Questions:

1. The ______ ______ defines the most distant point in the universe from which light can reach us in a finite amount of time. The sphere whose radius is this distance forms a horizon. Fill in the blanks.

2. The event horizon is an imaginary spherical surface surrounding a gravitational singularity. Matter that enters an event horizon can’t escape back out. What’s the 3D region extending for a short distance out from the event horizon of a rotating black hole called? Matter that enters this region can still escape by expending energy.

3. After the Big Bang, the universe was filled with a hot, opaque plasma. As the universe cooled, the plasma slowly condensed into the first hydrogen atoms and the universe became transparent. This horizon has a four-word name and defines the farthest that light can travel freely in the universe. What’s the name?

4. The temperature of a black hole is directly proportional to the surface gravity at X. X is a horizon that relates the structure of spacetime with the physics of black holes. For a non-rotating black hole, X coincides with the event horizon. Name X.

5. Although the sun becomes directly visible (from the earth’s surface) only after sunrise, its light becomes apparent a little earlier. The same thing happens in reverse at dusk. Name the common word that defines these periods of time as distinct from daytime and nighttime.

Visual:

Name this Dutch astronomer whose 1922 model of the universe turned out to resemble some aspects of a universe undergoing accelerating expansion.

Answers:

1. Hubble radius

2. Ergosphere

3. Surface of last scattering

4. Killing horizon

5. Twilight

Visual: Jacobus Kapteyn