What is the observable universe?

The observable universe includes everything we can, in principle, see or detect from Earth because light from those regions has had time to travel to us. Even though the universe is about 13.8 billion years old, expansion means the observable radius is much larger, about 46.5 billion light-years. Beyond that limit, light has not yet reached us, so we cannot observe those regions directly. This does not mean the universe ends there; it only marks the edge of what is visible to us. The concept is fundamental in cosmology and observational astronomy.