|Big Bang Nucleosynthesis|
None of the "counting" arguments described above are capable of telling us much about the nature of the dark matter. In particular, these arguments don't help us figure out whether the dark matter is baryonic matter (like gas or dust) or something more exotic. To decide that question we need more information, and one of the strongest pieces of evidence that the dark matter is exotic is Big Bang nucleosynthesis (BBN).
Some of the lightest chemical elements in the universe - in particular, deuterium (a heavy isotope of hydrogen), helium-3, helium-4, and lithium-7 - are created in the early moments of the universe, when the whole universe was hotter than the interior of a star. The amounts of each of these nuclei that were formed depends critically on the conditions in the early universe - in particular, the balance between baryonic matter (protons and neutrons) and non-baryonic matter (neutrinos and exotic particles). Based on these ratios, astronomers have concluded that, in the universe as a whole, dark matter outmasses baryonic matter by a factor of almost 10.
The basis for this line of argument comes down to a question: how did the various chemical elements of the periodic table form? It turns out that these elements are made in several different ways. Helium is made from hydrogen by nuclear fusion in the core of stars. In the most massive stars, heavier elements such as carbon, oxygen, and even iron are formed in later stages of the star's lifetime. Elements heavier than iron are formed by the heat of exploding stars (supernovae).
These processes do not, however, account for the very lightest elements - helium (not all of it can be accounted for by the stars), deuterium (a heavy isotope of hydrogen), lithium, and beryllium. The last three on this list are particularly troublesome because they are actually destroyed within stars, not formed. Where did these elements come from?
According to the Big Bang model, the universe began in an extremely hot and dense state and has spent the last 13 billion years expanding and cooling. For the first second or so of its history, the universe was so hot that atomic nuclei could not form - space was filled with a hot soup of protons, neutrons, electrons, and photons (as well as other, short-lived particles). Occasionally a proton and a neutron may collide and stick together to form a nucleus of deuterium (a heavy isotope of hydrogen), but at such high temperatures these clusters will be broken immediately by high-energy photons.
When the universe cools off a bit more, these high-energy photons become rare enough that it becomes possible for deuterium to survive. At this point, a race begins. These deuterium nuclei can keep sticking to more and more protons and neutrons, forming nuclei of helium-3, helium-4, lithium, and beryllium. This process of element-formation is called "nucleosynthesis". The denser protons and neutrons are at this time, the more of these light elements will be formed. As the universe expands, however, the density of protons and neutrons decreases and the process slows down.
It turns out, however, that neutrons are unstable (with a lifetime of about 15 minutes) unless they are bound up inside a nucleus. After a few minutes, therefore, the free neutrons will be gone and nucleosynthesis will grind to a halt. That's the race - there is only a small window of time in which nucleosynthesis can take place, and the relationship between the expansion rate of the universe (related to the total matter density) and the density of protons and neutrons (the baryonic matter density) determines how much of each of these light elements that are formed in the early universe.
Astronomers can use various techniques to study the amount of these light elements that are present in various distant parts of the universe. The abundances of these isotopes have led cosmologists to believe that in the universe as a whole, baryonic matter is far outmassed by some kind of exotic, non-baryonic matter.
Last updated April 28, 2007