INTRODUCTION TO COSMOLOGY

Cosmological Assumptions

Olber's Paradox

A simple observation is that the sky is dark at night? This simple fact has dramatic consequences! Olber's paradox is the question "Why is the sky dark at night?". The consequences of the sky being dark at night is that the universe is cannot be both infinite and unchanging.

Hubble's Law

In the 1920's it was observed that all external galaxies are moving away from our Galaxy. Hubble noticed that the farther away the galaxy is the faster it is moving away from us. Hubble's Law is the linear relationship between the the distances of galaxies and their speed of recession (motion away from us). The slope of this relationship is called Hubble's constant, which is written H0.

The fantastic implciation of Hubble's Law is that the universe is expanding. In order to visualize the behavior of the expansion of space, we discussed the "raisin bread analogy" and the "balloon analogy".

Cosmological Redshifting of Light

Since galaxies are moving away from us, their light is Doppler shifted to longer wavelengths. The spectra of galaxies appear redshifted. However, the physics of this redshifting is quite different than how the Doppler Effect normally works.

While traveling across space and time, light "stretches" as space expands. Recall that light is defined by its wavelength. As the light is stretched during its travels, the wavelength gets longer (the light gets "redder"). The more time the light travels, the more the universe has expanded and the more stretched out the light becomes.

Space is Curved

We use light to define the contours of space and time. Any light beam will follow shape of space as it travels. This can be seen as the gravitational lensing effect, which happens because a large mass concentration "bends" space and time. We discussed this when we talked about black holes. But it is not only black holes that bend space and time. All mass does. The bigger and more concentrated the mass, the more space can be curved and not flat.

Since the universe is filled with matter (mass), it must have an effect on the overall geometry (curvature of space) of the universe as a whole.

The Three Possible Geometric Shapes of the Universe

The three types of geometries for space are

Learning that curvature has been a long standing goal of astronomers. Up until the last decade, we were certain that the fate of the universe depended upon its curvature. As we shall see in the coming lectures, we were very wrong.