Summary of the lecture on 06-17-96
The course will be divided into a cosmology part which will deal with questions concerning the global properties of the universe, its creation and its evolution leading to the emergence of life, and a life part which will deal with the emergence and evolution of life on our planet as well as the possibility of life on other planets and how we might try to communicate with extraterrestrial intelligence.
One may define cosmology as "the study of the universe as a whole without worrying about the details".There are several aspects of cosmology:
- Theoretical (Mathematical) Cosmology: Describes the properties of the universe using mathematics.
- Astrophysics of the Early Universe: Describes the physics that went on in the moments immediately after the Big Bang under conditions that are too extreme (high temperature, pressure, density) to be reproduced in laboratories.
- Observational Cosmology: Uses observations of distant objects to constrain cosmological theories.
- Religious Aspect: Cosmology deals with issues (such as the beginning and the end of the universe) which are also dealt with by many religions.
- Philosophical Aspect: It is often necessary to accept certain assumptions which cannot be proven or disproven (e.g., see "cosmological principle" below) to be able to interpret cosmological theories and observations. In contrast to most other fields of science, these are often still debated by the cosmologists.
A fundamental problem of cosmology is that there is only one example of a universe, so it is hard to tell which of its properties are general properties and which are accidents. If we were able to turn the clock back to the beginning, which of the properties would recur almost every time we "restarted" the universe and which would we only encounter in, say, 0.1% of all "universes". The problem is similar for the emergence of life: Since life on Earth is the only type of life we know, it is hard to tell whether all life forms everywhere in the universe are based on water, carbon, etc.
There are 3 basic types of models (pictures) of the universe:
- Ad Hoc Models: Explain only what has already been observed and add new pieces to the theory whenever there is a new observation that cannot yet be explained by the theory. Do not make any predictions that can be verified or falsified by observations. The example given in lecture was that of the mouse living on the back-side of the moon causing the phases of the moon.
- "Supernatural" Models: Explain the universe by introducing supernatural powers that do not have to obey the laws of nature. Therefore, there is no way to falsify this kind of theory.
- Scientific Theories: A theory can be considered scientific if it makes predictions about things that have not been observed or measured in addition to those that have already been observed. This makes the theory falsifiable.
The Cosmological Principle is the assumption that the universe is homogeneous (the same everywhere) and isotropic (the same in all directions) on large scales (it is obviously not correct on "small" scales, like that of the solar system or galaxies). It is an extension of the Copernican Principle which says that we are not in a special place at the center of the solar system, but just one of nine planets.
The Cosmological Principle has not been proven or disproven, but it appears to be consistent with the parts of the universe that have been observed.
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