Galaxies and the Universe - Observational Cosmology

Galaxies and the Universe - Observational Cosmology -...

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1/15/12 Gala[ieV and Whe UniYeuVe - ObVeuYaWional CoVmolog\ 1/6 ZZZ.aVWu.Xa.edX/keel/gala[ieV/obVcoVmo.hWml Observational Cosmology: World Models and Classical Tests One of the fundamental questions of astronomy is "How did this all get here?" (Kleinmann 1988 private communication). This is deeply tied to observations bearing on the formation, evolution, and distribution of galaxies. First we develop some basic consideration on the "standard" big-bang picture, then deal with the relevant observations of the extragalactic universe. The Hubble e[pansion (conventionally interpreted as an expansion of spacetime with galaxies carried along for the ride) and cosmological principle together imply one of two kinds of universe: "Big Bang" - the universe is expanding from some initial configuration of (arbitrarily high) initial density. "Stead\ State" - involving a "perfect cosmological principle"; matter is being created and the expansion is driven by some long-range repulsive action. There are several immediately relevant data to help us choose a best bet scheme: Olbers' paradox Stellar and galaxy ages Microwave background log N - log S relation for QSOs and radio sources Color-magnitude distribution of high-redshift galaxies Relative light-element abundances The most useful description of cosmological models involves general relativity. Its geometric basis allows a natural treatment of light propagation along geodisics, which is how we get most of our information. Static solutions to Einstein's equations are unstable unless there is some repulsive force giving rise to nonzero cosmological constant Λ. A description of this kind has two locally observable parameters, in terms of the behavior of a scale factor R . This is defined so that the evolution of the distance between two comoving points (that is with no peculiar velocities superimposed on those from the cosmological expansion) evolves as r 12 = r 0 R(W) . These parameters are the Hubble ratio (not quite constant since it will in general change with cosmic time) and the density or deceleration parameter For both of these, a subscript 0 indicates their evaluation at the present epoch. q is related to the open or closed nature of the universe. The critical closure density is ȡ 0 = 3 H² / 8 ȡG as may be easily derived from Newtonian
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1/15/12 Gala[ieV and Whe UniYeuVe - ObVeuYaWional CoVmolog\ 2/6 ZZZ.aVWu.Xa.edX/keel/gala[ieV/obVcoVmo.hWml physics. Consider a volume element of gas in a uniform medium of density ρ at distance r from some arbitrary central point. Using a theorem of Newton, the gravitational attraction of a uniform medium outside r has no net effect, so the gravitational force due to the material within r generates a potential - GM(< r)ρ dV/ r = -4G ρ r ² ρ² dV /3 for the matter in the volume element dV . Its kinetic energy for a Hubble-like flow is mv ²/2 or ρ dV Hð rð /2 . At critical density, the net energy is zero or alternately the material in
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Galaxies and the Universe - Observational Cosmology -...

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