The Fate of the Universe

An expanding Universe is subject to two possible outcomes:

• 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.

Note the difference between the Physical Universe and the Observable Universe. This is related to horizons.

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 of mass-to-luminosity

How do we know its there?

• Motions of galaxies in clusters of galaxies
• 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)

• Beer cans
• Jupiter size objects (see microlensing project
• very low mass stars
• black holes and other stellar remnants

Non-baryonic (exotic matter):

• 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 early universe
• 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" cosmologies.

The above is currently a big huge debate with all sides yelling and screaming. Here is what Dr. DarkMatter has to say:

• 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 other 1/2
• The total mass density of the Universe is 10--20% of that required to close it
• Deal with it

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The Electronic Universe Project

e-mail: nuts@moo.uoregon.edu