An Infinity of Worlds - Cosmic inflation and the beginning of the universe by Will Kinney
On dark energy
Nonzero vacuum energy, or something very much like it, would neatly explain the observd cosmic acceleration as well as a number of otherwise difficult to explain features of the observed universe... The current evidence for the existence of dark energy is overwhelming and comes from multiple independent sources.
Since the energy is contained in the empty space itself, dark energy does not dilute in density at all, and the total amount of dark energy in the box increases as the box expands... you can create energy out of nothing by creating more empty space inside since nothing flows across the boundary of the expanding box in the process.
On quantum uncertainty and virtual particles
Consider a hydrogen atom - an electron in orbit around a proton... Maxwell's laws of electromagnetism implied that the moving electron would radiate, losing energy in the process, and spiral into the proton. Quantum uncertainty prevents this; the closer we squeeze the electron to the proton at the center of the atom, the higher the uncertainty in its momentum, which carries the electron away again.
Imagine a microscope of hypothetically infinite magnifying power; as you use the microscope to probe shorter and shorter distance scales, the associated uncertainty in momentum (and therefore energy) becomes larger and larger, eventually exceeding the mass (via E=mc2) of the Higgs boson, or any other particle in the standard model. Once the uncertainty in energy exceeds the threshold of twice the particle mass, it becomes possible to create-out of nothing-a particle/antiparticle pair. These quantum fluctuations of the vacuum are know as virtual particles. Even a perfect vacuum is not empty but instead is populated, via quantum uncertainty, with an infinite depth of virtual objects, whose existence only becomes definite when probed at sufficiently short distances or (equivalently) high energy.
... the quantum mechanical definition of empty space is observer dependent. The simplest example of this (although not the first discovered) is the Unruh effect, proposed by William Unruh in 1976. Unruh showed that an observer in an accelerated reference frame in a vacuum would observe virtual particles as real ones in a thermal distribution with finite temperature. All inertial (i.e. unaccelerated) observers agree on the definition of a vacuum, but noninertial observers do not. Similarly in the case of a black hole, it is impossible to define what you mean by a vacuum everywhere in space at once. A "vacuum" as defined by an observer near the event horizon of a black hole appears to be a source of radiation to an observer looking down on the black hole from a distance. Likewise, what appears to be empty space to one observer in an inflating space-time is no longer empty from the view of another observer, outside that observer's cosmic horizon.
...quantum uncertainty tells us that localizing any system in a radius smaller than the Planck length will immediately form a microscopic black hole.
On multiverse
This is a curious state of affairs; in constructing a picture of the early universe that explains its current observed properties, we find that almost any model results in the prediction that inflation runs out of control forever into the future and there should be an infinite number of universes like our own, embedded in a larger, eternally self-reproducing inflationary space-time.
An analogy to the eternally inflating universe is bubbles in a glass of beer: each bubble in the beer is a universe like our own, with the edge of the bubble universe expanding outward at the speed of light. In between bubbles is vacuum-dominated, inflating space, expanding exponentially and pulling the bubble universes apart from one another faster than the speed of light.
Remarkably, each bubble universe like our own appears finite to an outside observer, but appears infinite to an observer (like us) inside the bubble. We see the "wall" of the bubble not as a boundary in space but rather as a boundary in time - the moment in our past when eternal inflation ended and classical evolution commenced.
