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PARALLEL UNIVERSE PROPOSAL: Cyclic Multiverse

EXPLANATION: Collisions between braneworlds can manifest as big bang-like beginnings, yielding universes that are parallel in time.

PARALLEL UNIVERSE PROPOSAL: Landscape Multiverse

EXPLANATION: By combing inflationary cosmology and string theory, the many different shapes for string theory’s extra dimensions give rise to many different bubble universes.

PARALLEL UNIVERSE PROPOSAL: Quantum Multiverse

EXPLANATION: Quantum mechanics suggests that every possibility embodied in its probability waves is realized in one of a vast ensemble of parallel universes.

PARALLEL UNIVERSE PROPOSAL: Holographic Multiverse

EXPLANATION: The holographic principle asserts that our universe is exactly mirrored by phenomena taking place on a distant bounding surface, a physically equivalent parallel universe.

PARALLEL UNIVERSE PROPOSAL: Simulated Multiverse

EXPLANATION: Technological leaps suggest that simulated universes may one day be possible.

PARALLEL UNIVERSE PROPOSAL: Ultimate Multiverse

EXPLANATION: The principle of fecundity asserts that every possible universe is a real universe, thereby obviating the question of why one possibility—ours—is special. These universes instantiate all possible mathematical equations.

As with the metaphorical book of nature, so with the book you’re reading. In this last chapter, I’d be delighted to pull all the pieces together and answer the subject’s most essential question: Universe or multiverse? But I can’t. That’s the nature of explorations that brush the edge of knowledge. Instead, to catch a glimpse of where the multiverse concept might be headed, as well as to emphasize the essential highlights of where it now stands, here are five central questions with which physicists will continue to grapple in the years ahead.

Is the Copernican Pattern Fundamental?

Regularities and patterns, evident in observations and in mathematics, are essential to formulating physical laws. Patterns of a different sort, in the nature of the physical laws accepted by each successive generation, are also revealing. Such patterns reflect how scientific discovery has shifted humankind’s perspective on its place in the cosmic order. Over the course of nearly five centuries, the Copernican progression has been a dominant theme. From the rising and setting of the sun to the motion of constellations across the night sky to the leading role we each play in our mind’s inner world, experience abounds with clues suggesting that we’re a central hub around which the cosmos revolves. But the objective methods of scientific discovery have steadily corrected this perspective. At nearly every turn, we’ve found that were we not here, the cosmic order would hardly differ. We’ve had to give up our belief in earth’s centrality among our cosmic neighbors, the sun’s centrality in the galaxy, the Milky Way’s centrality among the galaxies, and even the centrality of protons, neutrons, and electrons—the stuff of which we’re made—in the cosmic recipe. There was a time when evidence contrary to long-held collective delusions of grandeur was viewed as a frontal assault on human worth. With practice, we’ve gotten better at valuing enlightenment.

The trek in this book has been toward what may be the capstone Copernican correction. Our universe itself may not be central to any cosmic order. Much as with our planet, star, and galaxy, our universe may merely be one among a great many. The idea that reality based on a multiverse extends the Copernican pattern and perhaps completes it is cause for curiosity. But what elevates the multiverse concept above idle speculation is a key fact that we’ve now repeatedly encountered. Scientists have not been on a hunt for ways to extend the Copernican revolution. They’ve not been plotting in darkened laboratories for ways to complete the Copernican pattern. Instead, scientists have been doing what they always do: using data and observations as a guide, they’ve been formulating mathematical