The Hidden Reality_ Parallel Universes and the Deep Laws of the Cosmos - Brian Greene [39]
Figure 3.7 Because fields can settle down to different values in different bubbles, the universes in the Inflationary Multiverse can have different physical features, even though the universes are all governed by the same fundamental physical laws.
Universes in a Nutshell
Because of their fundamental differences, the Quilted and Inflationary Multiverses might appear unrelated. The quilted variety emerges if the extent of space is infinite; the inflationary variety emerges from eternal inflationary expansion. Yet, there is a deep and wonderfully satisfying connection between them, one that brings the discussion in the previous two chapters full circle. The parallel universes arising from inflation generate their quilted cousins. The process has to do with time.
Of the many strange things Einstein’s work revealed, the fluidity of time is the hardest to grasp. Whereas everyday experience convinces us that there is an objective concept of time’s passage, relativity shows this to be an artifact of life at slow speeds and weak gravity. Move near light speed, or immerse yourself in a powerful gravitational field, and the familiar, universal conception of time will evaporate. If you’re rushing past me, things I insist happened at the same moment will appear to you to have occurred at different moments. If you’re hanging out near the edge of a black hole, an hour’s passage on your watch will be monumentally longer on mine. This isn’t evidence of a magician’s trickery or a hypnotist’s deception. The passage of time depends on the particulars—trajectory followed and gravity experienced—of the measurer.12
When applied to the entire universe, or to our bubble in an inflationary setting, this immediately raises a question: How does such malleable, custom-made time comport with the notion of an absolute cosmological time? We freely speak of the “age” of our universe, but given that galaxies are moving rapidly relative to one another, at speeds dictated by their various separations, doesn’t the relativity of time’s passage create a nightmarish accounting problem for any would-be cosmic timekeeper? More pointedly, when we speak of our universe being “14 billion years old,” are we using a particular clock to measure that duration?
We are. And a careful consideration of such cosmic time reveals a direct link between parallel universes of the inflationary and quilted varieties.
Every method we use to measure time’s passage involves an examination of change that occurs to some particular physical system. Using a common wall clock, we examine the change in position of its hands. Using the sun, we examine the change in its position in the sky. Using carbon 14, we examine the percentage of an original sample that’s undergone radioactive decay to nitrogen. Historical precedent and general convenience have led us to use the rotation and revolution of the earth as physical referents, giving rise to our standard notions of “day” and “year.” But when we’re thinking on cosmic scales, there is another, more useful, method for keeping time.
We’ve seen that inflationary expansion yields vast regions whose properties on average are homogeneous. Measure the temperature, pressure, and average density of matter in two large but separate regions within a bubble universe, and the results will agree. The results can change over time, but the large-scale uniformity ensures that, on average, the change here is the same as the change there. As an important case in point, the mass density in our bubble universe has steadily decreased over our multibillion-year history, thanks to the relentless expansion of space, but because the change has occurred uniformly, our bubble’s large-scale homogeneity has not been disrupted.
This proves important because just as the steadily decreasing amount of carbon 14 in organic matter provides a means of measuring time’s passage on earth, so the steadily decreasing mass density provides a means of measuring time’s passage across space. And because the change has happened uniformly,