Summary of the lecture on 07-10-96

This lecture was held by Prof. Bruce Balick who also provided the following outline/summary.

Review:

universe expands (in such a way that any observer sees more distant objects at ever larger redshifts) and cools Light elements freeze out of the soup of p's and n's leaving a tiny residue of D (the residue depends very sensitively on the density of the universe during the freezeout) Dark matter formed??? Evidence: deuterium abundance coupled with understanding of gravitational forces in the universe today Atoms form from ions as the free electrons recombine when the universe gets cold enough (after <~10^6 y). At this point universe turns from opaque to clear, releasing the microwave photons that we now see coming from huge distnaces (and large redshift ~ 1000)

State of today's universe:

Uncountable galaxies strewn throughout space (in clusters & superclusters), observed by the light from their dense stars. Between the galaxies are voids in the baryons (very low density). Background photons (about 10^9 of them for every atom) fill the universe uniformly, as we observe from Earth. Where is the dark matter? Hard to determine directly.

Issue: How do we get from the almost featureless universe at z ~ 1000 to today's highly structured disvtirbutions of stars, galaxies, clusters, etc? No completely successful picture, primarily because the seeds out of which galaxies could have grown have not been clearly identified.

Issue: Even if the details aren't clear, can we surmise the PROCESSes that would lead to the formation of galaxies? Yes: gravity (attraction), thermal pressure (expansion; fights against gravity), and rotation are probably all relevant.

How might galaxies form? Need to know the "initial conditions" which, of course, are no longer observable. So we ASSUME a fairly smooth distribution of material with a few mild lumps which may roate very slowly (that is, we IMPOSE the requirement of structure as an assumption) Gravity forces the lumps to collapse inward. The compression of infall causes heating, which slows down the collapse if the heat is trapped inside. As any spinning object shrinks, it spins up. In a fluid, this can cause the formation of a spinning disk supported by its angular momentum.

In this way we can imagine the formation of a halo of early-to-form stars, a central bulge of somewhat older ones, and a relatively dense rotating disk where stars are presently forming. Good news: that's what we see!

Stars and solar systems form much the same way -- just the size scales are different. That is a galaxy forms, concentrating lumpy matter out of which stars and their planets form from the fragments. The theory is said to be "hierarchical" - the same sort of process works both on very large and much smaller scales...at least until all of the gas available is dissipated or locked into dead solid masses (old stars & planets).

Since it was a beautiful day in Seattle, we spent the last 25 minutes of the class setting up our 10'' telescope on the obseving deck to take a look at the sunspots (and check out what people were having for lunch in the Space Needle Restaurant).

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bbeck@astro.washington.edu