Chapter 4: An Evolving Universe

The Cosmological Principle, Evolution, and Change

There is nothing exceptional about our place in the Universe; everything in the Universe looks the same. This was originally titled “The Copernican Principle,” but it was later renamed “The Cosmological Principle”. The Cosmological Principle states that regardless of where we are in the Universe, everything looks identical. The principle implies that the Universe is homogeneous. Any measured physical quantity at a random point in the Universe will always have that give value at the same time. It allows us to say that the universe is smooth.

It is inappropriate to assume that everything in the Universe is smooth because everything has a texture. Regardless of how smooth things may be, they will never be perfectly smooth. Even when we view the stars, the sky appears smooth, but it is not smooth- it is scattered with stars and astronomical clusters. Therefore, the Universe is definitely not smooth. Even though the Cosmological Principle insists that it is smooth, and we know that it isn’t, we can still study the properties of the Universe as if it were smooth on such a large scale.

Assisted by the Cosmological Principle, the Universe looks the same wherever we are in reference to the fact that the Universe does not have a centre, nor does it have edges. The Cosmological Principle pays not attention to the large-scale structure of the Universe, but it is a crucial factor in understanding the Universe’s evolutionary history.

When the word evolution is used, it means that there has been a change over time; not necessarily that something has evolved. What can be said about the Universe- has it evolved or has it merely changed over time? Take the stars, for example. When you look at the stars, it appears that there is no change- there is no motion. Now look at the Earth and the other planets, they are always moving in their orbits, and they sky appears different at different times of the year. But really, it maintains its normal state. Considering the state of the Universe at this point, each planet revolves around the Sun, situated at a specific distance from it, where they have been situated for the past five billion years. It is true that stars do move- they have velocities, which cause a shift in angular positions in the sky. However, despite the changes the Universe undergoes, it is simply changing with respect to time, not evolving.

Newton’s belief in a static universe caused him to reconstruct his theory of gravity. What wasn’t recognized was gravity’s attractive nature. A concentration of mass at the centre of space would tend to pull everything towards it. With this idea, Newton boasted, that in a finite universe all exterior matter would be attracted to this central matter and would fall towards this central matter. However, he refused to accept this for the entire Universe, so he then conjured that all matter was evenly disposed in an infinite space. This prevented the collapse of matter because it was evenly dispersed in the Universe and in a state of equilibrium. Although Newton was insightful, his new idea was not applicable to the concept of gravity. It was not until Einstein’s general theory of relativity that it became clear that the Universe was and is evolving.