The Cosmological Principle

The Cosmological Principle
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  • 0:02 Science,…
  • 1:04 The Cosmological Principle
  • 3:22 The Major Implications
  • 4:38 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson will dive into the famous cosmological principle by defining and explaining isotropy and homogeneity as well as their important implications for our universe.

Science, Experimentation, & Observation

In the world of science, researchers use controlled experiments to find out if something is true or not in order to understand its nature. For instance, we can test the way a person reacts to a drug to see if it's effective and how so. In a similar fashion, scientists can compare things, like a proposed drug and a placebo, to see if they have the same effect on a person or not. But in the world of cosmology, such experiments and comparisons are not possible.

There is only one universe, we have nothing to compare it to, and for a lot reasons we simply can't actually experiment on the universe like we can in other branches of science. Therefore, cosmologists simply have to accept certain assumptions about the universe in order to try to understand it: assumptions that actually help successfully describe the evolution and structure of our universe. These assumptions, involving the cosmological principle, will be described for you now.

The Cosmological Principle

The assumptions that are made about our universe include that it is isotropic and homogenous. Isotropic means the universe looks approximately the same in all directions. Homogenous means one large region of the universe is approximately the same as any other large region of the universe. The two terms aren't exactly the same.

Isotropy implies that on a large scale the universe looks the same in any direction and, thus, there are no special directions to our universe. Homogeneity implies that on a large scale, the distribution of matter (that is to say, the average density of matter) is approximately the same in any region of the universe and, hence, there are no special locations in the universe. No doubt there will be minor local variations, but these are purposefully overlooked in order for this principle to hold.

For example, some relatively small regions of space will have more galaxies than others, but on average, you'll see about the same number of galaxies no matter where you look so long as you look at a large enough region of space. Furthermore, one part of the universe will look essentially the same as another because it will have the same types of galaxies distributed through that region of space in basically the same kind of way as any other large region of space. Thus, the cosmological principle is then a fundamental principle and assumption of cosmology which says that on a large scale, the universe is both homogenous and isotropic. Now, this principle may be hard to understand, but we can simplify it to something familiar.

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