Cosmology is the study of the Universe itself. Here, we will study the principles that we believe govern the way the universe behaves, based on observation and assumption. We will study the geometry of spacetime and gain understanding ot what we mean by curvature of spacetime. We will analyze the evidence that supports out theory, the cosmic microwave background radiation. We will also focus on what makes up the universe and how dark matter and dark energy play roles in the behavior of the universe.
The cosmological principle is based on two observations and an assumption, that the universe is the same everywhere, it is the same in every direction, and that the laws of physics are the same everywhere.
The statement that the universe is homogeneous regards the universe on very large scales. If you look at the distribution of matter inside a galaxy and outside of a galaxy, the matter distribution is obviously not the same. If you look at the the Hubble Deep Field, it is easier to see what the cosmological principle means. A large expanse of the universe contains many galaxies. On an even larger scale, we see there are great clusters of galaxies with voids between. On an even larger scale, the pattern of clusters and voids begins to look like an overall pattern in the universe.
A pencil-beam survey takes a lot of narrow-beamed samples of galaxies. The main takeaway here is that no matter what direction the beam survey is pointed, it sees roughly the same distribution of galaxies over space. It is tool which allows us to assess the isotropic nature of the universe on cosmological scales.
observed wavelength = 6x greater than at time of emission
light was emitted when the universe was 1/6 present size