Three major assumptions of cosmology. What is Universality, Homogeneity, and Isotropy.

Three major assumptions of cosmology.  What is Universality, Homogeneity, and Isotropy.
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Assumptions of Cosmology

In life as it is with science, making general assumptions can get you into trouble. Sometimes assumptions are based on our predisposed view of a subject, sometimes it’s based on all available, though incomplete, information. The latter is what we will touch on in this article.

Astronomers, cosmologists, and theoretical physicists have had the lofty and worthwhile goal of trying to understand how the universe works. Big task to say the least. It’s like trying to duplicate a detective novel with only the last sentence available to you. Fortunately, the universe is kind enough to provide clues to how it works. Through years of observation, and amazing leaps in our understanding of physics (thanks Albert), we have made giant strides. We have moved from Newton, to Einstein, and beyond.

That brings us to the subject of this short article. There are three incredibly important assumptions that scientists use when peering into the night sky for understanding, or when they look below the covers of matter and energy, to see what makes it all tick, and to find out what makes nothing into something. We are not going to go into the long history, or the major issues with each of the three major tenets of cosmology, we are going to define them in simple terms to help us understand the basics of these building blocks of modern cosmology.

So fasten your seat belts.

Here we go.

The Cosmos


Ok, so this may be the most important assumption made by scientist. It is pretty simple to grasp, but it is also the linchpin to our understanding of the universe, and the way it works. We can use sports as an example that can help you relate to the importance of Universality. Take football. The Miami Dolphins can expect to play by the same rules whether they are in Florida, Texas, New Jersey, or even California. There is a standardization, and a uniformity to the rules that make up football, as well as other games like chess, tennis, or golf. Imagine what would happen if sporting rules changed based on the state you played in? Chaos would ensue. No team would want to play away from home because they would not know what to expect. Maybe a forward pass is a foul in Denver, and a Field Goal is worth 1 point in Seattle. Because no one would know what to expect, and there would be no way for teams to prepare for arbitrary changes in the rules, the games would also be unwatchable. The universality of the rules in football makes for a stable sporting environment where even competition can take place.

The same is the case with the universe. We assume that the laws of physics we observe are not unique to our neck of the woods. We assume that gravity works the same here, as it does in the Andromeda galaxy, and beyond. That electromagnetic waves have the same properties, and produce the same effects at home as they would in the far reaches of outer space.

How important is this? Well, if it were not true, we would not be able to discuss the physics of astronomy anymore. It would be a dead subject since you could not explain nor predict anything outside our small little corner of the universe.

Why do we feel confident that our assumption is right? Well, everywhere we point our telescopes, we see stars, solar systems, and galaxies that all play by the same rules. We have a flood of empirical evidence to support this most basic, and important assumption.



This is the principle that says that the distribution of matter does not vary with an observer’s position in the universe. Now this doesn’t mean that matter perfectly spread through out the universe like the peanut butter on a slice of bread. Even in our solar system, we know almost all the stuff is in the middle, the sun, and some nuggets (planets, moons, asteroids) are spread out around it. On the scale of solar systems, as well as galaxies, it would seem that things are not homogeneous, right? Then, in what sense do we make this assumption? Cosmologists are thinking big. Really big. They are not looking at it on the small scale, but on a grand scale. When taking into account the overall structure of the universe, evidence points to a homogeneous universe. Yes filled with irregularities, and variety in structure, but overall homogeneous. And even if our universe is somewhat inhomogeneous on the small scale, the homogeneity principle can still serve as simplified model science can work with with great accuracy.


Principle of isotropy says that the universe looks about the same in every direction you look. Sounds like homogeneity? Well it does, but it’s interesting to note that they are not interdependent. The universe can be homogeneous and not be isotropic, or vice-a-versa. Strange right? But, again, all the evidence we have points to this. Reminds me of that line from Hamlet: “There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy.” And isn’t that a good thing? What a bummer it would be if we figured it all out by now. What would we do with the billions of years to come?


The three cosmological principles we just discussed are the basic assumptions we use to help us explain the workings of the universe. Science bases this on the years of data that has accumulated from many different sources and fields of study. Because of these three assumptions we can make a simple model of the universe that we think we live in. Of course, it’s to much to expect that all assumptions are exactly right in reality. But, by comparing the models of the universe we postulate, and by examining the real universe, we can learn how and why they differ. That lets cosmologists go back to the “chalk board,” and work on updating the theories and models, to make them fit closer to the reality we see.

The quest for knowledge continues. Our journey to better knowing who we are, and where we are going, moves on. Along the way we will make mistakes, as well as discover new and unexpected wonders about our magnificent universe.

What fun!