A Radically Conservative Solution for Cosmology’s Biggest Mystery | Hacker News Reader

A Radically Conservative Solution for Cosmology’s Biggest Mystery

quantamagazine.org

45 pointscfadvan14 comments

TFA seems completely contradicted by its penultimate sentence? Or does this mean that the latter study doesn't consider the error possibilities raised by the former?

I would suggest that the antipenultimate sentence represents a partial ontological quandary. But if you peruse the former, and not the latter, you should be sifficiently convinced of any such probable suppositions.

I can't tell, is this a joke? Or are you being this obtuse on accident?

Poe's Law in action?

I think it's just the scientific reporting equivalent of FUD, this is news, but it doesn't fit the long term narrative they will need to keep selling scientific reporting. Hence some editor probably stuffed those last bits in there. The fact they are reporting it all is pretty decent for modern journalism.

Uh no, this is actually more like the man plastering the city with "all is well" posters: that the tension between CMB and SN determinations of the Hubble constant will be resolved by fixing some tricky detail in the analysis is the most mainstream expectation out there.

The tension has been there for a long time, but its only something like two years ago that the statistical errors got small enough you had to somewhat seriously consider there could be some important reason for the disagreement.

Right now the absolute calibration is done by matching nearby supernovae with Cepheids and then extrapolating to remote SNs which is where the cosmology lies (essentially).

Gaia gives better Cepheid distances, decreasing the total uncertainty somewhat. But the big story is the calibration of the intrinsics supernova brightness (and its variation), so while better Cepheids help, its mostly by further closing a already unlikely avenue of explanation.

The big crime of the article is portraying the field as thinking the tension is due to new physics, when in fact the difference between the CMB and SN determinations has been known for something like 20 years and for all that time been mostly viewed as something experimentalists get sorted. For a long time the values agreed within their stated uncertainties, and its only rather recently that they have pushed their statistical errors down enough that the difference can be said to be real. So its a while yet until the difference is truly trusted as something to be explained by new physics rather than some calibration error.

The Planck results are really the most problematic, so we will see if that changes over time.

"He and Bernal provide a meta-analysis of the disparate measurements with a “Bayesian” statistical approach. It separates measurements into separate classes that are independent from one another — meaning that they don’t use the same telescope or have the same implicit assumptions. It can also be easily updated when new measurements come out. “There’s a clear need — which you would’ve thought statisticians would’ve provided years ago — for how you combine measurements in such a way that you’re not likely to lose your shirt if you start betting on the resulting error bars,” said Peacock. He and Bernal then consider the possibility of underestimated errors and biases that could systematically shift a measured expansion rate up or down. "

I am not sure how to parse this quote, and seem unlikely that appropriate statistical models weren't available

They've made a framework for combining datasets from different sources, something that is tricky in general because of the need to guard against unknown differences between experiments.

It's also not obvious that there should be a statistical framework because combinations is usually something done carefully by people familiar with both experiments, so essentially it's on a case by case basis and not wholesale across the field.

Can someone ELI5 why the rate of expansion of the universe has to be a constant everywhere? Couldn't different parts of the universe be expanding at different rates? And couldn't that rate change over time?

If that's true then it needs to be explained, i.e. new physics.

General Relativity affords uniform expansion through the cosmological constant. Non-uniform expansion means new fields, so either dark matter or modified gravity.

This idea is known as Eternal Inflation. Here is the first in a series of 4 lectures about it: https://www.youtube.com/watch?v=xUZeZD9sytc

The main problem with eternal inflation is it gets us attempting to make statements about regions outside what we can observe directly.

Considering the enormous scale of the universe, I don't think it's unrealistic to have huge error bars on measurements at that scale. And in any event, estimation might be a better word than measurements.

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