Talk:Hubble volume

Relationship to age of the universe

"The Hubble length {\displaystyle c/H_{0}} c/H_0 is 14 billion light years in the standard cosmological model, somewhat larger than {\displaystyle c} c times the age of the universe, 13.8 billion years." Why is that? — Preceding unsigned comment added by 192.249.47.217 (talk) 14:36, 25 August 2017 (UTC)

Hubble parameter vs Hubble constant

I believe these are the same thing, but someone coming to this topic might have a hard time working that out. It might be best to use Hubble constant rather than parameter as that is the more common term.

I don't want to change it myself in case I'm wrong; and if I am wrong then "Hubble parameter" should be properly explained or linked to a topic which actually explains it. Gk007 (talk) 04:29, 2 June 2014 (UTC)

I'm not a cosmologist. From what I understand they are the same but "Hubble Constant" is a very confusing term because it is only constant in space at a given time, and it's not constant in time. It's continually varying, except in the very specific case of exponentially accelerating expansion.Joancharmant (talk) 02:46, 10 August 2018 (UTC)

Event Horizon?

The whole reason I'm reading about this sort of thing is that I find it very interesting (I suppose I have better things to do but Wikipedia is addictive) so forgive me for not understanding but I noticed that an Event Horizon is mentioned in this article. It's quite appropriate in the sense that objects moving out of range at a certain speed will no longer send information back to us, but isn't an event horizon typically a physical boundary from which objects can only pass in one direction? The linked article on event horizons discusses black holes to the exclusion of hubble volumes. I haven't changed anything because I'm not confident about which article is amiss. Cheers. SneakyWho am i (talk) 15:10, 4 March 2009 (UTC)

An event horizon is any surface from beyond which no information can ever reach the observer. Of course, that includes objects of all kinds - in fact, anything moving at or below the speed of light - so in a sense you're right. An important thing to note though is that the event horizon - in the strictest sense - belongs to the observer, rather than the thing (not) being observed. In the case of a black hole, the event horizon is the same for all outside observers, which I guess is what you mean by a 'physical boundary'. The Event horizon article is biased towards this, the most commonly encountered form of event horizon. The cosmological event horizon is different: since every point in the Universe is receding from every other point with a speed proportional to distance, the event horizon is always a sphere centred on a particular observer, so there is no single 'event horizon of the Universe' (although of course we tend to give a special place to our own event horizon).

So neither article is wrong, although the introduction to Event horizon gives a misleadingly narrow interpretation of the term, which I'll try to fix. The cosmological use is however mentioned in Event horizon#Event horizon of the observable universe. There are more details in Cosmological horizon and Particle horizon.

Cosmo0 (talk) 13:10, 6 March 2009 (UTC)

I'd like to add to those remarks that the above is true only for theories with expanding universe.

In theories with non expanding universe (like eg. Einstein's general realtivity from 1917) where the cosmological redshift and its apparent acceleration is caused by the flatness of spacetime (and the necessary compensation of the curvature of space through time dilation) there are no horizons and the universe complies with the Perfect Copernican Principle (it is seen approximately the same always). Jim (talk) 11:46, 30 October 2010 (UTC)

Factual inaccuracies?

Cosmo0, you removed the following claims with the comment "removed factual inaccuracies":

• "The Hubble volume is also known as the 'Hubble sphere,' its radius as the 'Hubble radius' or 'Hubble distance,' and the boundary of the volume as the 'Hubble limit.'"

• "Per Hubble's law, objects at the Hubble limit have an average comoving speed of c relative to the center. Contrary to common belief, this does not define the boundary of the observable universe. The observable universe has a much larger radius, about 46 billion light years, and the most distant object we can see, the primordial fireball (the source of the cosmic microwave background), has a comoving speed of about 3.3 times c."

Can you please explain what's inaccurate about these? -- BenRG 17:14, 1 October 2007 (UTC)

Apologies for the first one: you were right and I was confused. The problem with the second one is that it implies that the primordial fireball is receding from us at 3.3c, which is misleading. Objects in the region of space that we observe as the primordial fireball are currently receding from us at 3.3c, but the fireball itself (as far as it can be said to be receding at all when it no longer exists) appears to be receding at less than the speed of light. Similarly, objects at the Hubble limit are observed to be receding at a fraction of the speed of light, but are currently receding at the speed of light, which is what confused me over the first point. I hope what I've now written makes sense. Cosmo0 18:46, 1 October 2007 (UTC)

Actual Volume?

(4/3) * 13.8 billion light years^3 * pi = 11008442251073380127723994828776 light years^3

8.46732406*10^47 meters^3 per light year^3

1.10084423*10^31 * 8.46732406*10^47 = 9.32*10^78 meters^3

That's a lot of space :) Rm999 09:21, 4 October 2007 (UTC)

Merger proposal

Articles Hubble volume and Observable universe are written on the same principle and should therefore be merged.

I don't know exactly what the process for merging is, but I have placed tags on the pages involved . Beast of traal T C _ 13:14, 8 October 2007 (UTC)Beast of traal

This (or something similar) has already been discussed (see the talk page for observable universe). Although the term "Hubble Volume" is often used as a synonym for the observable universe, it also has a more strict definition, which is actually somewhat smaller than the size of the currently observable universe, although I agree the two are related. Cosmo0 21:10, 8 October 2007 (UTC)

Ok, I'll take the tags off. Thanks. Beast of traal T C _ 23:47, 1 November 2007 (UTC)Beast of traal

"The term "Hubble volume" is also frequently (but mistakenly) used as a synonym for the observable universe; the latter is larger than the Hubble volume.[2][3]"

• • • How can the observable universe be larger than the Hubble volume!?

It would seem that the observable universe is smaller than the Hubble volume, since we can see less than the limits of the Hubble volume as the "edges" recede away. Please reply, else I will make the correction "latter is larger than" to "latter is smaller than" —Preceding unsigned comment added by 70.52.148.201 (talk) 20:30, 23 April 2009 (UTC)

Read the references - that's what they're there for. Especially #2 (section 3.3 is the relevant section) which specifically says "For a discussion of why objects can be seen that are outside the Hubble sphere, see ..." Cosmo0 (talk) 23:40, 23 April 2009 (UTC)

decelerate due to gravity?

[Main article] "This is because 1 / H0 gives the age of the universe by a backward extrapolation which assumes that the recession speed of each galaxy has been constant since the Big Bang. In fact, recession speeds initially decelerate due to gravity, and are now accelerating due to dark energy, so that 1 / H0 is only an approximation to the true age."

There is the above silly statement in the main article that a guy or a gal responsible for it should fix a.s.a.p.

The problem with it is of course that "gravity" (some time ago we agreed rather on the term gravitation) couldn't cause neither deceleration (nor acceleration to be sure) since garavitation does not add any energy to the system of free falling bodies (since there is no action at a distance in a real world - or at least in Einstein's world, in which we might be living, or at least it was never proved otherwise).

In my opinion it might be better for readers of wikipedia if wikipedia diddn't promote prejudices from ancient times when gravitation was controlled only by Newtonian math. We already know a great deal more about the universe and wikipedia should follow rather the knowledge than the prejudices that even Newton didn't share. Jim (talk) 10:55, 30 October 2010 (UTC)

Always visible

I think that this article has slightly misrepresented the observability of distant objects. Any object that was ever within the Hubble sphere will always be observable, but the light from it will continue to be redder and more aged as the object appears to approach the speed of light. Their time appears to slow down more and more. (Similar to light from an object falling into a black hole.) A better way of saying it is that light emitted (now) from objects (now) beyond the Hubble sphere will never be seen. - Allyn

For instance the CMB appears to have a red shift of about 1100X. This means that it takes three years to observe one day of aging of the surface radiating the CMB. That surface continued to move away from us for those three years, but we can only see the next day's light. An observer that is on the 13 Billion year old galaxies that formed where we currently observe the CMB can also observe the CMB from 13 Billion year ago emitted from our current location. - Allyn — Preceding unsigned comment added by 97.65.82.66 (talk) 16:31, 20 December 2011 (UTC)

1/H₀

Is the 1/H₀ an error? It looks like both instances are supposed to be c/H₀, not 1. Or am I wrong? 137.150.14.176 (talk) 11:07, 3 December 2012 (UTC)

Comoving vs. proper speed

This may just be a lack of firm understanding of comoving vs. proper distance on my part, but this statement in the article:

• "Objects at the Hubble limit have an average comoving speed of c relative to an observer on the Earth ..."

seems to me to refer to proper not comoving speed: Comoving distance says "Comoving distance factors out the expansion of the universe, giving a distance that does not change in time due to the expansion of space", and "Proper distance ... can change over time due to the expansion of the universe": so shouldn't an expansion-caused speed be referred to as proper not comoving speed? KaiGetrost (talk) 20:39, 25 April 2013 (UTC)

I agree, "comoving speed" as a result of cosmological expansion is an oxymoron. The article makes the same mistake in the introduction. The statement:

• "The comoving radius of a Hubble sphere (known as the Hubble radius or the Hubble length) is c/H_0"

is wrong. Hubble's law is stated in terms of proper distances. The comoving Hubble radius is (aH)^-1, where a is the scale factor. Without the scale factor, it is only comoving at the present epoch.

I would welcome additional input before making changes, but I believe both these instances of "comoving" should be changed to "proper". 63x7r5 (talk) 15:24, 18 February 2014 (UTC)

Radiation (very long wavelength photons, but in the future energetic particles because its size isn't constant) with wavelength similar to the Hubble volume (decausality/discausality/causal disconnection) diameter

Actually nowadays the mean discausality electromagnetic wavelength is longer than the Hubble volume (due to the overlapping Hubble volumes which hold the universe as a whole with some hypernymic/superordinate causality that merely holds the universe as a whole and isn't related to interaction (common) causality). If we deem Hubble volumes as standalone marbles without to focus on spacetime as a whole, the mathematical description is wrong. — Preceding unsigned comment added by 2A02:2149:8200:6E00:1CD2:E750:1821:8A8A (talk) 13:27, 25 November 2021 (UTC)

It's a little bit disappointing

that the official name for this concept is not Hubble Bubble. 84.71.42.186 (talk) 12:43, 11 January 2022 (UTC)

Red links: pointwise superluminal expansion and expandatory cyclic cosmology

Where are these terms mentioned anywhere outside of Wikipedia / mirrors (WP:CIRCULAR)? Does anyone know? I can't find mention of these anywhere. El_C 13:58, 16 October 2022 (UTC)

Observations indicate that the expansion of the universe is accelerating,[6] and the Hubble constant is thought to be decreasing.[7]

Obviously, the both things cannot be true at the same moment. The second link cited says the universe is decelerating. Reciprocist (talk) 02:37, 24 July 2023 (UTC)

P.S. It seems, the second source is outdated, it is from 2003, when accelerating expansion was not yet known.--Reciprocist (talk) 02:40, 24 July 2023 (UTC)