Saturday, November 04, 2006

Big Bang Theory Saved

From the title you might think that the theory was in serious peril. But it wasn't, really.

Big Bang Theory Saved
An apparent discrepancy in the Big Bang theory of the universe's evolution has been reconciled by astrophysicists examining the movement of gases in stars.

Professor John Lattanzio from Monash's School of Mathematical Sciences and Director of the Centre for Stellar and Planetary Astrophysics said the confusion surrounding the Big Bang revolved around the amount of the gas Helium 3 in the universe.

The issue arises from what is actually one of the greatest successes of the big bang theory – quantitative calculations of the relative abundances of a few light element produced within the first 5-10 minutes of the universe by the process of "big bang" nucleosynthesis.

Detailed calculations depend on vaious factors, such as the temperature and rate of expansion of the universe during the time in question, as well as the densities and relative abundances of the "raw materials" (mostly protons, neutrons, photons, and neutrinos) when nucleosynthesis begins. The calculations are complex due to the dependence on so many factors, but they're no real sweat with modern computers. Even before modern computers, the first physicists to make the computations (George Gamow and associates) in the late 1940s were able to come up with surprisingly good results, all things considered. (They predicted a mass fraction of about 50% helium-4, when the correct figure is more like 25%.)

The objective is to compute the abundances of the light elements deuterium (hydrogen-2), helium-3, helium-4, lithium-6, and lithium-7 (relative to protons – ordinary hydrogen). In order to check the correctness of the calculations, the numbers have to be compared with actual measurements of the relative abundances of these elements, either at the present time, or at some known time in the past.

And that's where the difficulties lie. In the first place, it's necessary to be sure the relative abundances can be measured accurately. This is nontrivial, since, in the most extreme case, the predicted abundance of lithium-7 is a minuscule mass ratio on the order of 10-10. (Lithium-6 is an even smaller ratio, too small to measure.)

The second problem is that there's no way to observe the relative abundances right after nucleosynthesis is complete. At best, the measurement could be made a billion years or so after the big bang. And in practice, the best measurements are made on nearby objects, corresponding to more than 13 billion years after the big bang. So you have to make allowances for any changes that could have occurred in that time span, due to incremental production or destruction of isotopes (in stars, for example).

The most problematic case has been with helium-3. Until the research just reported, there has been much less helium-3 detected than should be expected, because this isotope can be produced in low mass star like our sun. But the discrepancy can now be accounted for, since this helium-3 should be destroyed near the end of a star's life:
Near the end of a star's life there is a 'core flash' and it was at around this time that the computer models revealed a small instability in the movement of the gases in the star. "When we looked at this in 3D we found this hydrodynamic instability caused mixing and destroyed the helium 3 so that none was released into space," Professor Lattanzio said.

Additional information:

Deep Mixing of 3He: Reconciling Big Bang and Stellar Nucleosynthesis – original research report in Science (subscription rqd for full access)

On the case of the "missing" helium – PhysicsWeb

Scientists crack open stellar evolution – Lawrence Livermore National Laboratory


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