Nucleosynthesis in the Early Universe
The term nucleosynthesis refers to the formation of heavier elements,
atomic nuclei with many protons and neutrons, from the fusion of lighter
elements. The Big Bang
theory predicts that the early universe was a very hot place. One second
after the Big Bang, the temperature of the universe was roughly 10 billion
degrees and was filled with a sea of neutrons, protons, electrons, anti-electrons
(positrons), photons and neutrinos. As the universe cooled, the neutrons
either decayed into protons and electrons or combined with protons to make
deuterium (an isotope
of hydrogen). During the first three minutes of the universe, most of the
deuterium combined to make helium. Trace amounts of lithium were also produced
at this time. This process of light element formation in the early universe
is called “Big Bang nucleosynthesis” (BBN).
The predicted abundance of deuterium, helium and lithium depends on the density of ordinary matter
in the early universe, as shown in the figure at left. These results indicate
that the yield of helium is relatively insensitive to the abundance of ordinary
matter, above a certain threshold. We generically expect about 24% of the
ordinary matter in the universe to be helium produced in the Big Bang. This
is in very good agreement with observations and is another major triumph
for the Big Bang theory.
However, the Big Bang model can be tested further. In order for the predicted
yields of the other light elements to come out in agreement with observations,
the overall density of the ordinary matter must be roughly 4% of the critical density. The WMAP satellite should be able to directly measure the ordinary matter density
and compare the observed value to the predictions of Big Bang nucleosynthesis.
This will be an important and stringent test of the model. If the results
agree, it will be a further evidence in support of the Big Bang theory. If
the results are in conflict, it will either point to 1) errors in the data,
2) an incomplete understanding of the process of Big Bang nucleosynthesis,
3) a misunderstanding of the mechanisms that produce fluctuations in the microwave background radiation, or 4) a more fundamental problem with the Big Bang theory.
Nucleosynthesis in Stars
Elements heavier than lithium are all synthesized in stars. During the late stages of stellar evolution,
massive stars burn helium to carbon, oxygen, silicon, sulfur, and iron. Elements
heavier than iron are produced in two ways: in the outer envelopes of super-giant
stars and in the explosion of a supernovae. All carbon-based life on Earth
is literally composed of stardust.
Last updated: Tuesday, 03-01-2005