A. The Fate of the Universe
Is the expansion being slowed by the gravity from all of the mass in the universe?
There are two important quantities needed to determine the overall fate of the universe:
the expansion rate, Ho, and the mean density, rho.
If the average density is high enough, the expansion will stop. Otherwise, it will
continue forever.
The ratio of the mean density to the critical density is Omega = rho/rhoo.
If Omega is high, the universe is younger and more likely to recollapse than if Omega is
low.
By adding up all of the measurable mass (including dark matter), we can find up to 1/10
of the critical amount.
B. The Early Universe
The Hubble Law implies that the universe started out as a single point. The age of the
universe is given by 1/Ho = 10 to 20 billion years.
We can model the conditions in the early universe through the study of the behaviour of
matter at high energies (i.e., in particle accelerators).
There are 2 important principles involved in models of the early universe:
1) The universe cools as it expands.
2) The photon energies depend on the temperature.
When the universe was 0.01s old, it was a hot (1011K), dense (1010g/cm3)
sea of protons, neutrons, electrons, positrons, and neutrinos. Photon collisions created
electron-positron pairs, and photons could destroy nuclei.
At time t = 1s, the density is 105g/cm3 and the temperature is 2 x
1010K, the neutrinos stop interacting with matter. These primordial neutrinos
are probably still travelling through the universe, but we haven't been able to observe
them.
From t = 1s to t = 1000s, the temperature T drops to 9 x 108K and the density
drops to less than 10g/cm3, and protons and neutrons combine to form deuterium
and helium.
Models predict that 25% of the mass of material became He, with virtually all of the
rest being hydrogen. This matches observations well.
The amount of deuterium left over is a sensative indicator of the average density of
protons, neutrons, and electrons in the universe.
At t = 700000 years, T = 3000K, density = 10-20g/cm3, the
electrons and protons can start combining to form hydrogen atoms. This ``recombination''
made the universe transparent, so photons produced then are still moving through the
universe now.
In 1966, these photons were detected as a background radiation to the universe. The
expansion of the universe has ``cooled'' (redshifted) the photons from 3000K (visible) to
3K (microwave).
At t = 109 years, galaxies and stars had started forming. These would have
resulted from small irregularities in the background density.
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