuclear force, which is one of the four fundamental forces of nature.

Physicists Heinz Oberhummer of Technical University in Vienna, Attila Csoto of Eotvos University in Budapest, and Helmut Schlattl with the Max-Plank Institute for Astrophysics, Garching, Germany, wanted to know how sensitive changes in the triple-alpha reaction rate were to variations in the nuclear force that binds particles together. How much of a change in the strong nuclear force would be needed to critically alter the production of carbon, which is so critical to producing life in the universe?

They found that, in red giant stars with low, medium, and high masses, a change of only 0.4 percent in the strong nuclear force would have made it impossible for carbon-based life to evolve. Given even this slight variation, all stars would have produced either carbon or oxygen, but not the necessary abundance of both elements so critical to organic life.