Wednesday, June 27, 2007

Supersymmetry and big bang nucleosynthesis

The general acceptance of big bang cosmology for the past four decades rests primarily on three solid lines of evidence. First, the observation of the general expansion of the universe (using distance measurements based on "standard candles" like supernovae), is very consistent with the Friedmann equations derived from general relativity. Second, very precise measurements of inhomogeneities in the cosmic microwave background are very consistent with what is to be expected of conditions present in the universe at the time photons decoupled from matter. Third, the abundances of several light nuclei are very close to what would be expected to be produced in the process of nucleosynthesis that should have occurred around five minutes after the big bang.

As good as the agreement between theory and observation has been where nucleosynthesis is concerned, there have been various discrepancies that required some creative thinking to resolve. One of these involves the abundance of helium-3. We discussed it here.

Another example involves lithium. Although both lithium-6 and lithium-7 are calculated to have been produced in very small amounts, there was definitely some. Yet some very old stars have been observed that seem to contain no lithium at all. Where did it go? The theory here is that such stars are the result of mergers between even older stars, in which all the lithium was destroyed in the cataclysmic merger. See this.

Now there is yet another anomaly involving lithium observed in certain very old stars. The interesting thing is that one theorist is viewing this as possible evidence for very heavy supersymmetric particles that may not have yet decayed out of existence at the time of primordial nucleosynthesis.

Catalyzing Primordial Nuclear Chemistry
But a remaining puzzle is the amount of primordial lithium; both Li-6 and Li-7 are unexpectedly abundant in metal-poor stars (those with very few heavier elements). For example, a much higher than expected level of Li-6 might be pointing to a primordial origin (that is, not made later in stellar cores or in supernovas), in which case the BBN model would need to be amended. Maxim Pospelov ... of the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, and University of Victoria, British Columbia suggests that the anomaly can be explained if early nucleosynthesis was aided---catalyzed---by the presence of charged heavy particles, which are common in many models of particle physics.

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