Big Bang Theory: New Simulations Solve an Old Issue

So what was the problem between the Big Bang theory and the observed amount of Helium3?

The Big Bang mostly produced isotopes of Hydrogen and Helium, and because carbon is a very difficult to synthesize element in the conditions of the Big Bang, no Carbon or heavier elements were produced there.  The same calculations that were successful in understanding how stars produced the Carbon, Hydrogen, Nitrogen, Iron, and other elements, those calculations robustly produced, particularly in low mass stars, lots of Helium 3. 

Normally, to test the Big Bang theory, you look at observed abundances and you subtract out the changes expected from the theory of stellar evolution (how the elements are produced).  When you subtract out the changes of what has been produced, you should be left with what the Big Bang originally had.  In comparing that, it was good except that the observed abundance of Helium 3 was actually a little below the predicted abundance of Helium 3 in the Big Bang theory.  That means that stars had to be destroying Helium 3, but instead, when you looked at the population of stars and you did the same calculations that successfully produced all the heavy elements, the stars seemed to have been producing huge amounts of Helium 3, not destroying it. 

That is a conflict between two very successful theories: the theory of stellar evolution and the Big Bang theory.

Your computer simulations [1] suggest that low mass stars do not release any Helium 3 into the Universe…

The low mass stars in particular were the ones that were the criminals in producing too much Helium 3.  The calculations showed that rather far out, in the envelopes, the stars should be producing Helium 3, and then when they become red giants, Helium 3 is mixed to the surface.  As stars evolved up the giant branch, large fractions of the envelopes are rejected and the result is a big increase in Helium 3.

We found that there was an instability mechanism that was not recognized before. We saw that there is an unusual nuclear reaction:  the Helium 3, which has a fairly fragile nucleus, burns with itself to produce Helium 4+2 protons.  That is an unusual reaction as most nuclear reactions actually raise the average weight of a particle in the gas while this one lowers the average weight of a particle in the gas.  The bubble of material that you produce is buoyant and starts to float up.  As it floats up, it is replaced by the heavier material above it, which then gets processed, which drives more bubbles going up. 

Has the apparent problem with the Big Bang been solved?

Yes, we are working on a somewhat more elaborate version.  We mention in Science that the mixing is robust enough that it destroys the bulk of Helium 3 that was made.  In fact, it probably destroys a little more Helium 3 than was made. It looks very promising too.  

[1] Peter P. Eggleton et al, Science, online 26 October (2006)

David Dearborn works at the Lawrence Livermore National Laboratory.

Interview by Thanh Tam Candice Vu and Gilles Prigent