NOTE: Assess your responses on the basis of 4 points total.
1. Describe the large-scale structure of the universe, and summarize what observational evidence led astronomers to this description.
On the largest of scales, astronomers have observed galaxies and measured their distances and mass, and they conclude that the density of mass (the kind we can observe - not including dark matter) is the same everywhere - ALL DISTANCES IN ALL DIRECTIONS - to within about 0.1%. This is remarkably uniform. These measurements began with Edwin Hubble around 1920 and continue to the present using large Earth-based and space-based telescopes. At all smaller scales, on the order of hundreds of millions of parsecs, the distribution of matter is notably clumpy - at the smallest scale, our solar system has very small, high density objects we call the Sun and planets. The matter in our galaxy is non-uniformly distributed, and the distribution of matter within the local group of galaxies is even more non-uniform. The local sector of the universe appears to have sheets of galaxy clusters and concentrations of clusters and regions almost devoid of galaxies. Accounting for this smaller scale lack of uniformity in the context of a universe that is remarkably uniform on the largest scale is one of the toughest challenges of cosmology. The microwave background, a sort of picture of the distribution of mass early in the universe (pre-galactic), also shows very small but significant non- uniformities. This snapshot corresponds to a time of about 380,000 years after the big bang. According to the most-accepted cosmology today, very early during the big bang a short period of inflation occurred during which the universe expanded WAY more rapidly (hugely faster than the speed of light) than before or after. At the beginning of this expansion, minute random quantum fluctuations in energy density were expanded in space enormously and eventually resulted in the tiny variations in background radiation and matter in the universe today. Speculation is that dark matter clumped first (because of the gravity from its larger mass), and that in turn attracted ordinary matter, and that combination is what we see in the form of galaxies today. The acceleration of the universal expansion apparently started a few billion years later, and therefore had little or no effect on clumping of mass at early times.
2. Describe how the Big Bang cosmology accounts for the presence of hydrogen and helium universe. What happened to all the anti-particles also created?
The earliest universe had high energy photons - high
energy gamma radiation with enough energy to create particles and
anti-particles. Protons formed in such reactions, as did electrons and
neutrons, along with anti-particles for all of them. At the
same time that particles and anti-particles were coming into existence,
particles were also annihilating with anti-particles.
Somehow, when the production of particles shut
down, only a tiny fraction of the particles (not anti-particles) were left (the
cause of this 'asymmetry' is still unknown). After formation of protons,
neutrons, and electrons, the universe expanded and cooled enough that no more
particle formation occurred, but temperatures were still high enough for a few
nuclear reactions to occur, but just for a few minutes. Surviving protons and
neutrons combined (fused) to form deuterium, helium, and tiny amounts of
lithium and beryllium - the most abundant of this list is helium, mass
After a few minutes, temperatures dropped below the point at which any
nuclear reactions could continue to occur, and no heavier elements such as
Carbon were produced. About 2/14 of the original protons fused with almost all
the neutrons to produce the helium, so the particles surviving this epoch are
protons, helium-4 nuclei, electrons, and traces of other light
nuclei from other nuclear reactions that also occurred during the epoch
(deuterium, helium-3, lithium-6, lithium-7). The mass split
between hydrogen (protons) and helium-4 nuclei
coming out of this epoch was 75% protons, 25% helium-4 - not far from today's
composition in our Sun's outer layers. Over the next 14 billion years, stellar
evolution has modified this mix by only a few percent. About one
particle (p, n, or electron) in a billion particle-antiparticle pairs survived
the earliest epoch, which included not only particle production but also
particle-antiparticle annihilation. The answer to the last question is that all
antimatter annihilated with matter, with a tiny amount of matter left over to
form the universe as we see it today. The asymmetry between particle production
and annihilation remains a major puzzle
and continues to be the target of research.
The best match between the data and the standard model of the Big Bang yields an age of 13.7 billion years, with an uncertainty of plus or minus 0.2 billion years.