7 ms·

New JWST data confirms, worsens the Hubble tension

108 points by ColinWright 3y ago

benj111 3y ago
So does this require new physics to solve? What should the layman take away from this? >distant galaxies have been speeding up in their recession, and the expansion rate, though still dropping, is not headed toward zero. If the expansion rate is dropping, surely it is headed towards zero? Or are they using expansion rate to mean acceleration and the zero refers to the recession. Or am I misunderstanding something?
- sornaensis 3y ago
  I took it to mean the rate of expansion is approaching a horizontal asymptote e.g. -x/ln(x), x > 1
- irishsultan 3y ago
  Imagine the expansion rate starts out at 2. Then 1 day later it's 1.5, after 1 more day it's 1.25 and after another day it's 1.125. Hopefully you can see that if this series continues then the expansion rate is always dropping, but it's headed towards 1, not 0. (And if expansion rate of 1 is too confusing in this context imagine if it starts out at 3 and goes to 2.5, 2.25, 2.125, ..., it still is always decreasing, but it will never be less than 2 which means the universe keeps expanding).
  - benj111 3y ago
    Ok, that makes sense. What's the mechanism that allows the acceleration to drop, without dropping to zero though? Is the dark matter not expanding with everything else
    - rout39574 3y ago
      Yes, that is indeed the question.
    - layer8 3y ago
      > What's the mechanism that allows the acceleration to drop, without dropping to zero though? The expansion rate drops because the energy density goes down due to the expansion. However, the dark energy density aka cosmological constant does not go down, therefore providing a nonzero floor. > Is the dark matter not expanding with everything else This has nothing to do with dark matter in particular, and it’s space itself that’s expanding, not matter. Also, the expansion has no center, the universe is expanding at every point. Some types of matter expanding and others not would imply a center.
      - denton-scratch 3y ago
        > The expansion rate drops because the energy density goes down due to the expansion. I thought the idea was that Dark Energy increased with the amount of empty space; so as the space between galaxies expands, the energy driving the expansion also expands - hence the acceleration.
        layer8 3y ago
        I wrote “density”, i.e. the amount of dark energy remains constant per unit volume. Since the volume increases due to the expansion, the total amount of dark energy increases accordingly, but its density remains the same. This is as opposed to the matter and radiation density, which decreases.
        denton-scratch 3y ago
        Thank you for clarifying (you said "energy density goes down due to the expansion"). Can you also clarify why galaxies don't expand, but empty space does? I suppose this is something to do with "vacuum energy", but it's not obvious to me that vacuum energy actually requires a vacuum; I thought it was present everywhere, but was only significant in the absence of other "stuff". I also understand vacuum energy to be related to Hawking Radiation, which is black-body. But black-body radiation is EM radiation; DE is neither black-body nor electromagnetic. Why does vacuum energy not produce observable EM radiation? Is it just too weak to observe?
- yk 3y ago
  > So does this require new physics to solve? Not necessarily. First the distance ladder could have another problem. Second to look at the early universe, for the early universe observations you are looking through the entire universe at the CMB and that requires foreground subtraction, which is very much a non trivial task. And finally from theory, General Relativity is a non-linear theory which means taking the average and then evolving the average does not necessarily yield the same result as evolving the initial state and then taking the average. Either of this could explain the tension, though an actually dynamic cosmological constant would be more fun.
- pdpi 3y ago
  > If the expansion rate is dropping, surely it is headed towards zero? Expansion rate could be dropping but converging to a non-zero value. That seems unlikely to me, but it's an answer that would fit that description just fine. The language is a bit imprecise, though, which I expect is the problem. The (to me) obvious technical interpretation of "expansion rate is headed towards zero" is that d size(t)/dt -> 0 as t -> infinity, but the (again, to me) obvious non-technical interpretation is "expansion will completely stop at some point". So "*not* headed towards zero" means "derivative isn't going to zero", or "expansion never quite stops", respectively. The derivative of ln(t) does go to zero, but it has unbounded growth, so it fails the first test but passes the second. The universe experiencing logarithmic expansion seems reasonable enough.
- venusenvy47 3y ago
  It's confusing because that quote about "expansion rate decreasing" is in the caption of a picture with a giant "Accelerating Expansion" label at present time.
fjfaase 3y ago
Article seems to be based on 'Crowded No More: The Accuracy of the Hubble Constant Tested with High Resolution Observations of Cepheids by JWST' [1]. [1] https://arxiv.org/abs/2307.15806 https://arxiv.org/abs/2307.15806
joshuahedlund 3y ago
Possibly a dumb question: can the tension between the two rates simply be resolved because one method measures an old historical rate and one method measures the current rate, and it’s been dropping over time? Or are they both measuring the same time periods? (I assume it’s not that simple, but it’s very hard for me to understand and reason about data that involves looking back in time and space simultaneously)
- venusenvy47 3y ago
  That idea is mentioned in the article, with the hypothesis that there was more dark energy in the past that has since dissipated. They label that idea as "New Physics" in the chart above that comment.
- privong 3y ago
  > Possibly a dumb question: can the tension between the two rates simply be resolved because one method measures an old historical rate and one method measures the current rate This is already generally being accounted for (assuming we have the correct cosmological model for the Universe). The CMB measurement is probing the Universe at an early state (when the Universe was ~380,000 years old) and the Hubble "constant", H(t=380,000 yr), was different at that point from what it is now, H_0 = H(t=13.7 billion years). The comparison to the current Hubble constant (e.g., determined from Cepheids) is made by evolving the CMB Hubble "constant" value to today, under the assumption of our best cosmological model model. Of course, if we have the wrong cosmological model, then that could be a reason for the discrepancy, and that might point to "new physics". Alternately there may be some systematic in the Cepheid technique that is causing our H_0 estimate to be somewhat off.
- mr_mitm 3y ago
  Both methods measure H_0, which is the Hubble parameter today. It's a bit confusing that it's sometimes called the Hubble constant, because it's not really a constant. The expansion rate H changes over time, so we write H(t) and by H_0 we mean H(t_today). It's probably the case that they didn't directly measure H_0 with the early universe probes, so they measured H at that time and then used the standard model to extrapolate this value to H_0, roughly speaking. This means, that if the measurements didn't suffer from some unknown systematic error, that the model is wrong. That's what they call "new physics" and could be the most exciting thing in cosmology since the 90s.
croes 3y ago
A little confusing that strengthening the Hubble tension is worsening it.
- enkid 3y ago
  The Hubble tension is "bad" (i.e. shows a problem with) for current models of cosmology, hence strengthening it is making it "worse." Its like a hurricane, a strengthening hurricane is worse.
  - croes 3y ago
    It's just a weird combination of something mostly considered positive with something negative. Like strengthening my weaknesses.
    - jdiff 3y ago
      Where's the positive? If you've got a strengthening tension headache, there's no mixing there. Tension in your head, tension in the room, tension in a Hubble, it's all bad.
_madmax_ 3y ago
>However, for the past ~6 billion years, distant galaxies have been speeding up in their recession, and the expansion rate, though still dropping, is not headed toward zero. Am I not getting something here or is this sentence not make sense?
- XzAeRosho 3y ago
  The rate of acceleration is slowing down, but still is accelerating.
  - bonzini 3y ago
    So the first derivative was already understood to remain nonzero forever[1], now we know that the second derivative probably will also remain nonzero, and the third is slowing down but still positive for the moment? [1] IIRC in the 80s the big bang/big crunch model was preferred
- vidarh 3y ago
  It makes sense if the expansion rate isn't dropping linearly and the rate of the drop is itself dropping towards zero.
- layer8 3y ago
  This article explains it in more detail: https://bigthink.com/starts-with-a-bang/universe-expansion-not-accelerating/ https://bigthink.com/starts-with-a-bang/universe-expansion-n...
- BurningFrog 3y ago
  The slowing down is speeding up. What could be simpler?
bmitc 3y ago
Is this an accurate summary of the current status quo? 1. The universe is expanding 2. The velocity at which the universe is expanding is increasing, since there is non-zero acceleration. 3. The acceleration is actually slowing down, but it is not heading towards zero. It is approaching some steady state acceleration. ---------- In Exploring Black Holes by Edwin Taylor and John Wheeler, published in 2000, I recently read the following passage by Wheeler: > An article by John Noble Wilford in the Science Times section of the New York Times for Tuesday March 3, 1998, reports observations by two separate groups of investigators which they interpret as showing that today the expansion of the Universe is speeding up rather than undergoing the slowdown expected for any approach to maximum expansion. Later that day I encountered a hard-bitten veteran gravitation physics colleague in the elevator of the Princeton physics building and asked him if he believed the purported evidence of accelerating expansion. "No," he replied. Neither do I. Why not? Two reasons: (1) Because the speed-up argument relies too trustingly on the supernovas being standard candles, (2) Because such an expansion would, it seems to me, contradict a view of cosmology too simple to be wrong. Such clashes between theory and experiment have often triggered decisive advances in physics. We can hope that some decisive advance is in the offing. Now, Wheeler was one of the most preeminent physicists of the 20th century and spearheaded several fields and was not afraid of wild ideas. I am curious if anyone knows if he had any inklings that further his thoughts here. Was he just wrong here, before new measurements came out? Or is there something missing in these calculations and the theory that we're missing? I'm also not sure what means in (1).
- mr_mitm 3y ago
  Yes to 1) and 2), however: > 3. The acceleration is actually slowing down, but it is not heading towards zero. It is approaching some steady state acceleration. No. The scale factor in LCDM (the current concordance model) is proportional to sinh^(2/3)(t/t_Lambda), which means it's asymptotically exponential. Acceleration will always increase. (Edit: The expansion rate, so a'/a, will go to a constant value, though.) Caveat: If the Hubble tension proves real, LCDM is wrong.
  - bmitc 3y ago
    Thanks! So the currently accepted model says the acceleration will grow indefinitely but our observations are saying that the acceleration levels out to a non-zero, positive constant?
denton-scratch 3y ago
Too many pictures, few of which add information. Too few words.
- fooqux 3y ago
  Agreed; the presentation was, IMHO, terrible. You had to read each image's relatively hard to read annotation to make sense of what they were trying to say. Hell, 95% of the entire article was image annotations, with single sentences between. Really difficult to follow along.
- EdwardDiego 3y ago
  That GIF of Puppis' light echoes is amazing though.
- dhuk_2018 3y ago
  THANK YOU. I was feeling dumber after reading it, not smarter.
m3kw9 3y ago
The first image depicting space expansion in that article: what happens if some is to travel beyond that space boundary? Is it because of speed of light that no one can catch up beyond it or is it something else?
- fluoridation 3y ago
  I don't believe the graphic is representing a boundary. I think it's representing the relative expansion of space over time. The observable universe does have an event horizon that can't be reached due to the finiteness of the speed of light, though.
  - fnordpiglet 3y ago
    I think the key point is the event horizon, or effective boundary, is relative to the observer. From the earth there’s a point beyond which we can never observe, even if we left today at the speed of light. However for a planet at that point everything looks “the same” to them - except the earth is at their boundary. The boundaries here aren’t absolute - they’re only relative to the observer.
- uoaei 3y ago
  PBS Spacetime is a wonderful rabbit (worm)hole if you have the time and inclination: https://youtu.be/AwwIFcdUFrE https://youtu.be/AwwIFcdUFrE
- nonameiguess 3y ago
  The graphic is an attempt to project the 3D observable universe down to 2D and then depict the time evolution of it to show that it has expanded in spatial volume, with the bell shape showing extremely rapid expansion during cosmic inflation, then slowing expansion, then accelerating expansion again as matter density became low enough for dark energy density to take over. But the observable universe is not the entire universe. There is no known boundary to space and there probably is no boundary at all. We'll never have any way of knowing, but space could easily be infinite in extent, while still expanding at every point. If you were on the boundary depicted (the cosmic horizon of the Earth-centered observable universe), or even if you were outside of that completely, the picture from your vantage point should look exactly the same. As for what the horizon represents, it's all light that could have reached Earth within 13.7 billion years, that is, Earth's past light cone. It isn't necessarily the case that nothing leaving Earth today could ever get past that horizon. Light can travel forever if nothing ever scatters it. Given some very large amount of time, it can beyond the boundary depicted there. Because of the expansion of space, however, that time is likely to be more than 13.7 billion years, and given the acclerating expansion of space, there will eventually come a time that no light leaving any point within our local supercluster can ever reach any other supercluster. They will become forever outside of each other's horizons.
earthboundkid 3y ago
I'm not saying MOND, but c'mon, MOND.

108 points by ColinWright 3y ago

So does this require new physics to solve? What should the layman take away from this? >distant galaxies have been speeding up in their recession, and the expansion rate, though still dropping, is not headed toward zero. If the expansion rate is dropping, surely it is headed towards zero? Or are they using expansion rate to mean acceleration and the zero refers to the recession. Or am I misunderstanding something?

sornaensis 3y ago
I took it to mean the rate of expansion is approaching a horizontal asymptote e.g. -x/ln(x), x > 1
irishsultan 3y ago
Imagine the expansion rate starts out at 2. Then 1 day later it's 1.5, after 1 more day it's 1.25 and after another day it's 1.125. Hopefully you can see that if this series continues then the expansion rate is always dropping, but it's headed towards 1, not 0. (And if expansion rate of 1 is too confusing in this context imagine if it starts out at 3 and goes to 2.5, 2.25, 2.125, ..., it still is always decreasing, but it will never be less than 2 which means the universe keeps expanding).
- benj111 3y ago
  Ok, that makes sense. What's the mechanism that allows the acceleration to drop, without dropping to zero though? Is the dark matter not expanding with everything else
  - rout39574 3y ago
    Yes, that is indeed the question.
  - layer8 3y ago
    > What's the mechanism that allows the acceleration to drop, without dropping to zero though? The expansion rate drops because the energy density goes down due to the expansion. However, the dark energy density aka cosmological constant does not go down, therefore providing a nonzero floor. > Is the dark matter not expanding with everything else This has nothing to do with dark matter in particular, and it’s space itself that’s expanding, not matter. Also, the expansion has no center, the universe is expanding at every point. Some types of matter expanding and others not would imply a center.
    - denton-scratch 3y ago
      > The expansion rate drops because the energy density goes down due to the expansion. I thought the idea was that Dark Energy increased with the amount of empty space; so as the space between galaxies expands, the energy driving the expansion also expands - hence the acceleration.
      - layer8 3y ago
        I wrote “density”, i.e. the amount of dark energy remains constant per unit volume. Since the volume increases due to the expansion, the total amount of dark energy increases accordingly, but its density remains the same. This is as opposed to the matter and radiation density, which decreases.
        denton-scratch 3y ago
        Thank you for clarifying (you said "energy density goes down due to the expansion"). Can you also clarify why galaxies don't expand, but empty space does? I suppose this is something to do with "vacuum energy", but it's not obvious to me that vacuum energy actually requires a vacuum; I thought it was present everywhere, but was only significant in the absence of other "stuff". I also understand vacuum energy to be related to Hawking Radiation, which is black-body. But black-body radiation is EM radiation; DE is neither black-body nor electromagnetic. Why does vacuum energy not produce observable EM radiation? Is it just too weak to observe?
yk 3y ago
> So does this require new physics to solve? Not necessarily. First the distance ladder could have another problem. Second to look at the early universe, for the early universe observations you are looking through the entire universe at the CMB and that requires foreground subtraction, which is very much a non trivial task. And finally from theory, General Relativity is a non-linear theory which means taking the average and then evolving the average does not necessarily yield the same result as evolving the initial state and then taking the average. Either of this could explain the tension, though an actually dynamic cosmological constant would be more fun.
pdpi 3y ago
> If the expansion rate is dropping, surely it is headed towards zero? Expansion rate could be dropping but converging to a non-zero value. That seems unlikely to me, but it's an answer that would fit that description just fine. The language is a bit imprecise, though, which I expect is the problem. The (to me) obvious technical interpretation of "expansion rate is headed towards zero" is that d size(t)/dt -> 0 as t -> infinity, but the (again, to me) obvious non-technical interpretation is "expansion will completely stop at some point". So "*not* headed towards zero" means "derivative isn't going to zero", or "expansion never quite stops", respectively. The derivative of ln(t) does go to zero, but it has unbounded growth, so it fails the first test but passes the second. The universe experiencing logarithmic expansion seems reasonable enough.
venusenvy47 3y ago
It's confusing because that quote about "expansion rate decreasing" is in the caption of a picture with a giant "Accelerating Expansion" label at present time.

fjfaase 3y ago

Article seems to be based on 'Crowded No More: The Accuracy of the Hubble Constant Tested with High Resolution Observations of Cepheids by JWST' [1]. [1] https://arxiv.org/abs/2307.15806 https://arxiv.org/abs/2307.15806

joshuahedlund 3y ago

Possibly a dumb question: can the tension between the two rates simply be resolved because one method measures an old historical rate and one method measures the current rate, and it’s been dropping over time? Or are they both measuring the same time periods? (I assume it’s not that simple, but it’s very hard for me to understand and reason about data that involves looking back in time and space simultaneously)

venusenvy47 3y ago
That idea is mentioned in the article, with the hypothesis that there was more dark energy in the past that has since dissipated. They label that idea as "New Physics" in the chart above that comment.
privong 3y ago
> Possibly a dumb question: can the tension between the two rates simply be resolved because one method measures an old historical rate and one method measures the current rate This is already generally being accounted for (assuming we have the correct cosmological model for the Universe). The CMB measurement is probing the Universe at an early state (when the Universe was ~380,000 years old) and the Hubble "constant", H(t=380,000 yr), was different at that point from what it is now, H_0 = H(t=13.7 billion years). The comparison to the current Hubble constant (e.g., determined from Cepheids) is made by evolving the CMB Hubble "constant" value to today, under the assumption of our best cosmological model model. Of course, if we have the wrong cosmological model, then that could be a reason for the discrepancy, and that might point to "new physics". Alternately there may be some systematic in the Cepheid technique that is causing our H_0 estimate to be somewhat off.
mr_mitm 3y ago
Both methods measure H_0, which is the Hubble parameter today. It's a bit confusing that it's sometimes called the Hubble constant, because it's not really a constant. The expansion rate H changes over time, so we write H(t) and by H_0 we mean H(t_today). It's probably the case that they didn't directly measure H_0 with the early universe probes, so they measured H at that time and then used the standard model to extrapolate this value to H_0, roughly speaking. This means, that if the measurements didn't suffer from some unknown systematic error, that the model is wrong. That's what they call "new physics" and could be the most exciting thing in cosmology since the 90s.

croes 3y ago

A little confusing that strengthening the Hubble tension is worsening it.

enkid 3y ago
The Hubble tension is "bad" (i.e. shows a problem with) for current models of cosmology, hence strengthening it is making it "worse." Its like a hurricane, a strengthening hurricane is worse.
- croes 3y ago
  It's just a weird combination of something mostly considered positive with something negative. Like strengthening my weaknesses.
  - jdiff 3y ago
    Where's the positive? If you've got a strengthening tension headache, there's no mixing there. Tension in your head, tension in the room, tension in a Hubble, it's all bad.

_madmax_ 3y ago

>However, for the past ~6 billion years, distant galaxies have been speeding up in their recession, and the expansion rate, though still dropping, is not headed toward zero. Am I not getting something here or is this sentence not make sense?

XzAeRosho 3y ago
The rate of acceleration is slowing down, but still is accelerating.
- bonzini 3y ago
  So the first derivative was already understood to remain nonzero forever[1], now we know that the second derivative probably will also remain nonzero, and the third is slowing down but still positive for the moment? [1] IIRC in the 80s the big bang/big crunch model was preferred
vidarh 3y ago
It makes sense if the expansion rate isn't dropping linearly and the rate of the drop is itself dropping towards zero.
layer8 3y ago
This article explains it in more detail: https://bigthink.com/starts-with-a-bang/universe-expansion-not-accelerating/ https://bigthink.com/starts-with-a-bang/universe-expansion-n...
BurningFrog 3y ago
The slowing down is speeding up. What could be simpler?

bmitc 3y ago

Is this an accurate summary of the current status quo? 1. The universe is expanding 2. The velocity at which the universe is expanding is increasing, since there is non-zero acceleration. 3. The acceleration is actually slowing down, but it is not heading towards zero. It is approaching some steady state acceleration. ---------- In Exploring Black Holes by Edwin Taylor and John Wheeler, published in 2000, I recently read the following passage by Wheeler: > An article by John Noble Wilford in the Science Times section of the New York Times for Tuesday March 3, 1998, reports observations by two separate groups of investigators which they interpret as showing that today the expansion of the Universe is speeding up rather than undergoing the slowdown expected for any approach to maximum expansion. Later that day I encountered a hard-bitten veteran gravitation physics colleague in the elevator of the Princeton physics building and asked him if he believed the purported evidence of accelerating expansion. "No," he replied. Neither do I. Why not? Two reasons: (1) Because the speed-up argument relies too trustingly on the supernovas being standard candles, (2) Because such an expansion would, it seems to me, contradict a view of cosmology too simple to be wrong. Such clashes between theory and experiment have often triggered decisive advances in physics. We can hope that some decisive advance is in the offing. Now, Wheeler was one of the most preeminent physicists of the 20th century and spearheaded several fields and was not afraid of wild ideas. I am curious if anyone knows if he had any inklings that further his thoughts here. Was he just wrong here, before new measurements came out? Or is there something missing in these calculations and the theory that we're missing? I'm also not sure what means in (1).

mr_mitm 3y ago
Yes to 1) and 2), however: > 3. The acceleration is actually slowing down, but it is not heading towards zero. It is approaching some steady state acceleration. No. The scale factor in LCDM (the current concordance model) is proportional to sinh^(2/3)(t/t_Lambda), which means it's asymptotically exponential. Acceleration will always increase. (Edit: The expansion rate, so a'/a, will go to a constant value, though.) Caveat: If the Hubble tension proves real, LCDM is wrong.
- bmitc 3y ago
  Thanks! So the currently accepted model says the acceleration will grow indefinitely but our observations are saying that the acceleration levels out to a non-zero, positive constant?

denton-scratch 3y ago

Too many pictures, few of which add information. Too few words.

fooqux 3y ago
Agreed; the presentation was, IMHO, terrible. You had to read each image's relatively hard to read annotation to make sense of what they were trying to say. Hell, 95% of the entire article was image annotations, with single sentences between. Really difficult to follow along.
EdwardDiego 3y ago
That GIF of Puppis' light echoes is amazing though.
dhuk_2018 3y ago
THANK YOU. I was feeling dumber after reading it, not smarter.

m3kw9 3y ago

The first image depicting space expansion in that article: what happens if some is to travel beyond that space boundary? Is it because of speed of light that no one can catch up beyond it or is it something else?

fluoridation 3y ago
I don't believe the graphic is representing a boundary. I think it's representing the relative expansion of space over time. The observable universe does have an event horizon that can't be reached due to the finiteness of the speed of light, though.
- fnordpiglet 3y ago
  I think the key point is the event horizon, or effective boundary, is relative to the observer. From the earth there’s a point beyond which we can never observe, even if we left today at the speed of light. However for a planet at that point everything looks “the same” to them - except the earth is at their boundary. The boundaries here aren’t absolute - they’re only relative to the observer.
uoaei 3y ago
PBS Spacetime is a wonderful rabbit (worm)hole if you have the time and inclination: https://youtu.be/AwwIFcdUFrE https://youtu.be/AwwIFcdUFrE
nonameiguess 3y ago
The graphic is an attempt to project the 3D observable universe down to 2D and then depict the time evolution of it to show that it has expanded in spatial volume, with the bell shape showing extremely rapid expansion during cosmic inflation, then slowing expansion, then accelerating expansion again as matter density became low enough for dark energy density to take over. But the observable universe is not the entire universe. There is no known boundary to space and there probably is no boundary at all. We'll never have any way of knowing, but space could easily be infinite in extent, while still expanding at every point. If you were on the boundary depicted (the cosmic horizon of the Earth-centered observable universe), or even if you were outside of that completely, the picture from your vantage point should look exactly the same. As for what the horizon represents, it's all light that could have reached Earth within 13.7 billion years, that is, Earth's past light cone. It isn't necessarily the case that nothing leaving Earth today could ever get past that horizon. Light can travel forever if nothing ever scatters it. Given some very large amount of time, it can beyond the boundary depicted there. Because of the expansion of space, however, that time is likely to be more than 13.7 billion years, and given the acclerating expansion of space, there will eventually come a time that no light leaving any point within our local supercluster can ever reach any other supercluster. They will become forever outside of each other's horizons.

earthboundkid 3y ago

I'm not saying MOND, but c'mon, MOND.