Schemes for the possible creation of a universe in a laboratory have received a decent amount of publicity in recent years. In comparison, laboratory-based schemes for the destruction of our universe have been sadly neglected. In an effort, then, to redress this inequality, let me explain how our universe may be destroyed.
The idea depends upon the concept of a false vacuum, introduced by inflationary cosmology. Inflation suggests that there is a scalar field, the 'inflaton', whose 'equation of state' is such that a positive energy density corresponds to a negative pressure. In general relativity, a matter field with negative pressure generates a repulsive gravitational effect. Inflationary cosmology suggests that at some time in the early universe, the energy density of the universe came to be dominated by the non-zero energy density of the inflaton field. A region of the universe in this so-called false vacuum state would undergo exponential expansion until the inflaton field dropped into a lower energy state. This lower energy state is conventionally considered to be the 'true vacuum' state, the lowest energy state of the inflaton field. However, inflation works just as effectively if the transition is from one positive energy state to another, lower, positive energy state. And it is this possibility which opens up the doomsday scenario.
It was originally proposed that the false vacuum state was a local minimum of the potential energy function for the inflaton field, and that inflation ended when the state of the inflaton quantum-mechanically tunnelled through the potential barrier from the initial minimum to another, lower, minimum of the potential energy function, possibly the global minimum (true vacuum) state (see diagram). It was suggested that inflation was ended locally by this quantum tunnelling, and a bubble of the lower-energy vacuum formed, surrounded by a region of the higher-energy vacuum. The walls of the bubble then expanded outwards at the speed of light, destroying all in their path. It was subsequently thought that such 'bubble nucleation' could not explain the observed homogeneity of our own universe, and the original inflationary proposal was duly superceded by the 'new' inflationary proposal, and the 'chaotic' inflationary proposal, which both suggested that inflation could occur without the need for the false vacuum to be a local minimum of the potential, and inflation could therefore end without quantum tunnelling and bubble nucleation.
This, however, does not mean that the bubble nucleation of a lower-energy vacuum is physically impossible, it merely entails that such a process was not involved in inflation. If the current state of the inflaton field is still not the lowest energy state of that field, and if the current state is a local minimum, it may be that the current state will be ended by quantum tunnelling and bubble nucleation. Moreover, it may be that particle accelerators or laser-fusion devices of the future, may generate sufficient energy density to perturb the current state of the inflaton field out of its local minimum, and over the potential barrier into a lower-energy state. A bubble of lower-energy vacuum could thereby form in the laboratory, and propagate outwards at the speed of light, destroying all in its path.