Unphysical(?) Cosmology: Alternative(?) Universes and the CMB. Please answer part (a), and at least one of parts (b) and (c). These questions are vaguely worded because they are really research problems and these are always vaguely worded. Don't let this throw you, just interpret the question in the way that seems most reasonable to you, then give the best answer you can and have fun with it. You need not do any calculations--a qualitative discussion is fine.
Antimatter. Imagine a universe which has an overall equal amount of matter and antimatter, but made of separate "domains" which contain only matter (e.g., protons, electons, etc) or only antimatter (positrons, antiprotons, etc). A sketch of such a universe appears in the left panel of this diagram. Necessarily, the domains will have interfaces at which matter and antimatter come into contact and annihilate.
Imagine the CMB sky contained one or more such
interfaces. Would the annihilations lead to observable effects
in the CMB? What would these be, and how could we detect them.
How might one the observed properties of the CMB to set
constraints on the sizes of the antimatter domains?
Would these be upper or lower limits on the average domain
size?
Dark Matter and Recombination. In our consideration of recombination in the homogeneous universe, we simply ignored dark matter. Yet it should be present. First, explain how we might justify our neglect of dark matter effects on recombination. But go on to imagine what might happen if dark matter had unexpected properties, and how these would affect recombination. You may invent or propose any dark matter property you think is likely or plausible or interesting. For example, you might imagine DM had electric charge (say with equal amounts of positively and negatively charged DM parcticles, perhaps with a charge Q << e). Or you might imgine dark matter as uncharged but able to scatter off of baryons. Or you might imagine the dark matter is uncharged but carries a magnetic moment. Or you might imagine the dark matter is (at least partly) in the form of neutrinos. In any of these cases, would recombination be affected by dark matter interactions with the plasma? If so, would this affect the observed CMB? How would we use the CMB to learn about (or constrain) the interactions of dark matter with ordinary matter?
Dark Energy and Recombination. We also ignored dark energy in our consideration of recombination in the homogeneous universe, yet it too should be present. First, explain how we might justify our neglect of dark energy effects on recombination. But go on to imagine how dark energy might have some influence. For example, consider dark energy which does not interact directly with the plasma, but has some energy density (assume, for the sake of argument, a tracker-type situation in which the dark energy density is of the same order as the matter density). How might this affect recombination? Would this leave any signature on the CMB? How might we use the CMB to learn about (or constrain) dark energy at recombination?
Breifly describe the main technique(s) used to directly detect WIMPs. Some experiments are already up and running--where are some of the these?
(Optional) This past December, the CDMS experiment announced an interesting result. What was the result? Why is it interesting? Why don't they use any form of the verb "to detect" in their paper? What are the future prospects for making this result more solid?
