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Background: For a few years now there is significant difference in the calculation of the cosmological constant between standard candles methods and CMB methods. CMB measurement comes from a very young universe (~400k years) and the resulting cosmological constant as calculated by the Planck collaboration is smaller than what is observed today.

Question: Is this so-called "tension" the main reason for talking about accelerated expansion?

I've found a relevant article by Hossenfelder. I understand that according to the article, the answer to this question is "yes; but we've also seen some other minor indications in support of accelerated expansion".

Note: The question is not about whether any of the measurements are plagued by uncertainties or how trustworthy they are in general.

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No. Evidence for the accelerating expansion of the universe comes from multiple angles: supernovae data, Baryon acoustic oscillations, the mass functions of galaxy clusters, etc. That the universe's expansion is accelerating is not in doubt; the question is by how much.

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There is a potential tension between low redshift probes of mass clustering and Planck data (CMB measurements). This ongoing speculation might be evidence of new physics or even modifications of general relativity. However, the author of the article you cited seems to have a confusion between the cosmological constant (no tension discussed in the literature) and the Hubble constant (that has $3\sigma$ differences), which becomes clear when you read the comments where a noted CMB researcher expresses his views. There is even no real evidence that the cosmological constant is not constant.

So it is really exciting if some tensions in CMB data grows into significant differences. But one should not confuse the cosmological constant with the Hubble constant which are completely different stuffs.

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    $\begingroup$ This answer doesn't address the main confusion on the part of the OP, which was the idea that the recent tension between experimental results was the reason for believing that cosmological expansion has accelerated (which was in fact a result from 2 decades ago). $\endgroup$
    – user4552
    Commented Jul 26, 2019 at 14:43
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    $\begingroup$ I also wondered about Hossenfelder's description of this as a tension in the values of $\Lambda$ rather than $H$. It seems unlikely that she just made a silly naive mistake here, since she's a well-known professional in the field. More likely this is a situation where the data analysis doesn't disentangle the two issues. $\endgroup$
    – user4552
    Commented Jul 26, 2019 at 14:52
  • $\begingroup$ @BenCrowell true and true; I should search Wikipedia first the next time :) As about Λ vs. H I'll try to understand it better and add an edit to the question. $\endgroup$
    – Helen
    Commented Jul 28, 2019 at 17:32
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Cosmologists believe the universe is expanding at an accelerating rate because the measured value of the cosmological is positive.

The "tension" is that the two different methods give different values of the Hubble parameter (from which the cosmological constant can be calculated). There is a low probability ($4.4 \sigma$ according to Wikipedia) that these results are due to chance, which leads some people to believe that the cosmological constant is not in fact constant and may vary in space and time. This would be an indicator of new physical effects beyond our current theories. However, all measured values (and data from other sources) still suggest an accelerated expansion.

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    $\begingroup$ Cosmologists believe the universe is expanding at an accelerating rate because the measured value of the cosmological is positive. This is tautological. Fitted values of $\Lambda$ are simply a measure of the rate of acceleration, the relation being $\ddot{a}/a=\Lambda/3$ for a vacuum-dominated spacetime. So this is sort of like saying that cosmologists believe $\Lambda/3$ to be positive because they measure $\Lambda$ to be positive. $\endgroup$
    – user4552
    Commented Jul 26, 2019 at 14:46
  • $\begingroup$ Thanks for the answer, but it is true that it is only repeating the question in this case... $\endgroup$
    – Helen
    Commented Jul 28, 2019 at 17:24

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