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A Closer Look

Dark Matter
ill the universe continue to expand of matter in that galaxy. The outer matter consists primarily of exotic undis-
Wforever, or will it be pulled back to- stars are observed to rotate too fast covered particles or known particles such
gether into a really big crunch? Whether for the amount of matter that can be as the neutrino. Neutrinos are electrically
the universe will continue expanding or seen in the galaxy. Again, measure- neutral, stable subatomic particles. This
gravity will pull it back together again ments indicate there is about 10 times school of thought is concerned with forms
depends on dark energy and the critical more matter in each galaxy than can of dark matter called WIMPs (after “weakly
density of the universe. If there is enough be accounted for by the light coming interacting massive particles”). In spite of
matter, the actual density of the universe from the galaxies. the name, particles under study are not
will be above the critical density, and grav- 3. Finally, there are estimates of the always massive. It is entirely possible that
ity will stop the current expansion and total matter in the universe that can some low-mass species of neutrino—or
pull everything back together again. On be made by measuring ratios such some yet unidentified WIMP—will be ex-
the other hand, if the actual density of the as deuterium to ordinary hydrogen, perimentally discovered and found to have
universe is less than or equal to the critical lithium-7 to helium-3, and helium the mass needed to account for the dark
density, the universe will be able to escape to hydrogen. In general, theoretical matter and thus close the universe.
its own gravity and continue expanding. Is calculations all seem to account for The other school of thought about dark
the actual density small enough for the uni- only about 4 percent of all the matter matter focuses on astrophysics and con-
verse to escape itself? in the universe. tends that dark matter is ordinary matter in
All the detailed calculations of astro- the form of brown dwarfs, unseen planets
Calculations from a variety of sources
physicists point to an actual density that is outside the solar system, and galactic ha-
all seem to agree that 23 percent or more
less than the critical density, meaning the los. This school of thought is concerned
of the matter that makes up the universe is
universe will continue to expand forever. with forms of dark matter called MACHOs
missing. The missing matter and the mys-
However, these calculations also show (after “massive astrophysical compact halo
terious variations in the orbits of galaxies
that there is more matter in the universe objects”). Dark matter considerations in
and stars can be accounted for by the pres-
than can be accounted for in the stars and this line of reasoning consider massive
ence of dark matter, which is invisible and
galaxies. This means there must be matter objects, but ordinary matter (protons and
unseen. What is the nature of this dark
in the universe that is not visible, or not neutrons) is also considered as the material
matter? You could speculate that dark
shining at least, so we cannot see it. Three making up galactic halos. Protons and
matter is simply normal matter that has
examples follow that show why astrophysi- neutrons belong to a group of subatomic
been overlooked, such as dark galaxies,
cists believe more matter exists than you particles called the baryons, so they are
brown dwarfs, or planetary material such
can see. sometimes referred to as baryonic dark
as rock, dust, and the like. You could also
1. There is a relationship between the speculate that dark matter is dark because matter. Some astronomers feel that bary-
light emitted from a star and the mass it is in the form of subatomic particles, too onic dark matter is the most likely candi-
of that star. Thus, you can indirectly small to be seen. As you can see, scientists date for halo dark matter.
measure the amount of matter in a have reasons to believe that dark matter In general, astronomers can calcu-
galaxy by measuring the light output exists, but they can only speculate about late the probable cosmological abundance
of that galaxy. Clusters of galaxies its nature. of WIMPs, but there is no proof of their
have been observed moving, and the The nature of dark matter represents existence. By contrast, MACHOs dark
motion of galaxies within a cluster one of the major unsolved problems in as- matter candidates are known to exist, but
does not agree with matter infor- tronomy today. There are really two dark there is no way to calculate their abun-
mation from the light. The motion matter problems: (1) the nature of the dance. MACHOs astronomers assert that
suggests that galaxies are attracted by a dark matter and (2) how much dark mat- halo dark matter and probably all dark mat-
gravitational force from about 10 times ter contributes to the actual density of the ter may be baryonic. Do you believe WIMPs
more matter than can be accounted for universe. Not everyone believes that dark or MACHOs will provide an answer about
by the light coming from the galaxies. matter is simply normal matter that has the future of the universe? What is the na-
ture of dark matter, how much dark matter
2. There are mysterious variations in the been overlooked. There are at least two
exists, and what is the fate of the universe?
movement of stars within an indi- schools of thought on the nature of dark
Answers to these and more questions await
vidual galaxy. The rate of rotation of matter and what should be the focus of
further research.
stars about the center of rotation of research. One school of thought focuses
a galaxy is related to the distribution on particle physics and contends that dark

368 CHAPTER 14 The Universe 14-18