The Fine-Tuning of the Universe

A short list of dials, and what happens when you turn them.

The universe has a small number of fundamental constants — numbers that the equations of physics do not predict but instead take as input. The gravitational constant. The cosmological constant. The mass ratio of the electron to the proton. The strength of the strong and weak nuclear forces. A few dozen others.

You can run the equations with any values you like. Almost every set of values produces a universe that is, in a strong technical sense, boring — either it collapses on itself in a fraction of a second, or it disperses into a thin gas that never forms stars, or it forms stars but no carbon, or it forms carbon but no stable atoms longer than a day.

The values we actually observe are not in the boring region. They are in a narrow band that allows for stars, planets, chemistry, life.

How narrow the band is, exactly, is what is sometimes called the fine-tuning problem. I want to be honest about both the strength and the limits of the argument that follows from it.

A few of the dials

The cosmological constant controls the expansion rate of the universe. The observed value is, in units that physicists use, roughly \(10^{-122}\) — meaning about one part in \(10^{122}\).¹ If it were larger by even a tiny fraction of that, the universe would have flown apart before galaxies could form. If it were smaller (or negative), the universe would have collapsed before stars could form. The window for a universe that permits structure is unimaginably small.

The ratio of electromagnetic to gravitational force is approximately \(10^{39}\). Roger Penrose has argued that a change of one part in \(10^{40}\) would mean either no stars, or stars that burn out before life can form.²

The mass of the neutron is just barely larger than the mass of the proton, by about 0.1%. If neutrons were lighter than protons, all the protons in the universe would have decayed into neutrons in the first seconds after the Big Bang, and there would be no hydrogen — and so no water, no stars in the way we know them, no chemistry.³

The strong nuclear force sets the binding of atomic nuclei. A change of 2% in either direction is enough to eliminate either hydrogen or every element heavier than hydrogen. Either way, no chemistry.⁴

I could keep going. Cosmologists generally count somewhere between 20 and 30 such fine-tuned constants. Stephen Hawking, who was not a theist, wrote in A Brief History of Time that “the laws of science, as we know them at present, contain many fundamental numbers… The remarkable fact is that the values of these numbers seem to have been very finely adjusted to make possible the development of life.”⁵

What the fine-tuning argument is, and is not

The fine-tuning argument is not the claim that the universe is elegant, therefore God. It is a sharper claim than that. It says:

1. The values of the fundamental constants lie in a band whose width, relative to the range of physically possible values, is vanishingly small.
2. Outside that band, no life of any kind — not just life as we know it, not just carbon-based life, but any complex chemistry at all — is possible.
3. Therefore the values we observe demand an explanation. Either they are an extraordinary brute fact, or there is a selection mechanism, or they are the product of an intentional choice.

The argument is for (3) — that some explanation is owed. Where it goes next depends on which of the three options you think is most plausible.

The three responses

Brute fact. “It just happens to be this way.” This is a coherent position, but it asks you to accept a coincidence whose improbability, on standard accounts, exceeds anything else in human knowledge. One part in \(10^{122}\) is not a number we should let slide as brute fact without examining the alternatives.

Multiverse. “There are very many universes, with random values for the constants, and we are observing the one whose values permit observers.”

This is the most popular response in physics today. It is, I think, the strongest naturalistic move available. But it has costs. It postulates an enormous and (currently) unobservable structure — many orders of magnitude more stuff than the observable universe — to explain the observation of a single universe. It also does not, on inspection, dissolve the fine-tuning problem: the multiverse- generating mechanism itself turns out to require its own fine-tuning, in the parameters of whatever law is producing the universes.⁶

It is, in short, trading one piece of fine-tuning for another, plus a vast unobserved ontology.

Design. “The values are not random; they were chosen.” This is the theistic option. It is the simplest of the three in the technical sense — fewer postulated entities, no unobserved structure, a unification of the fine-tuning of the constants with the fact that the universe is the kind of place that produces minds capable of noticing fine-tuning.

I do not say that the design inference is forced. I say it is a serious option, and that the burden of argument that it is not the right option has not, in my reading, been met.

What this gets you, and what it does not

Even taken at full strength, the fine-tuning argument gets you only to a cosmic mind — something with the intention and the power to choose the parameters of a universe. It does not, by itself, get you to the God of the Bible. It does not name him, locate him in history, or tell you anything about whether he is the kind of being you could know.

For the rest of the case — which mind, who he is, what he has said — you have to go to history. That is what the history section is for.

But the doorway is open. Something set the dials.