Graphs of cosmological variables, with life-permitting regions shown in white. From The Fine-Tuning of the Universe for Intelligent Life by Luke Barnes. Don’t worry – you don’t have to understand these graphs to understand this post!
Recently I was a participant in a discussion on the science of fine-tuning, on Howie’s blog, Truth is Elusive. The discussion got into some questions that I thought were worth looking at in more detail.
Let’s start with the science
There are two big questions relating to universal fine-tuning:
- The science – what cosmologists have discovered and calculated.
- The explanation – why it has turned out like this. And of course, the big question for many people is whether the science points to the actions of God, or not.
Here I want to just focus on the science. Only after we’ve got a handle on the science can we reasonably begin to draw conclusions.
Cosmologists use theoretical physics to analyse how our universe developed from the beginning of the big bang, and how it would be different if the laws of physics, some of the constants and the initial conditions were different. Constants include such basic things as the strength of gravity, the mass and charge of particles like electrons and protons, and the rate of expansion of the universe.
It could have been very different!
Cosmologists have found many cases where small changes in one or more of these constants would result in a very different universe. They have expressed this in three different ways:
- If many of the constants were changed just a little, intelligent life would be impossible because the basic building blocks and environment wouldn’t be there.
- Of all the possible universes allowed by theoretical physics, a very small subset would allow intelligent life to evolve.
- Some cosmologists estimate probabilities, assuming the processes were all random. Lee Smolin has estimated the probability of stars forming to be 1 in 10^229. Roger Penrose has estimated that the probability of a low entropy universe such as ours is 1 in 10^10^123.
How well-based is the science?
Astrophysicist Luke Barnes, in the latest scientific paper I can find (2011), outlines the scientific evidence, and lists 25 scientists, mostly working in cosmology or related disciplines, who accept that the universe is fine-tuned for life.1 I have read books by Rees, Susskind, Davies and Penrose which confirm this. My summary of the scientific facts is at Science and the design of the universe.
It seems there are not many cosmologists who disagree about the basic fact of fine tuning, though a few question some aspects of it (e.g. Lee Smolin says that the formation of stars requires a high degree of fine tuning whereas Fred Adams says it does not). Here are the main objections I have found.
Couldn’t a very different form of life evolve in a different universe?
Life evolves within the limits of its environment. If we had a different universe, it is said, a different form of life may have evolved. It is hard to define the requirements for any life. We shouldn’t only consider carbon-based life.
But we are not talking here about how life could have evolved differently if conditions on earth had been different, but something much more fundamental. Many of the possible universes would be unable to produce life because they do not provide a chemistry out of which to build life. For example, Luke Barnes says:
If the strong force were weaker, the periodic table would consist of only hydrogen. We do not need a rigorous definition of life to reasonably conclude that a universe with one chemical reaction (2H → H2) would not be able to create and sustain the complexity necessary for life.
It is likewise unlikely that life could form if there were no stars and planets to provide habitat and energy, or the universe was very short-lived and re-collapsed relatively quickly, or the matter in the universe was spread so thinly that few particles would interact – all likely outcomes if different parameters were changed. Paul Davies concludes:
There is now broad agreement among physicists and cosmologists that the Universe is in several respects ‘fine-tuned’ for life …. the conclusion is not so much that the Universe is fine-tuned for life; rather it is fine-tuned for the building blocks and environments that life requires.
We don’t know enough about the probabilities to make an estimate
There are clear difficulties with estimating probabilities for fine-tuning.
- If the range of the variable is infinite, then the probability can’t sum to one as it should.
- We have no idea what the probability distribution is, and have to make an assumption (e.g. the simple assumption that it’s flat).
So, it is argued, any probability calculation is meaningless. Christian mathematician William Dembski makes this argument.
However the following must also be considered:
We don’t need calculated probabilities
Calculating numerical probabilities is only a small part of fine tuning, and I’ve only seen a few cosmologists do it. The statement “Of all the possible universes allowed by theoretical physics, a very small subset would allow intelligent life to evolve” is descriptively probabilistic, but not numerical. It doesn’t depend on any assumption of the probability distribution of an parameter.
Most cosmologists conclude fine-tuning is real without estimating the probability.
Penrose has done it
Roger Penrose, who was Professor of Mathematics at Oxford University and worked with Stephen Hawking on the science of black holes, is one cosmologist who has made such an estimate. If anyone would know about this, you would expect it to be him.
Fine tuning the probabilities
Several plausible probability distributions have been tested and found to make little difference to the result – the universe is still fine tuned. It requires quite a remarkable distribution to make our universe more probable, but as Luke Barnes says:
to significantly change the probability of a life-permitting universe, we would need a prior [probability] that centres close to the observed value, and has a narrow peak. But this simply exchanges one fine-tuning for two — the centre and peak of the distribution.
So there are significant questions about probabilities, and any estimate must make some assumptions, but so far it appears none of these questions change the conclusion much. Whichever way you look at it, the universe appears to be fine tuned.
What if the laws themselves were changed?
Most of the fine-tuning discussion centres on a couple of dozen constants. But what if the laws were different, for example, if like electromagnetic charges attracted instead of repelled, and vice versa for unlike charges? It is said that this opens up a huge number of possible universes, and who could predict whether these could evolve intelligent life?
Cosmologists have looked at some of these questions, and Barnes outlines six alternative laws that don’t allow life. But what of all the other possibilities?
There is no complete answer to this. The huge number of hypothetical universes that have been examined point towards fine-tuning. That is the best evidence available at present. For fine tuning to be disproved, the cases that haven’t been examined would have to be enormously likely to lead to intelligent life in order to change the fine tuning conclusion.
So we can say that present knowledge points to fine tuning, the rest is presently speculation.
Perhaps an explanation will be found
Cosmologist Sean Carroll, in a recent debate with William Lane Craig, suggested that at least one physical constant once thought to be finely tuned can now be seen to have a quite explicable cause. The initial expansion rate of the universe, once thought to be fine-tuned to the 60th decimal place, can be derived from the equations of relativity, he says. Thus, he argues, other cases of apparent fine tuning might also be explained in the future.
I am having trouble assessing this claim (due to my limited understanding of cosmology). Several references show that the density of the universe is fine tuned to 10^60, but I can’t find any that give the same value for the fine tuning of the expansion rate. But as the density and the expansion rate are related, this may be what Carroll means.
Nevertheless, it seems that most cosmologists agree that to explain this apparently finely tuned density requires the hypothesised phenomenon of inflation to be even more finely tuned (perhaps to a degree of 1 in 10^120). And apparently even Carroll recognises this, for he writes: “The homogeneity of the early universe, however, does represent a substantial fine-tuning …. inflation only occurs in a negligibly small fraction of cosmological histories, less than 10^-6.6×10^7.”. 2
In addition, Barnes outlines 6 issues inflation has to resolve to be able to set up a life-permitting universe, and many of these require some form of fine tuning. He concludes by quoting Hollands & Wald: “although inflationary models may alleviate the “fine tuning” in the choice of initial conditions, the models themselves create new “fine tuning” issues with regard to the properties of the scalar field”
I don’t pretend to understand all of this, I am just trying to report what the experts say. But it seems clear that Carroll’s “explanation” doesn’t change the fine tuning conclusions.
Perhaps an underlying ‘theory of everything’ will explain it all
This would be the solution most cosmologists would probably prefer – the constants are this way because the universe couldn’t have been any other way, granted the laws of physics. But cosmologists such as Rees and Susskind reject this possibility. The maths is too diverse and other possibilities can easily be described, while Barnes says that the most plausible theory of everything (string theory) requires a high degree of fine tuning itself.
But most of the universe is hostile to life!
While astrophysicists continue to search for planets which might support carbon-based life, it is clear that the vast majority of our universe is not friendly to life – e.g. inside or close to stars, the cold emptiness of deep space, etc. Doesn’t this show that our universe is only barely suitable for life?
This may be an argument against the possibility that God created this universe (though I think it would be a weak one), but it is irrelevant to the scientific fine tuning claim I am discussing here. The fine tuning claim is simply that ours is one of the few possible universes that allows intelligent life, or observers, to evolve. This claim says nothing about how much of the universe that life could inhabit.
Victor Stenger’s ‘MonkeyGod’
Physicist Victor Stenger has developed a computer program called ‘MonkeyGod’, which repeatedly calculates whether randomly chosen variables produce a life-permitting universe, or not. The results show that the universe isn’t fine-tuned after all. Stenger has written a book based on these results.
However Barnes has shown, in the paper referenced earlier and a slightly abbreviated version published in Publications of the Astronomical Society of Australia, that Stenger has grossly oversimplified and misunderstood the physics. The simulation is based on only 8 life permitting criteria, and, Barnes says, “three are incorrect, two are irrelevant, and one is insufficient. Plenty more are missing.”
Barnes concludes: “MonkeyGod is so deeply flawed that its results are meaningless.” Meanwhile, Stenger has produced his own paper on the subject but I am unable to find anywhere that it has been accepted for publication. Read more about the Stenger vs Barnes debate on this blog.
The scientific case for fine tuning seems as strong as ever. There are of course uncertainties, but the fine tuning conclusion appears to be significantly more probable and well-based than any doubts.
1. He names Barrow, Carr, Carter, Davies, Dawkins, Deutsch, Ellis, Greene, Guth, Harrison, Hawking, Linde, Page, Penrose, Polkinghorne, Rees, Sandage, Smolin, Susskind, Tegmark, Tipler, Vilenkin, Weinberg, Wheeler, Wilczek.
2. Carroll concludes: “… analysis shows that inflation doesn’t really change the underlying problem — sure, you can get our universe if you start in the right state, but that state is even more finely-tuned than the conventional Big Bang beginning.”