Chapter 15 Questions

	I don't understand what made the tight coupling between matter and radiation cease, thus allowing the matter to clump.
	It might be easier to turn this around and ask why matter and radiation interacted in the first place. Neutral atoms interact only with photons of highly specific energies. Free electrons, on the other hand, interact very strongly with photons. Before recombination, there were plenty of free electrons around to scatter photons. At the densities of the early universe, both in matter and in radiation, the constant interaction of matter and photons kept them in thermal equilibrium with one another. A photon could not travel very far before encountering another free electron and being scattered again. Once the free electrons combined with the nuclei to create atoms, very few particles remained to scatter photons. Nearly all of the atoms present at this point in the history of the universe were hydrogen and helium, which scatter very little radiation.
	What are those WIMP and MACHO things?
	A WIMP is a Weakly Interacting Massive Particle. It is some unspecified type of collisionless particle that interacts via the weak interaction. A MACHO is a MAssive Compact Halo Object. It is a compact object made of baryons, inhabiting the halo of the Milky Way (and, by extension, other galaxies). The MACHOs' identity is undetermined, but they may consist mainly of white dwarfs and neutron stars.
	Are there any computer programs that simulate all of these different cosmological models and can they, given data, provide a statistical analysis of which model is most likely to be the correct one?
	Yes, a number of researchers are working on just that. At the moment, computer programs can simulate much of the universe, including expansion and the gravity of ordinary matter and any possible dark matter components. The difficulty is resolution, capturing all the scales correctly from hundreds of megaparsecs down to the sizes of galaxies.
	What is the great attractor? If the Virgo cluster is approaching it, could the great attractor be approaching something else?
	The Great Attractor is a hypothetical center of attraction for the Virgo cluster. It would presumably have to be an enormous concentration of mass, far greater than average. If the Great Attractor is itself approaching something else, that implies an even larger concentration of matter. Increasingly huge concentrations of matter cause difficulties for the cosmological principle (on what scale do things smooth out to average?) and for the idea of structure formation in the universe (how did such huge things form from a reasonably smooth cosmos at the time of recombination?) Measurements of large-scale galaxy motions is a very difficult business, so the existence of the Great Attractor remains, at present, somewhat uncertain.
	How do astronomers put borders on galaxies--if they do at all? For instance, is there a certain spot where the Milky Way ends and another galaxy begins? If so, how is that point determined?
	Typically astronomers adopt a definition of the "edge" of a galaxy based on how the light falls off as you run out of stars. But there is evidence of a mostly nonluminous "halo" surrounding the visible part of many galaxies, and we don't really know how far that halo goes. People tend to think of galaxies as isolated entities but maybe they are more like coral islands. We see the island where the coral pokes up above the reef, but the reef is continuous under the surface of the water. Maybe we just see the densest regions where there are stars, and but there is still a lot of "dark matter" mass between the visible regions.
	Since current estimates of Omega are around 0.01 or so, how likely is it that there is enough dark matter to make Omega = 1?
	The low estimates of Omega are for ordinary baryonic matter. The main reason for thinking that there must be things to add up to Omega=1 is from the WMAP data which shows that the geometry of the universe is flat (or nearly so) which requires that Omega=1. However, it doesn't require that all of that Omega be in the form of ordinary matter. A value of Omega = 0.23 comes from "dark matter" - a form of matter other than ordinary baryons. The balance is in the form of "dark energy" or Lambda.
	Is there an experimental way to test for the existence of dark matter in the universe?
	Look for its gravitational influences on things. How much gravity is required to hold that galaxy cluster together (for instance)? Also the MACHO observations of the Large Magellanic Cloud is a search for dark matter. (Dark matter doesn't have to be exotic matter. Just dark.) One can also try to detect the presence of dark matter through experiments in laboratories.
	In observing deep fields, do astronomers look in any particular direction or can they look in any direction to look "back in time"?
	They look in relatively empty regions of the sky away from the plane of the galaxy. You can look in any direction and look back in time, but you want to have a clear unobstructed view as far out as possible.
	To what extent is the Milky Way affected by neighboring galaxies? Do we belong to a cluster? Do galaxy clusters orbit anything, or move in a pattern?
	We are part of a little cluster of galaxies known as the local group. Our local group is falling toward the Virgo cluster of galaxies. The Virgo cluster may in turn be moving toward another more massive conglomeration of stuff. In clusters galaxies orbit around the center of the cluster, but not on simple orbits (like the planets around the sun). Mostly the have randomly oriented highly elongated orbits.
	What sort of impact does dark matter have on cosmological models?
	Considerable. Dark matter makes up the majority of the gravitating component of the universe, and gravity from the dark matter accounts for much of the overall large scale structure seen in galaxy clusters and superclusters.
	The universe apparently needs lots of dark matter if it is flat. If there was a lambda force would that increase the amount of dark matter needed to make the universe close to the flat model?
	The lambda term (or dark energy) reduces the amount of dark matter required for the universe to be flat.
	Copyright © 2005 John F. Hawley