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Some of the most remarkable findings of modern physics and cosmology are the "cosmic coincidences," namely indications that our particular universe and its laws seem remarkably tailored for the rise of intelligent life. For example, the expansion of the universe is finely tuned to the long-term existence of the universe -- if gravitation had been very slightly stronger in the early universe, the expansion would have stopped and even reversed long ago, ending the universe in a big crunch long before any sentient creatures would have arisen. On the other hand, if gravitation had been very slightly weaker, stars and galaxies might not have formed until matter was too dispersed, leaving the universe a cold and lifeless place.

A few of these cosmic coincidences that have been noted in previous years now have reasonable explanations. For instance, the knife-edge balance of gravitation and expansion is credibly explained by the "inflation" theory of cosmology, wherein the early universe underwent an incredible expansion in the first microscopic instances of time. But many other coincidences remain inexplicable, and, if anything, recent developments in physics and astronomy have compounded these mysteries. Such paradoxes have even led some leading scientists to resurrect the "anthropic principle": the reason that we see these cosmic coincidences is a selection effect of our very existence -- if the universe weren't constructed in a very, very special way, we nor any other conceivable sentient beings wouldn't be around to discuss the issue [Barrow1986]. Other scientists view the mere fact that scientists would even suggest the anthropic principle as an indictment of the entire enterprise of modern physics [Smolin2006]. For additional discussion on the anthropic principle, see Anthropic.

It is still to early to fully understand what these "cosmic coincidences" mean, but they are fascinating in any event. Here are just a few of the coincidences that have been noted in the scientific literature:

1. Carbon resonance and the strong force. Approximately 74% of the mass in the universe is hydrogen, another 24% is helium, and all other elements comprise less than 1%. The currently understood laws of physics are dramatically successful in explaining the abundances of the "light" elements, namely hydrogen, helium, lithium and beryllium -- these were formed in the first 100 seconds or so after the big bang. The synthesis of heavier elements, beginning with carbon, was something of a mystery until 1951, when astronomer Fred Hoyle hypothesized and then discovered a "resonance" that is just energetic enough to greatly increase the time for which a triple-helium nuclear reaction could occur and produce a carbon nucleus. The energy at which this resonance occurs depends sensitively on the interplay between the strong nuclear force and the weak nuclear force. If the strong force were slightly stronger or slightly weaker (by just 1% in either direction), then the binding energies of the nuclei would be different, and the resonance would not work. In that case, there would be no carbon or any heavier elements anywhere in the universe, and thus no carbon-based life forms to contemplate this intriguing fact [Davies2007, pg. 133-138].

2. The weak force and the proton-neutron balance. Had the weak force been somewhat stronger, primordial neutrons produced in the first few seconds after the big bang would have decayed faster and less helium would have been produced. Since carbon is crucially dependent on helium for formation, there may have been little if any carbon in our universe. On the other hand, if the weak force had been somewhat weaker, this would have significantly lowered the proton-to-neutron ratio beyond its current level of six-to-one. This would have significantly reduced the amount of hydrogen in the universe, starving stars of the fuel for nuclear energy [Davies2007, pg. 142-143].

3. The electromagnetic-gravitational strength ratio. In 1974, Brandon Carter noted an interesting relationship between the ratio of the strengths of the electromagnetic and gravitational fields, which is roughly 10⁴⁰, and the properties of stars. If gravity were slightly stronger (so that the ratio is lower), all stars would be radiative rather than convective, and planets might not form. But if gravity were somewhat weaker (so that the ratio was higher), then all stars would be convective and supernovas might not happen. Since all elements from carbon on up are synthesized in stellar explosions, we might not be here to discuss the issue [Davies2007, pg. 144].

4. The proton-to-electron mass ratio. The ratio of the mass of the proton to that of the electron is approximately 1836.15, according to latest measurements. The ratio of the mass of the neutron to the mass of the proton is approximately 1.0013784. In other words, the neutron's mass is slightly more than the combined mass of a proton, an electron and a neutrino. As a result, free neutrons (neutrons that are not tied up in the nucleus of an atom) spontaneously decay with a half life of about 10 minutes. If the neutron were very slightly less massive, then it could not decay without energy input. If its mass were lower by 1%, then isolated protons would decay instead of neutrons, and very few atoms heavier than lithium could form [Davies2007, pg. 145].

5. Uniformity of the cosmic microwave background. For many years after the discovery of the cosmic microwave background radiation, measurements indicated that it was isotropic (constant in all directions), except for a well-understood effect resulting from our galaxy's motion. In 1992, scientists discovered that there is a very slight anisotropy in this radiation, roughly one part in 100,000, which is just enough to ensure the formation of stars and galaxies. If this anisotropy had been significantly smaller, the early universe would have been too smooth for stars and galaxies to have formed. It it had been significantly smaller, galaxies would have been denser, resulting in numerous stellar collisions, so that stable, long-lived stars with planetary systems would have been very rare [Davies2007, pg. 146].

6. The cosmological constant. Perhaps the most startling "cosmic coincidence" is the fine-tuning of the cosmological constant. The paradox derives from the fact that when one calculates, based on known principles of quantum mechanics, the "zero-point mass density'" or the "vacuum energy density" of the universe, focusing for the time being on the electromagnetic force, one obtains the incredible result that empty space "weighs" 10⁹³ grams per cc. The actual average mass density of the universe is 10^-28 grams per cc [Susskind2005, pg. 70-78]. Stephen Hawking quipped that this is the most spectacular failure of a physical theory in history [Davies, pg. 147]. Physicists, who have fretted over this paradox for decades, have noted that calculations such as the above involve only the electromagnetic force, and so perhaps when the contributions of the other known forces are included (bosons give rise to positive terms, whereas fermions give rise to negative terms), all terms will cancel out to exactly zero as a consequence of some unknown fundamental principle of physics. But these hopes were shattered with the 1998 discovery that the expansion of the universe is accelerating [Panek2011], which implies that the cosmological constant must be slightly positive. But this means that physicists are left to explain the startling fact that the positive and negative contributions to the cosmological constant cancel to 120-digit accuracy, yet fail to cancel beginning at the 121-st digit. Curiously, this observation is in accord with a prediction made by physicist Steven Weinberg in 1987, who argued from basic principles that the cosmological constant must be zero to within one part in roughly 10¹²⁰, or else the universe either would have dispersed too fast for stars and galaxies to have formed, or else would have recollapsed upon itself long ago [Susskind2005, pg. 80-82]. For additional discussion of the cosmological constant, see Cosmological constant.

In short, numerous features of our universe seem fantastically fine-tuned for the existence of intelligent life. This has led some writers to claim that such features constitute iron-clad "proof" that our universe was designed. But others caution that the "design" notion in the area of living organisms, for instance, has proven to be very problematic. These debates will likely continue. Thus it is best to keep an open mind.

For additional discussion, see Anthropic principle, Cosmological constant , Multiverse and Universe-beginning.

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