Physical Cosmology, Lead: Eberhard Möbius

2/4/2003

Summary by Victoria Taibe:

Introduction to Physical Cosmology

The discussion on cosmology cannot begin without discussing the Big Bang theory, first. “The Big Bang Theory is the dominant scientific theory about the origin of the universe. According to the big bang, the universe was created sometime between 10 billion and 20 billion years ago from a cosmic explosion that hurled matter and in all directions”.

According to the big bang theory, the universe started as small as the size of an atom, and has been continuously expanding. Many different topics relating to the big bang and our growing universe were discussed this evening. Space, it is important to note, does not expand into existing space in the universe. Rather, the universe is continuously growing, and space is generated constantly.

Before further discussion on our growing universe, it may be helpful to put our universe into perspective. As discussed in the first class session, if our galaxy were to shrink down by a large factor, the sun may be represented by a 15 cm Styrofoam ball, and earth may be represented as the size of a bearing ball. There would be 15 meters of distance between the sun and the earth, at this scale. Our nearest star (besides the sun), _-Centauri, would be located in San Francisco, if the sun were located on the UNH campus. Another way to look at our galaxy is to compare the distance of planets and stars from the sun, to the distance from earth to the sun. If the earth is 1 AU from the sun, Saturn is 10 AU. a-Centauri will be 2*10^5 AU, which is equivalent to 4 light years. Andromeda Galaxy, our ‘next door neighbor,’ is approximately 2*10^6 light years away. (It is interesting to note that between galaxies, very strong vacuums are found.)

It is also helpful to remember that our observable, or known, universe, is only observable in the past. For instance, light reaching the earth from the sun is already 8 minutes old by the time we on earth can see it. In the same respect, visualized light from Saturn is 80 minutes old, light from a-Centauri is 4 years old, and light from Andromeda is 2 million years old. In mentioning our observable or known universe, this means we can observe and study the universe, but only with a radius of 15-20 billion light years. Beyond that, is the unknown, or unobservable, universe.

Scientists have been studying cosmology for only a few centuries. In this time, great advances have been made which have changed our thoughts of the origins of the universe, evolution, and creationist theory. We can now analyze (light) spectra to determine the distance the planet/star/particles’ light being analyzed is, from earth. Spectra are coined the “fingerprints of the elements,” as the light from each element is reflected in different colors, or wavelengths. Comparing spectra from different and distant planets and galaxies illustrate the Doppler effect—the red shift to the right. The longer wavelength (red) tells us how far away the light was, and therefore how far away the planet/galaxy is. More distant galaxies have a greater red shift to the right. It was mentioned that the spectral red shift is also deemed the cosmological red shift. It is easy to grasp this concept by thinking of in terms of energy—as the universe expands, it cools, and as the universe cools, its energy is decreasing. Lower light frequencies are equal to longer wavelengths, which in our visible spectra, means red wavelengths. Having said that, the constant expansion/cooling can be thought of as the cosmological red shift.

As mentioned earlier, the universe is constantly growing. This means planets and galaxies are constantly moving away from the center, and in relation to us, are constantly moving away from us. It was noted that gravity and mass are the forces that keep galaxies from being pulled apart from itself. These forces keep mass in galaxies, and it is galaxies in the universe that are constantly moving. As discussed, the more distant planets and galaxies are moving away at a faster velocity—there is a linear relationship between the distance and the speed of the galaxies. To create a standard for light and its distance, the Hubble telescope searches for supernovas, as these are the ‘standards’ of the sky. Many other telescopes are in use to map the sky. It must be remembered that these maps are of the universe in the past (as it can only be observed in the past), and that six months of observation go into each map of one corner or pocket of the universe.

Perhaps the most thought-provoking topic in this discussion, in my opinion, was the mention of the “big bang big crash” theory. For mass to move away from the center (for instance, a galaxy moving from the center of the universe following the big bang), it must have a speed faster than the ‘escape speed.’ Otherwise, the mass will fall back towards the center. In the big picture, this theory states that unless the universe is expanding at a speed greater than the ‘escape speed,’ it will eventually collapse back in on itself. Fortunately, this theory hasn’t held too seriously, and it is not thought that the universe will come crashing in on itself.

The last topic touched on in this session was the three main problems with the big bang theory. Briefly, they are:
• 1. The flatness (or fine-tuning) problem
• 2. The horizon problem
• 3. The matter problem
These problems focus on the subjects unexplained by the big bang theory, and alternative explanations must be sought out for said problems.

Mini-discussions brought up by class members included the conflicting ideas of space expanding into nothing, itself, or into other existing space; the big bang-big crash theory; the cosmological redshift and its applications; and how the universe may only be observed in the past—supernovas are seen after the fact, meaning the redshift ‘standards,’ are a historical artifact.

This evening’s discussion focused on the history of the universe, how certain things are gauged and measured, viewed, and studied, among other things. Next week’s discussion, Life as we know and its evolution, will focus on the genetic changes that have been occurring since the beginning of time, to put a biological dimension to cosmology.