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.
Bit More Detail
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. The heaviest elements (gold, uranium, etc.) 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
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
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
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.
For more details, see pages by Martin White
, Ned Wright
, and Kipp Penovich
. Images borrowed from these pages.
Jeff Filippini, UC Berkeley
Cosmology Group (August 2005)