Talk:Hubble's law

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)

Requested move 1 January 2023

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 discussion.

The result of the move request was: not moved. Numbers are moderately in favour of remaining in place (nb, two IP comments from very similar origin in Belgium were in support and counted as distinct). Support comments preferred to follow the IAU as authoritative, with expectation that WP:COMMONNAME would follow, while oppose comments held that the common name has not yet shifted, but may yet change in future. (non-admin closure) — HTGS (talk) 02:15, 9 January 2023 (UTC)

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Hubble's law → Hubble–Lemaître law – This is now the new WP:COMMONNAME, after it was renamed in 2018. It is used by the relevant academic literature, see https://link.springer.com/chapter/10.1007/978-3-030-80654-5_5 for example. Sadly Google Ngram is not yet usable since it only works up to 2019. PhotographyEdits (talk) 17:26, 1 January 2023 (UTC)

• See also previous discussion on this. Banedon (talk) 22:18, 1 January 2023 (UTC)
  • Same as previous. 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) 12:44, 4 January 2023 (UTC)
• Oppose. True common names are not determined by decree or by polls conducted only among specialist experts. —⁠ ⁠BarrelProof (talk) 04:48, 2 January 2023 (UTC)

  Disagree: specialists have thought this through. Common names can still be used in the article (as is currently the case).

  alex (talk) 13:05, 3 January 2023 (UTC)

• Oppose. WP:COMMONNAME. The purpose of a name is to place a relatively-permanent label on an object, not to ascribe credit from moment to moment. Tarl N. (discuss) 12:16, 2 January 2023 (UTC)

  Disagree: look at country names: they can change too. alex (talk) 13:06, 3 January 2023 (UTC)

• Oppose. This is still the common name for the relation. It will take some years before we know whether the IAU recommendation becomes the name in common use. Their recommendations are not always followed, though it is good to recognize Lemaître. Common use means more than just academic papers, but textbooks and popular science articles. StarryGrandma (talk) 19:47, 2 January 2023 (UTC)

  Disagree:

  1. Delaying the name change will make the common use more difficult, leading to a vicious circle.

  2. Fact is that the name change has been approved by a reference body.

  alex (talk) alex (talk) 12:58, 3 January 2023 (UTC)

• Support.

- This is now the correct name for this law, as decided by a reference organisation.

- It should be introduced as soon as possible to prevent memory to fade again.

- One should also take into account the risk of resistance to a name that could be seen as belonging to / originating from the English language region,

alex (talk) 13:03, 3 January 2023 (UTC)

Wikipedia does not make the news, and should only reflect what is going on in the real world. If not changing the article title causes the memory to fade again, then it was not the common name in the first place (or at the very least, can be shown to have not been a significant-enough part of history to merit changing the page name). In other words, we cannot make fetch happen. Primefac (talk) 13:43, 3 January 2023 (UTC) I also changed the indenting of your message mainly for formatting purposes.

One should also take into account the risk of resistance to a name that could be seen as belonging to / originating from the English language region @Axd: I'm not sure what you mean by that. Could you please explain. Maybe you are trying to say that we should be careful because refusing to adopt the non-English name "Lemaître" could be seen as cultural bias on our part? That's the opposite of what you wrote, so I apologize, but hence why I ask for clarification. Renerpho (talk) 21:32, 5 January 2023 (UTC)

Or maybe you mean that the name "Hubble", which is an English name, may run into resistance from those who oppose the influence of English/American culture? I am guessing. Renerpho (talk) 21:35, 5 January 2023 (UTC)

Support It is now the official name by the IAU. 2A02:A03F:AA2C:7200:D01A:6015:8D63:34BB (talk) 13:40, 3 January 2023 (UTC)

• Support. The International Astronomical Union is the internationally offically recognised body in charge of the nomenclature of everything in astronomy. And, although the International Astronomical Union cannot enforce the application of its resolutions, the fact that resolution B4 was adopted in October 2018, effectively means that the official name of the law is since then the "Hubble-Lemaître law". (by Thierry Pauwels, member of the IAU) — Preceding unsigned comment added by 2A02:A03F:F8CE:7300:F943:ECF2:918D:792A (talk) 15:42, 3 January 2023 (UTC)

  The officialness of IAU's role is overstated, both on this talk page, and in general. They have no mandate to name physical laws. All they can do is make recommendations, which they call resolutions. People will follow those recommendations when they are convenient, and ignore them otherwise. That is a democratic process, but there are no actual votes involved. Scientists vote with what they publish in scientific journals, not with what they vote on at IAU meetings. The ongoing discussion about the term dwarf planet (used in the IAU sense by some astronomers, not used in the IAU sense by most planetary scientists, and no consensus on Wikipedia as of November 2022) is a good example, as is IAU's recommendation to stop using the unit erg (see my comments here). And even if IAU were the most official body in the world, that still would not matter. There is a policy WP:COMMONNAME on Wikipedia, and there is WP:OFFICIALNAME. Only one of those is a policy saying what the title of a Wikipedia page should be (hint, it's not WP:OFFICIALNAME). Renerpho (talk) 16:26, 5 January 2023 (UTC)

• Oppose per WP:COMMONNAME. We use the most common English term for article titles, which is not necessarily the officially sanctioned term. Rreagan007 (talk) 19:49, 3 January 2023 (UTC)
• Support. The IAU carries sufficient weight for its resolutions to be considered common use in encyclopedias and other reference works. In fact, IAU resolutions are challenged so rarely that the exceptions (see: status of Pluto) become newsworthy on their own.--Lieven Smits (talk) 11:00, 4 January 2023 (UTC)
• Oppose per WP:COMMONNAME. -- Necrothesp (talk) 14:11, 4 January 2023 (UTC)
• Support. If anyone gets to decide the names of astronomy-related things, it's the IAU. - Parejkoj (talk) 15:53, 4 January 2023 (UTC)
• Comment What source are we using for determining the WP:COMMONNAME? I don't think it's fair to use sources from before the renaming, because that heavily skews it towards the old name. In newer academic sources, the longer name is being used. And it's not popular subject that newspapers and magazines write about, so purely using academic sources seems fine to me. PhotographyEdits (talk) 17:27, 4 January 2023 (UTC)
• Oppose per WP:COMMONNAME. Nothing changed since Talk:Hubble's_law/Archive_3#Requested_move_1_November_2018. Headbomb {t · c · p · b} 17:42, 4 January 2023 (UTC)
• Comment On another note, the WP:NAMECHANGES policy has not been mentioned yet, which says: "we give extra weight to independent, reliable English-language sources ("reliable sources") written after the name change. If the reliable sources written after the change is announced routinely use the new name, Wikipedia should follow suit and change relevant titles to match.". So that means my previous comment applies here. PhotographyEdits (talk) 18:24, 4 January 2023 (UTC)
• Oppose. Considering Google Scholar results since 2019, I found 344 results for "Hubble's law", 1040 results for "Hubble law", and 239 results for "Hubble-Lemaître law" (dash/hyphen and diacritic don't matter for the numbers). While there are significant caveats in taking those numbers too seriously, I would generally conclude that, although Hubble–Lemaître has increased in popularity in the past few years, it has not (yet?) turned into the common name, even if we accept that WP:NAMECHANGES applies. Separately, I think the level of officialness of the 2018 recommendation is overstated; there's neither formal authority nor de facto precedent. From Nature news coverage: "The move seems to be the first time an organization has voted to alter the name of a scientific law — although some scientists doubt whether the change will be noticed. The IAU has been the arbiter of planet and moon names since 1919, and oversees astronomers’ official catalogue of star names, but it has no formal mandate over the names of scientific laws. Piero Benvenuti, a former IAU general secretary who proposed the motion, says that the new terminology is a recommendation only. 'If people will continue to use the Hubble law naming, nobody will object,' he says." (See also [1] for other coverage of the 2018 vote.) Adumbrativus (talk) 06:46, 5 January 2023 (UTC)
• Oppose, for now Nothing significant has changed since I voted oppose on this in November 2018, and what I said in the previous discussion still applies.[2] IAU has given a recommendation, nothing more; they could not do more since they have no mandate to name physical laws. IAU has demonstrated in other instances that they often do not follow their own recommendations (see the ergs example I gave in the old discussion). While Hubble–Lemaître law has been used in a number of publications (20-30%, going by Adumbrativus's count above), that does not yet make it the common name. We are not here to set (or even to follow) trends, but to report on current knowledge. The common name may still change in the future, but we are not there yet. Renerpho (talk) 15:57, 5 January 2023 (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.

Hubble tension article

Would it be advisable to start a separate article on the Hubble tension? It has a lot of traction in science news and would be easier to follow the developments. A small section in this article does not seem enough. ReyHahn (talk) 11:20, 28 November 2023 (UTC)

I'd support a split, but someone who have to write the separate article. Banedon (talk) 00:44, 29 November 2023 (UTC)