The “Missing Mass” Problem

The “Missing Mass” Problem

The “missing mass” problem then becomes easy to understand. If the universe did get stretched
out such that the mass density of the universe is nearly the critical density of a flat universe, then
we should be able to find this much mass in our universe. But of ordinary matter, we only see
enough to account for at most approximately 10% of the critical mass. Ordinary matter consists
primarily of Hydrogen and Helium, as previously discussed. We can detect sources of both
simply by the radiation they emit. Some uncertainty is introduced by the difficulty of detecting
very cool sources, which radiate very little. But it seems clear that there is not enough ordinary
matter to add up to the critical mass predicted by inflation, and observed in the apparent flatness
of space. However, not all matter need be ordinary matter. Any kind of matter that has mass
will add to the overall mass density of the universe, even if it doesn’t radiate like ordinary matter
does.
So if there is non-ordinary matter or “dark matter” in the universe, how could we ever detect it?
Since dark matter has mass, it exerts a gravitational force on other matter. So we must detect it
by observing its gravitational influences on ordinary matter that can be observed. Actually, dark
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matter has already been detected in this manner. It was originally proposed by Fritz Zwicky in
the 1930’s to explain how the Coma Cluster of galaxies could be held together when the motion
of the galaxies appeared to be too great for the gravitational attraction of the observable mass to
hold it together [25]. Since then other indicators show there must be additional matter inside
galaxies. When observing the speeds with which stars revolve about the center of our galaxy, the
Milky Way, and other spiral arm galaxies such as the Andromeda, we find that they do not
exhibit the speeds expected on the basis of the gravitational influence of the matter that we can
observe. Most of the stars far away from the center of the galaxies are moving at much higher
speeds than expected, indicating that there is additional mass in the galaxy causing it [24]. One
suggestion is that the matter could all be inside a supermassive “Black Hole”, located at the
center of the galaxy. A Black Hole is a concentration of mass so dense that not even light can
escape its gravity. We can determine accurately how much mass is inside a Black Hole by
measuring the orbital speeds of nearby objects. In this way, we know that there exist several
such supermassive Black Holes at the center of many galaxies. However, these cannot account
for the orbital behavior of stars far out in the outer arms of the galaxies, since the gravitational
influence of even a supermassive Black Hole is much reduced at such great distances. There
must be dark matter distributed throughout the galaxies, in order to produce the spiral arm
rotations we observe. This may amount to enough matter to bring the total mass of the universe
up to the critical mass predicted by inflationary expansion.

Even though we know there is dark matter in the universe, which would account for the missing
mass expected from inflation, we still have not determined what this missing mass really is.
Another suggestion is that it might be the mass of neutrinos, of which there are three known
types. Recently we have discovered clear evidence that the neutrinos coming from our Sun
change from one type to another en route to our underground detectors on earth [26]. This
implies that the neutrinos must have non-zero masses. They also qualify as dark matter since the
elusive neutrinos do not have an electric charge, neither do they interact as do quarks, the matter
from which protons and neutrons are composed. They interact only gravitationally and via the
nuclear weak force, resulting in their elusive nature, which allows neutrinos to go through
millions of miles of lead with a very low probability of being absorbed. However, we have other
means of knowing that the mass of neutrinos is extremely small, from the study of β-decays of
unstable nuclei. Still the question remains how much of the dark matter can be accounted for by
neutrinos. If the bulk of the missing mass is not from neutrinos, as many physicists suspect, then
there exists dark matter of a nature not yet understood within the present model of fundamental
particles. However, many theories predict particles beyond what is presently known and could
account for the missing mass. Presently, we conclude that the missing mass problem is simply a
problem of not knowing what is contributing to the bulk of the mass of the universe, since it does
not emit radiation and beyond its gravitational influences, remains extremely difficult to detect.