What is the Observable Universe?

Redshift, The Reachable Universe and The Future Visibility Limit

Ed Noble
Thoughts And Ideas

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Photo by Josh Gordon on Unsplash

When astronomers talk about the universe, they almost always mean the observable universe. The word observable does not refer to the capability of technology to receive signals from distant galaxies, or even if there is anything out there to be detected. Rather, it is a limit based on the speed of light itself. No signal can travel faster than the speed of light, meaning that there is a maximum distance since the start of the universe beyond which nothing could have been detected; there simply hasn’t been enough time for light to reach us. This distance is called the particle horizon, which is analogous to the event horizon of a black hole.

The universe can be assumed to be isotropic in all directions, which means that every location has its own observable universe when considered as the centre (or observer). In 2016 the Hubble telescope detected a galaxy called GN-z11, found in the constellation Ursa Major. GN-z11 is amongst the furthest known galaxies from Earth ever discovered and is at the edge of the observable universe. Due to the isotropic nature of space, if you were an inhabitant of this fairly small galaxy, the Earth would be at the edge of your observable universe.

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Ed Noble
Thoughts And Ideas

I write about philosophy, psychology and ethics. I live and work in London, having previously studied physics. Started writing in lockdown.