6 ms·

The controversy is that we get 2 different numbers depending on which method (cosmic microwave background vs cosmic distance ladder) we use to calculate the pre

The controversy is that we get 2 different numbers depending on which method (cosmic microwave background vs cosmic distance ladder) we use to calculate the present rate of expansion. These numbers used to have their error bars overlapping, so we assumed they would eventually converge to the true value. But as we get more data the opposite is happening: the numbers are diverging and their error bars are shrinking such that they no longer overlap. This tells us that either our model of the universe is wrong (therefore the cosmic microwave background method is giving us an incorrect answer) or that something is wrong with how we're calculating the distances along the cosmic distance ladder. The latter was originally the assumption that should be proven true with more and better data from newer telescopes. This is now turning out not to be the case: our cosmic distance ladder calculations seem to have been very good, so it now seems more likely that our model of the universe is wrong.

detourdog 2y ago
Thank you for this explanation. I would like to emphasize that our model being wrong and not our numbers sounds like progress. It also sound like progress that we seem to have 2 “scales” to play with to try to develop a consistently measurable distance.
api 2y ago
Distance ladder seems much more error prone than the CMB.
- artemonster 2y ago
  there is no "seems" with provable error bars
  - feoren 2y ago
    ehh ... everything we measure relies on our understanding of the universe in some way. It's perfectly reasonable that our distance measurements could rely on a shakier foundation of assumptions than our understanding of the CMB. I don't know enough to say one way or the other, but GP's comment is not unreasonable on its face. Whereas talking about "provable" in cosmology, and certainly in this case, does seem unreasonable -- especially with error bars, which by definition have a small chance of not including the real value. Normally I'd say that we can generally refine our assumptions to be extremely good, and just take more measurements, and keep narrowing that error bar, until we hit a level of certainty that anyone reasonable would call "proven", but the entire point of TFA is that this isn't happening in this case. We seem on our way to "proving" two inconsistent things.
- db48x 2y ago
  All of the cosmologists expected that too!
- naasking 2y ago
  It's not just the CMB, it's the CMB + LCDM. You know, that theory that posits a form of matter that we have consistently failed to find and that has technically been falsified a number of times, but which was then adjusted to work around those problems, and yet problems still remain. You might have "slight" bit of unaccounted for error sneaking in there. I personally wouldn't trust the CMB calculation at all until the dark matter issue is resolved.
  - api 2y ago
    Isn’t there loads of evidence for dark matter including direct evidence like gravitational lensing? We’re just not sure what it is, though primordial black holes look like a strong candidate.
    - naasking 2y ago
      > Isn’t there loads of evidence for dark matter including direct evidence like gravitational lensing? There is an equivalent boatload of evidence for modified gravity that can't be explained by particle dark matter [1], but one set of observations is hand waved away as a minor inconvenience while the other set is taken as definitive proof. If gravity doesn't work the way we expect, then lensing in galaxy collisions isn't necessarily telling us what we think it is. But setting that aside, if we take even the most naive modified gravity, MOND, then lensing does require a form of particle dark matter, like sterile neutrinos, but the total amount of dark matter required to explain the evidence is only double the amount of visible matter, rather than almost 10x the amount of visible matter as in LCDM. This has significant implications for calculating the Hubble constant from the CMB [2]. [1] https://tritonstation.com/2024/06/18/rotation-curves-still-flat-after-a-million-light-years/ https://tritonstation.com/2024/06/18/rotation-curves-still-f... [2] TeVeS is wrong, but it shows that modified gravity significantly effects the H0 calculation, https://arxiv.org/abs/1204.6359 https://arxiv.org/abs/1204.6359
shepardrtc 2y ago
> so it now seems more likely that our model of the universe is wrong. Whenever a scientist says that it's not possible that the model is wrong, then I just roll my eyes. Of course models can be wrong - and isn't that exciting? Good on them for making sure that there are no errors in the measurements - that's incredibly valuable and absolutely necessary - but I'm really excited to see creative models being thought up that are drastically different. My personal hell is the universe being consistent and boring.
- immibis 2y ago
  Who said it was impossible? In fact, someone just said it was quite likely.
- mr_mitm 2y ago
  > Whenever a scientist says that it's not possible that the model is wrong, then I just roll my eyes. But no one said that. Im fact, scientists are known to say things like: all models are wrong, but some are useful.
  - shepardrtc 2y ago
    From the article: > That the three methods disagree “is not telling us about fundamental physics,” Freedman said. “That’s telling us there’s some systematic [error] in one or more of the distance methods.” Freedman is saying that the model is not wrong.
    - dotnet00 2y ago
      What she means is that the bar for proving that this is an error in physics is much higher than that of proving that it's a measurement error. Like, if you're measuring acceleration due to gravity, and your sensor/calculation gives you 5m/s^2 rather than the real ~9.81m/s^2 that everything else measures, you can't immediately resort to arguing that physics is wrong, you have to rule out that your sensor/calculation is wrong first. To argue that the physics is wrong, you are likely to be arguing that very well tested theories like general relativity, special relativity or electromagnetism are off in some way. That's a much higher bar than just the measurements of either the ladder or CMB being wrong in some way.
sounds 2y ago
I remember reading that the local group, Laniakea Supercluster and the great attractor [1] are new developments that helped us refine our understanding of H0 but didn't fundamentally remove the controversy. It's exciting to see how the question drives many new discoveries. [1] https://en.wikipedia.org/wiki/Great_Attractor https://en.wikipedia.org/wiki/Great_Attractor I'll try to paraphrase what it meant: measuring H0 comes down to measuring the relative velocity of galaxies around us. The great attractor was a relatively recent discovery that the "closer" galaxies, the ones we can use in the distance ladder, all have a common component in their velocities which we've recently begun to understand better.
sdenton4 2y ago
Interesting, though, that they're getting different numbers using different kinds of stars, which does suggest problems with the distance ladder.
pdonis 2y ago
> our cosmic distance ladder calculations seem to have been very good Not according to at least one research group described in the article: the Freedman group, which is only getting the higher answer using Cepheids, but gets a lower answer, one consistent with the CMBR calculations, by two other methods. Which raises the possibility that it's the Cepheid part of the cosmic distance ladder that's the problem.
- chongli 2y ago
  Right, but that's been Freedman's project all along. Riess is continuing to publish results consistent with a higher H0 including both Cepheids and TRGB stars. Perhaps we need some outside groups to take a look at this to see if either group is making systematic errors in their analyses.
  - pdonis 2y ago
    > Perhaps we need some outside groups to take a look at this to see if either group is making systematic errors in their analyses. I agree this would be a good thing.
    - pmayrgundter 2y ago
      Here's a 25 video critique on the CMB projects by Pierre Robiteille https://youtube.com/playlist?list=PLnU8XK0C8oTDaiwe8Us_YNl4Kjmt8ceRD&si=EysmA5zaH7MtQMs6 https://youtube.com/playlist?list=PLnU8XK0C8oTDaiwe8Us_YNl4K... He also has an alternative theory for stellar physics that might call into question the interpretation of Cepheids and SNs, tho I don't think he's done a major analysis there, at least afaik
naasking 2y ago
> so it now seems more likely that our model of the universe is wrong. Anyone with an ounce of sense should have concluded that LCDM was wrong long ago. Hopefully this will finally cause physicists to actually try something different.
- mr_mitm 2y ago
  You're not a cosmologist, are you? I'm not anymore either, but when I was in grad school, everyone agreed that LCDM is probably wrong. Most researchers pretty much wanted it to be wrong. All they did was try something different (quintessence, bigravity, modified gravity, f(R) theories, Horndeski gravity, phantom dark energy, chameleon cosmologies, coupled dark matter, etc. etc), but nothing fit the data as well as LCDM. When new data came in, and it pointed yet again at LCDM, it was considered a disappointment. Now the Euclid satellite has been launched, which I remember being the big hope that it will give us data that will finally enable us to tell which LCDM alternative could replace LCDM as the standard model of cosmology. You won't even get funding for such a mission unless you make a case for how it can improve our understanding of the universe. "We hope it will confirm our currently accepted view" will not get you one cent. So I'm not sure where you get the confidence that nothing different is being tried and that professional scientists wouldn't have an ounce of sense. I find it a bit disrespectful actually. I invite you to read the introduction (and introduction of part 1) of this review paper of the science behind the Euclid mission: https://arxiv.org/abs/1606.00180 https://arxiv.org/abs/1606.00180 It's understandable to laymen, co-authored by reputable, leading scientists in the field (disclaimer: I'm on the author list) and I hope that after reading it you would revise your opinion. Here is a short excerpt: > The simplest explanation, Einstein’s famous cosmological constant, is still currently acceptable from the observational point of view, but is not the only one, nor necessarily the most satisfying, as we will argue.
  - naasking 2y ago
    > but nothing fit the data as well as LCDM. I think you mean, "nothing could be made to fit the data as well as LCDM". See below. > When new data came in, and it pointed yet again at LCDM, it was considered a disappointment. This is highly revisionist, and exemplifies the kind of cherry picking that has kept LCDM alive when it has outright failed numerous predictions and had to be adjusted after the fact to fit the data: From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331 https://www.mdpi.com/2073-8994/14/7/1331 LCDM is a complete mixed bag when it comes to predictions, where MOND has a very slight edge. It's absolutely clear that MOND is not correct or complete either, but it is undeniable that it has had greater predictive success than anyone anticipated and with far fewer free parameters than LCDM, mostly in areas where LCDM is weak. If LCDM can't explain why MOND has been so successful, which it currently cannot, then it is incomplete at best. Basically, the only good, almost definitive, evidence for particle dark matter is gravitational lensing, but if we're considering modified gravity anyway, then lensing isn't necessarily telling us what we think. On the flip side, no possible cold dark matter halo is compatible with recent observations that rotation curves are flat past a million light years [1]. I can't wait to see what epicycles they add to LCDM to save it this time. [1] https://tritonstation.com/2024/06/18/rotation-curves-still-flat-after-a-million-light-years/ https://tritonstation.com/2024/06/18/rotation-curves-still-f...

detourdog 2y ago

Thank you for this explanation. I would like to emphasize that our model being wrong and not our numbers sounds like progress. It also sound like progress that we seem to have 2 “scales” to play with to try to develop a consistently measurable distance.

api 2y ago

Distance ladder seems much more error prone than the CMB.

artemonster 2y ago
there is no "seems" with provable error bars
- feoren 2y ago
  ehh ... everything we measure relies on our understanding of the universe in some way. It's perfectly reasonable that our distance measurements could rely on a shakier foundation of assumptions than our understanding of the CMB. I don't know enough to say one way or the other, but GP's comment is not unreasonable on its face. Whereas talking about "provable" in cosmology, and certainly in this case, does seem unreasonable -- especially with error bars, which by definition have a small chance of not including the real value. Normally I'd say that we can generally refine our assumptions to be extremely good, and just take more measurements, and keep narrowing that error bar, until we hit a level of certainty that anyone reasonable would call "proven", but the entire point of TFA is that this isn't happening in this case. We seem on our way to "proving" two inconsistent things.
db48x 2y ago
All of the cosmologists expected that too!
naasking 2y ago
It's not just the CMB, it's the CMB + LCDM. You know, that theory that posits a form of matter that we have consistently failed to find and that has technically been falsified a number of times, but which was then adjusted to work around those problems, and yet problems still remain. You might have "slight" bit of unaccounted for error sneaking in there. I personally wouldn't trust the CMB calculation at all until the dark matter issue is resolved.
- api 2y ago
  Isn’t there loads of evidence for dark matter including direct evidence like gravitational lensing? We’re just not sure what it is, though primordial black holes look like a strong candidate.
  - naasking 2y ago
    > Isn’t there loads of evidence for dark matter including direct evidence like gravitational lensing? There is an equivalent boatload of evidence for modified gravity that can't be explained by particle dark matter [1], but one set of observations is hand waved away as a minor inconvenience while the other set is taken as definitive proof. If gravity doesn't work the way we expect, then lensing in galaxy collisions isn't necessarily telling us what we think it is. But setting that aside, if we take even the most naive modified gravity, MOND, then lensing does require a form of particle dark matter, like sterile neutrinos, but the total amount of dark matter required to explain the evidence is only double the amount of visible matter, rather than almost 10x the amount of visible matter as in LCDM. This has significant implications for calculating the Hubble constant from the CMB [2]. [1] https://tritonstation.com/2024/06/18/rotation-curves-still-flat-after-a-million-light-years/ https://tritonstation.com/2024/06/18/rotation-curves-still-f... [2] TeVeS is wrong, but it shows that modified gravity significantly effects the H0 calculation, https://arxiv.org/abs/1204.6359 https://arxiv.org/abs/1204.6359

shepardrtc 2y ago

> so it now seems more likely that our model of the universe is wrong. Whenever a scientist says that it's not possible that the model is wrong, then I just roll my eyes. Of course models can be wrong - and isn't that exciting? Good on them for making sure that there are no errors in the measurements - that's incredibly valuable and absolutely necessary - but I'm really excited to see creative models being thought up that are drastically different. My personal hell is the universe being consistent and boring.

immibis 2y ago
Who said it was impossible? In fact, someone just said it was quite likely.
mr_mitm 2y ago
> Whenever a scientist says that it's not possible that the model is wrong, then I just roll my eyes. But no one said that. Im fact, scientists are known to say things like: all models are wrong, but some are useful.
- shepardrtc 2y ago
  From the article: > That the three methods disagree “is not telling us about fundamental physics,” Freedman said. “That’s telling us there’s some systematic [error] in one or more of the distance methods.” Freedman is saying that the model is not wrong.
  - dotnet00 2y ago
    What she means is that the bar for proving that this is an error in physics is much higher than that of proving that it's a measurement error. Like, if you're measuring acceleration due to gravity, and your sensor/calculation gives you 5m/s^2 rather than the real ~9.81m/s^2 that everything else measures, you can't immediately resort to arguing that physics is wrong, you have to rule out that your sensor/calculation is wrong first. To argue that the physics is wrong, you are likely to be arguing that very well tested theories like general relativity, special relativity or electromagnetism are off in some way. That's a much higher bar than just the measurements of either the ladder or CMB being wrong in some way.

sounds 2y ago

I remember reading that the local group, Laniakea Supercluster and the great attractor [1] are new developments that helped us refine our understanding of H0 but didn't fundamentally remove the controversy. It's exciting to see how the question drives many new discoveries. [1] https://en.wikipedia.org/wiki/Great_Attractor https://en.wikipedia.org/wiki/Great_Attractor I'll try to paraphrase what it meant: measuring H0 comes down to measuring the relative velocity of galaxies around us. The great attractor was a relatively recent discovery that the "closer" galaxies, the ones we can use in the distance ladder, all have a common component in their velocities which we've recently begun to understand better.

sdenton4 2y ago

Interesting, though, that they're getting different numbers using different kinds of stars, which does suggest problems with the distance ladder.

pdonis 2y ago

> our cosmic distance ladder calculations seem to have been very good Not according to at least one research group described in the article: the Freedman group, which is only getting the higher answer using Cepheids, but gets a lower answer, one consistent with the CMBR calculations, by two other methods. Which raises the possibility that it's the Cepheid part of the cosmic distance ladder that's the problem.

chongli 2y ago
Right, but that's been Freedman's project all along. Riess is continuing to publish results consistent with a higher H0 including both Cepheids and TRGB stars. Perhaps we need some outside groups to take a look at this to see if either group is making systematic errors in their analyses.
- pdonis 2y ago
  > Perhaps we need some outside groups to take a look at this to see if either group is making systematic errors in their analyses. I agree this would be a good thing.
  - pmayrgundter 2y ago
    Here's a 25 video critique on the CMB projects by Pierre Robiteille https://youtube.com/playlist?list=PLnU8XK0C8oTDaiwe8Us_YNl4Kjmt8ceRD&si=EysmA5zaH7MtQMs6 https://youtube.com/playlist?list=PLnU8XK0C8oTDaiwe8Us_YNl4K... He also has an alternative theory for stellar physics that might call into question the interpretation of Cepheids and SNs, tho I don't think he's done a major analysis there, at least afaik

naasking 2y ago

> so it now seems more likely that our model of the universe is wrong. Anyone with an ounce of sense should have concluded that LCDM was wrong long ago. Hopefully this will finally cause physicists to actually try something different.

mr_mitm 2y ago
You're not a cosmologist, are you? I'm not anymore either, but when I was in grad school, everyone agreed that LCDM is probably wrong. Most researchers pretty much wanted it to be wrong. All they did was try something different (quintessence, bigravity, modified gravity, f(R) theories, Horndeski gravity, phantom dark energy, chameleon cosmologies, coupled dark matter, etc. etc), but nothing fit the data as well as LCDM. When new data came in, and it pointed yet again at LCDM, it was considered a disappointment. Now the Euclid satellite has been launched, which I remember being the big hope that it will give us data that will finally enable us to tell which LCDM alternative could replace LCDM as the standard model of cosmology. You won't even get funding for such a mission unless you make a case for how it can improve our understanding of the universe. "We hope it will confirm our currently accepted view" will not get you one cent. So I'm not sure where you get the confidence that nothing different is being tried and that professional scientists wouldn't have an ounce of sense. I find it a bit disrespectful actually. I invite you to read the introduction (and introduction of part 1) of this review paper of the science behind the Euclid mission: https://arxiv.org/abs/1606.00180 https://arxiv.org/abs/1606.00180 It's understandable to laymen, co-authored by reputable, leading scientists in the field (disclaimer: I'm on the author list) and I hope that after reading it you would revise your opinion. Here is a short excerpt: > The simplest explanation, Einstein’s famous cosmological constant, is still currently acceptable from the observational point of view, but is not the only one, nor necessarily the most satisfying, as we will argue.
- naasking 2y ago
  > but nothing fit the data as well as LCDM. I think you mean, "nothing could be made to fit the data as well as LCDM". See below. > When new data came in, and it pointed yet again at LCDM, it was considered a disappointment. This is highly revisionist, and exemplifies the kind of cherry picking that has kept LCDM alive when it has outright failed numerous predictions and had to be adjusted after the fact to fit the data: From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331 https://www.mdpi.com/2073-8994/14/7/1331 LCDM is a complete mixed bag when it comes to predictions, where MOND has a very slight edge. It's absolutely clear that MOND is not correct or complete either, but it is undeniable that it has had greater predictive success than anyone anticipated and with far fewer free parameters than LCDM, mostly in areas where LCDM is weak. If LCDM can't explain why MOND has been so successful, which it currently cannot, then it is incomplete at best. Basically, the only good, almost definitive, evidence for particle dark matter is gravitational lensing, but if we're considering modified gravity anyway, then lensing isn't necessarily telling us what we think. On the flip side, no possible cold dark matter halo is compatible with recent observations that rotation curves are flat past a million light years [1]. I can't wait to see what epicycles they add to LCDM to save it this time. [1] https://tritonstation.com/2024/06/18/rotation-curves-still-flat-after-a-million-light-years/ https://tritonstation.com/2024/06/18/rotation-curves-still-f...