Is the universe fine-tuned?
A handful of numbers could have been almost anything. They landed in a narrow band that allows atoms, stars, and chemistry.
1 min read
Fine-tuning is the claim that several fundamental constants sit within surprisingly narrow ranges, and that small changes would leave a universe with no stars, no chemistry, and no observers.
Take the cosmological constant — the energy of empty space. Much larger, and the universe would have flown apart before galaxies could form. Much more negative, and it would have recollapsed in an eyeblink. The window that allows structure is, by some estimates, staggeringly thin.
Gravity, the strong nuclear force, the ratio of particle masses, the lumpiness of the early cosmos — each has a similar story. Martin Rees gathered six of them in his book Just Six Numbers, and those are precisely the dials in our Universe Tuner.
How surprised should we be?
Not everyone agrees the tuning is remarkable. Perhaps the constants are not free to vary. Perhaps life could take forms we haven't imagined, in universes we've dismissed too quickly. Perhaps our probability intuitions simply break down when applied to a sample size of one.
Still, the mainstream view is that at least some of these coincidences are real and call for explanation — even if the explanation eventually turns out to be no explanation needed.
Change a few numbers by a few percent and the cosmos is a sterile fog. Why aren't they a few percent off?