# the tricky business of universe counting

You can never accuse cosmologists of thinking small. To them even galaxies are like tiny little molecules and complex formulas govern the behavior of manifolds measuring hundreds of millions of light years. And if you thought that one universe was complicated and difficult enough, some cosmologists are actually tackling the idea of multiple universes branching off into infinity, or the multiverse. According to this theory, our universe is just one of countless others. Or, as two Stanford physicists are proposing, one of a very large, but ultimately finite set of recognizable classical regions of the cosmos we would define as their unique universes. The big question of course, is how exactly they came up with their numbers and what definitions they were using.

While their final tally comes in at an overwhelming 10^{1016} which is a number so big that it doesn’t really have a name of its own, you may be surprised to find that this number actually has far more to do with us than with how many distinct universes there are in the cosmos. The authors, Andrei Linde and Vitaly Vanchurin, actually tried to limit their initial results into a meaningful reference frame by creating a narrow definition of what types of universes they would count and considering how many of them could be recognized from the perspective of an observer looking out into the cosmic horizon. In their paper, the Big Bang is considered to have given birth to countless universes by creating quantum fluctuations in the fabric of space and time. As our universe grew, so did the other cosmic realms and some of them should have ended up like ours, though with a few slightly different rules of physics in low energy environments.

Its those 10^{10107} universes that the authors wanted to count since trying to incorporate strange, anisotropic, non-homogenous possibilities over the last 13.7 billion years would’ve pushed an already huge number into infinity. Of course the big question here is whether there’s any evidence that the Big Bang was a big quantum event producing a set of quantum fluctuations which sowed the seeds for new universes. So far, there’s none. Not only that, but according to the multiverse theory, the immense distances and complex physics involved in trying to find other universes would most probably make the whole process futile. That’s a big dilemma in the world of science because to be considered accurate, a theory must withstand repeated testing. Yes, we can replicate the formulas, but we also need to back the math with observations and when the observations aren’t there, the rest of the theory starts to look rather suspect.

And just to make matters more muddled, Linde and Vanchurin scrap their initial calculation at the end of their paper and focus on how many discrete universes a single observer could identify. Instead of working further with the constrains of general relativity or explaining how exactly those universes would be detected, which is probably going to be a major factor in identifying other cosmic realms, the authors state that the human brain can only handle a certain amount of information according to previous studies and that number of bits has to be the maximum number of universes an observer could identify. Every universe beyond that is considered to be irrelevant. Again, this is not a methodology that instills a great deal of confidence in the result. Why would they even bother to try and calculate the number of isomorphic, homogenous universes which expand in strict accordance to Friedmann equations if all they would end up doing is referring to a theoretical maximum of an observer’s mental capacity? Why bother to put bounds on conditions that won’t really matter for the final tally?

So let’s review. We’re given a mechanism by which new universes could form with no evidence of their actual existence. We’re taken through elaborate calculations of expansion and entropy to end up with a number for a purely hypothetical construct with rather arbitrary constraints. Then, the number is discarded in favor of taking an estimate for the capacity of a human brain and limiting the number of observable universes at that with no explanation of how we could actually see them. Oh and along the way we’re told that there could be an infinite number of universes out there but since we can’t see them or put them in a similar framework as ours, we’re just going to discard them. In the end, we’re left with an arbitrary result that seems to have very little relevance to our observable universe. Have physicists really come up with a number of universe in the multiverse as the press mentions declare in their headlines? Not really. Sure, they came up with a number but that big number doesn’t tell us anything we could verify outside a thought experiment.

See: Linde, A., Vanchurin, V. (2009). How many universes are in the multiverse? arXiv: 0910.1589v1