Cosmology and our View of the World
Physical Cosmology, Part II, Lead: Eberhard Möbius
Summary by Michelle Conroy:
We have only one universe in which to explore, discover and hypothesize. Hence, we have only one universe in which to test our models and theories. It is important that we are able to test our models through more than one independent technique. Yet we still will not know if the universe in which we live is unique or standard, because we have no other universe to compare it to.
Space Expansion versus Moving through Space:
When elemental spectra from distant galaxies are measured here on earth, it is found that the distinctive color bands from the elements are shifted. In other words each line has moved to a longer wavelength. This can be explained through the Doppler shift of light. As an object moves away from us, the frequency of the wave that we see is less than if that object were not moving relative to us. A lower frequency indicates a longer wavelength. Since space expands/stretches in all dimensions, it does not matter in which direction the wave is traveling. Likewise, the amplitude of the wave is not changed by expansion, since it is not a spatial element. Still this does not tell us, whether it is space itself that is expanding or whether it is the galaxies themselves moving through space away from us. Most likely, the shift that is seen on earth is a combination of these effects since undoubtedly the galaxies are moving due to gravity alone. It is also important to recognize, that just because we see galaxies moving away from us in all directions, this does not mean that the earth is the center of the universe.
Recent discoveries indicate that the universe is not only expanding, but it is expanding at an accelerating rate. Will the universe continually expand, or will it fold back in on itself? Again recent research suggests that the universe will continue to expand, growing infinitely large.
Expansion with Waves and Forces:
Are any waves, other than electromagnetic waves, affected by the expansion of space? Sound waves would be affected because as space expands, the density changes. Since, sound waves are dependant on the density of the medium they would be affected, although in a very complicated manner. Gravity waves should indicate the expansion of space, in the same way that electromagnetic waves do, since they travel at the speed of light. However, these waves have not yet been directly detected.
Gravity is the only force that has been shown to retard or prevent expansion. For example, space within our solar system, indeed within our galaxy is not expanding. This is due to the intense force of gravity, near large amounts of mass. Only at a certain distance from large masses will the driving force of expansion be able to overcome the effects of gravity. However, the curvature of space due to gravity has nothing to do with expansion. The curvature due to gravity is a geometrical concept describing gravity, not a physical reality. No other forces are capable of overcoming expansion, since although they are much stronger than gravity, their range is much smaller. Even electromagnetic interactions become insignificant, because any effect of an individual charge or a group of charges will be shielded by a large number of charges. This is because there is an even number of negative and positive charges, and there is no net effect.
Evolution of the Universe:
The universe began as an intensely hot, extremely dense quantum fluctuation. The temperature then began to cool adiabatically as it aged, and is still continuing to do so. In the beginning the entire universe had to maintain a very constant temperature, spatially, since if it did not the universe would not have evolved to its present state. The multi-directional uniformity of the background radiation suggests that even a tiny deviation would have made evolution to our present state impossible. In order, for this to have happened, we must assume that the heat information was communicated throughout the universe. A complication that arises from this is that that information would have had to have been communicated faster than the speed of light.
In the beginning the tiny quantum fluctuations eventually led to the formation of large amounts of mass and an expanding universe. Elementary particles, elements, stars, quasars and galaxies eventually formed as the universe cooled and expanded. We can look back toward this earlier time, because light travels at a finite speed. Therefore, when we look to distant galaxies and quasars (infant galaxies) we are actually looking back in time. Even though, looking deeper into space is the equivalent of looking back in time, we can never see any one galaxy in an earlier stage of formation. Also, there will be a distance, approximately 15 billion light-years, at which we are unable to see past, since the objects at this distance will be moving faster than the speed of light.
The Anthropic Principle:
Is there something unique about our form of life on this planet? Do we affect the universe in which we are trying to study? Is there some meaning behind our fundamental constants? For some reason all of the basic physical constants are the same wherever we have been able to measure them so far. For this reason the Plank length did not change with the expanding universe. Yet a larger question resides in the idea of John Archibald Wheeler’s self-reflecting universe. In his view, we see the universe through our own eyes, and therefore we force the results we see. Nature is inherently quantum.
March 20, 2001