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Some textbooks describe accelerated expansion with an H value, as shown in the two figure above. It is said that the H value is accelerated because it is smaller in the past and larger in the present. On the contrary, during the deceleration expansion period, the value of H was greater in the past.

However, a proper explanation explains acceleration expansion with the acceleration of the scale factor. In fact, the Hubble constant decreases with time, and in the distant future, the Hubble constant becomes constant, that is, the exponential expansion era is reached.

Why do many textbooks insist on explaining acceleration expansion with H? I think it's a wrong way of explaining.!

teacher
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  • It's descrbing an out-of-date model – John Davis Sep 11 '21 at 17:25
  • @John Davis Are you saying that the H-based explanation method is an old-fashioned method? Are you saying that it was right at the time but wrong now? You're talking about the first picture and the linked picture, right? – teacher Sep 11 '21 at 17:41
  • @john Davis I wonder if the explanation using H itself means a wrong approach or an outdated but acceptable explanation. – teacher Sep 11 '21 at 17:43
  • @ 빛나는밤 Have you considered the possibility that the universe is not expanding, but only appears to be due to us using the wrong redshift scale-factor relation? https://physics.stackexchange.com/questions/620794/cosmology-an-expansion-of-all-length-scales – John Hunter Sep 11 '21 at 18:24
  • I'm surprised at the fringe comments here. The right answer is that $\Lambda$CDM is well supported but if you find it easier to explain without referring to Hubble's constant, more power to you. – Connor Behan Sep 11 '21 at 18:26
  • Also, the (plausible) statement that $H$ approaches a constant in the distant future says nothing about whether it does so from above or below. – Connor Behan Sep 11 '21 at 18:27
  • @Connir Behan I just wonder if the explanation method of the two pictures is right or wrong. And please ignore comments that go against the general principles. Is it appropriate to use the hubble parameter as in the picture? – teacher Sep 11 '21 at 18:40
  • In its Jan. 2021 issue, Forbes Magazine ran on a pop.-sci. article on a possible slowing of the cosmic expansion. I believe the 1st that I (a layperson) had heard of it was in the 2012 book titled "The Arrows of Time: A debate in Cosmology", edited by Rudiger Vaas and Laura Mersini-Houghton: Mersini-Houghton's well-known for having explained a void in the Cosmic Microwave Background through inhomogeneity, which is not an aspect of the "Standard Model". – Edouard Sep 13 '21 at 15:44

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