Talk:Hubble's law/Archive 3

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Hubble constant discrepancy

I feel like there's enough material now about the discrepancy in measurements of the Hubble constant (CMB vs. astrophysics measurements) to have an independent article on it. Banedon (talk) 02:11, 24 September 2018 (UTC)

Agreed. The independent article, however, should also point to the fact that the Hubble diagram is mostly misinterpreted. To show the alleged proportionality of speed v and distance d (v1/d1 = v2/d2 = v3/d3 = H = constant) at least two measurement points of one and the same galaxy would be required. But the diagram shows only single measurement points of many different galaxies. These are scattered between the axes of the diagram, showing no order at all. The red middle line drawn in the diagram (by whom?) has no rational meaning whatsoever. It is not possible to deduce a constant H = v/d from the diagram. Ed Dellian2003:D2:9719:3088:2162:ED6F:B25B:D535 (talk) 16:30, 25 September 2018 (UTC)

The validity of Hubble's law isn't in dispute, only the precise value of the Hubble constant is. I don't think your comment is relevant. Also why would you need two measurements per galaxy? A single measurement, or multiple measurements of different galaxies at the same distance, is enough. Banedon (talk) 21:10, 25 September 2018 (UTC)

Since Hubble's asserted "constant", that is, the v/d of galaxies, is not really constant according to measurement, Hubble's law, which requires H to be constant, is of course at stake. This is why astronomers are desperated about the discrepancy in measurements of H. - Why two measurements per galaxy to see if h/d is constant? Because you need two measurement points at different times to see whether or not the quantity h/d changes in time, that is, whether or not the measurement points all the time will lie on the same straight line from the zero point to the measurement points. Only this outcome would indicate that h/d is constant. The "red line" added to Hubble's diagram tries to make the reader believe in such an outcome, but falsely. - Why do you speak of "multiple measurements of different galaxies at the same distance"? Would the distances of galaxies be the same - what would be the consequence? Actually the distances are different, of course, so it makes no sense to speak of "galaxies at the same distance". Ed Dellian 2003:D2:9719:3088:2162:ED6F:B25B:D535 (talk) 05:45, 26 September 2018 (UTC)

Hi Ed, your objections are off-topic here, as this is not the place to discuss the subject of the article. Here we can talk about the format and the structure of the article, and on adding/changing/renaming content, provided it is based on reliable sources. We cannot discuss (aspects of) the content here, based on our own opinions — see wp:Talk page guidelines. You can always ask at the wp:Reference desk/Science. For the origin of the (blue) line in the diagram of the section Hubble's law#Interpretation, see article Correlation. - DVdm (talk) 06:39, 26 September 2018 (UTC)

Thank you for this information. Please note that I do not discuss something "based on my opinions". It is not "my opinion" but a basic truth of mathematics that it is absolutely not possible to deduce a "law" from correlations of measurements of velocity and time, v/d, of different objects (this is corroborated by the article "Correlation"!). It is also a basic truth of mathematics that a v/d proportionality is simply impossible, since it would state that velocity increases instantaneously with space, which is not true: Everybody knows that velocity increases only in proportion to time. As a consequence, its proportion to distance is not linear v/d but a proportion of v to the square root of distance d. It is also not my opinion but a well-known fact that measurements of H since Hubble's time have always produced different values. Why did Hubble get an "age of the universe" of only 2 billions? Because he calculated it based on the data of quite "near" galaxies. Why do we get 14 billions today? Because we are using the data of the most distant galaxies which were not observable in Hubble's time. This fact demonstrates that the value of H depends not only on time but also on distance, so that H is certainly not a constant. You will know that the final conclusion is generally admitted, and even initially here! So I believe that the public should be informed not only about the fact of "discrepancy in measurements of Hubble's constant" but also about the consequences of H being a variable according to these discrepancies. Ed Dellian87.188.202.42 (talk) 14:23, 26 September 2018 (UTC)

Wikipedia is not here to inform the public of what you think is wrong with Hubble's constant. Wikipedia is here to report the current state of affairs in science, based on the established literature. Without a proper wp:reliable source your remarks and questions are off-topic here. I have put a second level warning at User talk:87.188.202.42. - DVdm (talk) 14:40, 26 September 2018 (UTC)

1. "Since Hubble's asserted "constant", that is, the v/d of galaxies, is not really constant according to measurement, Hubble's law, which requires H to be constant, is of course at stake."

   Hubble's law does not require that H remains constant. This is clearly explained in the lead and elsewhere in the article.

2. "This is why astronomers are desperated[sic] about the discrepancy in measurements of H."

   Astronomers aren't 'desperate' on this, there is no crisis about the validity of Hubble's law. The interest is because discrepancies are things of inherent interests to scientists, and understand the cause of the discrepancy and how to resolve it furthers knowledge and understanding of the world. The different methods make different assumptions, and knowing which assumptions need to be modified / which are invalid is of great interest.

Headbomb {t · c · p · b} 14:36, 26 September 2018 (UTC)

I have been discussing the matter with various experts. You are the first to assert that Hubble's law "does not require that H remains constant". O. k. That's your belief. It has nothing to do with science. Ed Dellian2003:D2:9719:3087:C027:F949:12FA:FD91 (talk) 17:07, 26 September 2018 (UTC)

Then I question the so-called 'experts' you've been talking to, because this is something covered in any introduction to astronomy/astrophysics class out there. Headbomb {t · c · p · b} 17:11, 26 September 2018 (UTC)

Back on topic, I don't see that the discrepancy merits more than a section in this article. I don't see the case that this is something so massively important that it needs more than that to adequately summarize and cover. Headbomb {t · c · p · b} 17:18, 26 September 2018 (UTC)

The discrepancy is quite big though, by several sigma. Whatever is causing it is a major headache for cosmologists, especially since this is a discrepancy in one of the most important parameters in Lambda-CDM. We have articles on other unsolved problems in physics, a similar one could be created for this. Banedon (talk) 07:29, 27 September 2018 (UTC)

Do we rename article to Hubble-Lemaitre law?

We're getting into something of an edit war over the first sentence as of time of writing. Do we rename the article to Hubble-Lemaitre law? If so, we should not just amend the first sentence, we should move the article entirely. Banedon (talk) 23:24, 31 October 2018 (UTC)

No, per WP:COMMONNAME. That the IAU recommends to call the Hubble's law the Hubble–Lemaitre law, does not change what most people call it, or what it's best known as, and also overlooks the fact that people are free to ignore or follow the recommendation. Headbomb {t · c · p · b} 01:28, 1 November 2018 (UTC)

I just did a symbolic revert, should be banned for a few hours at least, to be in on this great semi-official recognition by the International Astronomical Union that the cosmology that describes the expanding universe was published and accredited to Georges Lemaitre two years before Hubble published. Finally. I'm biased on this, or else I'd put up the RM, but please, someone go ahead and RM the Hubble-Lemaitre law, so it could be discussed fully. Thanks. Randy Kryn (talk) 02:59, 1 November 2018 (UTC)

I'd hold off on the rename until it comes into common usage. Right now it most definitely is not. Praemonitus (talk) 03:16, 1 November 2018 (UTC)

Reverted WP:POINT-y revert by Randy Kryn back to the long-standing, massively dominant WP:COMMONNAME of the phenomenon. The IAU does not decide how physical laws are named. Headbomb {t · c · p · b} 03:16, 1 November 2018 (UTC)

That's what an RM would discuss, how soon after an inside-baseball rename should a Wikipedia article title be changed. Corporations do it all the time, and although the IAU isn't the sole factor to take into consideration it should carry its due weight (as when Pluto was knocked off its high horse as a planet). Should be an interesting discussion at some point. It may end up that the common name stays as the title but that the duo name be entered first as the first mention, followed by the common name, as the preferred option by several editors suggests. Randy Kryn (talk) 03:26, 1 November 2018 (UTC)

The difference with a corporation is that a corporation controls its own name, and a name change usually happens after an acquisition, merger, or rebranding. A physical law belongs to no one, and the current reflects that, despite Lemaitre having a claim of precedence to the law, Hubble was the one that convinced the world. Snell's law isn't renamed the Sahl's law for the same reason, Snell was the one that convinced the world. Headbomb {t · c · p · b} 14:54, 1 November 2018 (UTC)

Wait to see if this becomes common usage in astronomical literature. I expect that will happen, as astronomers do tend to follow (most) IAU recommendations, but it's not a foregone conclusion. WP:NAMECHANGES is designed for names of people, but seems to work just as well in this situation. Right now we should just list it as an alternative name. Maybe reassess in a year. Modest Genius ^talk 11:26, 1 November 2018 (UTC)

Astronomers tend to follow IAU recommendations where it is convenient. There are examples where such recommendations have simply been impractical. In 1989, the IAU has decided that the unit erg should no longer be used in scientific publications. I don't know any astrophysicist who cared about that recommendation.[1] Renerpho (talk) 04:34, 3 November 2018 (UTC)

Hence why I said 'most'. Modest Genius ^talk 14:16, 5 November 2018 (UTC)

Requested move 1 November 2018

The following is a closed discussion of a requested move. Please do not modify it. Subsequent comments should be made in a new section on the talk page. Editors desiring to contest the closing decision should consider a move review after discussing it on the closer's talk page. No further edits should be made to this section.

The result of the move request was: Not moved as the policy based arguments against moving outweigh the arguments made in favour of the move. (non-admin closure) Iffy★Chat -- 16:06, 8 November 2018 (UTC)

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Hubble's law → Hubble–Lemaître law – 78% of professional astronomers voted in support of renaming the law. JeanLucMargot (talk) 13:54, 1 November 2018 (UTC)

References

• No, per WP:COMMONNAME. The IAU does not have the authority to decide what physical laws are called, and until the world accepts their recommendation, neither should Wikipedia. Headbomb {t · c · p · b} 14:39, 1 November 2018 (UTC)
• Yes, per the recommendation of the IAU and the overwhelming support by the relevant community. - DVdm (talk) 17:10, 1 November 2018 (UTC)
• Yes per nom and DVdm (IAU and relevant community). Randy Kryn (talk) 17:16, 1 November 2018 (UTC)
• No —but possibly later when others actually start using the name a lot. At the moment "Hubble's law" seems to be used 1000 times more often, according to ghits. And the "Hubble–Lemaître law" is only reporting the name change vote and not actually using it yet. Graeme Bartlett (talk) 22:19, 1 November 2018 (UTC)
• Lean yes since it's the IAU and it represents the consensus of most astronomers. Similarly when the IAU redefined Pluto as a dwarf planet in 2006 we didn't adopt a wait-and-see-if-it-becomes-common stance. However I don't feel strongly either way. It's just a name, and it's obvious what is being referred to regardless of whether it's called Hubble's law or Hubble-Lemaitre law. Banedon (talk) 23:50, 1 November 2018 (UTC)
• Yes, per IAU recommendation. De wafelenbak (talk) 10:15, 2 November 2018 (UTC)
• Yes, per IAU recommendation. The Cosmology section of the International Astronomical Union (IAU) overwhelmingly voted in favor of the change. Following this vote, the IAU brought it to all members and they approved it by 78%. As such, the change has the support of both the very specialists and the overall community of professional astronomers. Most people still refer to it by the old name and they will do so for several years to come, but this is because of the inertia that names have in society. In my opinion wikipedia's role on scientific subjects is to provide accurate information of the views of the science community. — Preceding unsigned comment added by Thanassiskatsiyannis (talk • contribs) 14:12, 2 November 2018 (UTC)
• Oppose for now. The IAU's recommendation is important, but I would say that it is still Wikipedia's place to follow common usage rather than try to change that usage. If referring to it as the "Hubble–Lemaître law" gains wide usage following the IAU's recommendation, then we should change our title, but I think that changing it right away is premature. Dragons flight (talk) 15:39, 2 November 2018 (UTC)
• Oppose per WP:COMMONNAME. A comparison of ghits shows the overwhelming preponderance of usage is the current form. When that changes, we can move the article. Wikipedia serves everybody, not just the IAU. Praemonitus (talk) 15:42, 2 November 2018 (UTC)

• Of course the common name is the present title, and that won't change for awhile, especially if going by ghits. But long-term significance is more important here, and the proposed title fits that criteria. Randy Kryn (talk) 15:46, 2 November 2018 (UTC)

• Actually no, this is an encyclopedia of knowledge not a trend setter. It's here to document what is known, not to be a WP:CRYSTALBALL. Not every decision by an official board gets widely adopted. Until that happens, there's no reason to be premature. Praemonitus (talk) 16:02, 2 November 2018 (UTC)
• "Long-term significance is more important". You appear to be assuming the IAU's recommendation will actually come into wide usage and gain significance. That seems likely, but it is hardly a sure thing. I can think of examples in both geology and biology where leading bodies made naming recommendations only to abandon them some years later when it became clear that few people were actually following the stated preference. Hence, I think we should wait to see if the new usage is actually widely adopted. Dragons flight (talk) 15:58, 2 November 2018 (UTC)

• "That seems likely, but it is hardly a sure thing." It does not even seem likely to me. Astronomers tend to follow IAU recommendations where it is convenient. There are examples where such recommendations have simply been impractical. In 1989, the IAU has decided that the unit erg should no longer be used in scientific publications. I don't know any astrophysicist who cared about that recommendation.[2][3] Renerpho (talk) 04:42, 3 November 2018 (UTC)

• How many astrophysicists do you know? No astrophysicist I know still uses ergs (SI units have long taken over). Banedon (talk) 04:59, 3 November 2018 (UTC)

Speaking as an astronomer: everyone working in AGN/quasar physics. erg/s/cm^2/Hz is a *very* common unit of flux. - Parejkoj (talk) 15:58, 3 November 2018 (UTC)

Of course it depends on the field... But a Google search for Astronomer's telegram erg will provide you with an arbitrary number of examples; some of them [4] in recent IAU publications. Renerpho (talk) 14:56, 4 November 2018 (UTC)

• Oppose, per WP:COMMONNAME, and the fact that the IAU has no mandate to name physical laws.[5] The IAU resolution is a recommendation, nothing else, and it remains to be seen if it will catch on. Renerpho (talk) 18:49, 2 November 2018 (UTC)
• Wait, see my reasoning in the section above. Why was a new discussion started when one was already underway? Modest Genius ^talk 20:21, 2 November 2018 (UTC)
  • This section is a formal move request which brings outside eyes to the topic of renaming per WP:Requested moves process. -- Netoholic @ 21:03, 2 November 2018 (UTC)
• Oppose per WP:COMMONNAME and others above. Seems to be WP:TOOSOON. -- Netoholic @ 21:03, 2 November 2018 (UTC)
• Oppose per WP:COMMONNAME. Rreagan007 (talk) 01:09, 4 November 2018 (UTC)
• Yes. The majority of professional astronomers voted in support of renaming the law. — Preceding unsigned comment added by 193.190.231.130 (talk) 08:53, 5 November 2018 (UTC)

Yes, per IAU recommendation. — Preceding unsigned comment added by 193.190.231.130 (talk) 10:35, 6 November 2018 (UTC)

Struck. You can't !vote twice. Modest Genius ^talk 11:51, 6 November 2018 (UTC)

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The above discussion is preserved as an archive of a requested move. Please do not modify it. Subsequent comments should be made in a new section on this talk page or in a move review. No further edits should be made to this section.

Graphic - Gravitational Inverse Square Law

I'm not familiar with Wiki editing but found this to be a something worth commenting on. When you click on the diagram at the top right of the Hubble's Law page, under Physical Cosmology, you are taken to a description of the graphic that is more reminiscent of Gene Ray's Time Cube website then a Wiki article. You only see the rambling description if you click on the graphic, if you try to open it in a new tab it won't display the information.

Hopefully someone finds this helpful. — Preceding unsigned comment added by 2601:100:4100:7CF0:B83E:2A97:47E6:7196 (talk) 02:29, 31 December 2018 (UTC)

Looks like vandalism. I'll revert. Primefac (talk) 15:47, 31 December 2018 (UTC)

Error in Analysis

It should be noted that the Fizeau-Doppler shift formula, which was used in the analysis of the redshift data, is only valid if the source and receiver were both stationary in the same reference frame at some point in their past and one of them remains so. Otherwise their relative velocity is given by the velocity addition formula as u = +/-( v2 - v1 ) / ( 1 - v1 * v2 / c^2 ) where v1 and v2 are reckoned in the context of that common reference frame and the Doppler shift factor is f1 / f2 = gamma1 / gamma2 / ( 1 + u / c ). If the common reference frame was that of the Big Bang, then the ratio of gammas is related to the ratio of masses. Hubble's law might need be revised if the analysis did not take that into account. --Relativity Guy (talk) 22:09, 26 May 2019 (UTC)

Do you have a reliable source to this effect? That is, that the Hubble Law is described incorrectly? Tarl N. (discuss) 22:34, 26 May 2019 (UTC)

I do not have access to the raw data or the analysis. I just noticed that the referenced Doppler formula is incorrect for this application. --Relativity Guy (talk) 01:32, 28 May 2019 (UTC)

Relativity Guy There is no need for the raw data or analysis. We just need independent verification of your claim. Can you provide a reliable source that states that the formula used here is not applicable to the context (and, ideally, for the allegedly correct formula, too)? The simplest solution would be to give the source you used (unless it is original research, in which case you need something from a different reliable source). Did you use a text book? A specific paper? Renerpho (talk) 02:02, 28 May 2019 (UTC)

I don't think this qualifies as original research. It's just the logical consequence of the principle of conservation of momentum since the reference frame in which the net 3-momentum is zero has the maximum possible lapse of proper time. --Relativity Guy (talk) 04:03, 28 May 2019 (UTC)

Please indent all your talk page messages as outlined in wp:THREAD and wp:INDENT — See Help:Using talk pages. Thanks.

Wikipedia needs reliable wp:secondary sources for all challenged new content. See wp:RS and wp:BURDEN. - DVdm (talk) 08:08, 28 May 2019 (UTC)

The derivation of the Hubble law from the Friedmann-Lemaître-Robertson-Walker metric is standard and to be found in many textbooks, among others for example .

It is my personal opinion that the challenge to the factual accuracy of the result is unjustified. The arguments put forward also have a peculiar nature, to the extent that they appear to imply that the Doppler effect is somehow frame dependent, which would go against special relativity. Falw (talk) 02:11, 18 August 2019 (UTC)

References

It should be stated that Hubble's Law is a mathematical inconsistency except for short distances

The redshift z always used, which came from prerelativity physics in the 1920s, is only valid over the shorter distances originally considered by Hubble. Over large distances it fails to add i.e. if A, B, C, are 3 objects (e.g. nebulae) in a line then (redshift B from A) + (redshift C from B) does not equal (redshift C from A) a condition essential for the linearity of the Hubble Law. It is also an essential condition for the objectivity of redshift measurements. Because if A and B are two observers, they will want to observe the same redshifts apart from a correcting factor. JFB80 (talk) 06:15, 6 November 2019 (UTC)

I don't see the problem. Hubble's Law is based off the distance, not the redshift, and converting redshift to distance isn't too hard. Banedon (talk) 10:46, 6 November 2019 (UTC)

This page conflates Hubble's empirical law for the galaxies with Lemaitre's prediction of cosmic expansion

The discussion of Hubble's law throughout most of this wikipage conflates two distinct discoveries, the observation of an empirical relation between redshift and distance for the galaxies by Edwin Hubble and the theoretical prediction of cosmic expansion from general relativity by Georges Lemaitre. As recently pointed out by several historians, only the first of these is known traditionally as Hubble's law and it is ahistorical to attempt to rename it. While Hubble's law constitutes important evidence for an expanding universe, it stands as an empirical law independent of theoretical explanation. See https://arxiv.org/abs/1809.02557 or https://arxiv.org/abs/1909.07731 for further information — Preceding unsigned comment added by Coraifeartaigh (talk • contribs) 15:57, 20 November 2019 (UTC)

Planck evidence for closed universe, leading to "possible crisis in cosmology"

This article should be updated, to reflect the recent findings of Di Valentino et al, who show that the Planck data is inconsistent with the LambdaCDM model at the 4.4-sigma level, preferring a closed universe. The derived value for the Hubble constant in that new model is much smaller than before (54.4 km/s/Mpc), which fits the CMB data considerably better than the old model, but which is inconsistent with other observations. The inconsistencies go beyond just the value of the Hubble constant. See here for a description of the problem in layman's terms. Maybe Hubble tension deserves its own article. Renerpho (talk) 17:46, 21 November 2019 (UTC)

Too soon that paper's only a few weeks old. There needs to evidence that it is the mainstream opinion first before inclusion. I'm in favor of removing the "measurement" because it's not really a measurement - it's a reinterpretation of Planck's results. Banedon (talk) 18:57, 21 November 2019 (UTC)

There can be no mainstream opinion as long as the crisis is unresolved. Renerpho (talk) 19:13, 21 November 2019 (UTC)

There can, however, be a mainstream opinion that the universe is not flat, or that Lambda-CDM is wrong and this paper is the reason. Banedon (talk) 19:14, 21 November 2019 (UTC)

Took a look at the paper, definitely in favor of removing this now. First it's not an observer - the observer remains Planck, and the authors only reanalyze Planck's results. Second, it's barely about the Hubble constant. Finally, it doesn't show that the universe is closed. The key point I got from the paper is, if we assume the universe is closed, then certain internal inconsistencies in Planck data disappear; however inconsistencies with local measurements of H0 remain. This doesn't get anywhere - a systematic bias in Planck data for example could still lead to the universe being flat. I say remove as too soon. Banedon (talk) 05:56, 26 November 2019 (UTC)

An anonymous cosmologist wrote this public review for the paper, with a response by one of the authors [6]. I think this reinforces what I wrote about above about the insertion being "too soon". I'm going to remove it. Banedon (talk) 02:39, 2 December 2019 (UTC)

@Banedon: agreed. This is a hot topic in cosmology, producing a flood of theoretical papers. Some will make attention-getting claims, but WP should not repeat those claims uncritically. Wait until they've penetrated cosmology enough to be repeated in secondary sources. (And "journalism" consisting of near-verbatim quoting from a University press release is not an independent review.)

That said, reanalyzing existing data sets is completely fair game in this case; that's one of the reasons why this is such a hot topic. The published results are contradictory, so there must have been an error in the analysis somewhere. Either in the data reduction of the raw observations, the estimation of errors, or in the theoretical model which connects the observations to the universe. People are also trying to collect more observations (a self-inconsistent data set would be a nice flashing light pointing to an error), but that's a multi-year process. Re-crunching the numbers can be done now. 196.247.24.22 (talk) 16:24, 4 January 2020 (UTC)

Burying the symbolic representation of redshift by "z"

The redshift is represented by "z" in equations as in "the redshift z = ∆λ/λ of its spectrum of radiation" in the section "Combining redshifts with distance measurements". For people who want to know what that "z" in equations is and what redshift is, would it not be better to put: "The redshift z = ∆λ/λ for a spectrum of radiation." in the lead section? - Fartherred (talk) 23:55, 30 March 2020 (UTC)

One could make that "The redshift is represented by z which is ∆λ/λ for a spectrum of radiation." and put it in the lead section of the Redshift article. - Fartherred (talk) 00:14, 31 March 2020 (UTC)

First Paragraph

I have updated the first paragraph because it was too technical. Vpab15 (talk) 17:23, 3 May 2020 (UTC)

I appreciate the simplification. One thing: The first sentence should give the official name, which is "Hubble–Lemaître law". This was removed, but should be added back. Renerpho (talk) 18:14, 3 May 2020 (UTC) -- I added it back. Renerpho (talk) 18:21, 3 May 2020 (UTC)

• Comment The references/notes sections appear to be broken, with the two references that make up the "notes" being defined but not used in the article. This needs to be repaired. Renerpho (talk) 18:20, 3 May 2020 (UTC)

I have removed the unused references. Regarding the official name, I didn't realise I had removed it. Sorry about that. Vpab15 (talk) 21:29, 3 May 2020 (UTC)

Improve coverage of Megamaser Cosmology Project

I added a table entry for the measurement of the Megamaser Cosmology Project. Several related changes should be made that I don't see myself capable of:

• The image above the table should be updated accordingly. It currently contains lots of unlabelled blue results that should be labelled, and the megamaser result should be added in blue.
• The caption says "blue are other techniques, whose uncertainties are not yet small enough to decide between the two". This should be updated when the image is updated. The abstract of the megamaser article says: "We corroborate prior indications that the local value of $H_{0}$ exceeds the early-universe value, with a confidence level varying from 95% to 99% for different treatments of the peculiar velocities."
• The treatment of the "peculiar velocities" (as indicated in the abstract) should be explained, as it's relevant for assessing the error bounds of the megamaser result.

Joriki (talk) 11:43, 13 June 2020 (UTC)

Age of the universe column

@Tomvassos: I'd like to discuss your recent edit. I don't think adding each study's computed age of the universe to the table is a good idea. As discussed in § Hubble time, the translation from Hubble constant to age of the universe is itself a tricky thing which depends on models of the expansion history of the universe. And questioning those models is one of the main ways theoreticians are trying to resolve the Hubble tension.

While I agree it would be very useful for lay accessibility, it's not something that's directly reported by most of the studies (e.g. Reid and Pesce's 2019 paper (third row in table) doesn't quote a figure), and given that ΛCDM is being called into question by this very data, there's no obviously correct way to convert an H₀ value to such a time.

We could report the Hubble time itself (does cosmology use Gy or Ga?), which would give (since the conversion factor is close to 1) a useful idea of the amount of spread in the estimates. In that case, I'd prefer to add it in parens as a second row in the Hubble constant column, to emphasize that it's the same value in different units. 196.247.24.22 (talk) 12:37, 20 January 2020 (UTC)

• Agree with IP. Banedon (talk) 13:14, 20 January 2020 (UTC)

Astronomy and Cosmology is likely to be the science that is the most behind and a lot has to do with the units used. One would not have to ask about Ga or Gy if one stuck entirely to seconds and applied an appropriate prefix. Instead of gigayears, one would simple use petaseconds (Ps), exaseconds (Es), zettaseconds (Zs) and yottaseconds (Ys) or whatever prefix works to reduce the number between 0 and 1000. Getting rid of lightyears and parsecs for metres with the proper prefixes would make the understanding of distances in the universe much more coherent and logical, but why be logical when you can make it all confusing?

The Hubble constant will then appear in units of reciprocal seconds.

--Ametrica (talk) 03:30, 2 October 2020 (UTC)

Comprehensible expansion measure

The article says expansion is measured in kilometres per second per megaparsec. This means the constant floats around 70, which is an easy to remember number. I understand that space is big ... really, really big, however megaparsecs are an incomparable measure. A parsec is a typical distance between stars in the Milky Way. This incomprehensible measure can be simplified to millimetres per second per parsec or mm/s/ps. Writing 70mm/s/pc immediately gives the sense of the expansion. I can show 70mm/s with my hands. It is my suggestion that even though cosmologists use km/s/Mpc, Wikipedia could use mm/s/ps. At least we can explain it for non-cosmologists. We could add that it's about 22mm/s/ly. Travelmite (talk) 11:20, 23 January 2021 (UTC)

...cosmologists use km/s/Mpc... and so that's what we use. We don't arbitrarily give another unit just because the original is inconvenient for some. Primefac (talk) 11:22, 23 January 2021 (UTC)

I agree with Primefac: km/s/Mpc is the standard. However, there is nothing wrong with improving usability. If there is a more intuitive equivalent unit then I have no problem with adding a note that gives the conversion factor (keeping km/s/Mpc for the main body of the article). Since I can't visualize a light-year or a parsec any better or worse than a Mpc, something like mm/s/ly is no improvement. And using mm/s/pc instead of km/s/Mpc is nonsensical in two ways: First, it hides the fact that the speeds in question are km/s, not mm/s (which is an immeasurable speed), and that the effect is only relevant on Mpc scales (there is no expansion on scales of parsecs). The claim that the Hubble constant is approximately 22mm/s/ly may be mathematically true, but it is misleading (because there is no expansion on light-year scales, and even if it were it could never be measured), and thus it is physically wrong. That something can be "simplified" mathematically doesn't mean it should be. Renerpho (talk) 16:59, 24 January 2021 (UTC)

mm/s is a measurable speed, and inflation occurs at every scale. Travelmite (talk) 01:46, 25 January 2021 (UTC)

Show me the telescope that can measure cosmological speeds in the mm/s range, please. You can measure such speeds on Earth in some contexts, but that is not what we are talking about. Regarding the other point: At scales smaller than Mpc, gravity dominates. That's why our galaxy stays together, atoms don't fall apart, and the Andromeda galaxy is moving towards us. Cosmological expansion decidedly does not occur at every scale, just at large scales. In particular, it only occurs at scales where the universe is approximately homogeneous and isotropic, which it is not at small scales. Renerpho (talk) 08:15, 25 January 2021 (UTC)

The telescopes are not measuring km/s either. If the universe is homogeneous and isotropic, then any scale is appropriate. Travelmite (talk) 15:22, 26 January 2021 (UTC)

The telescopes are measuring redshift, which can be directly translated into a speed (as a fraction of the speed of light). For cosmological redshift, those measurements commonly have uncertainties in the km/s range, and rarely less than that. Measuring a speed of m/s is nearly impossible for objects like galaxies, because the individual stars inside the galaxy are moving at speeds of hundreds of km/s relative to their host galaxy (smearing out your measurement). Regarding the nature of the universe, I completely agree with you: If we lived in such a universe then the literature may not bother with megaparsecs. However, that's not the universe we live in. Our universe is only approximately homogeneous and isotropic at scales of Mpc or larger. Renerpho (talk) 01:41, 27 January 2021 (UTC)

In theory, km/s/Mpc is the same unit as s⁻¹ (since kilometer can be converted to megaparsecs after which they cancel). Doesn't mean we should write it that way. km/s/Mpc is the standard and should be the units used in the article. Banedon (talk) 02:36, 25 January 2021 (UTC)

Yes, it could be written as a per second fraction of 2.2 x 10⁻¹⁸, and it can be written less sensibly as 14 billion years (how long to travel one parsec at 70mm/s). My argument is that while scientists are expected to work mathematically, communicating to a reader with no assumed knowledge, it is best to use the most comprehensible units. Travelmite (talk) 15:22, 26 January 2021 (UTC)

Only if those units are used in the literature. Remember that Wikipedia is an encyclopedia, not a textbook. We, contributors, have no standing ignoring, or knowing better than, the literature. - DVdm (talk) 16:01, 26 January 2021 (UTC)

Writing it as 14 billion years is very sensible, that's why the concept of the Hubble time exists (look, that's even part of this article!). It is a rough estimate for the age of the universe. There are good reasons to keep the concepts separate though, particularly, the Hubble time does not account for the effects of gravity and dark energy. communicating to a reader with no assumed knowledge, it is best to use the most comprehensible units I disagree. It is best to use the most sensible units. That includes keeping the scales at which those numbers are applicable. Anything else would be misleading, which is even worse than being incomprehensible. Renerpho (talk) 01:47, 27 January 2021 (UTC)

Of course we could add an explanation to the article for why km/s/Mpc is the most sensible unit, so the reader doesn't need to wonder. Renerpho (talk) 02:12, 27 January 2021 (UTC)

I'm sure there are ways to improve the readability of this article, especially for readers with no prior knowledge. Inventing units isn't one of them. Why don't we work on improving the sections that lack verification by reliable sources, or on illustrating the sections that have lots of formulas and little prose? Adding explanations for what the formulas do and why they are relevant may greatly increase the value of those sections. Where such illustrations can't be done in a way that suites an encyclopedia, we may discuss whether the formulas are worth keeping. We don't need to make up stuff to make the article easier to understand. Renerpho (talk) 02:04, 27 January 2021 (UTC)

Measurements of Hubble tension

Parejkoj when you write "Also, it's not a difference between observations and models, its between observations", are you sure? My understanding is that these measurements of the Hubble constant are dependent on Lambda-CDM as a framework assumption, so the discrepancy is a difference between observations and models. Banedon (talk) 07:25, 23 December 2021 (UTC)

TRGB and Cepheid/SN measurements don't depend on LCDM at all, whereas CMB measurements do involve fitting a model to produce LCDM parameter values, but the power spectrum data are independent of LCDM or other model. I think the way that paragraph is currently phrased is good. Proposed solutions are both on the model side, and on the measurement systematics side. - Parejkoj (talk) 18:10, 23 December 2021 (UTC)

Are you sure? I'm under the impression that these local universe measurements (TRGB/Cepheid/SN) measure the luminosity distance, from which to get the Hubble constant one still needs a cosmological model. If we agree that one can resolve the Hubble discrepancy with a new cosmological model, then it sounds fair to say "The universe is expanding faster than models predict". Banedon (talk) 02:25, 24 December 2021 (UTC)

Terminology

In general, the Hubble constant changes with time. However, that doesn’t mean that the term is a misnomer (and thus “Hubble parameter” should be used; nothing wrong with that term, but the justification is wrong). It is a constant in the sense that a and b are constants in y=ax+b. It has been known from the very beginning of modern cosmology that the Hubble constant in general changes in time. The text should be corrected to reflect this. — Preceding unsigned comment added by 2001:9E8:23A1:F300:3181:95BE:8AB1:AA14 (talk) 15:47, 14 July 2022 (UTC)

You've given basically a word-for-word quoting (though in a different order) of the third paragraph of the lead; why does it then need to be changed, and how would you suggest we change it? Primefac (talk) 12:57, 15 July 2022 (UTC)