Concepts of Cosmology 

Cosmology is the study of the universe as a whole.

A. The Cosmological Principle: any observer in any galaxy sees the same general features of the universe as any other. Based upon:

1. Homogeneity: Matter is uniformly spread throughout space.
2. Isotropy: The universe looks the same no matter in which direction one looks.
3. Universality: The laws of physics that work on Earth work the same in every part of the universe.

B. Red-Shifted light from galaxies means that the galaxies are receding from us.

1. Hubble's Law shows that the radial velocity of a galaxy is proportional to its distance.
2. This observation is explained by assuming that the entire universe is expanding.

A suitable metaphor for this expansion is the "raisin bread model." Assume that you are on a particular raisin. As the loaf of bread expands in baking, the raisins that are closest to you appear to recede. The farther raisins appear to recede even faster. Those furthest away, fastest of all. Yet the bread actually expands at a constant rate. This model, with stars and galaxies as the raisins, and the universe itself as the bread, explains the red shift.

C. Big Bang Theory: When the expanding universe picture is run backward, it leads to a point of common origin for all matter in the universe. This point of common origin is the Big Bang, where everything in the known universe was created in a single instant. Because time and space also originated in the Big Bang, it is not possible to state that any specific location of space is where the Big Bang occurred. Every point in the universe was there when the Big Bang happened. Thus, every point in the universe is where the Big Bang happened. The cosmic microwave background radiation is the echo of the Big Bang. When the Big Bang occurred, it sent an enormous electromagnetic wave out into the universe. As the universe expanded, the wave itself was stretched-out, much like a phone cord is stretched as you walk away from the base. The stretched-out wave would be very long and have very low energy. Two scientists, A. Penzias and R. Wilson, discovered this radiation, and we therefore have experimental verification of the Big Bang.

D. There are three alternative theories that deal with the future of the universe.

1. Big Crunch: If the universe has a density greater than the critical density, then the collective gravity of all the matter in the universe will cause it to stop expanding and begin to contract. One variation of the Big Crunch theory is that after the universe collapses, it bounces back in another Big Bang. This idea, known as the oscillating universe theory, is not generally accepted because the oscillations would lose a great deal of energy to entropy.
2. Critical Density: If the universe has the density 4x10(-30) g/cm(3), then the universe will continue expanding forever.
3. Expanding Universe: If the universe has a density less than the critical density, which it does according to current scientific theory, then the universe will keep expanding forever.

This leads to the not generally accepted notion of Heat Death; wherein, as the universe keeps expanding and cooling off, everything runs out of energy. All the suns will die, galaxies will be gone, etc.

E. Olber's paradox: If the universe is static, infinite, eternal and uniformly filled with stars, then why is the sky dark at night? If we look in any direction in the sky, we are bound to be looking at a star, no matter how distant. Hence the nighttime sky should be as bright as the surface of a star. The resolution of the paradox can be found in noting that the assumptions made in the paradox are incorrect. First, the galaxies are all receding from us. Much of their light is red-shifted to the extent that the energy of the light is undetectable. Second, the universe is not eternal; it was created in the Big Bang. Therefore, light from the most distant stars has not had enough time to reach us.