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.
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.
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.
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.
The low estimates of Omega are for ordinary matter. You need to hypothesize some new type of matter to make Omega=1. There is some indirect and uncertain evidence for larger Omegas, but the issue is far from decided. The main reason for thinking that there must be things to add up to Omega=1 is from the inflationary model (which requires that Omega=1; but it doesn't require that all of that Omega be in the form of ordinary matter).
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.)
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.
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.
A lot. Or maybe very little. Depends on how much dark matter there is.
If there is a positive lambda it reduces the amount of dark matter required.
Copyright © 1998 John F. Hawley