By adding a shock wave to the explosive origins of our universe, two mathematicians have come up with an alternative scenario for the Big Bang theory. In a paper in the most recent issue of Physical Review D, mathematics professors Joel Smoller, University of Michigan, and Blake Temple, University of California, Davis, linked two fundamental solutions of Einstein's gravitational equations in an exact solution with a shock wave. With the addition of a shock wave, the task of reconstructing the origin of the universe becomes much less certain for astronomers. A shock-wave explosion adds disorder to what cosmology theory has assumed was an orderly expanding universe. In addition, a shock wave removes the mathematical certainty that the entire universe burst from a single point. "This is the first physically meaningful exact solution of Einstein's equations that has a shock wave in it, and thus it is the first solution that is time irreversible," Temple says. "If someone doesn't like the idea that the universe burst from a single point, they could use this as a model for exploring another scenario for how the universe came to be." The theory of modern cosmology is based on a formula for an expanding universe that can be traced back to a single point of origin. In other words, before the Big Bang, nothing existed except a single point, according to a classic 1922 solution to Albert Einstein's gravitational equations. That solution models an explosion with no shock wave. In this well-established scenario, the Big Bang created all the energy and matter in the universe, which led to the formation of galaxies and eventually, by other processes, to the birth of our solar system and planet Earth. Smoller and Temple's solution also describes an expanding universe, but the shock wave opens the door for other plausible scenarios for our universe's explosive origins. The mathematicians began with the traditional solution now used to describe the Big Bang and added two other well-established mathematical parts. If the expanding universe began with an explosion, they reasoned, it likely would have generated a shock wave blasting outward. After all, as Temple says, "Have you ever heard an explosion without a shock wave before?" In the Smoller and Temple solution, the newly incorporated shock wave forms the outer edge of this expanding universe, much like the surface of a growing balloon being inflated. The third major component in the shock-wave solution comes from a well-known exact solution to Einstein's equations, developed in the late 1930s. It describes a nonexpanding universe beyond the shock-wave boundary. The shock wave links the two half-century-old exact solutions of the Einstein equations. (The new mathematical solution by Smoller and Temple also addresses a problem left unsolved in a famous 1939 paper of J. Robert Oppenheimer and his student Harlow Snyder.) The traditional solution to Einstein's equation upon which much of modern cosmology is based not only excludes a shock wave from the explosion, it also ignores "entropy," which in this case refers to the increasing disorder that accompanies a shock-wave explosion. This has implications for the problem of reconstructing the details about the origin of the universe. "Even though our solution shows a universe expanding inside the shock wave, we cannot say that one can trace the solution back to the usual Big Bang scenario," Smoller says. "Because the shock wave increases entropy, the solution is time irreversible, and this implies that information about the past has been fundamentally lost. A lot of things could have happened." Einstein's equations describe gravity problems on a grander scale than does Newton's 17th-century Earth-bound physics. In a similar way, Temple and Smoller's relativistic shock-wave solution applies to the grand scale of cosmology and, encouragingly, is analogous to a well-known, nonrelativistic scenario for star formation. This connection suggests to Temple and Smoller that their shock-wave explosion could have come about by an analogous process but on a grander scale. This research was funded by a Guggenheim fellowship (Temple), by the National Science Foundation, the U.S. Office of Naval Research and the UC Davis Institute of Theoretical Dynamics.