Summary of the lecture on 06-19-96
Hubble Expansion
It was discovered by Hubble in 1929/30, that galaxies in the distant universe recede from us at a rate that is proportional to their distance (it is important to note that this is not true for the stars we see in the night sky which all belong to the Milky Way galaxy which does not expand). The relation between the recession velocity and the distance of galaxies is called the Hubble Relation.
v=H0d
where v is the recession velocity, H0 is the Hubble constant and d is the distance. The easy part is to measure the recession velocity: all one has to do is measure the Doppler shift of spectral lines in the spectrum of the galaxies which is related to the recession velocity by a simple formula. It is, however, much harder to get accurate distances, which is the reason why there is still some dispute over the true value of the Hubble constant 66 years after Hubble discovered his relation. The currently best way to determine distances is to use a certain type of variable star called Cepheid which can be found in many galaxies. The period of its variation is related to its luminosity (the longer the period, the brighter the Cepheid), so one can determine its intrinsic brightness and therby its distance from measuring its period. The value of the Hubble constant currently accepted by most astronomers is 70-80 (km/s)/Mpc. If one assumes that the galaxies have always receeded from us at their current velocity, one can calculate the age of the universe fom the Hubble constant: since time = distance / velocity and
H0 = velocity / distance, the age of the universe is 1/H0. If the assumption of constant velocity is incorrect, this time (called the Hubble Time) is not the same as the age of the universe. If the expansion has been decelerated, the age is smaller than the Hubble Time, if it has been accelerated, it is larger then the Hubble Time.
Olbers' Paradox
A second basic cosmological observation is the fact that the night sky is dark. At first sight, this may seem to be a trivial statement. Its deeper meaning becomes clear, however, when one realizes that if the universe was infinite, the line of sight in every direction would eventually intersect the surface of a star. Therfore, the entire sky should be as bright as the surface of the sun. The fact that it is not is commonly referred to as the Olbers' Paradox named after the German scientist Heinrich Olbers. Olbers suggested that a way out of this may be intersecting dust clouds which block out starlight in certain directions. However, these dust clouds would eventually come into thermodynamic equilibrium with the stars and be heated to the same temperature as the stars, thus radiating as bright as the stars themselves. There is, however, a way out of this: since the universe has a finite age and light travels at a finite speed, there is a certain distance beyond which light couldn't have reached us, even if the universe were infinite.
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