There exist numerous cases of what look like very strong fine-tuning in our universe. Both fundamental constants and natural laws are arranged in a way that allows for us to exist. It seems that the probability of all of these favorable conditions existing by chance is incredibly low. It has been argued that the probability of you existing in a universe as fine-tuned as ours is like the chance of you surviving a firing squad (having 10 or more soldiers firing their rifles at you at close range). If you survived a firing squad, it is argued, you should assume there was some purpose involved in this, and that it wasn't just a lucky accident.
But physicist Sabine Hossenfelder disagrees. She has a recent post in which she attempts to debunk what she calls “the myth that our universe is 'finetuned for life.'” Her attempt, however, is a complete failure.
She starts out by giving a very general armchair argument:
The general argument against the success of anthropic selection is that all evidence for the finetuning of our theories explores only a tiny space of all possible combinations of parameters. A typical argument for finetuning goes like this: If parameter X was only a tiny bit larger or smaller than the observed value, then atoms couldn’t exist or all stars would collapse or something similarly detrimental to the formation of large molecules. Hence, parameter X must have a certain value to high precision. However, these arguments for finetuning – of which there exist many – don’t take into account simultaneous changes in several parameters and are therefore inconclusive.
This is not actually correct, as quite a few scientific papers about cosmic fine-tuning and the biological sensitivity of various fundamental constants do actually take into account the effects of simultaneous changes of more than one parameter. An example is the diagram below from a recent article by physicist Luke Barnes, in which he allows us to view simultaneous changes in the strong nuclear force and the fine-structure constant, showing only a tiny area that is compatible with living creatures such as ours.
Then Hossenfelder claims to have found “counterexamples” that weaken the case for cosmic fine-tuning. But she's shooting blanks – her counterexamples are all duds.
The first “counterexample” she cites is a 2006 paper called A Universe Without Weak Interactions. She says that this paper describes a “universe that seems capable of complex chemistry and yet has fundamental particles entirely different from our own.” The paper actually talks about a universe without a weak nuclear force, one of the four fundamental forces of the universe. But the weak nuclear force has never been a very important part of arguments that the universe is fine-tuned. There are strong reasons for believing that all of the other three fundamental forces of nature (the strong nuclear force, electromagnetism, and gravitation) are very fine-tuned, but no one has claimed that the weak nuclear force is very fine-tuned.
Moreover, the 2006 paper she refers to was emphatically refuted by a later paper in 2006, a paper entitled “Problems in a Weakless Universe.” The paper concluded the following:
We point out, however, that on closer examination the proposed "weakless" universe strongly inhibits the development of life in several different ways. One of the most critical barriers is that a weakless universe is unlikely to produce enough oxygen to support life. Since oxygen is an essential element in both water, the universal solvent needed for life, and in each of the four bases forming the DNA code for known living beings, we strongly question the hypothesis that a universe without weak interactions could generate life.
So Hossenfelder's first “counterexample” doesn't do anything to undermine the case for cosmic fine-tuning. Her second “counterexample” is no better. She cites Abraham Loeb's paper “The Habitable Epoch of the Early Universe.” In that paper Loeb imagined that in the early universe, the cosmological constant (or the energy density of space) might have for a while been sufficient to bathe the whole universe with a warmth suitable for life. He imagined the following scenario:
- The Big Bang occurs 13 billion years ago.
- After about 400,000 years the universe cools enough for atoms to form.
- About 10 million years later, planets and stars form.
- For a few million years it is warm enough for life to exist, because of the cosmological constant, which fills all of space with a pleasant warmth.
- Microbial life forms appear during this relatively brief period.
- A few million years later, the continued expansion of the universe causes the universe to cool sufficiently so that the cosmological constant is no longer sufficient to keep space warm.
- Any microbial life that may have arisen from the warmth of the cosmological constant then dies, as temperatures fall far below freezing.
But this scenario is nothing like an alternate way for the universe to be life-friendly, because this “habitable epoch of the early universe” lasts way too short a time (as the expansion of the universe quickly causes those early warm temperatures to fade away, being replaced by deadly cold). It actually lasts (under Loeb's assumptions, as discussed here) only about two million years, as Loeb admits by saying that this epoch would last only “a few Myr,” using an abbreviation for megayears (a million years). Since there was supposedly billions of years (thousands of millions of years) between the appearance of earthly microbes and the appearance of man, it is clear that two million years is not long enough for any intelligent life to evolve. It is almost certainly not a sufficient time for any life at all to develop. So the possibility discussed by Loeb is irrelevant.
When discussing whether the universe is fine-tuned to allow for intelligent life, we don't care whether there might have been some brief two-million year window in the very early universe (a short-lived period of warmth) that might have allowed mere microbes to appear before being wiped out when the universe becomes super-cold again. We care about the possibility of intelligent life appearing. In fact, it is overwhelmingly likely that the higher radiation and asteroid concentrations in any early universe would not even allow microbes to have appeared in the early universe.
Loeb's paper (discussing something both extremely improbable and irrelevant) did nothing to upset the idea that the cosmological constant is fine-tuned. The fact that the cosmological constant is enormously fine-tuned is reaffirmed by a recent scientific paper noting that the cosmological constant is 10123 times smaller than its “natural value,” and that “there is no satisfactory solution yet for this problem.” The paper's graphs suggest that there would no observers if the cosmological constant were a few hundred times smaller or larger. Given the natural value so much larger, having a cosmological constant within this range is like hitting the exact center of a target 1000 yards distant.
So Hossenfelder's second “counterexample” is a dud that does nothing to undermine the case for cosmic fine-tuning. Hossenfelder's third “counterexample” is no better. She cites a 2016 paper by Adams and Grohs discussing the “triple alpha process,” a case in which nuclear physics has to be just right in order for carbon to be produced, because of what is called a resonance fine-tuning. The authors imagine other universes that might not require this particular type of resonance fine-tuning.
Hossenfelder claims that this paper is a “demonstration that a chemistry complex enough to support life can arise under circumstances that are not anything like the ones we experience today.” That's wrong, because the paper actually describes universes very much like ours, but in which there are small changes in fundamental constants. And also, the paper does not actually describe an alternate universe with a chemistry complex enough to support life, because it fails to describe a universe in which oxygen is produced in sufficient quantities. This is made clear on page 26 of the paper, in which the authors say, “This set of simulations does not include nuclear reactions that produce oxygen, neon, and heavier elements.”
It has long been recognized that the fundamental constants have to be just right for nuclear reactions in stars to produce large quantities of both oxygen and carbon, both of which are requirements for life. By failing to discuss alternate reactions in which stars produce oxygen, the paper of Adams and Grohs does nothing to undermine that fine-tuning requirement. The fine-tuning requirement is stated in a 2014 scientific paper which tells us on page 16 that in order for you to have abundant quantities of oxygen and carbon, you need for the quark masses to be within 2 to 3 percent of their current values, and you also need for the fine-structure constant to be within 2.5% of its current value. You could therefore say nature has to hit two different “holes in one,” and these aren't the only “holes in one” nature has to hit in order to end up with intelligent life.
In short, all of Hossenfelder's “counterexamples” are empty duds. She has done nothing whatsoever to weaken the case that the universe is fine-tuned. Far from being a “myth” as she claims, the finding that the universe is incredibly fine-tuned for life is one of the fundamental achievements in the past 50 years of physics, and is something that has been acknowledged by many physicists and cosmologists.
As all of her “counterexamples” are failures, Hossenfelder doesn't actually succeed in providing even a single example of some alternate universe as life-friendly as ours. Even if she were to provide such a thing, it would do nothing to discredit the claim that our universe is fine-tuned for life. Arguments about cosmic fine-tuning never claim that there is only one possible universe consistent with life, but merely claim that it is incredibly improbable that any particular random universe would be compatible with the appearance of intelligent life. There are many possible universes that would allow intelligent life to appear, but the set of all possible universes that would allow intelligent life to appear is almost infinitely smaller than the set of all possible universes, making it almost infinitely improbable that any particular random universe would accidentally meet the many requirements for intelligent life. You do not damage such reasoning in the least by showing a few other possible universes that might allow intelligent life to appear.
It's rather like this. A man may point to a car, point out its fitness for a purpose, and say, “Wow, that sure is fine-tuned” or “that sure wasn't produced by some set of accidents.” You do not at all discredit such reasoning by demonstrating that there are other possible cars with a very different appearance.