What is the Universe? – Part 1

February 12, 2013

The universe is not just the entirety of matter and energy. It is also the entirety of space and time themselves. In this post series I will try to examine some conclusions that follow from this simple definition and point out few intuitive but mistaken notions that are often used in discussions involving the nature of existence (Ontology).

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.

Balloon vs. Universe Expansion

Balloon vs. Universe Expansion

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.

Balloon expanding into space (left) and space itself expanding within a container space (right)

Balloon in space (left) and space itself within a container space (right)

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, none of which makes intuitive sense. For example, one model predicts that travelling long enough in any direction will get you back to the same point (making the universe one big pac-man game!). The fact that our intuition fails to grasp explanations of this sort (even after being proven by formal reasoning) is no wonder: our brains did not evolve in environments in which solving problems concerning the nature of space was necessary for our survival.

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

March 2, 2014 @ 4:45 pm

Where is your definition of the universe coming from?

    March 2, 2014 @ 8:40 pm

    This is a personal definition.

March 11, 2014 @ 9:55 am

Interesting stuff! I read all 3 parts. I’ll have to think about things and get back to you.

However, one thing sticks out. I agree that the consequences you describe are indeed of the way the universe is defined. My question is: Is this a standard definition within the scientific community?

The reason this question is important is that when physicists say that the “universe is expanding”, they are probably using a certain definition of the universe in that instance.

So, if your definition and the standard definition don’t match, it’s not correct to state something like “It’s space itself that is expanding”, because saying that the “universe is expanding” and “space is expanding” are only equivalent if we use that particular definition.

    March 18, 2014 @ 11:49 am

    My understanding is that when physicists say “the universe is expanding” this is short for “the space of the universe is expanding”. I can’t think of any other definition of expansion that would be useful in this case.

    July 16, 2014 @ 8:27 am

    @Devil’smind Both are standard definitions and are expressed interchangeably. Because space is mainly composed of dark matter and dark energy, the later is expanding the universe with an increasing acceleration. Driving galaxies apart until the Great Rip.


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