False vacuum

thumb|upright=0.8|A [[scalar field φ (which represents physical position) in a false vacuum. The energy E is higher in the false vacuum than that in the true vacuum or ground state, but there is a barrier preventing the field from classically rolling down to the true vacuum. Therefore, the transition to the true vacuum must be stimulated by the creation of high-energy particles or through quantum-mechanical tunneling.]]

In quantum field theory, a false vacuum is a hypothetical vacuum state that is locally stable but does not occupy the most stable possible ground state. In this condition it is called metastable. It may last for a very long time in this state, but could eventually decay to the more stable one, an event known as false vacuum decay. The most common suggestion of how such a decay might happen in our universe is called bubble nucleation‌—if a small region of the universe by chance reached a more stable vacuum, this "bubble" (also called "bounce") would spread.

A false vacuum exists at a local minimum of energy and is therefore not completely stable, in contrast to a true vacuum, which exists at a global minimum and is stable.

Definition of true vs. false vacuum

A vacuum is defined as a space with as little energy in it as possible. Despite the name, the vacuum still has quantum fields. A true vacuum is stable because it is at a global minimum of energy, and is commonly assumed to coincide with the physical vacuum state in which we live. It is possible that a physical vacuum state is a configuration of quantum fields representing a local minimum but not global minimum of energy. This type of vacuum state is called a "false vacuum".

Implications

Existential threat

If our universe is in a false vacuum state rather than a true vacuum state, then the decay from the less stable false vacuum to the more stable true vacuum (called false vacuum decay) could have dramatic consequences. The effects could range from complete cessation of existing fundamental forces, elementary particles and structures comprising them, to subtle change in some cosmological parameters, mostly depending on the potential difference between true and false vacuum. Some false vacuum decay scenarios are compatible with the survival of structures like galaxies, stars, while others involve the full destruction of baryonic matter In this more extreme case, the likelihood of a "bubble" forming is very low (i.e. one in 10868 or false vacuum decay may even be impossible).

A paper by Coleman and De Luccia that attempted to include simple gravitational assumptions into these theories noted that if this was an accurate representation of nature, then the resulting universe "inside the bubble" in such a case would appear to be extremely unstable and would almost immediately collapse:

where <math>\hbar</math> 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 <math>dS</math> (de Sitter space) to <math>AdS</math> (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

A bubble wall, propagating outward at nearly the speed of light, has a finite thickness, depending on the ratio between the energy barrier and the energy gain obtained by creating true vacuum. In the case when the potential barrier height between true and false vacua is much smaller than the energy difference between vacua, the bubble wall thickness becomes comparable to the critical radius.

False vacuum decay in fiction

The hypothesis of a false vacuum decay event is occasionally deployed as a plot device in works picturing a doomsday event.

1980 by Jack L. Chalker in his science-fiction novel The Return of Nathan Brazil, the fourth book in the Well of Souls series (although not named as such in the novel).
1988 by Geoffrey A. Landis in his science-fiction short story Vacuum States
2000 by Stephen Baxter in his science fiction novel Time
2002 by Greg Egan in his science fiction novel Schild's Ladder
2002 by Liu Cixin in his science fiction short story Heard It in the Morning
2008 by Koji Suzuki in his science fiction novel Edge
2015 by Alastair Reynolds in his science fiction novel Poseidon's Wake
2018 by System Erasure in their video game ZeroRanger
2020 by Phillip P. Peterson in his science fiction novel Vakuum

References

External links

calculates the Euclidean action for the bounce solution that contributes to the false vacuum decay.

– Joel Thorarinson.