**Horizon**

- (Past) Horizon:
- Future Horizon:

Explanations:

**Past Horizon:** the distance the light travels since the beginning of
the universe to the present moment.

**Future Horizon:** the distance the light travels since the present
moment until the infinite future (it is finite if the expansion of the
universe accelerates).

**Legend:**
- Mpc = million parsecs
- kpc = thousand parsecs
- Mlyr = million light years
- klyr = thousand light years