What is the observable universe?

The observable universe refers to the portion of the universe that can be seen from Earth, and it is defined by the distance that light has traveled since the Big Bang, approximately 13.8 billion years ago. The light emitted by distant galaxies and other cosmic phenomena reaches us only up to a certain distance due to the finite speed of light and the expansion of space. This creates a spherical region around Earth from which we can receive information, limited by the speed of light and the age of the universe.

This concept is distinct from the entire universe, as there are likely regions beyond the observable universe that are not reachable or visible to us due to this limitation. It also does not restrict visibility to the naked eye or to specific regions like those near black holes, which are not a part of this definition. The observable universe includes all the galaxies, stars, cosmic microwave background radiation, and other celestial objects visible through telescopes and other instruments.

Understanding the observable universe helps astronomers study the structure, evolution, and overall characteristics of the cosmos, while acknowledging that what exists beyond that boundary is not currently accessible to observation.