Why is the Cosmological Constant So Very Small?

A blatant discrepancy between theory and observation can be seen as a promising spur toward deeper understanding. But the gap between theory and measurements of the cosmological constant was too much of a good thing. If one expresses this fundamental parameter of cosmic geometry as the inverse square of a length, it is clear from observation of distant galaxies that this length is, at the very least, a billion light years. The quantum field theory of the elementary particles, however, could not allow a length much larger than $10−33 cm$—the Planck length—unless one invoked a seemingly miraculous and totally implausible cancellation of elementary‐particle parameters.