17 June 2017

Nautilus: “The Not-So-Fine Tuning of the Universe”

Astrophysicists have discussed fine-tuning so much that many people take it as a given that our universe is preternaturally fit for complex structures. Even skeptics of the multiverse accept fine-tuning; they simply think it must have some other explanation. But in fact the fine-tuning has never been rigorously demonstrated. We do not really know what laws of physics are necessary for the development of astrophysical structures, which are in turn necessary for the development of life. Recent work on stellar evolution, nuclear astrophysics, and structure formation suggest that the case for fine-tuning is less compelling than previously thought. A wide variety of possible universes could support life. Our universe is not as special as it might seem.

As it turns out, stars are remarkably stable entities. Their structure adjusts automatically to burn nuclear fuel at exactly the right rate required to support themselves against the crush of their own gravity. If the nuclear reaction rates are higher, stars will burn their nuclear fuel at a lower central temperature, but otherwise they will not be so different. In fact, our universe has an example of this type of behavior. Deuterium nuclei can combine with protons to form helium nuclei through the action of the strong force. The cross section for this reaction, which quantifies the probability of its occurrence, is quadrillions of times larger than for ordinary hydrogen fusion. Nonetheless, stars in our universe burn their deuterium in a relatively uneventful manner. The stellar core has an operating temperature of 1 million kelvins, compared to the 15 million kelvins required to burn hydrogen under ordinary conditions. These deuterium-burning stars have cooler centers and are somewhat larger than the sun, but are otherwise unremarkable.

Fred Adams

From the beginning of civilization, there’s been a long trend in scientific thought moving from the assumption that we humans find ourselves in special place in the world to proofs to the contrary (and to the fact that the world is much larger than we previously imagined). At first the Earth was believed to be the center of the solar system; then it was discovered that the Sun was just one of billions of stars in a relatively unremarkable corner of the Milky Way; and that our galaxy itself is small and insignificant compared to the vastness of the Universe. With this study, it looks like the final bastion of anthropocentrism, that out universe is somehow specially tuned for the emergence of life, is eroding, and there are multiple configurations which permit similar universes to evolve, even if the life there would be barely recognizable to us.

It’s a good occasion to revisit one of the more innovative SF novels, The Gods Themselves by Isaac Asimov, which plays on this idea of a universe with different parameters in the fundamental laws of physics, but still supporting life.

Universe fine tuning parameters for life
Setting The Parameters: The universe would have been habitable even if the forces of electromagnetism and gravity had been stronger or weaker. The crosshatched area shows the range of values consistent with life. The asterisk shows the actual values in our universe; the axes are scaled to these values. The constraints are that stars must be able to undergo nuclear fusion (below black curve), live long enough for complex life to evolve (below red curve), be hot enough to support biospheres (left of blue curve), and not outgrow their host galaxies (right of the cyan curve). Fred C. Adams

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