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:
- Homogeneity: Matter is uniformly spread throughout space.
- Isotropy: The universe looks the same no matter in which
direction one looks.
- 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.
- Hubble's Law shows that the radial velocity of a galaxy is
proportional to its distance.
- This observation is explained by assuming that the entire universe
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
- 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.
- Critical Density: If the universe has the density 4x10(-30)
g/cm(3), then the universe will continue expanding forever.
- 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.