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The current observations and estimates of dark matter is that 20% of dark
matter is probably in the form of massive neutrinos, even though that mass
is uncertain. The another 5% to 10% is in the form of stellar remnants
and low mass, brown dwarfs. The rest of dark matter is called CDM (cold
dark matter) of unknown origin, but probably cold and heavy. The
combination of all these mixtures only makes 20 to 30% the amount mass
necessary to close the Universe. Thus, the Universe appears to be open,
i.e. ΩM is 0.3.
With the convergence of our measurement of Hubble's constant and ΩM, the end appeared in site for the
determination of the geometry and age of our Universe. However, all was
throw into turmoil recently with the discovery of dark  energy. Dark
energy is implied by the fact that the Universe appears to be
accelerating, rather than decelerating, as discovered by observations of distant
supernovae.
The most direct cosmological observation you can make is to find some standard
candle, an object with a known luminosity, and follow its change in apparent
luminosity with distance (like watching the headlights of a distant car).
The problem is most objects, like galaxies, change in brightness from the
past till now. One object which is constant is the brightness of a supernova,
but until recently the technology to capture them at high distances was not
avaliable. Below is the results of the high redshift supernova project, a
unique combination of space and ground-based telescope work.
- the SN observations require a cosmological constant, one that dominates the very early Universe and the far
future
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This new observation implies that something else is missing from our
understanding of the dynamics of the Universe, in math terms this means
that additional cosmological constant in Friedmann's equation, Λ.
The implication here is that there is some sort of pressure in the fabric
of the Universe that is pushing the expansion faster. A pressure is
usually associated with some sort of energy, we have named dark energy.
Like dark matter, we do not know its origin or characteristics.
With a cosmological constant, there are many possible types of Universes,
almost any kind of massive or light, open or closed curvature, open
or closed history is possible. Also, with high Λ's, the Universe could race away. |
- SN, cluster and CMB observations produce a narrow range of values for Omega M, Lambda and k
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Fortunately, observations, such as the SN data and measurements of the cosmic microwave background constrain the
possible values for the density of matter and the cosmological constant. The following diagram displays the
results and error ellipses for these three observations. The density of matter is constrained by cluster
observations to be around 0.3. The flattness of the CMB forces a k=0 Universe, as expected by inflationary
cosmology (see next lecture). Lastly, the SN data contrains the value of the cosmological constant. |
- the Benchmark model has values of 0.7, 0.3 and 0 for cosmological constant, matter and curvature
- thus, we live in an open, flat Universe
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SN data gives ΩΛ=0.7 and
ΩM=0.3. This results in Ωk=0, or a flat curvature. This is sometimes referred to as the Benchmark Model which gives
an age of the Universe of 12.5 billion years.
Is this the end of cosmological work? Unlikely, continued information still flows in on the
conditions near the Big Bang. For example, the following is a list of the current
models being considered to explain the first moments of the Universe: Quasi-steady
state (non-cosmological redshifts), NUT space (microlensing space), Brans-Dicke gravitation
(dark energy couples to dark matter and baryons), Godel cosmology (inclusion of shear and
rotation), Lyra geometry (2D singularities or domain walls), complex topologies (dodecahedral
shaped universe), Cardassian expansion (non-linear Hubble's constant), Quantized everything
(particles based on radius of universe), self-creation comsology (no horizons), cosmological
synchronization (cosmological factors affect fluctuation processes), backward universe
(evolution proceeds to less order). |
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