Most of the quantum fields that fill our universe have one, and only one, preferred state, in which they will remain for eternity. Most, but not all.
True and false gaps
In the 1970s, physicists came to appreciate the importance of a different class of quantum fields whose values prefer not to be zero, even on average. This “scalar field” is like a collection of pendulums hovering at, say, a 10-degree angle. This configuration may be the ground state: pendulums prefer this angle and are stable.
In 2012, experimenters at the Large Hadron Collider showed that a scalar field known as the Higgs field permeates the universe. In the beginning, in the hot, early universe, their pendulums pointed down. But as the cosmos cooled, the Higgs field changed state, just as water can freeze into ice, and its pendulums rose to the same angle. (This non-zero Higgs value is what gives many elementary particles the property known as mass.)
With scalar fields around, the stability of the vacuum is not necessarily absolute. Pendulums in a field can have multiple semi-stable angles and a propensity to change from one configuration to another. Theorists aren’t sure if the Higgs field, for example, has found its absolute favorite configuration: the true vacuum. Some have argued that the current state of the field, despite having persisted for 13.8 billion years, is only temporarily stable or “metastable”.
If so, the good times won’t last forever. In the 1980s, physicists Sidney Coleman and Frank De Luccia described how a false vacuum of a scalar field could “decay”. At any moment, if enough pendulums somewhere move to a more favorable angle, they will drag their neighbors to meet them and a true vacuum bubble will fly outward at nearly the speed of light. It will rewrite physics as it goes, shattering atoms and molecules in its wake. (Don’t panic. Even if our vacuum is only metastable, given its staying power so far, it will likely last for billions of years more.)
In the potential mutability of the Higgs field, physicists identified the first of a virtually infinite number of ways that nothingness could kill us all.
More problems, more gaps
As physicists have tried to fit the confirmed laws of nature into a larger whole (filling giant gaps in our understanding in the process), they have prepared candidate theories of nature with additional fields and other ingredients.
When the fields pile up, they interact, mutually influence each other’s pendulums, and establish new mutual configurations in which they like to get stuck. Physicists visualize these gaps as valleys in an undulating “energy landscape.” The different angles of the pendulum correspond to different amounts of energy, or altitudes in the energy landscape, and a field seeks to reduce its energy in the same way that a stone seeks to roll down. The deepest valley is the ground state, but the stone might rest—for a while, anyway—in a higher valley.
A couple of decades ago, the landscape exploded in scale. Physicists Joseph Polchinski and Raphael Bousso were studying certain aspects of string theory, the main mathematical framework for describing the quantum side of gravity. String theory only works if the universe has about 10 dimensions, with the extra ones rolled into shapes too small to detect. Polchinski and Bousso calculated in 2000 that these extra dimensions could be folded in many ways. Each way of folding would form a different vacuum with its own physical laws.