# Talk:Particle horizon

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## Merge With Observable Universe

It should be merged with the above article because it adds nothing that cannot otherwise be explained in a one or two sentence statement in that article as it is done here. And in fact, it is more contextually appropriate in that article as well. Anyone have any objections to merging it? Astrobayes 20:41, 23 June 2006 (UTC)

Hi, IMHO this should stay a separate article. For one, you can define a particle horizon even for universes that are not ours (eg, for general spacetime metrics), so it's not necessarily related only to our universe, but really to any spacetime you like. Also, I imagine that "observable universe" should really refer to properties of the universe as observed.
That being said, however, taking a peek at the "observable universe" article it seems to be in a rather confused state at the moment. As you say it's mostly talking about the particle horizon in the context of the standard cosmological model. Perhaps what this article needs is a more general discussion of what particle horizon means in spacetimes more general than the FRW metric. Wesino 18:42, 23 November 2006 (UTC)
Perhaps you could simply mention somewhere in the article that you can define a particle horizon even for universes that are not ours. I wondered about the difference between the particle horizon and the cosmological horizon until I read this discussion. I would make the edits myself but I am not fully versed :) [nagualdesign - 79.70.42.84 (talk) 08:04, 14 February 2009 (UTC)]

## Major edits

I did a bit of heavy editing. There were a few factual inaccuracies (or fuzzy statements), for example

• what was defined previously was not the conformal time but the change in conformal time,
• the particle horizon is only equal to the conformal time when we're measuring it in comoving distance (otherwise there is a factor of ${\displaystyle a(t)}$)
• conformal time doesn't describe a universe that is expanding (this is the job of the metric, which can use the c.t. as one coordinate)
• the article seemed to suggest that just because the overall scale of the universe is expanding, then there are no horizons, which is false.

Also it wasn't really clear why things had to be a list. Anyway, I hope that the edits kept the sense and intention of the original information. If you don't think so let's talk and come up with a better solution. Wesino 18:59, 23 November 2006 (UTC)

## event horizon defs

I'm concerned about our event horizon definition, which right now reads --

while the event horizon is the largest comoving distance from which light emitted now can ever reach the observer at any time in the future.

I don't think that "now" really has any meaning in this context as there is no simultineity in GR, so it seems this def could be cleaned up. Wesino 22:20, 5 May 2007 (UTC)

## geeks =

Of course, I'm sure its me but I failed to see any (estimated) numerical value for the particle horizon. Hence my conclusion that this article needs work. If you didn't answer the question any 6 year old would ask (namely "but how far IS it?") then you probably aren't being very helpful to most people (and are directing your writing towards the geeks). —Preceding unsigned comment added by 69.40.243.166 (talk) 21:28, 2 February 2008 (UTC)

Eebster the Great (talk) 18:07, 18 March 2008 (UTC)The observable universe page puts this at around 14 GPc, but I'm not sure if that's correct (Since I thought it was more like 14 ly). If somebody can verify that, then they should put it in.

## Layman explanation lacking?

Generally, I found it very odd to land here from the Dark Flow page (this is linked from its 3rd paragraph) and to be almost immediately faced with a wall of mathematics that are unclear to a poor layman who would find this indecipherable. It might help to either include a link to a main article explaining the particle horizon in more general terms or to expand this article with an opening section. To clarify a bit, the Dark Flow article states a Particle Horizon of 46 billion light years. Given the Universe is ~14 billion years old, there's a 32 billion light year gap to leave the public scratching their heads over. The article as it stands does nothing to resolve such confusion. 86.46.117.189 (talk) 13:11, 8 June 2014 (UTC)

I've added a contents box with a lead-in before it that should clarify this point - the 32 billion light year gap comes from the expansion of the universe, so that we use the conformal time rather than the age of the universe to work out the particle horizon. In other words light travelling for 14 billion years actually travels a lot further than just 14 billion lightyears (which it would if the universe was static). HannahFord428 (talk) 09:55, 5 August 2014 (UTC)