Evolution of the Universe

Has the Universe always been around? If not, how did it begin? There were two theories about the evolution of the Universe to describe these two ideas: the Steady State Model and the Big Bang Model.


The Steady State Model tells us that the universe has always been around and will always be around. It just is. The Steady State Theory was championed by Fred Hoyle. The idea is that even though the Universe is expanding, as demonstrated by Edwin Hubble and the Hubble law, in order to see the universe as homogeneous, galaxies must be continuously created in the empty space between existing galaxies. In other words, matter must be continuously created and based on Einstein mass-energy equivalency, energy must also be continuously created. The theory requires that the universe looks the same in all directions at all times. This is referred to as the Perfect Cosmological Principle - the Universe looks the same in all places at all times. If we were able to see the universe 5 billion years ago or 10 billion years ago, or even 50 billion years in the future, the universe would look the same. However, we can actually observe the universe from 10  billion years ago and 5 billion years ago and can see that the universe does not look the same as it does today. The universe appears more crowded (i.e. denser) in the past than today. So what does this tell us?


The Big Bang Model is the theory that the Universe began about 14 billion years ago (the age of the Unverse is under debate with the prevailing idea that the Universe is about 13.8 billion years old, give or take a few hundred million years) with a sudden explosion of matter, energy, and space into nothing. Note, that the Big Bang does not say that matter and energy exploded into space, but that the singularity that started the Universe included space with the matter and energy. This means that all the matter and energy that exists in the Universe now equals the matter and energy at the beginning and that the amount of matter and energy will be the same in the distant future. This also tells us that as the Universe expands, the distance between galaxies will also increase which is what is observed. The Big Bang Theory was actually coined sarcastically by Fred Hoyle who as mentioned above was an advocate for the Steady State Theory. The Big Bang Theory tells us that while the Universe may change with time, it does not change with space and is a feature of the Cosmological Principle. We will talk more about the Big Bang Theory in the next post.