of the Universe
An expanding Universe is subject to two possible outcomes:
Note the difference between the Physical Universe and the Observable
Universe. This is related to horizons.
- If there is enough mass in the Universe then its mutual
gravitational attraction will eventually halt the expansion and
cause a contraction leading to --> THE BIG CRUNCH. In this case
the Universe is said to be closed
- If there is insufficient mass in the Universe then the expansion
continues to slow down and approach, but never reach, an expansion
rate of zero. In this case
the Universe is said to be open -- open Universes bother some
people because an open Universe has a definite beginning (when the
expansion started) but will never have an end.
Horizons and the Expansion Age of the Universe:
V = HD --> c = HD ==> D =c/H ==> This is our causal horizon -
beyond this distance something would have to travel faster than the
speed of light in order to communicate with us. All observers
are surrounded by such a horizon.
Horizons are okay. Our assumption about homogeneity means that the
stuff beyond the horizon is the same stuff we already know about. This
assumption must be correct due to horizon overlaps and causality.
The observable Universe is the one which is defined by our horizon.
This is always finite. Note however, that as H approaches zero,
which it must since gravity is slowing down the expansion rate of
the universe, the horizon size approaches infinity and hence the
physical universe and the observable universe become the same.
Whether the Universe is open or closed depends upon the amount of
matter or mass that is in it. This includes any Dark Matter that
might be present.
What is Dark Matter?
- Dark matter is material that gravitates but does not emit
very much light.
- More specifically, it is material which has a high ratio
How do we know its there?
- Motions of galaxies in clusters
- Gravitational Lenses
- How a Lens basically works
- Theory: if the Universe is closed then all the luminous matter
seen in galaxies contributes only 0.5% of the mass required to eventually
halt the expansion
- Best argument: baryonic density fluctuations will not produce
galaxies --> you need a heavy particle that is not effected by radiation
pressure in order to eventually form galaxies
- This means that galaxies are surrounded by dark matter halos which
trapped baryonic gas (e.g. hydrogen) and turned it into stars. So
a galaxy like this is surrounded by
an invisible dark matter halo.
What are the candidates
Baryonic: (e.g. made of protons and neutrons)
Non-baryonic (exotic matter):
- Beer cans
- Jupiter size objects (see microlensing project
- very low mass stars
- black holes and other stellar remnants
- neutrinoes with mass --> ratio of MWB photons to neutrinoes
is 4 to 1 therefore even a tiny neutrino mass has cosmological significance
- WIMPS - weakly interacting massive particles created in the extremely
- Particles associated with symmetry breaking
How much dark matter is there?
- Theory --> 99% of the Universe is Dark Matter. This is
required so that the Universe is spatially flat and homogenous.
This is predicted from the inflationary
theory of Cosmology
- Observations --> 90% of the Universe is Dark Matter and the
Universe is open
There is a big difference between Theory and Observation with good
arguments coming from both sides. The situation is unresolved
and has been for 20 years. Whomever solves the "dark matter"
problem will likely win the Nobel prize. However, its important
to realize that the "dark matter" problem exists only in the context
of one known long range force (gravity). Suppose there is another
long range force that we are ignorant of. If this is discovered
by future physicists then they will look back at this "dark matter"
cosmology much the same way we now view the early "geocentric"
The above is currently a big huge debate with all sides yelling and
screaming. Here is what
Dr. DarkMatter has to say:
The Electronic Universe Project
- neutrinoes have a small mass (3-6 eV) and contributes 1/2
the mass of the Universe
- dim galaxies , stellar remnants, and low mass stars make up the
- The total mass density of the Universe is 10--20% of that required
to close it
- Deal with it