Today, I was debating my roommate about some quite existential matters such as: where should we go for lunch today?, what should we do tomorrow?, but amongst these, perhaps having the most existential flavor: what’s the nature of our universe??

The Anthropic Principle

No, this isn’t related to Anthropic, the startup.

Instead, it refers to the selection bias of our observations of the universe because we’re the ones observing it. For example, consider the age of the universe. Amongst all intelligent species that could ever have been born on a uniformly random planet, at a uniformly random point in time in the history of the universe, it must necessarily be the case that intelligent life is born in the instant where intelligent life could be supported! Otherwise, we wouldn’t be there to observe it! Thus, our observation of the age of the universe (and other intelligent life, perhaps elsewhere, or in another time, observing it) is skewed! In other words, the types of universes we are allowed to observe are precisely the ones that permit our existence. We can’t observe universes where life is unhabitable, precisely because we would not be there to observe it!

I think some physicists believe that the anthropic principle is a dangerous idea, since the usage of it could dissuade research into the very nature of our universe itself: Why does gravity behave this way? “Because if it didn’t, we wouldn’t be here to observe it, and therefore it couldn’t have been any other way”¹.

What’s the nature of our universe?

But let’s now gently ask: what’s the nature of our universe? We think that life might be rare. But of course, it’s hard to estimate how rare, since 1) we haven’t detected intelligent life and 2) we don’t really understand the conditions for life. So instead, let’s assume that human life is extraordinarily rare. I’d like to propose two hypotheses of the nature of our universe, and we’ll try to use anthropic reasoning to figure out which hypothesis is more likely.

The first hypothesis

The first hypothesis is my favorite. We shall call it the “Egg” hypothesis, or EH for short. It goes as such. We think the universe started with a big bang, but what was before the big bang? EH says that the universe doesn’t have a start or an end, but instead, it lives in a repeated cycle of big bang, expansion, collapse, and big bang, over and over and over. From the EH, we could be living in the million’th iteration of this cycle (or the gazillion’th), but we would never know for certain. However, for mathematical convenience, we’ll say that in an EH world, there are only \(n\) iterations of the big bang, where \(n\) can be arbitrarily large.

The second hypothesis

The second hypothesis is the usual: the universe started with the big bang, it will expand, and then “die” (insert your favorite death of the universe here). We will call this “Permanent death” hypothesis or PD.

Big Bang

Let’s suppose that the probability that a single cycle generates intelligent life under EH is some extraordinarily small number \(p\), and likewise for PD. Now we can return to our original question: which hypothesis should we believe? Do we live in EH or do we live in PD?

At first glance, it seems that EH should be far more likely. Why? If intelligent observers are incredibly unlikely to arise in any given cycle, then the fact that I find myself existing might be evidence that the universe had many opportunities to produce observers. In other words, perhaps my existence is evidence in favor of EH over PD.²

However, there is another way to think about it. Although intelligent life may be exceedingly improbable, it isn’t *impossible under PD. Perhaps I simply happened to live in the one universe where the unlikely occurred. If so, then the mere fact that I exist should not, by itself, make EH any more likely than PD.

So, given that you exist, how should you update your beliefs about which type of universe we live in?

As it turns out, this question has no universally accepted answer. The conclusion depends on which framework of anthropic reasoning you adopt. The two most prominent are the Self-Sampling Assumption (SSA) and the Self-Indication Assumption (SIA)!