Description: The observable universe refers to the patch of the Universe we can see, which is a sphere with us at the center. The radius of the observable universe is determined by how far light has been able to travel towards us since the beginning of the Universe. Regions at the boundary of the observable universe are so far away that their light has just had enough time to reach us over the past 14 billion years; in other words: over the age of the Universe.

The most distant regions of the Universe that we can see are now over 40 billion light years away. This is because the Universe has expanded a lot since the light reaching us from those regions was emitted. Light from objects outside the observable universe has not yet had enough time to reach us.

The longer we wait, the more time light has to reach us, and the larger the observable universe grows. Other observers in the cosmos have their own observable universes: a sphere with them at the center, its radius the greatest distance over which light from other regions has had time to reach them.

Related Terms:

• Accelerating Universe
• Electromagnetic Radiation
• Light
• Light Year
• Space Telescope
• Universe

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".