Talk:Observable universe

Size of Universe

Under this statement under 'Cosmology and general relativity' ; "Size of universe: The diameter of the observable universe is about 93 billion light-years, but what is the size of the whole universe? Does a multiverse exist?"

I don't quite understand what this question is about. Is this questioning if the Universe finite or infinite or actually asking about its size. Universe#Size and region does not say 93 but 91 million light-year, 46 billion light years to 'edge'. Also this says "The size of the Universe is somewhat difficult to define." All here is not cited.

However, on this link[1] under Observable universe#Size says "93 billion ly (light years) (It is not however known if the size of the entire universe is finite or infinite.)"

Another is Observable universe#Misconceptions on its size, assuming "Many secondary sources have reported a wide variety of incorrect figures for the size of the visible universe. Some of these figures are listed below, with brief descriptions of possible reasons for misconceptions about them."

It seems this size/question seemingly all hinges on this single and likely dubious web cite.

(I have also posted this on Observable universe) Arianewiki1 (talk) 00:52, 6 February 2018 (UTC)

Extraterrestrial objects receding faster than light

@Ruslik0: Please explain why this is "crap". [2] Banedon (talk) 20:53, 25 February 2018 (UTC)

Because none such objects exist. Ruslik_Zero 21:00, 25 February 2018 (UTC)

Well, it depends on what one means, I think. Here's my vague understanding of what it means: Suppose f(t) is the distance between galaxy A and galaxy B, in the sense that t is a time coordinate in an inertial coordinate system in galaxy A, and then you find the distance between the event points "galaxy A at time t" and "galaxy B at the same comoving time that galaxy A has at time t". The notion of "distance" also needs to be appropriately specified, but I believe this can be done — the idea is that you're summing up small distances in frames of reference that are at rest with respect to the Hubble flow.

Then, I think, df(t)/dt can be greater than c.

The notion appears often enough that it probably ought to be discussed, together with caveats. We need high-quality sources and expert editors for this one. --Trovatore (talk) 21:08, 25 February 2018 (UTC)

"None such objects exist" is incorrect. They can not only exist, they can potentially be observed (for a while at least). Read the source and state what you don't understand, because this is a standard concept. Here's another source that discusses it: [3] Banedon (talk) 21:45, 25 February 2018 (UTC)

I've reverted since it's been a few days with no response, and this really is a standard concept. See also sources 73-76 in the dark energy article. Banedon (talk) 04:23, 28 February 2018 (UTC)

@FlightTime: if you're going to revert can you say why on the talk page? This is getting ridiculous. Banedon (talk) 04:56, 28 February 2018 (UTC)

@Banedon: Because you do not have consensus or a source for that matter. - FlightTime (open channel) 04:59, 28 February 2018 (UTC)

Banedon does have sources, but they don't say exactly what the disputed text says. In fact they partially contradict it; as Banedon him(?)self says, it doesn't follow from the recession velocity being greater than c that you can never detect the object.

This topic is treated in considerable detail at faster-than-light#Universal expansion. I think it might well be reasonable to mention it here as well. But it needs to be done carefully. There doesn't seem to be any simple relationship between faster-than-light recession and being in the observable universe, or being eventually detectable. This really needs expert attention. --Trovatore (talk) 05:32, 28 February 2018 (UTC)

@FlightTime: There are sources, it was in the reverted text and I listed at least five on this page. Trovatore is right though that the sentence is actually slightly inaccurate. I will fix the inaccuracy. Banedon (talk) 08:48, 28 February 2018 (UTC)

The subject is already mentioned in the section "The Universe versus the observable universe", where it says

However, due to Hubble's law, regions sufficiently distant from the Earth are expanding away from it faster than the speed of light....

It seems to me that the content is already present in the article (before Banedon's edits). The question is what (if anything) should be said in the lead section.

Lead sections are supposed to summarize the body, and this material is reasonably important, so I do think something should probably appear in the lead. However the lead of this article is already substantially too long, so the wording should be chosen carefully, getting the most important facts across with no wasted words. (Also the lead needs to be trimmed down in general.)

So I'm not sure Banedon's addition:

It is also possible some extraterrestrial objects are moving away from Earth faster than the speed of light, in which case will eventually not be detectable from Earth

is really ideal. It is not just "possible"; it is apparently established that some objects are receding faster than light. And it does not seem to be clear whether that is the dividing line to whether they will be eventually detectable from Earth, as that depends on whether the expansion will accelerate. Also the meaning of "move away faster than light" is not completely specified. And I'm not sure this is the appropriate place to say this, anyway, as the text up to this point is talking about light emitted in the past that is reaching us now, whereas this text is talking about light emitted "now" ("now" meaning at the same cosmological age, I guess) and whether it could reach us in the future.

So something should probably be said, but I'm not sure it's this. --Trovatore (talk) 09:09, 28 February 2018 (UTC)

This is a misconception that any object can move faster than light. Any apparent speed as measured by its redshift will always remain subluminal. The expansion of metrics does not equal any physical speed. The effect that Banedon was trying to insert into the lead again has nothing to do with the superluminal expansion of the metrics. After all in a flat matter dominated universe the metric expands exactly 3 times faster than speed of light but any object will remain observable indefinitely. The effect is more subtle than that. It is related to the exponential future expansion of the metrics. This is actually described in the first two paragraphs of the first section. I also should say that it is not good to use some blog-like source which looks like a salad of words to me. Ruslik_Zero 18:53, 28 February 2018 (UTC)

Ah, so that is the point you're sticking on. I thought it was but you hadn't made it completely explicit.

The disputed text doesn't say "moving faster than light". It says "moving away faster than light". No object can move faster than light, in an inertial reference frame. But there is no inertial reference frame that includes both us and the distant galaxies. The proper distance between us and those galaxies (at the same comoving time) is increasing faster than the speed of light. I think that situation is reasonably summarized by saying they're moving away faster than the speed of light. However there is definitely potential for confusion if not carefully explained. --Trovatore (talk) 19:11, 28 February 2018 (UTC)

Ok, I updated the lead and removed some bloat from it. Again the limited temporal visibility of all objects is related to the exponential expansion of the metrics not just to the superluminal expansion. This is basically sort of an event horizon (see De_Sitter_space). Ruslik_Zero 19:29, 28 February 2018 (UTC)

Observable universe misconception in the article

Throughout the article a misconception is regularly pointed out about the observable and not observable Universe in the sense that we do not see the entire Universe.

We always see the entire Universe, but not the complete history of each region as the information from that region did not or cannot reach the Earth.

However, even those regions whose history we cannot see are present. We see them at least as they were at the time of the Big Bang, as cosmic background radiation. In that sense, no galaxy ever disappears just because the Universe is expanding, for example.

This misconception is very common, but it is still a misconception nevertheless.

95.34.4.122 (talk) 16:07, 14 July 2018 (UTC)