B Which rate of Universe expansion are we currently using?

[HankDorsett]

TL;DR Summary

Including the most recent study I've heard about 3 different rates

The rates I've come across are 67, 71 and 74. Obviously it's not 74 as it was just announced. Is it one of these other numbers, something else? I'm only asking out of curiosity.

 

[member 656954]

Obviously it's not 74 as it was just announced.

That's not necessarily correct as the latest Hubble data lowers the possibility that the recently calculated discrepancy is only a fluke to 1 in 100,000.

 

[HankDorsett]

That's not necessarily correct as the latest Hubble data lowers the possibility that the recently calculated discrepancy is only a fluke to 1 in 100,000.

it's quite possible I have an incorrect assumption here. For some reason I thought we use this rate to help calculate other things. Age of the universe? Distance?

 

[member 656954]

Mostly distance as I understand the objective, as part of the "cosmic distance ladder" methods: https://en.wikipedia.org/wiki/Cosmic_distance_ladder

 

[HankDorsett]

Mostly distance as I understand the objective, as part of the "cosmic distance ladder" methods: https://en.wikipedia.org/wiki/Cosmic_distance_ladder

That's partially where my question was coming from. I think we got 67 from an original study that measured CMB. I think a 2001 study using stars gave us 71. After the release of the 2001 study did we switch over to it or stick with the original calculations?

 

[Jorrie]

That's partially where my question was coming from. I think we got 67 from an original study that measured CMB. I think a 2001 study using stars gave us 71. After the release of the 2001 study did we switch over to it or stick with the original calculations?

For large scales I think we are still using the 2018 Planck Collaboration data set, with $H_0=67.4$ +- 0.5 km/s/Mpc. One must remember that the whole dataset as a unit is consistent with the LCDM model and one cannot just change one parameter, like $H_0$, without changing at least some of the rest.

 

[HankDorsett]

For large scales I think we are still using the 2018 Planck Collaboration data set, with $H_0=67.4$ +- 0.5 km/s/Mpc. One must remember that the whole dataset as a unit is consistent with the LCDM model and one cannot just change one parameter, like $H_0$, without changing at least some of the rest.

Thanks for your response and providing the link. Although I wasn't able to understand much of what the link talked about it did show me something. The more you learn the more you realize you need to learn.

 

[kimbyd]

Summary: Including the most recent study I've heard about 3 different rates

The rates I've come across are 67, 71 and 74. Obviously it's not 74 as it was just announced. Is it one of these other numbers, something else? I'm only asking out of curiosity.

It really depends. Usually the way this is done in actual data analysis is to simply combine the error constraints of the different experiments together to provide a combined result. When there's a disconnect between different data sets, they may publish separate results which are combined with each.

Honestly, I think the best answer is, "We don't know, so we don't try to settle on either result."

And if you are only using the $H_0$ constraint, usually redoing some data analysis with a different input value of $H_0$ is as simple as pushing a button, so there's no real reason to only consider one of them.

 

[mfb]

It depends on who you mean by "we". Some people use the Hubble result, some people use the Planck result, and the people involved in this topic use both (plus the other measurements) and compare the results. For the expansion now the Hubble results should be more reliable, for things that happened in the early universe the Planck results will be better.

 

[HankDorsett]

Thanks for all of your replies. By chance, can someone provide links to the studies surrounding this rate? As I was catching up to this post I was wondering how cool would it have been to be a fly on the wall as they went through these studies.

 

[mfb]

Wikipedia has a table with references.