When the false vacuum decays, the lower-energy true vacuum forms through a process known as
bubble nucleation. In this process,
instanton effects cause a bubble containing the true vacuum to appear. The walls of the bubble (or
domain walls) have a positive
surface tension, as energy is expended as the fields roll over the potential barrier to the true vacuum. The former tends as the cube of the bubble's radius while the latter is proportional to the square of its radius, so there is a critical size R_c at which the total energy of the bubble is zero; smaller bubbles tend to shrink, while larger bubbles tend to grow. To be able to nucleate, the bubble must overcome an energy barrier of height {{NumBlk|:|\omega \approx \frac{1}{\gamma}\sqrt{\frac{2\Phi_c}{\hbar}} e^{-\Phi_c/\hbar},|}} where \hbar is the
reduced Planck constant. As soon as a bubble of lower-energy vacuum grows beyond the critical radius defined by , the bubble's wall will begin to accelerate outward. Due to the typically large difference in energy between the false and true vacuums, the speed of the wall approaches the speed of light extremely quickly. The bubble does not produce any gravitational effects because the negative energy density of the bubble interior is cancelled out by the positive kinetic energy of the wall. although required energy densities are several orders of magnitude larger than what is attained in any natural or artificial process. Bubble wall has a finite thickness, depending on ratio between energy barrier and energy gain obtained by creating true vacuum. In the case when potential barrier height between true and false vacua is much smaller than energy difference between vacua, shell thickness become comparable with critical radius.
Nucleation seeds In general, gravity is believed to stabilize a false vacuum state, at least for transition from dS (de Sitter space) to AdS (Anti-de Sitter space), while topological defects including
cosmic strings and
magnetic monopoles may enhance decay probability. According to this study, a potentially catastrophic vacuum decay could be triggered at any time by
primordial black holes, should they exist. However, the authors note that if primordial black holes cause a false vacuum collapse, then it should have happened long before humans evolved on Earth. A subsequent study in 2017 indicated that the bubble would collapse into a primordial black hole rather than originate from it, either by ordinary collapse or by bending space in such a way that it breaks off into a new universe. In 2019, it was found that although small non-spinning black holes may increase true vacuum nucleation rate, rapidly spinning black holes will stabilize false vacuums to decay rates lower than expected for flat space-time. If particle collisions produce mini black holes, then energetic collisions such as the ones produced in the
Large Hadron Collider (LHC) could trigger such a vacuum decay event, a scenario that has attracted the attention of the news media. It is likely to be unrealistic, because if such mini black holes can be created in collisions, they would also be created in the much more energetic collisions of cosmic radiation particles with planetary surfaces or during the early life of the universe as tentative
primordial black holes. Hut and Rees note that, because
cosmic ray collisions have been observed at much higher energies than those produced in terrestrial particle accelerators, these experiments should not, at least for the foreseeable future, pose a threat to our current vacuum. Particle accelerators have reached energies of only approximately eight
tera electron volts (8×1012 eV). Cosmic ray collisions have been observed at and beyond energies of 5×1019
eV, six million times more powerful – the so-called
Greisen–Zatsepin–Kuzmin limit – and cosmic rays in vicinity of origin may be more powerful yet. John Leslie has argued that if present trends continue, particle accelerators will exceed the energy given off in naturally occurring cosmic ray collisions by the year 2150. Fears of this kind were raised by critics of both the
Relativistic Heavy Ion Collider and the
Large Hadron Collider at the time of their respective proposal, and determined to be unfounded by scientific inquiry. In a 2021 paper by Rostislav Konoplich and others, it was postulated that the area between a pair of large black holes on the verge of colliding could provide the conditions to create bubbles of "true vacuum". Intersecting surfaces between these bubbles could then become infinitely dense and form micro-black holes. These would in turn evaporate by emitting Hawking radiation in the 10 milliseconds or so before the larger black holes collided and devoured any bubbles or micro-black holes in their way. The theory could be tested by looking for the
Hawking radiation emitted just before the black holes merge. == Bubble propagation==