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Another is that cosmic expansion means that the universe must be expanding into something outside the universe itself—a misconception that is fueled by the commonly used analogy of cosmic expansion as the surface of an inflating balloon or interior of a rising loaf of raisin bread. A
This is defined by a critical density that corresponds to about 10–26 kilograms,
we know that the overall density of the universe is equal to the critical density to within an uncertainty of about two-tenths of 1 percent.
the universe is almost exactly flat. Exactly why this is true is something of a mystery,
Quantum Void!
but even an infinitesimal moment after the initial singularity the universe is unbounded, and an infinite space springs into being all at once. The big bang happens everywhere at once in an infinite universe!
This leaves a loophole: 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.
If we travel backward in time to the very early universe, these numbers win; the energy density of the universe close to the big bang was overwhelmingly dominated by radiation, with matter and dark energy constituting trace quantities.
Thus the physics of the very early universe is the physics of elementary particles and fields,
Nevertheless, there is a crucial difference between the Higgs and a pencil or ferromagnet: the Higgs is a field, which means it takes a value at every point in space.
Think of the symmetric state of the Higgs field as an infinite number of pencils balanced on their tips, one for each point in three-dimensional space, and when the unstable Higgs symmetry breaks, all the pencils fall at once (figure 4.6).
The Higgs field, or another field with similar properties, is exactly what we need to provide the time-dependent vacuum energy necessary for inflation; the vacuum energy originates in the unstable symmetry, and the dynamics is provided by the breaking of that symmetry.
Any quantum field will produce the symmetry breaking dynamics that create inflation. It does suggest that quantum fields are the underlying reality, possibly the most fundamental of all constructs. A primitive quantum field plus time? Eternal and infinite?
The basic model for symmetry breaking in inflation is, similarly, a fundamental field like the Higgs, which undergoes symmetry breaking—an unstable symmetric equilibrium decaying into a stable state with broken symmetry—which can be modeled by exactly the same sombrero-like potential surface as a pencil falling from the vertical, or the Higgs field.
The identity of this field responsible for inflation is unknown and referred to generically as the inflaton field. The inflaton could even be the standard model Higgs, but it need not be, since any symmetry breaking transition will have the same physics.
Shortly after inflation sets in, the universe approaches an extremely simple state of almost perfect homogeneity and temperature of nearly absolute zero, described by a coherent quantum state called a Bose-Einstein condensate.
Instead, the potential energy of the field in the unstable state is turned into kinetic energy near the minimum, and the field oscillates about the minimum.
But if the inflaton field interacts with other particles such as those of the standard model, something else altogether happens.
A new universe begins!
The zero-point energy created by quantum uncertainty applies to all systems, no exceptions. This includes fields like the Higgs and inflaton, which permeate all space.
This malleable nature of who sees a vacuum and who does not is the key to how inflation sets the initial condition for the formation of structure in the universe.
I did not know this.
Gravitons, like photons or the Higgs boson, have zero-point fluctuations, and exist in a vacuum as virtual particles that can be stretched and frozen by inflationary expansion.
If Bruno was right and we are one of an infinity of worlds, we are not special since in an infinity of worlds, there will certainly be an infinity of civilizations, even an infinity of other Earths, identical to our own.
Nice story idea.
Quantum uncertainty ensures that unstable equilibriums, even in perfect isolation, have finite lifetimes.
A primitive underlying quantum field existing in spacetime is the match that lights everything else.
But for each universe in which inflation ends, there will be an infinite number of new inflating horizons generated via the self-reproduction caused by the size doubling of the inflating space.
An analogy to the eternally inflating universe is bubbles in a glass of beer (figure 7.1): 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.
Question sent to Kinney on bubble boundaries.
there should be an infinite number of universes like our own, embedded in a larger, eternally self-reproducing inflationary space-time.
More disturbing still, the inflationary multiverse lies outside our cosmic horizon, forever out of reach of any conceivable observation or experiment.
Not true if space can be compressed.
This “rolling up” of extra dimensions is called compactification.
The standard model of particle physics contains nineteen “free” parameters, which are not explained by the theory and must be determined by experiment.
Inflation provides a compelling explanation for the observed geometric flatness and overall homogeneity of the universe
Distant galaxies, which we see today receding from us at less than the speed of light, will eventually be swept out of our cosmic horizon by ever-accelerating expansion.
We may well speculate that the origin of inflation itself lies in quantum uncertainty acting on some primordial, inherently quantum gravitational state in which neither space nor time nor causality have meaning—a universe ex nihilo and uncaused. This is the explanation I favor. But can we know that this is true?
