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Glossary term: Observable Universe

Description: All of the objects and phenomena that ground and space-based instruments can observe and make measurements of are considered part of the observable Universe. The finite speed of light (electromagnetic radiation) places a physical limit on how far back (in space and time) we can observe. Using data from various ground and space-based instruments, the observable Universe is inferred to have a radius of over 40 billion light years; any light from objects beyond this radius will not have had time to reach our detectors here on Earth, and so will be outside the observable Universe.

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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".