One of the most startling scientific discoveries of recent decades is that physics appears to be fine-tuned for life. This means that for life to be possible, certain numbers in physics had to fall within a certain, very narrow range.
One of the examples of fine-tuning which has most baffled physicists is the strength of dark energy, the force that powers the accelerating expansion of the universe. If that force had been just a little stronger, matter couldn't clump together. No two particles would have ever combined, meaning no stars, planets, or any kind of structural complexity, and therefore no life.
If that force had been significantly weaker, it would not have counteracted gravity. This means the universe would have collapsed back on itself within the first split-second—again meaning no stars or planets or life. To allow for the possibility of life, the strength of dark energy had to be, like Goldilocks's porridge, "just right."
This is just one example, and there are many others.
The most popular explanation for the fine-tuning of physics is that we live in one universe among a multiverse. If enough people buy lottery tickets, it becomes probable that somebody is going to have the right numbers to win. Likewise, if there are enough universes, with different numbers in their physics, it becomes likely that some universe is going to have the right numbers for life.
For a long time, this seemed to me the most plausible explanation of fine-tuning. However, experts in the mathematics of probability have identified the inference from fine-tuning to a multiverse as an instance of fallacious reasoning—something I explore in my new book, Why? The Purpose of the Universe. Specifically, the charge is that multiverse theorists commit what's called the inverse gambler's fallacy.
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