Flat universe

Definition

A flat universe is a model of the cosmos where space-time is not curved. In this model, the geometry of the universe follows Euclidean principles and parallel lines remain parallel infinitely.

5 Must Know Facts For Your Next Test

1. The concept of a flat universe comes from Einstein's theory of general relativity and has been supported by observations of the cosmic microwave background (CMB).
2. Measurements of the CMB by missions like WMAP and Planck indicate that the universe is very close to being flat with only a small margin of error.
3. In a flat universe, the total density of matter and energy is exactly equal to the critical density.
4. A flat universe will expand forever, but at a rate that slows down over time without ever actually stopping.
5. The shape and geometry of the universe are crucial for understanding its fate, including whether it will continue expanding or eventually collapse.

Review Questions

• What observational evidence supports the idea that our universe is flat?
• How does a flat universe affect the fate of cosmic expansion?
• What role does critical density play in determining if the universe is flat?

Related terms

Cosmic Microwave Background:

Radiation left over from an early stage in the development of the universe, which provides evidence for its overall geometry.

General Relativity:

Einstein's theory describing gravity as a curvature of space-time caused by mass and energy.

Critical Density:

The exact density needed for the geometry of the universe to be flat; it separates models where the universe will expand forever from those where it will eventually collapse.