What Is the "Particle Horizon

Multiple Choice

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.