What is the Universe? (Part 1)

What lies outside the universe?

According to the big bang theory, the universe started as an infinitesimal and unimaginably-dense collection of hot matter. This collection (the matter content of the universe) has since been expanding, cooling down and forming all the familiar objects we see in existence today. This includes all galaxies, stars, planets, buildings, cars and humans. When we first learned about the big bang, and perhaps even today when we think about it, we probably picture it as an expanding object, much like a balloon being inflated by blowing air into it.

This is a useful analogy as it allows us to make sense of why things in the universe get further apart with time. Galaxies can be pictured as dots lying on the surface of the balloon and moving away from each other as the balloon gets bigger. However, there is a hidden intuitive pitfall within this analogy; the balloon is a physical object which is expanding to occupy more space while, in the case of the universe, it is space itself which is expanding.

Confusing the two leads to several interesting reasoning errors, most notably asking the question: what lies outside the universe? Here the question assumes the existence of a "container space" inside which the space of our universe exists as illustrated below.

This assumption contradicts our definition of the universe as the entirety of space and is therefore invalid. The universe is all of space and no more space exists "outside" or "elsewhere".

The question "what lies outside the universe?" is a loaded question (see Complex Question Fallacy) in which the assumption of a container space is implicitly made. To answer this with "something", "nothing", "possibly something or nothing", or "I don’t know" is to be wrong in all cases. The correct answer is that the phrase "outside the universe" does not refer to anything meaningful. Such is the case with all phrases that attempt to apply spatial adjectives and relationships to the universe as a whole (including "inside the universe", "bigger than the universe" and the more common "beyond the universe").

This makes sense logically but is very counter-intuitive. For instance, what happens if I travel to the boundary of the universe and try to stick my hand "out"? Will the atoms of my hand "be lost into the void" or will they hit an invisible wall onto which is inscribed "Boundary of the Universe – No matter shall pass"?

Several theoretical models of space give answers to the above. For example, three-torus models (T3) predict that travelling long enough in any direction will get you back to the same point, making the universe one big pac-man game. This is an attractive property from a modeling point of view as it means that each point in space has a symmetric outlook at other points and that there are no special cases (e.g. edges or corners). Despite this and other favorable predictive characteristics, these models defy our intuitive view of the universe as a three dimensional Euclidean space (R3). This is not a surprise however; our brains did not evolve in environments in which solving problems concerning the nature of space was necessary for our survival.

See this paper for an accessible overview of universe topology (including the distinction between topology and geometry).

In the following parts of this series I will describe two more intuitive traps that await the unwary when reasoning informally about the universe.

Continue to What is the Universe? (Part 2)