What Type of Universe do We Live In?

The last three posts describe the three possible universe types: open, flat, and closed. The question is what will happen to our universe? The real answer is that we really don't know. But we do know this: the universe is very close to being flat if it isn't flat. How do we know this?


The first way is by looking at the cosmic microwave background radiation. We know by looking at the data, we can see that the CMBR is relatively uniform. It could only be that uniform if not only the universe went through inflation, but also if the universe is flat. Any other type of universe would result in a lumpy universe.


Another way we know is by induction. If the density of the universe was just a tad bit smaller than the critical density (say 95% of the critical density), gravity would have lost out to expansion very near the beginning of the universe and would have expanded too rapidly for any galaxies or stars to have formed. Obviously, it must be have more mass and energy than 95% of the critical density because we are here.


Likewise, if the density was 105% of the critical density, then gravity would have overcome the expansion quickly. The universe would have collapsed too rapidly for anything to have formed.


We know then that the universe is relatively flat. The question is, which side of the critical density does the actually density lie? Again, we just don't know. The only thing we know is that we know that the mass density (both dark matter and baryonic matter) that has been measured is about 26% of the critical density but we don't really what the other 74% of the density is. Since we don't know what it is, cosmologists call it dark energy. The dark energy is believed to be a driving force to the expansion of the universe, but what it is made up of is still unknown.

The Flat Universe

A flat universe may be the easiest type of universe to imagine. We define a flat universe as a universe that has its mass/energy density equal to the critical density. The critical density is the density required to cancel out the expansion of the universe with gravity. In other words, a flat universe will expand at an ever slowing rate as the universe will reach a maximum size at some time in the infinite future. We call this asymptotic expansion. It will never quite reach the maximum size but approaches it without ever shrinking.


Flat universes are also defined as having zero curvature. Imagine a piece of paper that is infinitely large. The paper is flat in 2D, while the universe is flat in 3D, but they are similar. In a flat universe, if you start out at any location, there is no way to reach your original location without turning. If you start out in a straight line, you will remain on that straight line forever.


Another thing about flat universes is that if you draw two lines parallel to each other, the lines will always remain parallel. Two spacecraft travelling parallel to each other will always remain the same distance apart no matter how far they travel.


The sum of the angles in a triangle will always equal 180°, no matter how big you make the triangle. If you know anything about geometry, you know this is true. However, when we get into open and closed universes, you'll see how this "constant of geometry" will vary.

1. Flat universes have actual densities equal to the critical density, i.e. the ratio between the two is always equal to one.
2. Flat universes have zero curvature, hence the term flat.
3. The ultimate fate of a flat universe is it will asymptotically approach a maximum size based on the critical density, but will never reach the maximum size, yet will never shrink in size.
4. Parallel lines will always remain parallel, no matter how far the lines reach from their starting points.
5. Triangles always have the sum of its angles equal to 180°.

Next time, we'll discuss an open universe.