View Full Version : How accurately will GAIA measure the Hubble constant and will it be model dependent?

john hunter

2018-Mar-19, 11:10 PM

GAIA DR2 is due out in about a month.

Planck has measured the Hubble constant to be about 67-68km/s/Mpc. Riess and team have found about 73.2km/s/Mpc from the distance ladder, but uses the assumption of a deceleration parameter of q=-0.55. Another possibility which is ATM and won't be discussed much here is that the q=-1 leading to an estimated 75.5km/s/Mpc from Riess' analysis.

Does anyone know the answers to...How accurately will GAIA determine the Hubble constant? Will it be model dependent or model independent? Is GAIA likely to be able to help resolve the tension between the first two values above?

Thankyou.

glappkaeft

2018-Mar-20, 01:54 AM

As far as I know GAIA will not measure the Hubble constant at all. It will however provide a better foundation of the cosmological distance ladder by more accurately measuring the distance to nearby Cephids.

StupendousMan

2018-Mar-20, 02:54 AM

glappkaeft has Bingo. Gaia will provide improved distances to Cepheids, RR Lyraes, and other stars in the Milky Way, which can in turn be used to calibrate the first steps on the cosmological distance ladder; and that improved calibration can, in turn, provide more accurate values for the Hubble Constant, based on a variety of indicators which are too distant for Gaia to measure them directly.

A somewhat detailed discussion of parallax measurements, and the improvement that Gaia will provide, can be found in

http://spiff.rit.edu/classes/ladder/lectures/parallax/parallax.html

John Mendenhall

2018-Mar-20, 02:59 AM

As far as I know GAIA will not measure the Hubble constant at all. It will however provide a better foundation of the cosmological distance ladder by more accurately measuring the distance to nearby Cephids.

The acronym is GAIA Data Release 2 and the relevant link is here:

https://www.cosmos.esa.int/web/gaia/dr2

John Mendenhall

2018-Mar-20, 03:10 AM

glappkaeft has Bingo. Gaia will provide improved distances to Cepheids, RR Lyraes, and other stars in the Milky Way, which can in turn be used to calibrate the first steps on the cosmological distance ladder; and that improved calibration can, in turn, provide more accurate values for the Hubble Constant, based on a variety of indicators which are too distant for Gaia to measure them directly.

A somewhat detailed discussion of parallax measurements, and the improvement that Gaia will provide, can be found in

http://spiff.rit.edu/classes/ladder/lectures/parallax/parallax.html

Thanks, great link.

glappkaeft

2018-Mar-20, 03:40 AM

The acronym is GAIA Data Release 2 and the relevant link is here:

https://www.cosmos.esa.int/web/gaia/dr2

I was speaking about GAIA in general and not about DR2.

When we are sharing good sources on the subject I really recommend "Measuring the Universe: The Cosmological Distance Ladder" by Stephen Webb. It's an academic overview on the history and current thinking about cosmological distances. Its a bit old (1999) so it doesn't cover the latest research and is rather expensive but I found it so good I borrowed it twice from different university libraries. Hopefully something similar will come out a few years after GAIA final data release.

john hunter

2018-Mar-20, 07:27 AM

Thanks, John, StupendousMan and Glappkaeft,

So, is it likely that there is an improved, more reliable distance ladder as a result of GAIA - and that teams such as Riess' can then improve on the accuracy of their Hubble constant value, but will still have to use a deceleration parameter assumption?

Apparently DR1 confirmed the distance ladder value, so if DR2 does the same is it likely just to reduce the error in the 73.2km/s/Mpc value, and perhaps increase the tension problem between distance ladder and Planck determinations? If so, could be interesting for cosmology...

tusenfem

2018-Mar-20, 07:55 AM

GAIA DR2 is due out in about a month.

Planck has measured the Hubble constant to be about 67-68km/s/Mpc. Riess and team have found about 73.2km/s/Mpc from the distance ladder, but uses the assumption of a deceleration parameter of q=-0.55. Another possibility which is ATM and won't be discussed much here is that the q=-1 leading to an estimated 75.5km/s/Mpc from Riess' analysis.

Does anyone know the answers to...How accurately will GAIA determine the Hubble constant? Will it be model dependent or model independent? Is GAIA likely to be able to help resolve the tension between the first two values above?

Thankyou.

If you KNOW that it is ATM, then WHY would you put it into Q&A?

infraction

John Mendenhall

2018-Mar-20, 12:00 PM

I was speaking about GAIA in general and not about DR2.

When we are sharing good sources on the subject I really recommend "Measuring the Universe: The Cosmological Distance Ladder" by Stephen Webb. It's an academic overview on the history and current thinking about cosmological distances. Its a bit old (1999) so it doesn't cover the latest research and is rather expensive but I found it so good I borrowed it twice from different university libraries. Hopefully something similar will come out a few years after GAIA final data release.

Yes, of course. The DR2 reference was Hunter's in post #1. Sorry.

john hunter

2018-Apr-28, 10:19 AM

DR2 was released on Wednesday.

Does anyone know if a group has done the analysis yet on the DR2 data, which would give a better foundation for the distance ladder method. If so is there an improved value for the Hubble constant yet from the distance ladder?

antoniseb

2018-Apr-28, 12:38 PM

DR2 was released on Wednesday.

Does anyone know if a group has done the analysis yet on the DR2 data, which would give a better foundation for the distance ladder method. If so is there an improved value for the Hubble constant yet from the distance ladder?

I did not see such a paper, but if you'd like to employ your big data skills, the data is out there waiting for such research to be done on it.

john hunter

2018-May-01, 06:25 AM

It seems that, only about a week after the GAIA DR2 release, there has now been an analysis by Riess et al.

https://arxiv.org/abs/1804.10655

In the abstract there is:

"Including the DR2 parallaxes with all prior distance ladder data raises the current tension between the late and early Universe route to the Hubble constant to 3.8 sigma (99.99 %)."

Their value from the distance ladder is more or less unchanged but possibly amended to 73.52 (1.62) from 73.24 (1.74), and as the error is smaller that's what's probably increased the tension to 3.8 sigma...which represents a more serious problem, and a need for reconciliation with Planck.

Interesting time for cosmology!

Jean Tate

2018-May-01, 07:07 PM

It's not just through getting a better handle on the bottom rung(s) of the distance ladder that Gaia will contribute to cosmology.

Krone-Martins+ (2018) (link (https://arxiv.org/abs/1804.11051)): "Gaia DR2 Gravitational Lens Systems I: New lensed quasar candidates around known quasars" points to an indirect contribution: the more lensed quasars the more opportunities to estimate H0 by a method that's independent of the formal distance ladder ...

john hunter

2018-May-01, 09:41 PM

Good link Jean,

It might take a while, but it will be interesting to see how the H(0) values from the different methods, including the GAIA data, compare.

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