The title is a lie: I still worry about the multiverse.

However, the recent post by Ashutosh Jogalekar at Scientific American is a complete mischaracterization of the multiverse and what it’s all about, so I feel kind of honor-bound to correct the record. From Jogalekar’s essay:

This treads dangerously into the territory of pseudoscience and even crackpot science; after all, every New Age guru and preacher says the same thing, that the world that we can observe is somehow controlled by unobservable entities like spirits, Gods, “chakras” and the like. I can come up with an explanation right now for the simultaneous occurrence of identical thoughts in two individuals on opposite sides of the planet based on a “theory” of telepathy. Basically when you start making connections between untestable and unobservable phenomena and the real world, anything goes.

Jogalekar was responding to a ScienceNews essay by Tom Siegfried, which itself was an exegesis on a technical review article by Frank Wilczek, but we don’t need to follow every river back to its source.

Instead, let’s consider what the multiverse does — and doesn’t — mean. The multiverse is a broad term referring to the existence of other “pocket” or “parallel” universes isolated from the observable part of the total Universe since the Big Bang. These pocket universes may or may not have the same physical properties as ours; more on that shortly. In many ways, the existence of parallel universes isn’t terribly revolutionary: nobody sensible thinks our observable universe is all there is, so there are galaxies we’ll never see, and possibly rare phenomena in distant parts of the cosmos that haven’t happened in our corner. (One student in a cosmology class I taught told me this thought made her sad, knowing there are wonderful, beautiful things we’ll never observe.)

However, this cosmological multiverse is not the same thing as the “many-worlds” interpretation of quantum mechanics. (It’s a common conflation, so I don’t blame non-specialists for thinking they mean the same thing.) The latter is an attempt to reconcile the probabilistic nature of quantum theory and a deterministic worldview. The many-worlds view does indeed produce a plethora of “parallel universes”, but these exist in a kind of separate reality from ours, rather than being regions of space-time that are isolated from ours by dint of distance. One can accept the multiverse while rejecting the many-worlds interpretation, or vice-versa. (I’m not a fan of many-worlds, but that’s a topic for another day.)

The flavors of inflation

The simplest, vanilla multiverse is a natural consequence of inflation, the incredibly rapid expansion of the cosmos in the first tiny fraction of a second following the Big Bang. Inflation is something we can only see indirectly, but its explanatory power is enormous: among other things, it tells us why our Universe is nearly flat and why things look roughly the same whichever direction we observe. However, that same rapid expansion also would lead to regions of the Universe that have never been in causal contact with each other since the beginning of time. To rephrase: the expansion was so fast that these pockets were pushed apart, and nothing (light, information, matter particles) could have traveled between them in the 13.8 billion-year history of the cosmos.

If inflation is correct, then it’s nearly impossible to avoid a multiverse. It’s not a separate prediction, but a natural side-effect of a theory mooted to explain observed properties of the Universe we inhabit. That’s why I personally (reluctantly) accept the existence of pocket universes we can’t and will never be able to observe. There are alternative theories to inflation out there, and we’re still not in a position to decide between them yet. As I wrote for BBC Future,

Many cosmologists regard inflation as being the worst model we have, except for all the alternatives. Inflation’s generic properties are pretty nice, thanks to its usefulness in solving difficult problems in cosmology, but the specifics are slippery. What caused inflation? How did it begin, and when did it end?

(My money’s on inflation, but ask me again when the various observations of cosmic microwave background polarization come of age — something that should happen soon.)

Inflation by itself says little about the properties of those pocket universes: they could all have the same basic parameters as ours. However, they could also obey modified forms of the physical laws governing our corner of the cosmos. As Wilczek and others have pointed out, the multiverse could help resolve something known as the “fine-tuning problem”. If you’ll pardon another bit of self-referencing, I wrote about that in an essay for Nautilus:

Many multiverse models include the idea that the other pocket universes separate from ours might have different physical parameters, meaning they might be completely unable to support life. The bit of the cosmos we inhabit has just the right values for certain physical properties—the relative strengths of forces, for example—for life to exist. The question of why our universe seems so perfectly set up for us to be here is called the “fine-tuning” problem. A slight alteration to some parameters would result in a universe incapable of bearing life, or devoid even of stars and galaxies.

The multiverse, in other words, could help explain why our particular pocket universe is the way it is. (I’m rehashing a lot of the arguments I already made in that Nautilus piece, so I probably could’ve saved some time just by linking that. Oh well.)

What goes for fine-tuning also applies to the string theory “landscape”, which is a consequence of the lack of a single string theory. Instead, there are a huge number of possible quantum vacuums in string theory, which (at least according to some in the community) could each correspond to a possible pocket universe.

However, both the fine-tuning and landscape problem are independent of the basic multiverse hypothesis, and their  resolutions could be as well. Wilczek correctly points out that our current ideas might be due to a lack of imagination rather than excess of it. As often is the case, the existence of debate among physicists is the consequence of paucity of data.

An epistemological conundrum

As I see it, Jogalekar conflated several issues in his critique of the multiverse, some of which are fairly straightforward to address. First, the existence of pocket universes has no direct causal consequences on our corner of the cosmos. Something happening in a distant pocket universe is unobservable by us, but it also has no effect on us. That’s in stark contrast to telepathy (which has been effectively ruled out by a number of experiments) or to use a less frivolous example, dark matter (which we haven’t detected directly, but which has observable consequences in shaping the structure of the Universe). Telepathy fails as an explanation for physical phenomena, dark matter succeeds, and the status of the multiverse — whether in its vanilla or more exotic flavors — is currently uncertain.

However, that doesn’t entirely resolve the epistemological conundrum. In cosmology, unlike smaller-scale science, there’s  room for phenomena that we can’t observe, yet may still happen. That’s the case whether the multiverse is correct or not: the Universe is too big and too young for us to have seen everything, since light only travels so fast. Anyone who isn’t bothered by that hasn’t thought about it, yet to quote my college philosophy professor, “the world doesn’t conform to your wishes, schnookie-poo.” It’s important to separate the general consequences of the multiverse from the specific roles which it might play in resolving conundrums like the fine-tuning problem.