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to point out that one is dealing here with an evolutionary process akin to that in biology. According to Charles Darwin, the immense multitude of species of life on earth emerged through evolution founded, in simple terms, on the two principles of mutation and selection. Mutation means that offspring differ slightly from their parents; selection then unflinchingly chooses those individuals with the best properties for a given niche and strikes out others. This happens not haphazardly, but by an apparently rational process: In this sense, the most successful properties are those that promise the individual the highest amount of offspring making it into adulthood, for this ensures the survival of the individual’s genetic “self” beyond its own death.

Space-time could be ruled by similar principles. If properties of each new daughter universe change slightly, one has mutation. Selection occurs, too, because the reproduction rate via black holes in a daughter universe is subject to the physical laws that are valid there. (In this context, selection is understood in a statistical rather than existential sense, since individual universes cannot directly compete for resources. Rather, they compete for the statistical dominance of their properties in the multitude of all daughter universes.) For instance, the Chandrasekhar limit, the maximal mass of a white dwarf, depends on constants of nature such as Planck’s constant of quantum theory or Newton’s constant of gravity. When this limit is lowered by changing the constants, black holes could form at even smaller masses. Daughter universes with suitable parameter values—values that lead more easily to black hole formation—will produce more granddaughter universes than others and dominate as matrons in the metaphysical cosmos. With these lines, a picture of cosmic evolution is drawn that may account for the astrodiversity of objects in our universe—not only directly for black holes but also for related phenomena such as galaxies or possibly gamma ray bursts.

Smolin’s proposal has a notable feature: It is testable, at least in a statistical manner, and thus it is of scientific quality. In this regard it differs from other versions of so-called multiverses, patchworks of several space-time regions separated from one another and mutually inaccessible. Statistically, we should, after all, find ourselves in a typical part of the multiverse, in a daughter universe of a kind appearing especially often. Analogously to the biological example, the fittest individual universes, those with the most offspring, are most common. In this scenario, offspring come through black holes, the branching points of the multiverse into new daughter universes. If our universe is a typical one, it must produce many black holes, something it apparently does: Most galaxies carry an extremely massive one in their center and many smaller ones outside the center. One can get a more quantitative handle on this picture by estimating the characteristics that constants of nature must possess in order to produce black holes in large numbers. If, for instance, Newton’s constant were too small and gravity too weak, then the clumping of matter in the early universe would proceed too slowly, leaving just a widely spread-out gas left in the universe. Or neutron stars could perhaps remain stable for all masses, not allowing the further gravitational collapse into black holes.

In principle one can see whether our universe is indeed a contender in the interuniversal fertility contest, if the number of black holes is the measure of success. Calculations and statistical estimates, however, are, to put it mildly, difficult. The few derivations done so far remain disputed. Regardless, Smolin’s conjecture is based on the assumption that the singularity of a black hole in quantum gravity is indeed eliminated by the universe’s branching off into a daughter universe. This assertion is as hotly debated as it is difficult to test, but after the following section it will bring us back to the topic of black hole quantum physics.

HAWKING