UNC-CH Physics and Astronomy Colloquium
Fred Adams, University of Michigan
“STARS IN OTHER UNIVERSES: HOW MUCH FINE-TUNING IS NECESSARY”
Motivated by the possible existence of other universes, with different values for the fundamental constants, this talk considers stars and stellar structure with different values for the fundamental constants of nature. Focusing on the fine structure and gravitational constants, we first enforce the following constraints: [A] long-lived stable nuclear burning stars exist, [B] planetary surfaces are hot enough to support chemistry, [C] stellar lifetimes are long enough to allow biological evolution, [D] planets are massive enough to maintain atmospheres, [E] planets are small enough to remain non-degenerate, [F] planets are massive enough to support complex biospheres, [G] planets are less massive than stars, and [H] stars are less massive than galaxies. The parameter space that satisfies these constraints is relatively large — viable universes can exist when the structure constants vary by several orders of magnitude. Next we consider the triple alpha fine-tuning problem. Stars in our universe would have difficulty producing carbon and other elements in the absence of the
well-known carbon-12 resonance at 7.6 MeV. This resonance is necessary because beryllium-8 is unstable in our universe, so that carbon must be produced via the triple alpha reaction. Although a moderate change in the resonance energy (300 keV) would affect carbon production, an even smaller change in the binding energy of beryllium-8 (100 keV) would it to be stable. This change would obviate the need for the triple alpha process in general, and the carbon-12 resonance in particular. In such cases, beryllium can be synthesized through helium burning, and carbon can be produced later through nuclear burning of beryllium. The talk concludes by considering other types of fine-tuning issues.