This lesson will describe three models of the universe, including closed, open, and flat, as well as critical density and which model is most likely the correct one.

## The Shape of the Universe

You’ve probably learned by now, or heard on one of many TV programs, that after the Big Bang occurred, the universe expanded. But what shape did it become? Did it dilate like a balloon into a sphere? Maybe it simply rolled out into a flat sheet? Perhaps it became shaped like a saddle? This lesson will go over three major models of the universe, which one is believed to be correct, and why.

## The Three Models of the Universe

One model of the universe, which was considered by many in the 20th century, is known as the **closed universe**. The closed universe is a universe where space-time is curved back on itself, one that has a finite size but no center and no edge. It is said to have positive curvature.So, this model is like an orange.

It’s spherical, and it clearly curves around. But like an orange, it has a finite volume. However, because it curves back on itself, there can be no edge and no center. The latter point may be difficult to understand. You’re probably thinking: an orange does have a center, how could this model not?To help you understand this concept, you have to imagine that you are actually a 2-dimensional being walking on the surface of a closed spherical shape, like the earth, moon, or even our orange.

Because you are 2-dimensional, you don’t know of an up or a down. You can only go left, right, forwards, and backwards.If you were to walk around on the surface of the moon in this fashion, you’d eventually cover the moon with footprints and realize that you live in a finite universe since you’ve already been everywhere! However, precisely because you’ve walked everywhere without finding an end, you have found no edge to the universe. Without an edge, you cannot find a center to it, either. The closed model of the universe predicts that the expanding universe will begin to contract matter back into a Big Bang state, something called the ‘Big Crunch.

‘Another model of the universe is known as the **open universe**. This is a model of an infinite universe involving curved space-time, which does not curve back on itself. It is said to have negative curvature. As seen below, it looks like a saddle of sorts:

An infinite and un-curved space-time model of the universe is called the **flat universe**. It is seen as one having zero curvature.

Not surprisingly, the flat universe looks like a flat sheet since space-time is not curved. Both the flat and open models of the universe are considered to be infinite, with no center and no edge, and are ones that expand forever.

## Which Model is Correct?

The biggest clue to the actual shape of our universe came thanks to the understanding of **critical density**, the average density of matter and energy that is needed to make space flat, as opposed to curved. It is equal to approximately six atoms of hydrogen per cubic meter.Really simply summarized:

- If the average density of the universe is greater than the critical density, then the universe will curve back on itself and result in a closed universe.
- If the average density of the universe is less than the critical density, this will result in an open universe.
- If the average density of the universe is equal to the critical density, then a flat universe will result.

The most widely held theories astronomers have come up with predict that the density of our universe is very close to critical density. This means the curvature of the universe is very close to zero, and thus, our universe is nearly or completely flat.Just as a parting side-note here, there is a subtype of the closed model of the universe called the oscillating model that states that after the Big Crunch, another Big Bang will occur and a new universe will come to be of the same matter as before, and this kind of cycle will continue forever.

## Lesson Summary

Nevertheless, our lesson went over the three major models of the universe.

- The
**closed universe**, which is a universe where space-time is curved back on itself, one that has a finite size but no center and no edge. - The
**open universe**, which is a model of an infinite universe involving curved space-time, which does not curve back on itself. - The
**flat universe**, which is an infinite and un-curved space-time model of the universe.

Both the flat and open models of the universe are considered to be infinite, with no center and no edge, and are ones that expand forever.The main clue to understanding which model is correct is **critical density**, the average density of matter and energy that is needed to make space flat, as opposed to curved. Current theories predict that the average density of the universe is close to the critical density, meaning our universe is likely flat.

## Learning Outcomes

Following this lesson, you’ll have the ability to:

- Describe the three major models of the universe
- Explain how critical density relates to the shape of the universe
- Identify which model is most likely correct