Why Is The Expansion Of The Universe Always Drawn Like A Cylinder?

Why is the shape of the universe depicted as a cone?

If the universe is expanding with all points moving away from each other, how come it is depicted as cylindrical with the expansion outward mostly in one direction?

I have gotten a lot of questions about diagrams of the Universe's expansion. I must say the number of questions on this topic is of great credit to how widely circulated one particular diagram from the WMAP team has been. After my recent article about tracing the Big Bang back to its original location, there was another burst of questions about the setup of this particular diagram:

It’s true that while we had a long discussion about how the Big Bang was an even expansion of space itself in every possible direction, the diagrams usually give us a more directional vision of the evolution of the universe.  It’s not just this one diagram, either, though the WMAP image is probably the most familiar - if you’ve seen any of these diagrams, it’s probably that one.

The fundamental issue is that the Universe is an evolving four dimensional entity, and an artist has two dimensions to work with, and compressing by two dimension is really hard to do. Artists are pretty good at compressing three dimensions into two dimensions - we can imply a lot of depth with clever use of perspective.  And in fact the artist who’s constructed the WMAP image is doing just that by giving you a cylinder of space, which we have all successfully parsed as “has some volume”.

Here’s the issue: how do you draw and illustrate a changing three dimensional object? You can draw it at different stages, like a biologist’s illustrations of a jellyfish in different stages of life. You could make a video out of it, of course, but if your aim is to make an illustration, you’re stuck with a single image. The other option is to try and take a slice of the whole object, and show how that section evolves over time.  It’s definitely incomplete, but it might give you a better sense of the changes going on, particularly if you can make the assumption that every other section you might have chosen is doing pretty much the same thing.

That’s what’s happened with the cylinder view.  We’ve taken, effectively, a narrow cylinder of current-day space, and shown you how that evolves backwards in time.  In this case, the circular sliver of space that we’re looking at slowly shrinks, and the galaxies that lived in that space in earlier times become smaller and brighter, and less separated, and if we trace that region of space even further backwards, we hit the Cosmic Microwave Background - the oldest light in the Universe.  If we were to keep going, we’d expect this sliver of space to shrink rapidly as we go backwards in time through inflation, and would eventually become infinitely small, as it joins with all other pieces of space we could have selected at the start, in the singularity.  It’s because we’re showing time along the long direction of the cylinder that it looks like there’s directionality here, but in actual fact the expansion is evenly distributed within that cylinder - the expansion of the Universe isn’t “off to the right.”

This diagram, and the others like it are giving you a small slice of the universe to look at, rather than attempting to show the evolution of the entire universe, if such a thing were possible. This is a simplification of how the entire Universe has changed and evolved over time, but you could make a similar slice of any other piece of space that exists today - in tracing it back, you’d see the same sorts of changes.