At Quanta Magazine, a well-reported article by Jennifer Ouellette is a reminder of how arguments for the multiverse are built by stacking conjecture upon conjecture upon conjecture ("Multiverse Collisions May Dot the Sky"). Cosmologists Hiranya Peiris and Matt Johnson seek evidence in patterns in the cosmic microwave background (CMB) for past collisions of bubble universes with our own. They think there may be as many as four such patterns identifiable in the CMB. But it all depends…

Let the multiplication of wishful speculations begin. Some of those are alluded to below, indicated in bold:

Without some means of gathering experimental evidence, the multiverse hypothesis will be untestable by definition. As such, it will lurk on the fringes of respectable physics — hence the strong interest in detecting bubble collision signatures in the CMB.

Of course, "just because these bubble collisions can leave a signature doesn’t mean they do leave a signature," Peiris emphasized. "We need nature to be kind to us." An observable signal could be a rare find, given how quickly space expanded during inflation. The collisions may not have been rare, but subsequent inflation "tends to dilute away the effects of the collision just like it dilutes away all other prior ‘structure’ in the early universe, leaving you with a small chance of seeing a signal in the CMB sky," Peiris said.

"My own feeling is you need to adjust the numbers rather finely to get it to work," [Columbia University physicist Erick] Weinberg said. The rate of formation of the bubble universes is key. If they had formed slowly, collisions would not have been possible because space would have expanded and driven the bubbles apart long before any collision could take place. Alternatively, if the bubbles had formed too quickly, they would have merged before space could expand sufficiently to form disconnected pockets. Somewhere in between is the Goldilocks rate, the "just right" rate at which the bubbles would have had to form for a collision to be possible.

Researchers also worry about finding a false positive. Even if such a collision did happen and evidence was imprinted on the CMB, spotting the telltale pattern would not necessarily constitute evidence of a multiverse. "You can get an effect and say it will be consistent with the calculated predictions for these [bubble] collisions," Weinberg said. "But it might well be consistent with lots of other things." For instance, a distorted CMB might be evidence of theoretical entities called cosmic strings. These are like the cracks that form in the ice when a lake freezes over, except here the ice is the fabric of space-time. Magnetic monopoles are another hypothetical defect that could affect the CMB, as could knots or twists in space-time called textures.

Weinberg isn’t sure it would even be possible to tell the difference between these different possibilities, especially because many models of eternal inflation exist. Without knowing the precise details of the theory, trying to make a positive identification of the multiverse would be like trying to distinguish between the composition of two meteorites that hit the roof of a house solely by the sound of the impacts, without knowing how the house is constructed and with what materials.

Should a signature for a bubble collision be confirmed, Peiris doesn’t see a way to study another bubble universe any further because by now it would be entirely out of causal contact with ours. But it would be a stunning validation that the notion of a multiverse deserves a seat at the testable physics table.

The urgent agenda here is to get the multiverse notion into that "seat at the testable physics table," so that it no longer "lurk[s] on the fringes of respectable physics," recognizing that without "gathering experimental evidence," the idea "will be untestable by definition." Why the urgency?

A hypothesis with no evidence to speak of, whose supporters nevertheless feel a burning need for it to be true. What does that remind you of? Of course, it reminds you of another worldview-supporting scientific notion, that the diversity of species in biology is fully and satisfactorily explained by unguided Darwinian evolution, an extrapolation from microevolution (observable, factual) to macroevolution (speculation).

The comparison entered our head, which you might expect, but something like it also entered Ms. Ouellette’s:

Peiris acknowledges that this argument [for the multiverse] has its critics. "It can predict anything, and therefore it’s not valid," Peiris said of the reasoning typically used to dismiss the notion of a multiverse as a tautology, rather than a true scientific theory. "But I think that’s the wrong way to think about it." The theory of evolution, Peiris argues, also resembles a tautology in certain respects — "an organism exists because it survived" — yet it holds tremendous explanatory power. It is a simple model that requires little initial input to produce the vast diversity of species we see today.

A multiverse model tied to eternal inflation could have the same kind of explanatory power. In this case, the bubble universes function much like speciation. Those universes that happen to have the right laws of physics will eventually "succeed" — that is, they will become home to conscious observers like ourselves. If our universe is one of many in a much larger multiverse, our existence seems less unlikely.

The multiverse "explains" only in the sense of explaining away. Darwinian evolution "explains" only on the condition that we accept its story as true a priori.

Jennifer Ouellette doesn’t point out, but she might, that Darwinian evolution and the multiverse are key elements in the scaffolding of a materialist, reductionist picture of reality, where destiny and providence are anathema. Sibling theories, fraternal twins, Darwinism and the multiverse both seek to dissolve the common-sense intuition that there is something very special about our species, about life itself, and about our planet.

For the mediocrity principle to be true — the subject of Daniel Bakken’s current cover story for us — so must they.

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Image: Grand Prismatic Spring, Yellowstone National Park/Wikipedia.