For the next part of my series about the ‘metaphysics’ in metaphysical naturalism I will be analyzing how modern scientific theories about cosmology fit in to the naturalist worldview. Since I am not a professional scientist, I will be quoting authorities for all critical information. The purpose of this article is more philosophical than scientific, in that it does not seek to advance a particular scientific theory, but rather to demonstrate how modern cosmological theories align with the definition of metaphysical naturalism discussed earlier in this series. Feedback is welcome from professionals, if any of the scientific discussion below has factual errors or is unclear. I have worked to quote scientific authorities in their own words as much as possible, in order that their theories be represented as close as possible to their own views.
The study of cosmology has a long history (see here), but since around the end of the 20th century scientists have reached a generally cohesive view of what our universe looks like. The observable universe that we live in is a sphere with a radius of about 46 billion lightyears. Beyond that the unobservable universe is much larger, and is still inflating rapidly. Within this observable part of the universe alone there are at least 100 billion galaxies (and possibly 500 billion galaxies in the whole universe), and, if modern observational estimates among astronomers are correct about there being 17 billion Earth-sized planets in our galaxy, then the rest of the universe no doubt contains many, many more.
In the 1920’s American astronomer Edwin P. Hubble discovered that our universe is not static. Instead, space is expanding rapidly. Current estimates calculate the rate of expansion at 74.3 (plus or minus 2.1) kilometers per second per megaparsec (a megaparsec is roughly 3 million light-years). Since, looking towards the future, the universe is expanding out of control, looking towards the past, we would expect the universe to have emerged from a much smaller point.
The modern theory of the Big Bang answers this question. Scientists have traced the expansion of our universe to a very small initial state about 13.82 billion years ago. Before the expansion, our universe, including its matter and radiation, was compressed into a very hot and very dense point of mass just a few millimeters across. This state is theorized to have existed for a fraction of the first second of time, before a massive blast caused the universe’s matter and energy, and even space and time, to expand rapidly. In the trillionth of a trillionth of a second after the Big Bang, the universe expanded at an unfathomable rate from merely the size of a pebble to being of astronomical scale.
This sequence of events has left us with a vexing question: what caused the Big Bang?