Imagine this scenario: You’re walking down a random street and suddenly feel tired, hungry, and thirsty. So, since you live in a safe and friendly neighbourhood where everyone trusts each other, you turn up the first driveway you see, open the front door without knocking, and stroll in. To your surprise, you find a living room decorated in your preferred style, with a plush couch parked in front of a giant TV that happens to be showing your favourite TV program. A bowl on the coffee table is filled with your favourite potato chips. The stereo is tuned to your favourite radio station, your favourite song is playing, and the bass and treble controls are set just as you like them. On the wall, in giant letters, in English, a sign says “Welcome! Everything is fine.”
If you had this extraordinary experience, a thought would naturally occur to you: someone knew I was coming.
This, roughly speaking, is what lies behind what is known in astronomy and cosmology as the “fine-tuning problem.” The core idea is that the universe as we see it appears ideally suited for bringing forth stars, and, along with stars, galaxies, people, ladybugs, giraffes, couches, and everything else in our cosmic living room. The known physical forces, laws, constants, and initial conditions of our universe seem delicately arranged to support the glorious life-supporting ecosystem you see from your back yard at night whenever you look up. Or, for that matter, whenever you sit on your couch watching TV and reach for your potato chips.
For example, we now know, thanks to Edwin Hubble, Fritz Zwicky, Vera Rubin, and others, that the earth is not flying all alone through a featureless inky void, if it would exist at all, because there is just the right amount of Dark Matter tucked around each galaxy to hold planets like ours in just the right type of gravitational hug. The entire universe is expanding at just the right speed to keep space flat—so that parallel lines stay parallel—without either collapsing on itself or expanding so quickly that stars, and hence us, never have a chance to begin. Like Goldilocks and her bowl of just-right stars, the force of gravity presses you into your couch just enough to allow the kind of nuclear fusion that created the atoms of your body to begin with. At the smallest scales, there’s a tiny pirouette of beryllium and helium that allows for an energetic state of carbon without which you wouldn’t have a body at all. All told, there are at least 31 physical parameters poised on a knife-edge to kit out our borrowed cosmic living room. Explaining that arrangement is the fine-tuning problem. It is one of the deepest mysteries in science, joined at the root with parallel questions in philosophy and religion.
There are a number of solutions to the fine-tuning problem with varying degrees of persuasiveness and supporting evidence. The oldest is, of course, an external agency like one or more gods or super-smart aliens existing outside the universe. Fringier versions place us inside an alien laboratory or an inconceivably advanced computer simulation.
A second class of solutions involves some form of multiverse. Back in the 70s and 80s, theories of a multiverse seemed as speculative as alien laboratories seem today. And yet the fine-tuning problem, in tandem with discoveries like the Cosmic Microwave Background, the Higgs Boson, and gravitational waves, has exerted a constant pressure that has steadily pushed the multiverse farther into the mainstream.
There are a number of multiverse models based in container theories like string theory or eternal inflation, but they tend to share a common feature. In answer to the question “why is this living room so nice?” they propose that there are many, perhaps an infinite number, of living rooms, and each is slightly different from the others. Most are not suitable for life, or perhaps one thing is slightly off. Maybe there’s a universe that has the right couch but the wrong flavour of chips, or the right chips but no TV, or the right chips and TV but the TV only gets FOX News. If we find ourselves in a living room we like, maybe it’s not because someone knew we were coming but because it’s the only living room where we would (or, rather, could) stick around to talk about it. All the other living rooms might be empty, but they became permanently inaccessible the instant we walked into this one, so we will never know.
Well, pick your solution. It is the most profound mystery in astronomy and perhaps in all of science. I have no idea what the answer will turn out to be. I’m just glad we live in a universe where strangers are welcome.
Stargazer by Dennis Vanderspek
