Hubble Constant.

[Derek Glover]

I have just been watching the latest Sky at Night programme about the Hubble Constant. The programme was looking at the two different speeds of the expanding Universe. One result using the Microwave background measured the speed at 67 Km megaparsec and the other measurement using Galaxies had the Universe expanding at a speed of 73 Km megaparsec.

Yes the Universe is expanding and I think the two measurements could be correct, but with the two different results could we looking at two different parts of the Universe moving at two different speeds with the microwave background moving slower then the galaxies?

[Tim Haymes]

I didnt see the program, but a quick search of the internet indicate you are not alone thinking this:

https://www.space.com/25179-hubble-constant.html

“As of January 2018, measurements from multiple telescopes showed that the rate of expansion of the universe is different depending on where you look. The nearby universe (measured by the Hubble Space Telescope and Gaia space telescope) has a rate of expansion of 45.6 miles per second (73.5 kilometers per second) per megaparsec, while the more distant background universe (measured by the Planck telescope) is a bit slower, expanding at 41.6 miles per second (67 km per second) per megaparsec”

This could be a significant observation. 

[Grant Privett]

Does the measurement depend on how you look, or where you look? They are quite distinct things.

[Bill Barton]

So the Hubble Constant isn’t constant?

[Nick James]

No, it’s not constant! More correctly it should be called the Hubble parameter and it evolves with time but the odd thin is that we have two discordant measurements at the current epoch.

These are two very accurate measurements made in two different ways that don’t agree at a significant statistical level. Planck gives 67.7 km/s/Mpc from observations of the Cosmic Microwave Background and Gaia/HST gives 73.5 km/s/Mpc from more direct observations of Supernova redshifts. The two are very different measurements. I think the Planck measurement assumes a particular cosmological model and fits a power spectrum to the variations in the CMB and then infers H from that. The Gaia/HST measurements use a conventional redshift v distance approach but, with Gaia’s parallax measurements and HST’s Cepheid measurements, we now have a much better distance ladder to the distant SNe.

All interesting stuff. Cosmologists and astronomers will never be out of a job.

[Dr Andrew Smith]

Both measurements of H0 the current value if the Hubble parameter rely on physical models which have a fair degree of room for uncertainty. It is fun to conclude new physics maybe required but there is still room for them to converge within our current understanding. 

Regards Andrew 

[Andy Wilson]

I only saw then end of the program so I am not sure if they mentioned this as a possible cause.

The standard theory is that Type Ia supernova are all about the same luminosity as they are caused by the explosion of a white dwarf when it reaches the Chandrasekha mass limit. This holds true if matter is slowly added to the white dwarf so that it goes bang at exactly Chandrasekha mass. Hence a standard mass for the explosion, a standard luminosity, and so you can work out how far away it is.

I’ve heard of the possibility that you could also get a Type Ia supernova by the merger of two white dwarfs. They could have various masses, so the mass and luminosity of a Type Ia would not be a set value. This would cause problems with the determination of the Hubble Constant using this approach. I am not sure if this has now been discounted, or something that is quietly overlooked.

Andy

[Robin Leadbeater]

Yes the white dwarf merger scenario for Ia supernovae is still very much alive and I believe is suspected by many to the the primary mechanism. The empirical relationship between Ia supernova distances and other measurements of distance still seems to hold though regardless of the mechanism.  It is worrying though that we are relying on a standard candle without a clear understanding of the mechanism. 

A shameless plug here – studies of SN2018oh, which I classified, contributed to this Ia mechanism controversy  though without any clear conclusion.

https://www.skyandtelescope.com/astronomy-news/60-second-astro-news-helium-exoplanets-and-a-supernova-surprise

https://arxiv.org/abs/1811.09635

Robin

[Dr Andrew Smith]

The last time I looked at this in detail we could not get any models of supernova to explode! The art of 3D supernova modeling may have solved this issue but I am not sure. We still have a way to go to pin  this all down.

Regards Andrew 

[Andy Wilson]

Robin, congratulations on contributing to the debate! I would be surprised if varying the mass for the explosion in a 2 WD merger scenario did not lead to a variation in luminosity, though maybe the mass range is not significant. I thought there was a lack of good quality measures to any great distance where Type Ia’s are used, but I must admit that I’ve not looked into this in great detail.

Andrew, I think there might be difficulties in getting supernova explosions in core collapse supernova models but not a WD that reaches the Chandrasekha mass limit. I have a recollection of reading somewhere in the past couple of years that modelling revealed that certain stellar mass ranges could lead to explosions while others may collapse to a black hole without creating a supernova.

Cheers,

Andy

[Dr Andrew Smith]

Hi Andy, I think the issue with WD is stopping the accreting  material doing a hydrogen flash on the surface before the mass builds up to the limit. I need to research this area again though to be sure.

Regards Andrew 

[Robin Leadbeater]

Hi Andy,

I suppose it depends on exactly what happens at  the point of merger or perhaps just before. We do know that there are double degenerate systems with a total mass well beyond the Chandrasekhar limit eg

https://arxiv.org/abs/1609.00178

Although we do not have good independent distance measurements, the tight correlation between redshift and Ia maximum brightness (after correction for the light curve shape) does not suggest two obvious populations.

A mix of single and double degenerate progenitors giving different luminosity supernovae could be problematic for cosmological models though as the ratio of single and double degenerate systems might be dependent on the look back time. This has been even used in an argument supporting the double degenerate model as being the dominant mechanism.

https://academic.oup.com/mnras/article/417/2/916/983573

Cheers

Robin

[Dr Andrew Smith]

I am out of date! A simulation of a WD Type Ia explosion was achieved in 2007.

See here https://crd-legacy.lbl.gov/DOEresources/2008highlights/ASCR_accomplishment_Flash_Center.pdf

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