An alternative fine-tuning argument

This page last updated September 23rd, 2020
Remnants of supernove

Cosmologist Luke Barnes has developed this modified form of the fine-tuning argument for the existence of God.

The argument is based on Luke’s expertise in cosmology, the science of fine-tuning and probability, and avoids some of the objections to the most common form of the argument.

He argues that a universe like ours is much more likely if there’s a God (theism) than if there’s not (naturalism). Well-established probability methods and current scientific knowledge about the universe show the existence of God is much more likely than not.

The science of fine-tuning

According to the standard models of theoretical physics, the equations which describe our universe contain 31 fundamental constants. Cosmologists have discovered that many of these constants have to be within a very narrow range for the universe to be complex and long-lived enough to support life. In other words, of all the possible universes allowed by theoretical physics, only an amazingly small number would allow life.

In science, the words “fine-tuning” have no theistic implications, but are a way of describing a fundamental scientific understanding of our universe. Almost all cosmologists accept the science that our universe is statistically very unlikely.

However philosophers have constructed theistic arguments based on the science.

Different forms of the fine-tuning argument

The Teleological argument, as argued for example by WL Craig, says that a life-permitting universe can only be the result of chance, necessity or design. It then argues that chance and necessity are very unlikely, making design the most logical option.

Cosmologist Luke Barnes, in a paper published in Ergo follows another form of the argument, based on probability. Rather than using formal logic as in WL Craig’s version, Barnes uses Bayesian probability to show that a life-permitting universe is much more probable if theism is true than if naturalism is true.

Here is a brief outline of his argument.

Testing hypotheses using probability

Essentially, there are two competing hypotheses about the universe – either naturalism is true and there is no God or anything like a god, or theism is true and there is a God or gods.

Science is built on testing hypotheses. For instance, in medicine, does a new medication lead to better results than the medication currently in use? So Luke uses the same procedures used in science to test these two hypothesis.

For this, he uses Bayes Theorem, which calculates the likelihood of each hypothesis being correct based on the cosmological evidence.

Two questions

Barnes then formulates the “big question” he wants to try to answer:

“Of all the possible ways that a physical universe could have been, is our universe what we would expect on naturalism?”

But, he says, this question is “too big”. It requires thinking about every possible physical law that can be imagined.

If we want to test our hypotheses using Bayesian probability, something more manageable is needed. For this, he looks to the standard models of the universe which represent the current scientific understandings.

  • The standard model of particle physics defines fundamental particles and forces;
  • The standard model of cosmology describes the big bang, the General Theory of Relativity and the composition of the universe.

So he formulates a more answerable “little” question that reflects the big question:

“Of all the possible ways that the fundamental constants of the standard models could have been, is our universe what we would expect on naturalism?”

The probability of our universe if naturalism is true

Theoretical physics allows the estimation of the probability that the 31 parameters of the standard models occurred by chance, as is required if naturalism is true. In fact, he says, physicists have already assessed those probabilities.

This isn’t an easy calculation, he points out (and I believe him!) but having made appropriate assumptions, it has been done. Not all the 31 parameters have life-permitting limits on their values, and in the end he uses only three:

  • the likelihood of a life-permitting value of the cosmological constant is at most 10-90;
  • the likelihood of a life-permitting value of the Higgs vacuum expectation value (more or less the probability that a Higgs particle is at a point) is at most 10-33;
  • the likelihood of life-permitting up-quark, down-quark and electron Yukawa couplings (more or less forces between fundamental particles) is at most 10-13;

Understanding these parameters isn’t really important – we can take his word for it! But putting this together, he calculates the probability of our life-permitting universe occurred by chance (i.e. if naturalism is true) is less that 10-136. This is an amazingly small probability.

The probability of our universe if theism is true

For this estimation, Luke points out that we may be able to think of many (=n) reasons why God might choose to create. If we assume that just one of these reasons is to permit life, then the probability that the universe is life-permitting is 1/n. And so he argues, unless we can think of 10136 reasons why God might create, then this probability will be much larger than the probability given naturalism.


He poses, and answers, six objections.

Perhaps there are deeper laws that will explain the fine-tuning

Luke admits this may be so – he certainly hopes cosmology will make further progress on this. But, he says, it is just as likely that further knowledge will make the fine-tuning greater and less likely under naturalism, than it is to make it more likely.

Thus this objection doesn’t change his estimate of the two probabilities.

The multiverse

The multiverse is the idea that there are zillions of universes like ours, each with different laws and parameters. It is agreed that some cosmological models would allow new universes or domains of the whole to be created, but this idea cannot be tested. Some cosmologists believe it is the best explanation of fine-tuning; others believe it is just speculation.

It is an important idea because, if it was true, it might explain fine-tuning as a result of one universe out of zillions having life permitting properties. And thus make our universe much more likely under naturalism than Luke has calculated it to be.

However, Luke points out, the “little question” he is examining requires established science to define paramters, equations and ranges, so that a probability can be calculated. This isn’t possible for a multiverse – the probability could be just about anything.

So, he argues, the multiverse does nothing to change the probability he has calculated. One day, this calculation may be possible, but right now it is speculation.

Probability and infinity

How can we estimate a probability, it is argued, when we don’t know the probability distribution and the range of possible values could be infinite?

Luke’s answer is simple. Cosmologists estimate such probabilities all the time, and the same methods can be used in this case. It is open to anyone to try to improve the estimations, but not to disallow the approach.

God Wouldn’t Fine-Tune

This objection is more theological than scientific. It is argued that an omnipotent God wouldn’t need to fine-tune – he or she could create exactly what they wanted.

Barnes addresses several versions of this objection, and finds them all irrelevant. In the end, if theism is true, all other possible sets of parameters are irrelevant. Luke’s argument allows for all sorts of possibilities in God’s actions, and none of these objections seem to change that.

Too Fine-Tuned for God?

Essentially this argument says that the universe is more fine-tuned than it needs to be to allow life to form. For example, the initial entropy of the universe is extraordinarily low, and a higher figure could still have allowed time for life to form.

So, it is argued, if God created, he wasn’t efficient. He didn’t need to make things like they are. So perhaps there is a physics-based reason for the fine-tuning after all.

Luke’s responses are:

  • This doesn’t change the probabilities. And if the objector can find a physics-based reason for some aspect of fine-tuning (e.g. the low entropy), then the probability calculation can be re-done.
  • This objection is like saying if there was enough evidence to make the probability of the fine-tuning very low given naturalism, then making the probability even lower should make naturalism more likely! It is illogical.

The probabilities here cannot be known

It is argued that we cannot have any idea what God would do, how she might do it or why, so any estimate of probability is impossible.

But Luke responds that his argument doesn’t require us to make any such guess. All it requires is that the possibility God might create is significantly greater than 10-136. Further, if this objection works, it would equally well work against anything God might do to prove his existence, even making all the stars in the Milky Way spell out the Nicene Creed. If it would be absurd to reject that kind of proof because we can’t know what God would do, it is equally absurd to reject the fine-tuning argument.


So, Luke argues, we have found that using known physics, we’d expect any universe to be lifeless if naturalism is true. But we have a life-permitting universe. So naturalism doesn’t predict what we actually experience and observe. We have answered “the little question”.

And so it seems likely that including whatever other forms of physics we might imagine (“the big question”) would lead to a similar conclusion.

Using known science and probability, the fine-tuning of the universe makes theism much more likely than naturalism.

Final comment

That, in brief, is Luke Barnes’ argument. How good is it?

Doubtless it will be discussed, both positively and negatively, so time will tell.

But it seems to me that it forms a useful part in a larger discussion of the probability that God exists. Some facts about the universe seem to make God’s existence more probable. The existence of the universe rather than nothing and the almost universal belief in objective ethics are two examples.

Other facts seem to make God’s existence less likely. The suffering and evil in the world is a significant example.

If we want to use evidence to assess whether God exists or not, we can investigate each of these many facts and look at the cumulative results. There is a test on this site that allows you to do this, using the same Bayesian probability that Luke uses here (see The probability of God test).

Read more

Photo: remnants of the gigantic (150 light years across) supernova HBH 3, thought to have exploded somewhere between 80,000 and a million years ago (NAS)

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