Talk:Hubble's law

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

text: In theory, km/s/Mpc is the same unit as  (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)
replies: text: 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)
replies: text:  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)
replies: text:  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.  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)
replies: text: 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)
thread_title:  Comprehensible expansion measure 

message: 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)
replies: text: 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)
replies: text: 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)
thread_title:  Measurements of Hubble tension 

message: 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)
replies: text: 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)
thread_title:  Terminology 

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