What is the term for the portion of the universe that we can observe from Earth?

• A. Cosmos
• B. The Universe
• C. Observable Universe
• D. Milky Way

Answer: (C)

The observable universe is the part of the cosmos we can detect from Earth because light has a finite speed and the universe has a finite age. Light travels about 300,000 kilometers per second, and since the universe is about 13.8 billion years old, we can only receive light from objects up to roughly 46 billion light-years away in distance, accounting for the expansion of space. That boundary defines the region we can observe. Beyond this horizon, there may be more universe we simply can’t see yet.

The other terms don’t fit as precisely. The Milky Way refers to our own galaxy, not the entire observable region of space. Cosmos and The Universe are broad terms for all of space, time, matter, and energy—much larger concepts than the specifically observable portion.