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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 metastability. 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.
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:
In a 2005 paper published in Nature, as part of their investigation into global catastrophic risks, MIT physicist Max Tegmark and Oxford philosopher Nick Bostrom calculate the natural risks of the destruction of the Earth at less than 1/109 per year from all natural (i.e. non-anthropogenic) events, including a transition to a lower vacuum state. They argue that due to observer selection effects, we might underestimate the chances of being destroyed by vacuum decay because any information about this event would reach us only at the instant when we too were destroyed. This is in contrast to events like risks from impacts, , and , the frequencies of which we have adequate direct measures.
Chaotic inflation theory suggests that the universe may be in either a false vacuum or a true vacuum state. Alan Guth, in his original proposal for cosmic inflation, proposed that inflation could end through quantum mechanical bubble nucleation of the sort described above. See history of Chaotic inflation theory. It was soon understood that a homogeneous and isotropic universe could not be preserved through the violent tunneling process. This led Andrei Linde and, independently, Andreas Albrecht and Paul Steinhardt, to propose "new inflation" or "slow roll inflation" in which no tunnelling occurs, and the inflationary scalar field instead graphs as a gentle slope.
In 2014, researchers at the Chinese Academy of Sciences' Wuhan Institute of Physics and Mathematics gave an actual mathematical demonstration of the already existing idea that the universe could have been spontaneously created from nothing (no space, time, nor matter) by quantum fluctuations of a metastable false vacuum causing an expanding bubble of true vacuum.
The diagrams show the uncertainty ranges of Higgs boson and top quark masses as oval-shaped lines. Underlying colors indicate if the electroweak vacuum state is likely to be stable, merely long-lived or completely unstable for given combination of masses.
If measurements of the Higgs boson and top quark suggest that our universe lies within a false vacuum of this kind, this would imply that the bubble's effects will propagate across the universe at nearly the speed of light from its origin in space-time. A direct calculation within the Standard Model of the lifetime of our vacuum state finds that it is greater than years with 95% confidence.
where is the difference in energy between the true and false vacuums, is the unknown (possibly extremely large) surface tension of the domain wall, and is the radius of the bubble. Rewriting gives the critical radius as
A bubble smaller than the critical size can overcome the potential barrier via quantum tunnelling of to lower energy states. For a large potential barrier, the tunneling rate per unit volume of space is given by
Small bubbles of true vacuum can be inflated to critical size by providing energy, although required energy densities are several orders of magnitude larger than what is attained in any natural or artificial process. It is also thought that certain environments can catalyze bubble formation by lowering the potential barrier.
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.
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- Electronvolt (8×1012 eV). Cosmic ray collisions have been observed at and beyond energies of 5×1019 Electronvolt, 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.
Elementary particles entering the wall will likely decay to other particles or black holes. If all decay paths lead to very massive particles, the energy barrier of such a decay may result in a stable bubble of false vacuum (also known as a Fermi ball) enclosing the false-vacuum particle instead of immediate decay. Multi-particle objects can be stabilized as , although these objects will eventually collide and decay either into black holes or true-vacuum particles.
Other decay modes
Bubble nucleation
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 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
e^{-\Phi_c/\hbar},|}}
where 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.
Nucleation seeds
In general, gravity is believed to stabilize a false vacuum state, at least for transition from (de Sitter space) to (Anti-de Sitter space), while topological defects including and magnetic monopoles may enhance decay probability.
Black holes as nucleation seeds
In a study in 2015, it was pointed out that the vacuum decay rate could be vastly increased in the vicinity of black holes, which would serve as a nucleation seed. 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.
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
False vacuum decay event is occasionally used as a plot device in works picturing a doomsday event.
See also
Further reading
External links
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