Victor Stenger is a physicist, philosopher and prolific author, and has recently published the book The Fallacy of Fine-tuning: Why the Universe is Not Designed for Us. It contains perhaps the best currently existing response to the creationist argument from fine-tuning from the perspective of physics. Now, other philosophical and mathematical responses exists, but this is a comprehensive overview of the scientific case against the fine-tuning argument. I will summarize some of the more interesting parts of Stenger’s case below by paraphrasing certain parts of the last part of the final chapter in the book (pp. 293-294) as well as mentioning other problems mentioned in other parts of the book.
1. Many proponents of the fine-tuning argument quote Stephen Hawking out of context to try and show that Hawking thinks that the expansion rate of the universe is fine-tuned. In reality, Hawking just lists this problem as a problem for the big bang theory before cosmological inflation is taken into account. When it is, the fine-tuning problem of the expansion rate goes away.
2. Many proponents of the fine-tuning argument appeal to the singularity theorem proved by Hawking and Penrose in order to try and established that the universe began in a singularity. However, a singularity would be very massive and have infinitesimal volume. This is forbidden in quantum mechanics due to Heisenberg’s uncertainty principle, which states (in one of its versions) that the uncertainty in momentum times the uncertainty in position cannot be less than a specific non-zero number. Thus, the theorem proved by Hawking and Penrose is not applicable anymore.
3. Claims about fine-tuning are made against the backdrop of our particular form of life, yet other forms of life may be possible.
4. Certain physical constants, such as the speed of light in vacuum (c), the constant in Newtons law of gravity (G) and Planck’s constant (h), are claimed to be fine-tuned, yet their values depends arbitrarily on the system of units used. In cosmology, for instance, c is usually set to 1 to make calculations easier.
5. Fine-tuning claims are made for ratio between electrons and protons, expansion rate of the universe an the mass density of the universe, but these values are set by cosmological physics and do not need to be fine-tuned.
6. The relative strength of electromagnetic and gravitational forces are not fine-tuned as this quantity cannot be defined universally, as it depends on what the mass and charge of the particles you are comparing have.
7. Proponents of the fine-tuning argument most often keep all other parameters as constants, and just varying one of them. However, change in one parameter can in many cases be compensated by a change in another parameter, making so that a larger set of possible parameters are consistent with life-permitting universes.
8. The Rare Earth hypothesis states that many parameters of the earth and our solar system needs to be fine-tuned for it to be hospitable to life. However, this does not take into account the billions and billions of planets that exist in the universe, which means that a planets that are life-permitting should occur several times.
Stenger lists many more arguments, and goes into the physics of it all at a pretty detailed level and concludes that the fine-tuning argument can be dismissed based on the science without postulating a multiverse.