Horizon distance

Definition

Horizon distance is the maximum distance from which light has had time to reach an observer since the beginning of the universe. It defines the boundary beyond which events cannot be observed because their light hasn't had sufficient time to travel to the observer.

5 Must Know Facts For Your Next Test

1. The horizon distance increases over time as the universe expands.
2. During the inflationary period, the horizon distance grew exponentially, causing widely separated regions of space to come into causal contact.
3. The concept helps explain why distant parts of the universe appear homogeneous and isotropic.
4. At present, the observable universe extends approximately 46.5 billion light-years in all directions from Earth.
5. The horizon distance is not fixed; it changes depending on both the age and expansion rate of the universe.

Review Questions

• How does inflation affect horizon distance?
• Why can't we observe events beyond our current horizon distance?
• What role does horizon distance play in explaining the uniformity of cosmic background radiation?

Related terms

Inflation: A rapid exponential expansion of space in the early universe lasting for a tiny fraction of a second.

Cosmic Microwave Background (CMB): The thermal radiation left over from the Big Bang, providing evidence for an expanding universe.

Observable Universe: The region of space from which we can receive information via light or other signals since the beginning of cosmological time.