Cosmological principle definition11/29/2023 ![]() I will briefly discuss the “horizon problem” here, before talking about the “anisotropies” in the CMB in later posts (these are the 1 part in 100,000 fluctuations that you can’t see in the above picture because they are too small). And they all look exactly alike (to 1 part in 100,000)! The first measurement of the very smooth spectrum of the CMB provided strong supporting evidence to the foundational hypothesis of cosmology, as the universe truly does look the same in all directions (it’s slightly harder to convince yourself of homogeneity, that the universe looks the same at every point, but Copernicus can help here - if we proceed under the conservative assumption that we do not live in a particularly special place in the universe, we can conclude that since the universe is isotropic around us, it should be isotropic everywhere. ![]() Viewed in that context (I originally wrote “viewed in that light” but didn’t want anyone to think I was making a pun), this rather boring picture of the CMB (taken from the COBE satellite in the early 1990s) becomes much more exciting - as already discussed, the CMB photons are coming from all corners of the universe. Our universe is like that, if you believe the cosmological principle). From far away, though, one section of the canvas looks much like any other section, as they are all one color. ![]() Up close, you can see individual brush strokes with a great variation from place to place. This makes it hard to test the hypotheses, as we need to go to larger and larger length scales to really see this principle in action, by averaging large volumes (using painting again as an example, imagine a canvas entirely of one color. Looking at the Milky Way is clearly different from looking at other parts of the sky. Over short scales, this is obviously not true. These two hypotheses are together known as the “cosmological principle,” without which much of our presumed understanding of the workings of the universe would be invalid. Homogeneity contends that the universe is the same at all points. Isotropy is a statement that the universe is the same in all directions (the universe looks the same whether you are looking directly outward from the North Pole or the South Pole). This is a difficult task and would probably not be possible without a basic assumption about the universe - that it is spatially homogeneous and isotropic on large scales. In modern physics, cosmology begins with the application of Einstein’s theory of gravity, or General Relativity, to the universe. The definition of cosmology is the study of the structure and evolution of the universe.
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