Overview on Cosmology

Lead: Eberhard Möbius

1/31/2005

Summary by Andrew Russell:

Readings:

"God for the 21st Century" Part 1, by Russell Stannard ed.

"Curtains of the Universe", Astronomy, March 1995

In this, the second class meeting, Dr. Eberhard Moebius lectured on the physical dimensions of the known universe, so as to give the students a sense of scale. To give a sense of the enormity of size of significant objects on the cosmological scale, a scaled down version of the solar system was presented and the scaled down distances were discussed. It was useful to orient us in terms of the distances discussed to see it demonstrated that if the sun were 14 cm in diameter, most of the planets would be smaller that the head of a pin, the biggest would be the size of a large marble, and the whole solar system would still take up all of downtown Durham when the orbits are taken into consideration. The nearest star to our solar system (excluding our star, the sun, of course), Alpha Centauri, would be approximately as far away as California. It is hard to imagine that stars within a galaxy would be so far apart, and yet, that is the case. It was also mentioned that while stars cluster into galaxies, galaxies themselves also cluster.

The use of light as a mechanism for peering into the past was also discussed. Since the speed of light is finite (3x10^8m/s), the light we are receiving must have taken a calculable amount of time to reach the observer here on earth. Looking greater and greater distances, then, allows us to see events that happened further and further back in time.

Astronomer Edwin Hubble’s controversial observation of red-shifted light emitted by distant galaxies was discussed. By noticing that the emitted light spectra of distant stars’ burning hydrogen were shifted toward the red end of the visible light spectrum, and combining this with the Doppler effect, Hubble realized that other galaxies are moving away from us, and furthermore, that the more distant they are, the faster they are moving away from us. This observation disproved the Static model of an unchanging universe, but left us with two possible interpretations: either we are at the center of an expanding universe, or that the whole universe is expanding, and all of the galaxies are moving away from all the other galaxies.

This observable, expanding universe, can be run backward in time by physical models to calculate a time since the beginning of the universe: 13.7 billion years. This limited age of the universe gives a kind of answer to the Olbers paradox (If the universe is infinite, then stars would fill every inch of the sky and be bright white.) by showing that an infinite universe that is not infinitely old would not have the time it takes for the light from it’s limitless number of stars to reach the Earth.

Looking out in space, and therefore back in time, we observe that the galaxies were not always as they are now. Before recognizable galaxies were around, things called quasars (powerful radio wave emitting quasi-stars) dominated the landscape (spacescape?). Back in the time dominated by quasars, the universe was also seen to be much hotter than it is now (at 300,000 years old, the average temperature was 3000 degrees K, while today it is about 2.7 K). Further back than quasars, we see an opaque barrier from the time when the universe was too hot to permit the transmission of photons through space.

The NASA project CoBE was launched to measure background radiation in the universe to see if, in fact, we were moving relative to space, and indeed we are moving. More importantly than this is the realization that the background radiation present in the universe is not uniform, but “clumpy.” This is reminiscent of the observation that galaxies also group together, implying that an even distribution of anything in the observable universe would be an exception, not the rule.

The Big Bang model of the origin of the universe is, sadly, not capable of explaining everything.

• For example, there is a problem with the rate of expansion, called the Flatness, or Fine-tuning, Problem. If the universe had expanded too slowly, it would have collapsed back on itself under the weight of it’s gravity, which clearly didn’t happen. If the universe had expanded too fast, it would cool too quickly and spread out too much for most of the observable phenomena we see to exist. The rate of expansion would have to have been very precise, accurate to 0.000,000,000,000,001% at 3 minutes after the Big Bang.

• The Horizon Problem, or the idea that while the observable universe is the same in every direction, no communication would be possible between ends due to the limited time in the universe and the immense distance involved.

• The Matter Problem is the idea that according to our understanding of how matter came into existence in our universe, there is no good reason why there should be any matter left in the universe. If matter and antimatter both came into existence in equal amounts, why haven’t they cancelled each other out already?

These problems, while troubling, don’t disprove the Big Bang model, but offer it a few challenges. Incorporating some of these problems into the Big Bang model, we end up with a newer model, the Inflation Model. In the inflation model, we theorize that the universe must have expanded more quickly in the beginning and slowed it’s rate of expansion later in its development.

Lastly, we discussed what is known as the Anthropic Principle. The question at hand is: why should this universe be so particularly suited for us to be here? This question is answered in a couple of ways. The Strong Anthropic Principle is the idea that the universe was correctly designed for life, and that it was no accident. This perspective of a specific design at work in the universe implies an intelligence of some sort at the controls. The Weak Anthropic Principle answers the question by saying that it didn’t have to be this way. Any of a multitude of universes were possible, and while we ask why it ended up this way, we forget that if it didn’t we couldn’t be here to ask why. It is difficult to judge the factors that made this universe when the only universe we can observe is this one. It seems we can’t say too much about universe making because we live in one and have only known one. The universe is only able to contemplate itself through us, as John Archibald Wheeler’s Self-Reflecting Universe idea has stated.