Chapter 6: Chapter 6: A Hot Beginning and the Cosmic Photosphere

The Energy Density of Matter and Radiation in the early Universe

Interestingly enough, chapter six deals with the cosmic photosphere. The beginning of the chapter outlines the basic fundamental structure of the atom.

The Universe we observe is a combination of a series of chemical elements, each one with a unique atomic structure. An atom has a nucleus, which is orbited by many electrons. The nucleus holds most of the mass of the atom, however, compared to its mass ratio with the rest of the atom, it only occupies a very minute amount of space in the atom. For example, hydrogen’s nucleus is 99.5 per cent of its total mass, and it is orbited by one electron. The radius of its nucleus however, is 100,000 times smaller than the radius of the entire atom. Personally, I think that it is amazing that something so small is even a measurable quantity. It’s pretty fascinating to know that scientists are even able to measure values that are that small.

As it turns out, although the Universe is made up of many well-known identifiable elements, 99 per cent of the visible universe is actually made up of hydrogen and helium, the two simplest elements. It is well known that the evolution of the Universe is a consequence of the way radiation and atoms interact at high temperatures. Scientists have a general idea of how much energy in the Universe is in atomic form, and how much energy is in the form of radiation.

A quick estimate says that there are roughly one billion galaxies in the Universe, each containing about 100 billion stars. It is assumed that many of these stars and galaxies are composed of hydrogen atoms. From this knowledge, we can estimate that there are a few hydrogen atoms per every cubic metre. This concept is hard to imagine considering that we are and everything else is composed of an infinite number of atoms- there are not that many atoms around compared to the size of the Universe.

In comparison, the Universe is littered with photons. Roughly one billion photons can be found in a cubic metre- this number is much larger than the number of atoms we find in the observable Universe. Now when we picture the Universe, we can picture it consisting mostly of radiation, with a scattered amount of atoms.