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"researchers started using Cepheids to calibrate the distances to bright supernovas, enabling more accurate measurements of H0." It seems like if there were so

"researchers started using Cepheids to calibrate the distances to bright supernovas, enabling more accurate measurements of H0." It seems like if there were some error in the luminosity measurement for cepheids, it would propagate to the measurements with supernovas... I would expect that stacking measurement techniques (as is common with cosmology, where distances are vast and certainty is rare) would also stack error, like summing the variance in gaussians...

FredPret 2y ago
It'd be cool if we launched several space telescopes on Voyager-like trajectories. In 50-100 years they'd get a much better angular fix on stars that are too distant for Earth-orbit-sized angular measurements. https://en.wikipedia.org/wiki/Stellar_parallax https://en.wikipedia.org/wiki/Stellar_parallax
- AprilArcus 2y ago
  They'd need some very big RTGs to last that long, and I don't think we manufacture plutonium at the necessary volumes for that anymore.
  - KennyBlanken 2y ago
    Last I remember, RTG manufacturing was very constrained, period. And that was before Russia took a massive shit in Ukraine and got themselves embargo'd by most of the rest of the developed world.
  - ars 2y ago
    I would use Americium-241 instead, longer half life and much more availability. Lower power, but a telescope like this does not need constant power, so some kind of short term power storage (capacitor I would assume, or some kind of ultra long life battery) could handle that.
    - fsckboy 2y ago
      in terms of the potential plutonium shortage, wikipedia: Americium-241 is not synthesized directly from uranium – the most common reactor material – but from the plutonium isotope 239. The latter needs to be produced first
- HPsquared 2y ago
  That'd be a nice use for Starship.
  - jvanderbot 2y ago
    Well, no it wouldn't actually. It'd be a nice use for falcon heavy - to get them the necessary delta-v. But the constraint isn't cargo space on launch. This isn't a starlink constellation, the orbits are necessarily massively different, meaning each spacecraft needs its own large delta-v so a single-launch, multiple spacecraft option is less attractive. The constraint is budget, fuel, and ambition. Now what you could do is get a telescope out around a far outer planet and use the orbital parallax like we do from earth. A Starship might have a bunch of extra cargo space for this. But I just don't see how it is better than a big faring on falcon heavy. EDIT: You know what? I am completely mistaken here. I was not thinking about the diff b/w FH and starship correctly.
    - HPsquared 2y ago
      More lift capacity means more fuel capacity onboard the telescope(s).
    - baq 2y ago
      You can easily put a third stage bigger than the whole F9 second stage in the payload bay of the Starship and you likely wouldn't need a super complicated unfolding deployment procedure for the payload thanks to the enormous volume. Once the thing becomes reliable there are zero missions the FH can do that starship can't with a payload equipped with a third stage motor.
      - jvanderbot 2y ago
        OK I'll bite: What does starship add here? It's a very large re-useable faring that carries a stage + payload into orbit? Why not attach to the top of FH? Why: FH + Starship w/( rocket + payload) And not: FH + (rocket + payload)
        2y ago
- XorNot 2y ago
  I think the problem with such a mission right now is the high probability we could launch a faster mission in the very near future - i.e. with NASA looking at spaceborne nuclear propulsion again, we could send much more capable telescopes out faster - which is not just an "I want it now" benefit: time in space is time you run potentially having components wear out or break. So getting them onto their missions ASAP is a huge de-risking element.
  - Bluestein 2y ago
    I wonder - and I am sure this has been examined to death - if there's some calculation that can be performed to find the "optimum" wait-or-release-now pattern, given a certain rate of technological development vs. distance/years ... I am sure there are calculations for this ... PS. Of course, the rate of technological development is the unknown variable hete, I am sure.-
    - TeMPOraL 2y ago
      I imagine such calculations immediately break down when you make the input (funding, interest) depend on the output. Which is the case in reality. For example, say your calculations say that the optimal time for the mission is 10 years from now, once a currently in-development propulsion technology matures. You publish that, and the investors, government and the public, all motivated to support you by the dream of your ambitious mission, suddenly lose interest. Your funding dries out, as you're repeatedly told to call back in 10 years. The fact that the 10 year estimate, having been dependent on existing funding, is now "literally never", escapes them. See also: "nuclear fusion is always 30 years away". It is, because original 30 year timeframe assumed continued funding that never happened, and it's not happening because "it's always been 30 years away".
      - Bluestein 2y ago
        Literally a moving target ... > You publish that, and the investors, government and the public, all motivated to support you Interesting how PR/culture indeed is a factor - a tangible factor in this. Indeed optimizing for "PR Goodwill" might be a thing ...
- ianburrell 2y ago
  The other solution is to increase the accuracy of parallax. This is what the Gaia project is doing. It can measure distance to stars in galactic center to 20%. It will measure distance to 2 billion stars and be super accurate within 300 ly.
- silverquiet 2y ago
  New Horizons has taken some star pictures from the Kuiper Belt and you can easily spot the parallax of some nearby stars just by eyeball. I'm not sure that it has a good enough camera for any kind of precision measurement, but it was really cool to see that.
- jerjerjer 2y ago
  Why compromise? Might as well launch a solar gravitational lens [1] telescope. [1] https://en.wikipedia.org/wiki/Solar_gravitational_lens https://en.wikipedia.org/wiki/Solar_gravitational_lens
  - shagie 2y ago
    https://www.centauri-dreams.org/2020/12/10/developing-focal-mission-concepts/ https://www.centauri-dreams.org/2020/12/10/developing-focal-... (and its critical analysis) https://arxiv.org/pdf/1604.06351 https://arxiv.org/pdf/1604.06351 I believe that work on such a project would be interesting, even if it isn't ever done. It is something that if you worked on it as a grad student, and then became a professor and your grad students worked on it ... and they became professors and their grad students worked on it - they'd be the ones seeing the results. Not so much a "this is a way to do things..." but rather a "thinking about research that spans generations and the problems that they solve in the process of doing that great project has useful spinoffs." A mission out to 550 AU at New Horizons speed of 2.9 AU / year (it still boggles the mind of talking about those speeds and distances) is nearly 200 years from launch to primary science objective. It's "only" been 45 years since Mariner 2 to New Horizons.
antognini 2y ago
These uncertainties in Cepheid luminosities are accounted for in Type Ia distance measurements. Particularly with Gaia we can now calibrate the luminosities of Cepheids in our galaxy using parallax observations. (Knowing this field I'm sure there are some astronomers who argue that there are still some systematic uncertainties that are not fully being accounted for, but from what I understand it's pretty hard to account for it with the Gaia results at this point.)
hindsightbias 2y ago
"But according to Freedman, the galaxies’ supernovas seemed to be intrinsically brighter than the ones in farther galaxies. This is another puzzle cosmologists have yet to understand, and it also affects the H0 value. "

FredPret 2y ago

It'd be cool if we launched several space telescopes on Voyager-like trajectories. In 50-100 years they'd get a much better angular fix on stars that are too distant for Earth-orbit-sized angular measurements. https://en.wikipedia.org/wiki/Stellar_parallax https://en.wikipedia.org/wiki/Stellar_parallax

AprilArcus 2y ago
They'd need some very big RTGs to last that long, and I don't think we manufacture plutonium at the necessary volumes for that anymore.
- KennyBlanken 2y ago
  Last I remember, RTG manufacturing was very constrained, period. And that was before Russia took a massive shit in Ukraine and got themselves embargo'd by most of the rest of the developed world.
- ars 2y ago
  I would use Americium-241 instead, longer half life and much more availability. Lower power, but a telescope like this does not need constant power, so some kind of short term power storage (capacitor I would assume, or some kind of ultra long life battery) could handle that.
  - fsckboy 2y ago
    in terms of the potential plutonium shortage, wikipedia: Americium-241 is not synthesized directly from uranium – the most common reactor material – but from the plutonium isotope 239. The latter needs to be produced first
HPsquared 2y ago
That'd be a nice use for Starship.
- jvanderbot 2y ago
  Well, no it wouldn't actually. It'd be a nice use for falcon heavy - to get them the necessary delta-v. But the constraint isn't cargo space on launch. This isn't a starlink constellation, the orbits are necessarily massively different, meaning each spacecraft needs its own large delta-v so a single-launch, multiple spacecraft option is less attractive. The constraint is budget, fuel, and ambition. Now what you could do is get a telescope out around a far outer planet and use the orbital parallax like we do from earth. A Starship might have a bunch of extra cargo space for this. But I just don't see how it is better than a big faring on falcon heavy. EDIT: You know what? I am completely mistaken here. I was not thinking about the diff b/w FH and starship correctly.
  - HPsquared 2y ago
    More lift capacity means more fuel capacity onboard the telescope(s).
  - baq 2y ago
    You can easily put a third stage bigger than the whole F9 second stage in the payload bay of the Starship and you likely wouldn't need a super complicated unfolding deployment procedure for the payload thanks to the enormous volume. Once the thing becomes reliable there are zero missions the FH can do that starship can't with a payload equipped with a third stage motor.
    - jvanderbot 2y ago
      OK I'll bite: What does starship add here? It's a very large re-useable faring that carries a stage + payload into orbit? Why not attach to the top of FH? Why: FH + Starship w/( rocket + payload) And not: FH + (rocket + payload)
      - 2y ago
XorNot 2y ago
I think the problem with such a mission right now is the high probability we could launch a faster mission in the very near future - i.e. with NASA looking at spaceborne nuclear propulsion again, we could send much more capable telescopes out faster - which is not just an "I want it now" benefit: time in space is time you run potentially having components wear out or break. So getting them onto their missions ASAP is a huge de-risking element.
- Bluestein 2y ago
  I wonder - and I am sure this has been examined to death - if there's some calculation that can be performed to find the "optimum" wait-or-release-now pattern, given a certain rate of technological development vs. distance/years ... I am sure there are calculations for this ... PS. Of course, the rate of technological development is the unknown variable hete, I am sure.-
  - TeMPOraL 2y ago
    I imagine such calculations immediately break down when you make the input (funding, interest) depend on the output. Which is the case in reality. For example, say your calculations say that the optimal time for the mission is 10 years from now, once a currently in-development propulsion technology matures. You publish that, and the investors, government and the public, all motivated to support you by the dream of your ambitious mission, suddenly lose interest. Your funding dries out, as you're repeatedly told to call back in 10 years. The fact that the 10 year estimate, having been dependent on existing funding, is now "literally never", escapes them. See also: "nuclear fusion is always 30 years away". It is, because original 30 year timeframe assumed continued funding that never happened, and it's not happening because "it's always been 30 years away".
    - Bluestein 2y ago
      Literally a moving target ... > You publish that, and the investors, government and the public, all motivated to support you Interesting how PR/culture indeed is a factor - a tangible factor in this. Indeed optimizing for "PR Goodwill" might be a thing ...
ianburrell 2y ago
The other solution is to increase the accuracy of parallax. This is what the Gaia project is doing. It can measure distance to stars in galactic center to 20%. It will measure distance to 2 billion stars and be super accurate within 300 ly.
silverquiet 2y ago
New Horizons has taken some star pictures from the Kuiper Belt and you can easily spot the parallax of some nearby stars just by eyeball. I'm not sure that it has a good enough camera for any kind of precision measurement, but it was really cool to see that.
jerjerjer 2y ago
Why compromise? Might as well launch a solar gravitational lens [1] telescope. [1] https://en.wikipedia.org/wiki/Solar_gravitational_lens https://en.wikipedia.org/wiki/Solar_gravitational_lens
- shagie 2y ago
  https://www.centauri-dreams.org/2020/12/10/developing-focal-mission-concepts/ https://www.centauri-dreams.org/2020/12/10/developing-focal-... (and its critical analysis) https://arxiv.org/pdf/1604.06351 https://arxiv.org/pdf/1604.06351 I believe that work on such a project would be interesting, even if it isn't ever done. It is something that if you worked on it as a grad student, and then became a professor and your grad students worked on it ... and they became professors and their grad students worked on it - they'd be the ones seeing the results. Not so much a "this is a way to do things..." but rather a "thinking about research that spans generations and the problems that they solve in the process of doing that great project has useful spinoffs." A mission out to 550 AU at New Horizons speed of 2.9 AU / year (it still boggles the mind of talking about those speeds and distances) is nearly 200 years from launch to primary science objective. It's "only" been 45 years since Mariner 2 to New Horizons.

antognini 2y ago

These uncertainties in Cepheid luminosities are accounted for in Type Ia distance measurements. Particularly with Gaia we can now calibrate the luminosities of Cepheids in our galaxy using parallax observations. (Knowing this field I'm sure there are some astronomers who argue that there are still some systematic uncertainties that are not fully being accounted for, but from what I understand it's pretty hard to account for it with the Gaia results at this point.)

hindsightbias 2y ago

"But according to Freedman, the galaxies’ supernovas seemed to be intrinsically brighter than the ones in farther galaxies. This is another puzzle cosmologists have yet to understand, and it also affects the H0 value. "