# What's going on with the Hubble constant?

I'm doing a small project on LIGO for a third-year project at University. I want to write something about how LIGO can be used to measure $$H_0$$, which would be useful because the current state of the literature is that local measurements (e.g Riess 2016) give values roughly 3.5 sigma above measurements that use CMBR or Baryon Acoustic Oscillations (e.g. Planck collaboration et al 2018). But they should be the same - unless there's new Physics we don't understand yet.

I've seen this called 'the biggest crisis in astrophysics' in popular science videos and articles. I've seen it spoken about by people seemingly at the top of the field.

Then I find this paper;

https://arxiv.org/abs/1811.02376

"First Cosmological Results using Type Ia Supernovae from the Dark Energy Survey: Measurement of the Hubble Constant"

Which seems to sort everything out - if these results are good, all that remains is to determine what the systematic error in the old experiments is and move on.

However, I can't see many other people celebrating this as the important article I think is. Why are people not paying attention to it?

Sorry, I'm very new to this stuff, I'm still an undergrad, not really sure how the 'politics' of writing papers is and what being a professional scientist is like.

• Why do you say this article sorts everything out? Does it explain why there are conflicting measurements? – Javier Mar 21 '19 at 2:27
• Please note that you should link to the abstract page on arxiv, rather than the PDF (consider mobile users, for instance). – Kyle Kanos Mar 21 '19 at 14:34
• Conflicting measurements because the method is slightly different, "Whereas traditional measurements of H0 with SNe Ia use a distance ladder of parallax and Cepheid variable stars, the inverse distance ladder relies on absolute distance measurements from the BAOs to calibrate the intrinsic magnitude of the SNe Ia." I'll too naive to know the significance of this - it seems to me as long as both methods are generally OK they should give the same values, but they do not. – T. Smith Mar 21 '19 at 14:46
• The confliction is intensified according to the last word: arxiv.org/abs/1903.07603. "The difference between H0 measured locally and the value inferred from Planck CMB+LCDM is 6.6+/-1.5 km/s/Mpc or 4.4 sigma (P=99.999% for Gaussian errors) in significance, raising the discrepancy beyond a plausible level of chance. We summarize independent tests which show this discrepancy is not readily attributable to an error in any one source or measurement, increasing the odds that it results from a cosmological feature beyond LambdaCDM". – MadMax Mar 28 '19 at 14:41

What is Hubble tension? In a nutshell: the local measurement (via SNe Ia) of Hubble parameter $$H_0$$ favors a higher value than the one measured by Planck (inferred from CMB + $$\Lambda CDM$$). I would bet that the local measurement is more reliable since it's less model-dependent.
Although our $$H_0$$ value is in excellent agreement with Planck Collaboration et al. (2018), we emphasise that the use of an $$r_s$$ prior from Planck does not imply that our measured value of $$H_0$$ will inevitably agree with the value of $$H_0$$ derived from Planck cosmological parameters assuming a $$\Lambda CDM$$ cosmology. The value of $$r_s$$ is informed by only the baryon and matter densities at z = 1090; there are many viable cosmological models which are consistent with only these two quantities (or, in other words, this value of $$r_s$$) that have wildly different values of the Hubble constant at z = 0.
As long as I can tell, the $$r_s$$ prior in the cited paper is model-dependent.