What is the "particle horizon"?
A) The particle horizon is the distance beyond which astronomers cannot see because light cannot have traveled any farther in the finite age of the universe.
B) The particle horizon is the distance beyond which astronomers cannot see because no stars or galaxies had yet formed at those distances (because astronomers see back in time as they look out into space).
C) The particle horizon is the distance beyond which signals from Earth radio and television signals cannot have traveled through the universe.
D) The particle horizon is the distance beyond which astronomers cannot see because of absorbing matter in the universe.

Question

Quizplus · Accepted Answer

The particle horizon represents the maximum distance from which particles could have traveled to the observer in the age of the universe. It defines the boundary of the observable universe, beyond which events cannot be observed because the light from them has not had enough time to reach us since the beginning of the cosmological expansion.

What Is the "Particle Horizon