True vacuum

In quantum field theory, a false vacuum is a metastable sector of space that appears to be a perturbative vacuum, but is unstable due to instanton effects that may tunnel to a lower energy state. This tunneling can be caused by quantum fluctuations or the creation of high-energy particles. Simply put, the false vacuum is a local minimum, but not the lowest energy state, even though it may remain stable for some time. This is analogous to metastability for first-order phase transitions.

Stability and instability of the vacuum

For decades, scientific models of our universe have included the possibility that it exists as a long-lived, but not completely stable, sector of space, which could potentially at some time be destroyed upon 'toppling' into a more stable vacuum state.[2][3][4][5][6] The Standard Model of particle physics opens the possibility of calculating from the masses of the Higgs boson and the top quark, whether the universe's present vacuum state is likely to be stable or merely long-lived.[7][8] (This was sometimes misreported as the Higgs boson "ending" the universe[12]). A 125 - 127 GeV Higgs mass seems to be extremely close to the boundary for stability (estimated in 2012 as 123.8 - 135.0 GeV[1]) but a definitive answer requires much more precise measurements of the top quark's pole mass.[1] This catastrophic bubble of "true vacuum" (per quantum models) could theoretically occur at any time or place in the universe, which means (because the bubble of "true vacuum" will expand at the speed of light) the end of such a false vacuum could occur at any time.[13] At present therefore, there are "too large uncertainties which do not allow to draw a firm conclusion on the important question whether the electroweak vacuum is indeed stable or not".[1]


If measurements of these particles suggests that our universe lies within a false vacuum of this kind, then it would imply - more than likely in many billions of years[14][Note 1] - that it could cease to exist as we know it, if a true vacuum happened to nucleate.[14]

This is because, if the Standard Model is correct, the particles and forces we observe in our universe exist as they do because of underlying quantum fields. Quantum fields can have states of differing stability, including 'stable', 'unstable', or 'metastable' (meaning, long-lived but capable of being "toppled" in the right circumstances). If a more stable vacuum state were able to arise, then existing particles and forces would no longer arise as they presently do. Different particles or forces would arise from (and be shaped by) whatever new quantum states arose. The world we know depends upon these particles and forces, so if this happened, everything around us, from subatomic particles to galaxies, and all fundamental forces, would be reconstituted into new fundamental particles and forces and structures. The universe would lose all of its present structures and become inhabited by new ones (depending upon the exact states involved) based upon the same quantum fields.

It would also have implications for other aspects of physics, and would suggest that the Higgs self-coupling λ and its βλ function could be very close to zero at the Planck scale, with "intriguing" implications, including implications for theories of gravity and Higgs-based inflation.[1]:218 A future electron-positron collider would be able to provide the precise measurements of the top quark needed for such calculations.[1]

Vacuum metastability event

Further information: Relativistic Heavy Ion Collider § Critics of high energy experiments and Safety of particle collisions at the Large Hadron Collider

A hypothetical vacuum metastability event would be theoretically possible if our universe were part of a metastable (false) vacuum in the first place, an issue that was highly theoretical and far from resolved in 1982.[2] A false vacuum is one that appears stable, and is stable within certain limits and conditions, but is capable of being disrupted and entering a different state which is more stable. If this were the case, a bubble of lower-energy vacuum could come to exist by chance or otherwise in our universe, and catalyze the conversion of our universe to a lower energy state in a volume expanding at nearly the speed of light, destroying all that we know without forewarning.[3] Chaotic Inflation theory suggests that the universe may be in either a false vacuum or a true vacuum state.

A paper by Coleman and de Luccia notes that the resulting universe would be extremely unstable and collapse almost immediately:[3]

The possibility that we are living in a false vacuum has never been a cheering one to contemplate. Vacuum decay is the ultimate ecological catastrophe; in the new vacuum there are new constants of nature; after vacuum decay, not only is life as we know it impossible, so is chemistry as we know it. However, one could always draw stoic comfort from the possibility that perhaps in the course of time the new vacuum would sustain, if not life as we know it, at least some structures capable of knowing joy. This possibility has now been eliminated.

Sidney Coleman & F. de Luccia

Such an event would be one possible doomsday event. It was used as a plot device in a science-fiction story in 1988 by Geoffrey A. Landis,[16] in 2000 by Stephen Baxter,[17] and in 2002 by Greg Egan.[18]

In theory, either high enough energy concentrations or random chance could trigger the tunneling needed to set this event in motion. However an immense number of ultra-high energy particles and events have occurred in the history of our universe, dwarfing by many orders of magnitude any events at human disposal. Hut and Rees[19] note that, because we have observed cosmic ray collisions at much higher energies than those produced in terrestrial particle accelerators, these experiments will not, at least for the foreseeable future, pose a threat to our current vacuum. Particle accelerations have reached energies of only approximately eight tera electron volts (8×1012 eV). Cosmic ray collisions have been observed at and beyond energies of 1018 eV, a million times more powerful – the so-called Greisen–Zatsepin–Kuzmin limit - and other cosmic events may be more powerful yet. Against this, John Leslie has argued[20] 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.

Bubble nucleation

In the theoretical physics of the false vacuum, the system moves to a lower energy state – either the true vacuum, or another, lower energy vacuum – through a process known as bubble nucleation.[4][5][21][22][23][24] In this, instanton effects cause a bubble to appear in which fields have their true vacuum values inside. Therefore, the interior of the bubble has a lower energy. The walls of the bubble (or domain walls) have a surface tension, as energy is expended as the fields roll over the potential barrier to the lower energy vacuum. The most likely size of the bubble is determined in the semi-classical approximation to be such that the bubble has zero total change in the energy: the decrease in energy by the true vacuum in the interior is compensated by the tension of the walls.

Joseph Lykken has said that study of the exact properties of the Higgs boson could shed light on the possibility of vacuum collapse.[25]

Expansion of bubble

Any increase in size of the bubble will decrease its potential energy, as the energy of the wall increases as the area of a sphere 4 \pi r^2 but the negative contribution of the interior increases more quickly, as the volume of a sphere \textstyle\frac{4}{3} \pi r^3. Therefore, after the bubble is nucleated, it quickly begins expanding at very nearly the speed of light. The excess energy contributes to the very large kinetic energy of the walls. If two bubbles are nucleated and they eventually collide, it is thought that particle production would occur where the walls impact.

The tunnelling rate is increased by increasing the energy difference between the two vacua and decreased by increasing the height or width of the barrier.

Gravitational effects

The addition of gravity to the story leads to a considerably richer variety of phenomena. The key insight is that a false vacuum with positive potential energy density is a de Sitter vacuum, in which the potential energy acts as a cosmological constant and the Universe is undergoing the exponential expansion of de Sitter space. This leads to a number of interesting effects, first studied by Coleman and de Luccia:[3]

Development of theories

Alan Guth, in his original proposal for cosmic inflation,[26] 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[27] and, independently, Andreas Albrecht and Paul Steinhardt,[28] to propose "new inflation" or "slow roll inflation" in which no tunnelling occurs, and the inflationary scalar field instead rolls down a gentle slope.

See also



Further reading

External links

  • Free pdf copy of ISBN 3-87144-889-3.
  • Alan Guth
  • Sten Odenwald
  • Simulation of False Vacuum Decay by Bubble Nucleation by Joel Thorarinson
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