Cosmology and QSO's

Part I

Edwin Hubble made a major contribution to the understanding of the structure of the whole universe. He measured the spectra of distance galaxies. He used independent means to determine the distances to the galaxies. Two methods he used were to look at supergiant elliptical galaxies that tend to dominate clusters of galaxies. These galaxies, he found, were about the same brightness and size regardless of where they appeared in the universe. When they appear fainter or smaller, it was because they were farther away.

Upon taking the spectra of these galaxies, Hubble found that the each one was redshifted - the galaxies were moving away from us. Even more he found that the more distant galaxies were moving away faster. The information was presented in the form of a graph of recessional velocity vs. distance which we call Hubble's Law. The conclusion we draw from Hubble's Law is that the universe is expanding. Using the graph we can get the distance of anything in the universe as follows:

• Obtain the spectrum of a galaxy and measure its redshift
• Using the Doppler equation, compute the recessional velocity
• Using Hubble's Law, read off the distance that corresponds to that recessional velocity

The use of Hubble's Law does rely on the assumption that the redshifts are caused by the universal expansion. The relationship, however, would not be nearly so good if this conclusion were not the fact. Hubble's Law stands today as one of the pillars of Modern Cosmology. The question still remains, however, as to how the universe began.

Two cosmologies developed from Hubble's work:

• The Big Bang Cosmology assumes that the expansion can be drawn back in time to the time when the universe occupied a point. Some process began the expansion (the Big Bang), which continues to this day. Based on the presently observed rate of expansion, we can judge that the universe is 12- 15 billion years old.
• The Steady State Cosmology assumes that the universe always looks more or less the same. This idea accounts for the expansion of the universe by assuming that matter is continually created to fill in the voids left by the expansion. The amount of material required would be entirely too small to be observed.

Prior to the mid-1960's observations could not distinguish between these two cosmologies. Arno Penzias and Robert Wilson at Bell Labs changed all that with their discovery that the universe is emitting blackbody radiation indicative of a blackbody at 3 K. At the same time the Big Bang Cosmologists had determined that the temperature of the Big Bang was sufficient to make Hydrogen and Helium only and, therefore, was limited. Starting with that temperature and expanding the universe for 12 - 15 billion years at the observed rate yielded a universe that was, by now, much cooler. Recall that ideal gases cool when expanded. The present temperature of the Big Bang is 3 K, in exact agreement with the observed microwave background radiation discovered by Wilson and Penzias. Everyone now agrees that the universe began as a Big Bang. But many variations of the basic theory still exist to be sorted out. The 3 K background radiation is another of the pillars of cosmology today.