The Believing Brain - Michael Shermer [176]
4. The multidimensional string theory multiverse. A multidimensional multiverse may come about when a three-dimensional “brane” (a membranelike structure on which our universe exists) moves through higher-dimensional space and collides with another brane, the result of which is the energized creation of another universe.34 A related multiverse is derived through string theory, which by at least one calculation allows for 10500 possible worlds, all with different self-consistent laws and constants.35 That’s a 1 followed by 500 zeros possible universes (recall that 1 followed by 12 zeros is a trillion!). If true, it would be miraculous if there were not intelligent life in a number of them. Victor Stenger created a computer model that analyzes what just 100 different universes would be like under constants different from our own, ranging from five orders of magnitude above to five orders of magnitude below their values in our universe. Stenger found that long-lived stars of at least one billion years—necessary for the production of life-giving heavy elements—would emerge within a wide range of parameters in at least half of the universes in his model.36
5. Quantum foam multiverse. In this model, universes are created out of nothing, but in the scientific version of ex nihilo the nothing of the vacuum of space actually contains quantum foam, which may fluctuate to create baby universes. In this configuration, any quantum object in any quantum state may generate a new universe, each one of which represents every possible state of every possible object.37 This is Stephen Hawking’s explanation for the fine-tuning problem that he himself famously presented in the 1990s:
Why is the universe so close to the dividing line between collapsing again and expanding indefinitely? In order to be as close as we are now, the rate of expansion early on had to be chosen fantastically accurately. If the rate of expansion one second after the big bang had been less by one part in 1010, the universe would have collapsed after a few million years. If it had been greater by one part in 1010, the universe would have been essentially empty after a few million years. In neither case would it have lasted long enough for life to develop. Thus one either has to appeal to the anthropic principle or find some physical explanation of why the universe is the way it is.38
Hawking’s collaborator Roger Penrose layered on even more mystery when he noted that the “extraordinary degree of precision (or ‘fine tuning’) that seems to be required for the Big Bang of the nature that we appear to observe … is one part in 101023 at least.” Penrose suggested two pathways to an answer: either it was an act of God, “or we might seek some scientific/mathematical theory.”39 Hawking opted for the second with this explanation: “Quantum fluctuations lead to the spontaneous creation of tiny universes, out of nothing. Most of the universes collapse to nothing, but a few that reach a critical size, will expand in an inflationary manner, and will form galaxies and stars, and maybe beings like us.”40
6. The natural selection multiverse. For my money the best multiverse model is that proffered by American cosmologist Lee Smolin, who adds a Darwinian component to an evolving cosmos in which there is a “natural selection” of differentially reproducing bubble universes. Smolin thinks that, like its biological counterpart, there might be a selection from different “species” of universes, each containing different laws of nature. Universes like ours will have lots of stars, which means they will have lots of black holes that collapse into singularities, a point at which infinitely strong gravity causes matter to have infinite density and zero volume. Many cosmologists today believe