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University of Delaware
PHYS146
Quarks, Gluons, and the Big Bang
Maurice Barnhill
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Class Notes XIII. Dark Matter
Class Notes on Dark Matter
Definition: "Dark matter" is any form of matter that cannot be
detected by the light it emits.
Predictions of Big-Bang Models
- Meaning of "critical density"
If the cosmological constant vanishes, the fate of the universe is
determined by the density of matter it contains. If the density is
high enough, the universe is finite and will eventually collapse. If
the density is low enough, the universe is infinite and will expand
forever. The density where the behavior of the universe changes from
one case to the other is called the critical density, denoted
OMEGA0.
- The simple versions of inflation predict that the matter density
in the universe is equal to the critical density.
- Almost any big-bang model predicts that the density of matter
made up of baryons is now around 0.05 * OMEGA0 .
The prediction is made by examining how much deuterium (hydrogen with
one neutron in its nucleus) and lithium are made by nuclear reactions
in the early universe. The amounts are quite sensitive to the baryon
density at the time when the temperature permits these reactions to
occur. Comparing the amount of each that is observed to be present
now to the amount that theory gives for each density tells us what
the total baryon density has to be.
Methods for determining the amount of matter actually present
- Amount of matter in the "solar neighborhood"
- Can be found by direct observation
- Detecting mass by galactic rotation curves
- The speed of a star or cloud of luminous gas in its orbit around
the galaxy is determined by the total mass closer to the center of
the galaxy than the object itself. We can determine the speed by
using the Doppler frequency-shift of the light coming from the
object and hence determine the amount of mass in the galaxy as a
function of the distance from the center of the galaxy.
- Detecting mass between the galaxies in a cluster
- We also use the speeds of the galaxies to estimate the mass
density in the cluster. However, the galaxies are moving in random
direction rather than revolving around the center of the cluster, so
the determination is less direct and less accurate.
- Observational evidence for the total density of matter
- The
Supernova data described in the section on Expansion of the Universe
gives direct evidence for the total matter density in the Universe,
using the effect of the density of matter on the curvature of space.
Mass-density numbers
Estimates
from Joe Silk [accessed May 5, 1996], assuming zero cosmological
constant. OMEGAbaryon is the big-bang estimate of the
density of baryons in the Universe, 0.05 OMEGA0. One
value inferred from verious recent supernova data is also included.
| Object | M/L solar units | OMEGA/OMEGAbaryon | OMEGA/OMEGA0 |
| Sun | 1 |
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| Solar neighborhood | ca. 2 | 0.02 | 0.001 |
| Typical galaxy | 50 | 0.7 | 0.03 |
| Baryon limit | 75 | 1 (definition) | 0.05 [0.02 to 0.1] |
| Supercluster | 300 | 4 | 0.2 |
| Supernova data | 300 |
4 | 0.2 - 0.5 |
| Closed universe | 1500 | 20 | 1 (definition) |
Given these numbers, it appears that both baryonic and nonbaryonic
dark matter exists. Most of the Universe, in fact, is composed of
unknown types of matter, and very likely most of the mass of the
Universe is not baryonic.
What could the dark matter be?
- "Jupiters"
- Large planet-like bodies not attached to visible stars. There is
little evidence for their existance and little theoretical guidance
for their expected numbers, but these objects are not generally
expected to make a significant contribution to the overall mass
density.
- Brown Dwarfs
- Gaseous objects large enough to shine using
energy obtained by gravitational contraction but not large enough to
generate energy by nuclear reactions. In general, the smallest stars
are most abundant, so it is possible that there are large numbers of
Brown Dwarfs. A few, but not many have been detected, but they are so
faint that detection is difficult. These are perhaps the best
candidates for large amounts of unseen baryonic matter.
- Black Holes
- Collections of matter so large in mass and small in volume that
light itself cannot escape from the object. Many and perhaps most
galaxies are believed to have a large black hole at their centers, but
there are not enough known black holes to make much impact on the
total amount of matter.
- Intergalactic gas
- There is known to be a significant amount of intergalactic gas but
not enough to match the observed amount of matter.
- Neutrinos
- No experimental evidence is
available for or against the possibility of neutrinos explaining the
amount of dark matter. Calculations of the distribution of visible
matter in the Universe tend to work well if neutrinos are responsible
for the nonbaryonic dark matter in the Universe.
- Really exotic particles
- Speculative but not completely implausible possibility.
Conclusion?
We don't know yet.