Doesn't the multiverse explain the fine-tuning of the universe?

What is fine-tuning?

Physics describes the universe with laws that contain certain numbers: the strength of gravity, the strength of the strong nuclear force, the ratio of the masses of the proton and electron, the cosmological constant, the initial distribution of matter and energy. These values are not dictated by the laws themselves. Over the last fifty years physicists have found that if many of them were even slightly different, there would be no stable stars, no chemistry, no planets and no life.

Two widely discussed examples:

• The cosmological constant, which drives the accelerating expansion of space, is vastly smaller than simple estimates from quantum theory predict. Had it been much larger, the universe would have flown apart before galaxies could form. Estimates of how finely balanced this is vary, but they are commonly put at many tens of orders of magnitude.
• The initial entropy of the universe. Roger Penrose calculated that the low-entropy state required at the beginning of a universe like ours is a target of roughly one part in 10^(10^123) of the possible initial states (The Emperor's New Mind, 1989).

The fact of fine-tuning is not a Christian invention. It is widely acknowledged by physicists, including many who do not believe in God (see Rees, Just Six Numbers). The debate is about how to explain it.

Three options

1. Physical necessity. The constants had to be this way. But no known theory requires these values, and the leading candidate for a deeper theory appears to allow a vast range of possible values.
2. Chance. We were simply very lucky. On its own this is not an explanation, given the odds involved. This is where the multiverse comes in: it multiplies the number of throws of the dice.
3. Design. A mind set the values with life in view.

Problems with the multiverse explanation

The multiverse is a serious hypothesis, discussed by serious cosmologists, and Christians should not dismiss it. But as an alternative to design it has significant weaknesses.

1. It is speculative

Other universes are, by most proposals, unobservable in principle. Some cosmologists hold that inflationary theory and string theory make a multiverse likely. Others think this pushes physics beyond what can be tested. Either way, the multiverse is a theoretical extrapolation, not an observation.

2. The generator needs fine-tuning

A multiverse does not come for free. The most popular proposal, eternal inflation combined with the many possible vacuum states of string theory, requires a mechanism with its own specific laws and conditions. Robin Collins argues that such a "universe generator" would itself need just the right ingredients to produce any life-permitting universes. The fine-tuning is pushed back one step, not removed.

3. The Boltzmann brain problem

In a vast random multiverse, small fluctuations are far more probable than large, ordered ones. A single disembodied brain, with false memories of a world, fluctuating briefly into existence should vastly outnumber observers in a large, orderly, 13.8-billion-year-old cosmos like ours. If the multiverse were the right explanation, we should expect to be such observers. Cosmologists, including atheist physicist Sean Carroll, treat this "Boltzmann brain" problem as a serious challenge for any theory that predicts it. The fact that we observe a vast, coherent, orderly universe fits design better.

4. It doesn't explain this universe

Philosophers Ian Hacking and Roger White have argued that positing many other universes does not make it any more likely that this universe, the one we actually observe, is fine-tuned. Inferring other rolls of the dice from one lucky roll is a version of what Hacking called the "inverse gambler's fallacy". (This point is debated among philosophers.)

Comparing the explanations

Why it matters

The Bible does not present the order of nature as an accident but as the faithful work of the Word through whom "all things were made" (John 1:3) and in whom "all things hold together" (Colossians 1:17). The precision of the cosmos is not only an argument. It is an invitation to worship the One whose wisdom it reflects, and who made a universe fit for creatures he wanted to know and love.