The Mystery of the Cosmological Constant

W hat determines the struc­ ture of space and time in the universe? According to Ein­ stein's general theory of relativity, the geometric properties of space are related to the density of energy (and momentum) in the universe. To un­ derstand the structure of spacetime, therefore, we must identify potential­ ly relevant sources of energy and evaluate their contributions to the to­ tal energy (and momentum) density. The most obvious energy sources that come to mind are ordinary mat­ ter and radiation. A much less obvi­ ous source of energy that can have an enormous impact on the structure of the universe is empty space itself: the vacuum. The notion that the vacuum can be a source of energy may seem coun­ terintuitive. But present theories of elementary particles and forces not only allow for a nonzero vacuum en­ ergy density but also strongly sug­ gest that it should have a large value. Is the vacuum energy density really as large as these theories appear to suggest it is? The answer is most emphatically no. The geometric structure of the universe is extremely sensitive to the value of the vacuum energy density. So important is this value that a con­ stant proportional to the vacuum en­ ergy density has been defined. It is called the cosmological constant. If the vacuum energy density, or equivalently the cosmological con­ stant, were as large as theories of ele­ mentary particles suggest, the uni­ verse in which we live would be dra­ matically different, with properties we would find both bizarre and un­ settling. What has gone wrong with our theories? We do not know the an-