Talk:Observable universe#Size - epoch of decouping.3F

The article starts with

• {{tq|The observable universe is a spherical region of the universe consisting of all matter that can be observed from Earth;}}

A plain and simple reading of this sentence contradicts reality. It implies an agency for observers that physics denys us. The matter at 45.7 billion light-years cannot be observed "now", we can only observe the light from matter that would have been emitted 13 Gya.

I think a clearer starting sentence would be

• The observable universe is a spherical region of the universe consisting of all light that has ever been emitted since the Big Bang.

This includes the critical issue of time and is more consistent with particle horizon. Johnjbarton (talk) 19:15, 10 April 2025 (UTC)

::I agree that the current first sentence has issues, but your proposal does not seem to be correct. Suppose for example that the full universe is infinite and the Big Bang happened everywhere at once; then your sentence would imply that the observable universe is the full universe, which is not true. --Trovatore (talk) 20:15, 10 April 2025 (UTC)

:::Of course you are right. How about:

:::* The observable universe is a spherical region of the universe consisting of all light emitted since the Big Bang that arrives on Earth at one time.

:::Johnjbarton (talk) 21:35, 10 April 2025 (UTC)

::::The definition currently given here [note 1] is actually an imprecise definition of the Particle Horizon.

::::My understanding is that the Observable Universe is different from the Particle Horizon.

::::The correct definition [Note 2] should therefore be:

::::"The Observable Universe is the region of the universe consisting of everything that in principle could ever be observed from the reference location"

::::Note 1: To avoid future confusion, the Wikipedia definition at the time of writing is: "The observable universe is the region of the universe consisting of all matter that can be observed from Earth"

::::Note 2: The major change is the change of the time frame. The other alterations are more technical, but also needed. There may be better ways to express this definition...

::::I have removed "spherical", because this is only an approximation (albeit a rather accurate one), and

::::I have similarly removed "matter" on the basis that matter almost certainly did not exist in the very early stages of the universe, but precursors of light and gravity almost certainly did.

::::I have also generalised the definition so that it does not contradict the statements about "overlapping".

::::Some related explanations should probably be added as notes, as should a clarification of the words "in principle". PhysicistQuery (talk) 10:53, 10 June 2025 (UTC)

The term "observable universe" has a specific meaning. It is not a synonym for the universe. So some of the content seems off topic. I think most of the composition and structure content does not belong here, other than as a summary. Johnjbarton (talk) 23:46, 10 April 2025 (UTC)