1. Bell Ringer!
What objects in the night sky did Hubble look at to see the red shift that gave us evidence for an expanding universe?
What rule did Hubble come up with about a object’s movement in space?

2. Is the Universe Expanding Forever?
It is not known whether the universe will continue expanding.
Two opposing forces govern the expansion:
the momentum of expansion outward, and the force of gravity inward.

3. Astronomers can determine the mass of objects they can see, but this total would not include objects that are difficult to see.
Objects like white dwarves, black holes and other dead stars do not emit much light, but can be very massive.
The existence of dark matter cannot be seen and the mass cannot be calculated.
Until astronomers can better determine the mass of the universe, they will not know how strong the inward pull of gravity is.

4. Three Fates of the Universe
Astronomers have calculated the amount of mass needed to exactly balance the expansion, called the critical density.
What happens to the expansion depends on whether the true average density of the universe is equal to, higher than, or less than the critical density.

5. Fate 1: Open Universe
If the average density is less than the critical density, gravity will not be strong enough to stop the expansion, creating an open universe.

6. Fate 2: Closed Universe
If the average density is more than the critical density, gravity will slow down and eventually reverse the expansion, creating a closed universe.

7. Fate 3: Flat Universe
If the average density is equal to the critical density, gravity will balance the expansion and the universe will reach a stable size, creating a flat universe.
This is also called the Inflationary Model because the universe expanded at first and the reached a maximum size.