Talk:Cosmological horizon

How big is a quarter?

Is that what they meant by "Quarter sized"? I was a little confused by that. As for the size... 24.26 mm in diameter.

"In any case, it is interesting to note that the cosmological horizon is a maximal limit of perception and not an actual boundary" ... I'm not sure this is a safe statement: the nature of reality is always relative to an observer. for example, doesn't this in-principle boundary of visibility form an event horizon with an equivalent effect to hawking radiation?Snaxalotl (talk) 03:47, 11 June 2009 (UTC)snaxalotl

Size of the Universe

I would like to see some mention here of the size of the Universe, such as can be found on the observable universe article.

I would also like to know what the difference is between the/a Cosmological Horizon and the/a Particle horizon.

I copied some text over myself from the OU article, but it didn't quite fit so I undid it. Nagualdesign (talk) 04:16, 16 February 2009 (UTC)

Particle horizon ?

Isn't the particle horizon the distance at which are now objects from which we receive the light now, that's to say roughly 50 billion light years, not to confuse particle horizon with Hubble's horizon at 13.7 Gly ? (imagine an object emitting on the Hubble's horizon, the time its light comes to us, it would have moved (faster than light)) —Preceding unsigned comment added by 129.175.69.214 (talk) 08:56, 31 March 2010 (UTC)

Lineweaver and David

The above seems to conflict with the cited article by Lineweaver and Davis. In the case of a positive cosmological constant, an even horizon exists. — Preceding unsigned comment added by 193.29.76.37 (talk • contribs)

Some errors in this article

This article needs a review from an expert. As was stated in another comment, the article by Lineweaver and David points out clearly that the Hubble radius is not an event horizon when the Hubble parameter is not a constant in time (which, in the current accepted cosmological models, it is not). Light emitted from particles at the Hubble radius will still reach us in a finite time. Light emitted from the cosmic event horizon will reach us at infinite time. There is a very subtle difference between the two, but nevertheless an important one. The definitions in the article are partly right, mixed with wrong elements. I am no expert myself, so I will not alter too much. Someone with cosmological authority should. — Preceding unsigned comment added by 78.23.25.41 (talk) 09:04, 13 June 2017 (UTC)

I agree. But I'm a bit hesitant to edit it because I'm not confident enough. But for example, "In an ever-accelerating universe, if two particles are separated by a distance greater than the Hubble radius, they cannot talk to each other from now on (as they are now, not as they have been in the past)" contradicts what is mentioned a bit before that "It is a common misconception that light from the Hubble radius can never reach us. It is true that particles on the Hubble radius recede from us with the speed of light, but the Hubble radius gets larger over time (because the Hubble parameter H gets smaller over time), so light emitted towards us from a particle on the Hubble radius will be inside the Hubble radius some time later." I think the later is true (or at least thought to be true by most scientists according to our current best guest).

Also, this might seem like a detail, but I don't think it's quite right to say that "The particle horizon [...] is the maximum distance from which particles could have traveled to the observer in the age of the universe." Rather, the particle horizon is the current distance at which those particles are, not the distance at which they were when they emitted the photons we're now receiving (if I'm not mistaken). --Mati Roy (talk) 11:27, 16 February 2019 (UTC)

Yeah, this article's a mess. In addition to what was pointed out above, the current definition of "Future horizon" is vague but seems to be synonymous with the event horizon. Maybe it was intended to refer to the far-future limit of the Hubble distance (which is also the limit of the event-horizon distance)? If so, it's very unclear. It would also be good to add values, e.g. 14 Gly for Hubble distance, 16 Gly for event horizon, 17 Gly for "future horizon"(?), 47 Gly for particle horizon. Patallurgist (talk) 08:04, 27 February 2019 (UTC)

"at the same location in space that happened in the distant past"

Links to the 'spacetime' article. What the heck is the point of this link? How does it explain the link text or the rest of the sentence that precedes it? It seems to me this link is spurious and distracting and should be removed. Thanks. Comiscuous (talk) 20:10, 19 April 2021 (UTC)

Merge proposal

The following discussion is closed. Please do not modify it. Subsequent comments should be made in a new section. A summary of the conclusions reached follows.

No consensus to merge, with stale discussion; agreement to remove a misleading hatnote, and perhaps to make the scope of the articles clearer. Klbrain (talk) 17:39, 9 November 2025 (UTC)

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I propose to merge the content of Particle horizon into this article and redirect Particle horizon->Cosmological horizon.

This article already has a section on "particle horizon" almost as long as the "main" article. Neither article is great: we'd be better off focusing on the broader one. If someone comes long with a lot of energy expand the "particle horizon" section we can always split again. Johnjbarton (talk) 03:05, 6 April 2025 (UTC)

I suppose "Cosmological horizons" (plural) better matches the page or perhaps "Horizon (cosmological)" if we want to go for wiki-isms. Johnjbarton (talk) 03:09, 6 April 2025 (UTC)

Oppose. The particle horizon is an important concept in its own right distinct from the cosmological horizon. Also considering that the particle horizon article is approx 750 words and the section in cosmological horizon is 200, I would not say that they are "almost as long". If the particle horizon page is inadequate, expand it. As it stands its already not the worst (but can definitely be better).

I also disagree with renaming it to horizons plural. Usually articles are not prualized when dealing with a single concept that has multiplet options. For example quark is not pluaralized. We also don't delete the up quark page just cause there is a quark page (and that page is not even longer than the particle horizon page!).

OpenScience709 (talk) 13:41, 6 April 2025 (UTC)

Support. At least change the top of the page so that it doesn't say "Not to be confused with Particle Horizon" then immediately has a section called Particle Horizon! Either Particle horizon is a type of Cosmological horizon or it's something else. If it is then put the details here, otherwise this page is just a list of types of CH. Btljs (talk) 08:46, 16 April 2025 (UTC)

I mean, it could be the case (as it is) that cosmological horizon is a class of things, of which one of the most important one is the particle horizon and it is sometimes refered to as the cosmological horizon. But they are not equivalent; other cosmological horizons exist. The "not to be confused with" part is a bit confusing so we can delete that bit. But it is useful to have a page to bring together the various horizons, especially since there are some without pages. What we now need to add is an explanation for their differences and similarities, which the article is missing. But it is still also useful to have a dedicated page on the particle horizon. OpenScience709 (talk) 10:30, 17 April 2025 (UTC)

Oppose Per OpenScience709. 2001:8003:9060:601:D83C:D39:3E37:D16D (talk) 09:39, 12 July 2025 (UTC)

Support – No need to have two articles if they both talk about the same thing. The length isn't a problem at all. FaviFake (talk) 17:43, 3 October 2025 (UTC)

But they literally aren't about the same thing... Cosmological horizon is more general than the particle horizon. OpenScience709 (talk) 09:48, 4 October 2025 (UTC)

Which is why I support merging it into Cosmological horizon, where it's the very first section. FaviFake (talk) 10:23, 4 October 2025 (UTC)

But particle horizon is important enough to deserve its own article. In the same way up quark being a category of quarks, yet we dont merge the two. Just cause something has a subsection that correspond to another article doesn't mean that the two should be merged. Or more explicit examples, quantum field theory has plenty of subsections that are themselves articles. OpenScience709 (talk) 20:36, 4 October 2025 (UTC)

The discussion above is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

Sources for "cosmological horizon"

Many reliable sources only give two "cosmological horizon"s and many only one.

• Margalef-Bentabol, Berta; Margalef-Bentabol, Juan; Cepa, Jordi (8 February 2013). "Evolution of the cosmological horizons in a universe with countably infinitely many state equations". Journal of Cosmology and Astroparticle Physics. 015. 2013 (2): 015. arXiv:1302.2186. Bibcode:2013JCAP...02..015M. doi:10.1088/1475-7516/2013/02/015. ISSN 1475-7516. S2CID 119614479.
  • Very clear: there are two, "particle horizon" and "event horizon".
• Kolb, Edward; Turner, Michael S. (2018). The Early Universe. Boulder: Chapman and Hall/CRC. ISBN 978-0-201-62674-2.
  • Only particle horizon
• Dodelson, Scott (2003). Modern cosmology. San Diego, Calif: Academic Press. p. 146. ISBN 978-0-12-219141-1.
  • "the horizon" or "the comoving horizon".
• Peacock, J. A. (1998-12-28). Cosmological Physics (1 ed.). Cambridge University Press. doi:10.1017/cbo9780511804533. ISBN 978-0-521-41072-4. Page 85
  • Particle horizon and event horizon. Despite this distinction, cosmologists usually say the horizon when they mean the particle horizon.
• Ryden, Barbara (2016-11-17). Introduction to Cosmology (2 ed.). Cambridge University Press. doi:10.1017/9781316651087. ISBN 978-1-107-15483-4.
  • "particle horizon distance" or "horizon distance" for short.
• A., Zee (5 May 2013). Einstein gravity in a nutshell. Princeton. ISBN 9780691145587. OCLC 820123453.{{cite book}}: CS1 maint: location missing publisher (link)
  • particle horizon for inflation and event horizon for black holes.
• Weinberg, Steven (1971). Gravitation and Cosmology. John Wiley. pp. 740, 815. ISBN 978-0-471-92567-5.
  • particle horizon and event horizon.

Johnjbarton (talk) 02:49, 13 November 2025 (UTC)

Yup. Most older sources refered to the particle horizon as the cosmologial horizon, but the more modern parlance refers to cosmological event horizon as the cosmological horizon. This latter statement is supported by for example examining the recent arXiv particles which mention "cosmological horizon" in the abstract. Going back at least 10, every single one of them was refering to the event horizon. For example 2511.07525 also gives an explicit definition of a "null hypersurface at finite distance in the bulk"; corresponds to cosmic event horizon. Usually this is done in the context of dS spacetimes, where the event horizon coincides with the Hubble horizon while the particle horizon is divergent, although broader examinations are undertaken as well (such as 2505.07934). Considering that currently there is no dedicated article towards the "cosmic event horizon", I would suggest making this one that. But this should be done while also mentioning/discussing particle horizons as cosmological horizons in older parlance (but ofc particle horizon page should remain the main one there). One will also have to discuss the relation of cosmic event horizons to other horizons such as Hubble horizon and apparent horizon since these sometimes coincide with the event horizon. OpenScience709 (talk) 11:59, 13 November 2025 (UTC)