Known Contents of the Universe
- Large objects: Various types
of stars (luminous matter); Star remnants including black holes,
both star-sized and supermassive.
- Smaller objects: Planets; Meteoroid-
or asteroid-sized objects; Gas and dust, loose in the interstellar
or intergalactic medium, or in denser clouds.
- Particles and radiation: Electromagnetic
and gravitational waves; cosmic rays; neutrinos; ...
- Are we missing anything? Yes,
about 95% of what the universe is made of! There is matter whose
only detectable effect is gravitational and cannot otherwise
be seen (yet?); This has happened before (think of how Neptune
or Sirius B were discovered), so this "dark matter"
may or may not be of a new type.
Evidence for Dark
- Inside a galaxy: The rotation
curve problem for stars (spiral and elliptical galaxies) and
H clouds (spiral galaxies, like NGC 5746) [and the bobbing motion
problem in the Milky Way].
- In clusters: Evidence for more
matter from motion of binaries and around the center; X-ray emission
from hot (many millions of K) intergalactic medium (e.g., in
the Hydra cluster); and gravitational lensing (images are distorted
by the lens), which even allows us to map the dark matter.
- Conclusion: The mass-to-light
ratio indicates that 90% of the matter may be invisible (dim,
and/or of a new type), at least for spiral galaxies; We are seeing
only the tip of the iceberg.
- Is there an alternative way out?
Maybe the laws of gravity need to be modified; This is a possibility
one should consider carefully; Most scientists think it is unlikely,
and it is not clear whether it fits observations.
What Is the Dark Matter?
- Ordinary matter: Is it known
matter? Subatomic particles (like baryons or neutrinos), gas
clouds, MACHOS (like dwarf stars or planet-sized objects)? Whole
- Non-ordinary matter: Is it new,
unkown kinds of matter (neutralinos, WIMPS, Q-balls made of supersymmetric
particles)? Hot or cold?
- Search: Use microlensing to
look for MACHOS; only part of the missing mass can be accounted
for; Search for new types of particles on Earth, using detectors
deep underground, and for information from space that is not
provided by electromagnetic waves.
- Dark energy: The most distant
supernovas show that the expansion of the universe is accelerating;
Dark matter is not the whole story, and there is an even bigger
repulsive "dark energy", possibly Einstein's "cosmological
- Composition: There is probably
more than one kind of dark matter ("hybrid model",
with possibly 80% cold dark matter in galaxies, and additional
hot dark matter at the galaxy cluster level); Total about 23%
of the content of the universe.
- Does it make a difference? The
type of matter affects our views on when stars and galaxies form,
and the way clusters, superclusters and voids evolve; The total
amount of matter also affects our predictions for the future
of the universe.
- Predictions: Recent measurements
by satellites confirm ideas about the composition of the universe,
and that it will expand forever (regular + dark matter together
add up to only 27% of what is needed for it to eventually recollapse),
possibly accelerating (dark energy is repulsive, and is 73% of
the content of the universe!)
- Plans: The proposed SNAP (Supernova/Acceleration
Probe) mission will study the recent expansion history of the
universe, and should be able to distinguish even better between
page by luca bombelli <bombelli at olemiss.edu>,
modified 29 sep 2012