The Way of Science

Cosmology: Birth and Death of the Universe

We began this semester with some extraordinary and fantastic claims of the "paranormal." The material on cosmology may strike many of you as equally bizarre, but there is a vast difference in the empirical evidence behind these two topics. In the case of the "psychics," homeopathy, etc. we would have to throw out many of the natural laws of physics and chemistry; for cosmology, the insights come from applying those laws to distant times and places. The evidence for the "Big Bang" model is very strong, and in the next two weeks you will become familiar with the "old" Big Bang, and then see how this original version is being modified in rather radical ways. Besides giving you an opportunity to understand the basics of cosmology and relativity, this section opens up vast new horizons of thought. For most of human history, the question of origins has been the domain of theologians and philosophers. Darwin's Origin of Species crossed that line for biology; this century has, for the first time, seen physics and astronomy push firmly into the origins of everything: all matter and energy, all space and time. As you might expect, the effect on religion and philosophy has been enormous. Some people (including a few scientists) see evidence for a God in the "Big Bang" model. Others see exactly the opposite, and the debate gets more interesting with each new book on the subject.

I. Reading assignments

Read the following material in Hazen and Trefil as we cover the topics in class. Begin with Chapter 3, Electricity and Magnetism (particularly pp. 45-53, the EM spectrum); Chapter 9, Particle Physics (particularly for the four forces. Treat pp. 129-133 as optional reading); Chapters 10, 11 and 12 (Astronomy, The Cosmos, Relativity. These are particularly important).

There are many excellent and recent books available on cosmology, some of which are in the college library. We particularly recommend any by Timothy Ferris (Coming of Age in the Milky Way, The Whole Shebang, etc.).

II. The Universe

In order to "explain" everything, we must have an accurate picture of what the Universe looks like, its size, and what it is doing. As in all good science, a model is built on many lines of observation and experiment, all cross-checking each other. We will concentrate on one such major path (triangulation to Cepheids to red shift); remember, though, that the conclusions are corroborated (or perhaps contradicted) by other approaches. Let us now begin a sketch of the Universe.

First, let us put in place a working definition for the term "Universe." The Universe consists of all matter and energy ("stuff"), plus all continuous and connected spacetime (location) in which they are embedded.

You already know (from the section on thermodynamics) that matter and energy are basically the same "stuff." Where matter and energy is located is spacetime. Note that this is a single word; just as one must consider both matter and energy when talking about all the "stuff" in the Universe, so must one talk about the three dimensions of space and one of time when considering location. With these Einsteinian definitions in place, we must now add activity to the mix.

Matter and energy and spacetime are not passive; they interact in apparently endless ways. Like any good science, physics has reduced all this frenzied activity to just four basic patterns. These fundamental patterns are called interactions, or the four forces. (Be aware that the four "force" is used here in a very different sense from the common one of a "push.") Two of these four (gravity, electromagnetism) are quite familiar to you from your daily experience. The other two (weak nuclear force, strong nuclear force) are not directly accessible to your senses, because they are active only on a very small scale. If we view these four forces in the manner of modern physics, we would say that gravity and EM have fields that extend indefinitely (think back to the demonstration of the magnetic field in the paleomagnetism section), but the other two forces' fields are limited to within atoms.

The strong nuclear force holds the nucleus and its components together; the weak force is part of the mechanism for certain kinds of radioactive decay (remember radiometric dating?). Of the two forces with effectively infinite fields, gravity is very weak, and EM much, much stronger. You can intuitively "understand" gravity, in that you are large enough to feel its weak pull at all times. What, then, is your direct perception of electromagnetism? That's an easy one: everything else that is happening to you and around you that isn't attraction of mass for mass (gravity) is a result of EM interactions. Obviously anything connected with electricity and magnetism belongs here, but also in this package are chemical reactions, plus anything involving light, or radio waves, or X-rays, etc. Friction and surface tension phenomena belong here. (Think of a fly walking on the ceiling; gravity is nearly non-existent for such a small mass, and surface phenomena take over.)

We will return to gravity later, when Einsteinian relativity introduces a rather odd way of looking at this force. In the meantime, it is very important that you understand some details of the EM spectrum, since most of our information about the Universe comes to us via these channels.

Here, then, is what you need as background from careful reading of your text.

1. Be able to describe waves in terms of frequency and wavelength. Know the location (high/low frequency, long/short wavelength, high/low energy) of X-rays, UV, IR, radio waves, microwaves, gamma rays, and the spectrum of visible light from violet to red.
   
   
2. File in your memory bank the concept/definition of red shift and blue shift, as follows. Suppose a source of EM waves is emitting light of a given frequency. A detector, however, says that the wavelength received is not what is being emitted. If the "new" waves have a longer wavelength than the original source, one can say that the radiation has been red shifted, regardless of where the starting point was. (In other words, red light can undergo red shift, and perhaps be detected as IR.) Similarly, an apparent shift toward shorter wavelength, or higher frequencies, is called blue shift. Understanding these shifts (that will come soon) is critical for understanding our modern picture of the expanding Universe.

Most of our information about the past and present Universe comes to us via the EM spectrum. The days of peering through a telescope, using nothing but visible light, are long gone. Detectors now use radio waves, X-rays, gamma rays, IR and UV. Given enough input, it is possible to describe not only what is happening now, but what the past was like. (Recall how we did the same sort of approach with biological Evolution. Cosmology and Evolution have, in fact, much in common. It's very hard to do experiments with galaxies or extinct organisms. Nevertheless, a comparative/observational approach can yield vast quantities of information. Both disciplines use the same basic assumption - a good one - about the laws of physics, chemistry and biology: they are the same everywhere and everywhen.)