Anthropic principle

 The anthropic principle is the principle that there is a restrictive lower bound on how statistically probable our observations of the universe are, given that we could only exist in the particular type of universe capable of developing and sustaining sentient life. Proponents of the anthropic principle argue that it explains why this universe has the age and the fundamental physical constants  necessary to accommodate conscious life, since if either had been different, we would not have been around to make observations. Anthropic reasoning is often used to deal with the notion that the universe seems to be fine tuned .

There are many different formulations of the anthropic principle. Philosopher Nick Bostrom  counts them at thirty, but the underlying principles can be divided into "weak" and "strong" forms, depending on the types of cosmological claims they entail. The weak anthropic principle (WAP), such as the one defined by Brandon  Carter , states that the universe's ostensible fine tuning is the result of selection bias (specifically survivorship  bias. Most often such arguments draw upon some notion of the multiverse for there to be a Statistical population  of universes to select from. However, a single vast universe is sufficient for most forms of the WAP that do not specifically deal with fine tuning. The strong anthropic principle (SAP), as proposed by John D. Barrow  and Frank Tipler, states that the universe is in some sense compelled to eventually have conscious and sapient life emerge  within it.

In 1961, Robert  Dicke noted that the age of the universe , as seen by living observers, cannot be random. Instead, biological factors constrain the universe to be more or less in a "golden age", neither too young nor too old. If the universe were one tenth as old as its present age, there would not have been sufficient time to build up appreciable levels of metallicity  (levels of elements besides hydrogen and helium) especially carbon , by nucleosynthesis . Small rocky planets did not yet exist. If the universe were 10 times older than it actually is, most stars would be too old to remain on the main sequence  and would have turned into White  dwarfs, aside from the dimmest red dwarfs , and stable planetary systems would have already come to an end. Thus, Dicke explained the coincidence between large dimensionless numbers constructed from the constants of physics and the age of the universe, a coincidence that inspired Dirac's varying-G theory .

Dicke later reasoned that the density of matter in the universe must be almost exactly the critical density  needed to prevent the Big crunch  (the "Dicke coincidences" argument ). The most recent measurements may suggest that the observed density of baryonic  matter, and some theoretical predictions of the amount of dark matter  account for about 30% of this critical density, with the rest contributed by a cosmological constant . Steven   Weinberg gave an anthropic explanation for this fact: he noted that the cosmological constant has a remarkably low value, some 120 orders of magnitude  smaller than the value particle physics  predicts (this has been described as the "worst prediction in physics"). However, if the cosmological constant were only several orders of magnitude larger than its observed value, the universe would suffer catastrophic inflation , which would preclude the formation of stars, and hence life.

The observed values of the dimensionless physical constant  (such as the fine-structure constant ) governing the four fundamental interaction  are balanced as if fine-tuned to permit the formation of commonly found matter and subsequently the emergence of life. A slight increase in the strong interaction  would bind the dineutron  and the diproton and convert all hydrogen in the early universe to helium; likewise, an increase in the weak interaction also would convert all hydrogen to helium. Water, as well as sufficiently long-lived stable stars, both essential for the emergence of life as we know it, would not exist. More generally, small changes in the relative strengths of the four fundamental interactions can greatly affect the universe's age, structure, and capacity for life.