The Hidden Reality_ Parallel Universes and the Deep Laws of the Cosmos - Brian Greene [94]
For this line of thinking to be credible, though, we need at a bare minimum to know not only that there are bubble universes in which the cosmological constant has the right value, but also that in at least one such bubble the forces and the particles agree with what scientists in our universe have measured. We need to be sure that our universe, in all its detail, is somewhere in the landscape. This is the goal of a vibrant field called string model building. The research program amounts to hunting around the string landscape and examining possible forms for the extra dimensions mathematically, in search of universes that most resemble ours. It’s a formidable challenge, because the landscape is too large and intricate to be fully studied in any systematic way. Progress requires sharp calculational skills as well as intuition regarding which pieces to assemble—the extra-dimensional shape, its size, the field fluxes cycling its holes, the presence of various branes, and so on. Those who lead this charge combine the best of rigorous science with an artistic sensibility. To date, no one has found an example that reproduces the features of our universe exactly. But with some 10500 possibilities awaiting exploration, the consensus is that our universe has a home somewhere in the landscape.
Is This Science?
In this chapter we’ve turned a logical corner. Until now, we’ve been exploring the implications for reality, writ large, of various developments in fundamental physics and cosmology research. I delight in the possibility that copies of the earth exist in the far reaches of space, or that our universe is one of many bubbles in an inflating cosmos, or that we live on one of many braneworlds constituting a giant cosmic loaf. These are undeniably provocative and alluring ideas.
But with the Landscape Multiverse, we’ve invoked parallel universes in a different way. In the approach we’ve just followed, the Landscape Multiverse is not merely broadening our view of what might be out there. Instead, an array of parallel universes, worlds that may be beyond our ability to visit or see or test or influence, now and perhaps always, are directly invoked to provide insight into observations we make here, in this universe.
Which raises an essential question: Is this science?
*One point of language. For the most part, I use the terms “cosmological constant” and “dark energy” interchangeably. When I need a little more precision, I take the value of the cosmological constant to denote the amount of dark energy suffusing space. As noted earlier, physicists often use the term “dark energy” a bit more liberally, to mean anything that can look like or masquerade as a cosmological constant over reasonably long time scales, but might slowly change and hence not truly be constant.
*It’s also how 3D movie technology works: by suitably choosing the spatial offsets on the screen of nearly duplicate images, the filmmaker causes your brain to interpret the resulting parallaxes as different distances, creating the illusion of a 3D environment.
*If space is infinitely big, you might wonder what it means to say that the universe is larger now than it was in the past. The answer is that “larger” refers to the distances between galaxies today compared with the distances between those same galaxies in the past. The expansion of the universe means the galaxies are now farther apart, which is reflected mathematically in the universe’s scale factor being larger. In the case of an infinite universe, “larger” does not refer to the overall size of space, since once infinite always infinite. But for ease of language, I will continue to refer to the changing size of the universe, even in the case of infinite space, with the understanding that I’m referring to the changing distances between galaxies.
*The Cambridge astrophysicist George Efstathiou was also one of the early pioneers who argued strongly and convincingly for a nonzero cosmological constant.
*In Chapter 7, we will examine more thoroughly and more generally the challenges of testing theories that