6 ms·

A little background based on a few articles about this plus my recollection of PBS Space Time videos on this: There are at least two ways to try to figure out t

by tzs 3y ago

A little background based on a few articles about this plus my recollection of PBS Space Time videos on this: There are at least two ways to try to figure out the rate of expansion of the universe (which is called the Hubble Constant). • From variations in the cosmic microwave background (CMB) which are the result of certain conditions in the early universe it is possible to figure out what the expansion rate should be now. • From looking at very distant galaxies and noting how far away they are and how fast they are receding from us the expansion rate can be calculated. Theory says that these should give the same expansion rate. When the rates were first found using those two methods they gave different results, but the error bars on both were large enough to overlap. People expected then that further refinement of both methods to decrease the error bars would converge to some common value. That did not happen. Refinement of the CMB measurements got to 67 +/- 0.5, and refinement of the galaxy distance/speed method got to 73 +/- 1. Those do not overlap. This non-overlap between the possible ranges given by the two methods is called the Hubble tension, and it is one of the most irksome problems in cosmology. Possible explanations include: • Some sort of error in how we measure the variations in the CMB. • Some sort of error in the distant galaxy distance or speed measurements, which until the James Webb telescope were almost entirely Hubble telescope measurements. • We're missing something in our understanding of the physics. These new results add a bunch of data from the James Webb telescope, which observes in different wavelengths than Hubble. These results fit with the Hubble measurements. They do not resolve the Hubble tension. What they do is remove most doubt that the distant galaxy results involve some sort of Hubble measurement error. I believe cosmologists are pretty confident of the CMB measurements, and so this will be interpreted as telling us that the Hubble tension is not just a problem with our measurements. There is either physics that we got wrong or physics we need to discover.

rf15 3y ago
This, together with the endless philosophising around dark energy, dark matter and whatnot paints a pretty strong arrow towards our models having some flaws when it comes to their large-scale application. I hope to live long enough to see where we made our mistake and get a better model.
- pishpash 3y ago
  Almost every model we build of more trivial things based on observation always turns out to be not really right. I cannot imagine why one of the universe that has had multiple version updates in the last 100 years to not also be grossly mistaken. I also don't expect the full model to be simple or beautiful. We may be thinking wishfully based on massive extrapolation and cutting corners to suit our narrow view into the world.
  - CuriouslyC 3y ago
    I think the truth is likely that we live in a very complex universe that can be approximated by these sweeping laws in general, but is very "messy" close up. Consider that while the universe roughly obeys probabilities, it also has will and intent (to a degree we can debate in another conversation).
    - kaashif 3y ago
      What do you mean by will and intent? I guess if humans have will and are part of the universe, then the universe has it too, but I don't think that's what you meant.
      - CuriouslyC 3y ago
        Hopefully we can agree that at least part of the universe "chooses" outcomes, rather than them occurring randomly. You could assume that "choosing" is what random outcomes feel like but I don't think that's productive. Given that we have evidence of physical systems evolving their states by conscious choice, that begs the question "Why would large, complex physical systems have a governing force that doesn't exist in smaller physical systems?" Occam's Razor suggests that consciousness and will drive the evolution of all physical systems, and our elegant equations are merely statistical approximations following the law of large numbers given a population with a natural variance. Of course we can debate what it means for a low information system to be conscious and have will, but I think that's a more meaningful debate than trying to pinpoint the exact moment that a system becomes complex enough for the magical emergence fairy to sprinkle consciousness dust on inert matter.
michaelsbradley 3y ago
"Humason assembled spectra of the nebulae and I attempted to estimate distances." So wrote Hubble of his colleague Milton Humason in 1935 by which time spectra had been obtained for over 150 nebulae. Hubble was a stern warner of using the Doppler effect for galaxies and argued against the recessional velocity interpretation of redshift, convincing Robert Millikan, 1923 recipient of the Nobel Prize for Physics and director of physics at the California Insitute of Technology, that the redshift interpretation as an expanison of the universe was probably wrong, the year before both of their deaths in 1953. Hubble ended his book Observational Approach to Cosmology[+] with the statement:..."if the recession factor is dropped, if redshifts are not primarily velocity-shifts, the picure is simple and plausible. There is no evidence of expansion and no restriction of time-scale, no trace of spatial curvature, and no limitation of spatial dimensions. Moreover, there is no problem of internebular material. The observable region is thoroughly homogeneous; it is too small a sample to indicate the nature of the universe at large. The univers might even be an expanding model, provide the rate of expansion, which pure theory does not specify, in inappreciable. For that matter, the universe might even be contracting." [+] https://ned.ipac.caltech.edu/level5/Sept04/Hubble/paper.pdf https://ned.ipac.caltech.edu/level5/Sept04/Hubble/paper.pdf source: https://plasmauniverse.info/people/contributors.html https://plasmauniverse.info/people/contributors.html
throwawaymaths 3y ago
There's a 'fringe' theory that the CMB is not the echo of the big bang but rather the redshifted black body radiation of intergalactic dust (apparently the numbers are about right). Although the primary proponent of this theory is trying to justify some sort of cyclic universe model with it, if my understanding is correct it would still be compatible with a 'standard' big bang that has the "longer" (aka galactic expansion) timeframe. And in any case physics still needs to explain what happened to that blackbody radiation.
- sebzim4500 3y ago
  How could that be true? Wouldn't that smear the spectrum so that it no longer so matches the black body radiation of a single object of a set temperature?
  - throwawaymaths 3y ago
    No. According to the standard big bang theory the CMB should look like black body radiation. As for smearing, IIRC black body curve has the central limit property
    - sebzim4500 3y ago
      > According to the standard big bang theory the CMB should look like black body radiation. Yes, that's my point. >As for smearing, IIRC black body curve has the central limit property Surely that can only be true for a very specific distribution of temperatures? To go to an extreme, if you combine the spectrum of two objects of different temperatures you do not get anything like a black body curve.
      - raattgift 3y ago
        I agree with you; you can't build up a perfect blackbody spectrum by superposing different near-blackbody spectra, and that led to the cosmological thermalization problem. Furthermore, baryonic intergalactic dust isn't like dark matter: we see redshifted spectral features of components of such dusts imprinted on all sorts of backgrounds from quasars to the CMB. The dusts tend to be greybodies. (I'm ignoring the warm-hot intergalactic medium here). The CMB has narrow spectral lines imposed on it by galaxies, dust clouds, and similar objects. Those lines can be used to measure the CMB effective temperature at an object's redshift. A specific example of how this works is linked at the bottom of this comment. If a similar highly dusty object is found in the foreground of a bright quasar (merely nearby quasars were used in the case below), we could get an even tighter measurement of the CMB effective temperature at the dust, and perhaps look in new ways at how the CMB deviated (at the dusty object) very slightly from a blackbody spectrum. This amounts to a new line of evidence in the study of the expansion history. More generally, constraining the evolution of the CMB temperature in the matter-dominated era (T_cmb > 4 K, z > ~ 0.4) will prove fatal to ideas like some in this thread (which is already in trouble because of other spectroscopy e.g. the Lyman-alpha forest and Gunn-Peterson trough). I don't really understand the summary of the "'fringe' theory" at the top of the thread (and I can't identify which specific theory throwawaymaths means). I guess the idea could be that some unknown mechanism might homogenize the emissive components of baryonic intergalactic dust but not circumgalactic medium spectral features. I suppose one could try to investigate a non-adiabatic expansion to explain those features found in the CMB in a nonstandard way. Non-adiabatic components in the expansion have been examined by various theoretical teams in the context of quantum cosmology and/or "fifth force" dark energy, although mostly at much larger lookback times than the the standard surface of last scattering. I don't understand what was meant by '"longer" (aka galactic expansion) timeframe'. Finally, "echo of the big bang" isn't really helpful in understanding the origin of the CMB photons, why they're now filling space with a thermal Planck spectrum, and why we detect small variations in that spectrum in narrow views along different directions, but like "fabric of spacetime" I guess we're kinda stuck with the expression. - -- (Open Access) Riechers, D.A., Weiss, A., Walter, F. et al. Microwave background temperature at a redshift of 6.34 from H2O absorption. Nature 602, 58–62 (2022). https://doi.org/10.1038/s41586-021-04294-5 https://doi.org/10.1038/s41586-021-04294-5
xwolfi 3y ago
https://www.sciencedaily.com/releases/2023/12/231201123626.htm https://www.sciencedaily.com/releases/2023/12/231201123626.h...
xeonmc 3y ago
Could it be that nearby galaxies are akin to small angle approximations in spacetime trajectory, but as you get really far away (e.g. CMB) the perspective distortion increases hyperbolically? I notice that if you normalize the Hubble data by their Lorentz factor you get back a constant expansion rate: https://www.desmos.com/calculator/llhnja1ocb https://www.desmos.com/calculator/llhnja1ocb
- rhelz 3y ago
  For real? That is a very interesting observation.
- scotty79 3y ago
  Probably accidental? Redshifts can be way higher than 1 that in your scaling is a magic value. https://en.wikipedia.org/wiki/List_of_the_most_distant_astronomical_objects https://en.wikipedia.org/wiki/List_of_the_most_distant_astro...
deleted 3y ago
[deleted]
lagrange77 3y ago
Interesting, thanks! Out of curiosity: Can we actually differentiate between the expansion of space itself and the drifting of objects within it in the same direction?
- dmd 3y ago
  You would have to come up with an explanation for such drifting to be occurring across the entire visible universe in precisely the same way, across billions and billions of light years.
  - deleted 3y ago
    [deleted]
  - xeonmc 3y ago
    Just a fringe speculation without any physical basis: maybe the Big Bang emits a 4-dimensional spherical shell of matter at c? With the direction in which we are traveling at c called “time”, and the other three which we're not moving through called “space”; diverging rays having spatial velocity relative to each other equal to tan(alpha) i.e. flat projection; and at any given point on the 4-sphere's 3D surface everything appear to be diverging in 3D because the emission is spherically symmetric; and trajectories sufficiently paraxial to us fits the small angle approximation but those diverging widely has much more apparent perspective distortion, with an error ratio of tan(alpha)/sin(alpha) = sec(alpha) = 1/Lorentz_Factor?
    - dmd 3y ago
      [flagged]
      - deleted 3y ago
        [deleted]
      - dvsfish 3y ago
        While its probably totally wrong, I think your tone and general attitude to people throwing around imaginative and creative ideas is extremely unproductive and frankly rude. If you want to stifle creative discussions, this is exactly how you'd do it. I found the post to be very stimulating regardless of its validity. And it's not like they're trying to spread it as gospel. There was a lot of disclaimer.
Kerbonut 3y ago
Could it be the rate of expansion is not constant?
- mr_toad 3y ago
  https://en.wikipedia.org/wiki/Quintessence_(physics) https://en.wikipedia.org/wiki/Quintessence_(physics)
- diedyesterday 3y ago
  Of course the rate of expansion is not constant; That's the whole deal with dark energy which is understood to drive accelerated expansion of space due to its negative pressure (when you make something with negative pressure larger in volume you have to do positive work on it or give it energy and that energy is dark energy). But the Hubble tension is something else: It's about measurements of H0 (the current value of the Hubble parameter) using two different methods (early universe and CMB OR recent universe and supernovae, etc.) that do not agree. The disagreement is about two estimations of the current rate of expansion called H0; Everyone agrees H(t) has been increasing (=accelerating expansion); But accelerating toward which recession velocity is the question.
bryango 3y ago
> They do _not_ resolve the Hubble tension. Thank you! They should have clarified this up front in the title or the abstract. The original article makes it sound like it has been resolved: > (title) Webb & Hubble confirm Universe’s expansion rate > (abstract) ... However, a persistent difference, called the Hubble Tension, is seen between the value of the constant measured with a wide range of independent distance indicators and its value predicted from the afterglow of the Big Bang. The NASA/ESA/CSA James Webb Space Telescope has confirmed that the Hubble Space Telescope’s keen eye was _right all along_ (emphasis mine), erasing any lingering doubt about Hubble’s measurements. This feels like bad reporting to me. A better title would be "Webb confirms Hubble's measurement of Universe’s expansion rate", but that would probably be less exciting...
naasking 3y ago
> Some sort of error in how we measure the variations in the CMB. IIRC, the prediction that comes from the CMB depends on the lambda cold dark matter model, and there are plenty of independent reasons to be skeptical of that model already. If I were betting which one's most likely to be wrong, that's where I'd put my money.

by tzs 3y ago

rf15 3y ago

This, together with the endless philosophising around dark energy, dark matter and whatnot paints a pretty strong arrow towards our models having some flaws when it comes to their large-scale application. I hope to live long enough to see where we made our mistake and get a better model.

pishpash 3y ago
Almost every model we build of more trivial things based on observation always turns out to be not really right. I cannot imagine why one of the universe that has had multiple version updates in the last 100 years to not also be grossly mistaken. I also don't expect the full model to be simple or beautiful. We may be thinking wishfully based on massive extrapolation and cutting corners to suit our narrow view into the world.
- CuriouslyC 3y ago
  I think the truth is likely that we live in a very complex universe that can be approximated by these sweeping laws in general, but is very "messy" close up. Consider that while the universe roughly obeys probabilities, it also has will and intent (to a degree we can debate in another conversation).
  - kaashif 3y ago
    What do you mean by will and intent? I guess if humans have will and are part of the universe, then the universe has it too, but I don't think that's what you meant.
    - CuriouslyC 3y ago
      Hopefully we can agree that at least part of the universe "chooses" outcomes, rather than them occurring randomly. You could assume that "choosing" is what random outcomes feel like but I don't think that's productive. Given that we have evidence of physical systems evolving their states by conscious choice, that begs the question "Why would large, complex physical systems have a governing force that doesn't exist in smaller physical systems?" Occam's Razor suggests that consciousness and will drive the evolution of all physical systems, and our elegant equations are merely statistical approximations following the law of large numbers given a population with a natural variance. Of course we can debate what it means for a low information system to be conscious and have will, but I think that's a more meaningful debate than trying to pinpoint the exact moment that a system becomes complex enough for the magical emergence fairy to sprinkle consciousness dust on inert matter.

michaelsbradley 3y ago

"Humason assembled spectra of the nebulae and I attempted to estimate distances." So wrote Hubble of his colleague Milton Humason in 1935 by which time spectra had been obtained for over 150 nebulae. Hubble was a stern warner of using the Doppler effect for galaxies and argued against the recessional velocity interpretation of redshift, convincing Robert Millikan, 1923 recipient of the Nobel Prize for Physics and director of physics at the California Insitute of Technology, that the redshift interpretation as an expanison of the universe was probably wrong, the year before both of their deaths in 1953. Hubble ended his book Observational Approach to Cosmology[+] with the statement:..."if the recession factor is dropped, if redshifts are not primarily velocity-shifts, the picure is simple and plausible. There is no evidence of expansion and no restriction of time-scale, no trace of spatial curvature, and no limitation of spatial dimensions. Moreover, there is no problem of internebular material. The observable region is thoroughly homogeneous; it is too small a sample to indicate the nature of the universe at large. The univers might even be an expanding model, provide the rate of expansion, which pure theory does not specify, in inappreciable. For that matter, the universe might even be contracting." [+] https://ned.ipac.caltech.edu/level5/Sept04/Hubble/paper.pdf https://ned.ipac.caltech.edu/level5/Sept04/Hubble/paper.pdf source: https://plasmauniverse.info/people/contributors.html https://plasmauniverse.info/people/contributors.html

throwawaymaths 3y ago

There's a 'fringe' theory that the CMB is not the echo of the big bang but rather the redshifted black body radiation of intergalactic dust (apparently the numbers are about right). Although the primary proponent of this theory is trying to justify some sort of cyclic universe model with it, if my understanding is correct it would still be compatible with a 'standard' big bang that has the "longer" (aka galactic expansion) timeframe. And in any case physics still needs to explain what happened to that blackbody radiation.

sebzim4500 3y ago
How could that be true? Wouldn't that smear the spectrum so that it no longer so matches the black body radiation of a single object of a set temperature?
- throwawaymaths 3y ago
  No. According to the standard big bang theory the CMB should look like black body radiation. As for smearing, IIRC black body curve has the central limit property
  - sebzim4500 3y ago
    > According to the standard big bang theory the CMB should look like black body radiation. Yes, that's my point. >As for smearing, IIRC black body curve has the central limit property Surely that can only be true for a very specific distribution of temperatures? To go to an extreme, if you combine the spectrum of two objects of different temperatures you do not get anything like a black body curve.
    - raattgift 3y ago
      I agree with you; you can't build up a perfect blackbody spectrum by superposing different near-blackbody spectra, and that led to the cosmological thermalization problem. Furthermore, baryonic intergalactic dust isn't like dark matter: we see redshifted spectral features of components of such dusts imprinted on all sorts of backgrounds from quasars to the CMB. The dusts tend to be greybodies. (I'm ignoring the warm-hot intergalactic medium here). The CMB has narrow spectral lines imposed on it by galaxies, dust clouds, and similar objects. Those lines can be used to measure the CMB effective temperature at an object's redshift. A specific example of how this works is linked at the bottom of this comment. If a similar highly dusty object is found in the foreground of a bright quasar (merely nearby quasars were used in the case below), we could get an even tighter measurement of the CMB effective temperature at the dust, and perhaps look in new ways at how the CMB deviated (at the dusty object) very slightly from a blackbody spectrum. This amounts to a new line of evidence in the study of the expansion history. More generally, constraining the evolution of the CMB temperature in the matter-dominated era (T_cmb > 4 K, z > ~ 0.4) will prove fatal to ideas like some in this thread (which is already in trouble because of other spectroscopy e.g. the Lyman-alpha forest and Gunn-Peterson trough). I don't really understand the summary of the "'fringe' theory" at the top of the thread (and I can't identify which specific theory throwawaymaths means). I guess the idea could be that some unknown mechanism might homogenize the emissive components of baryonic intergalactic dust but not circumgalactic medium spectral features. I suppose one could try to investigate a non-adiabatic expansion to explain those features found in the CMB in a nonstandard way. Non-adiabatic components in the expansion have been examined by various theoretical teams in the context of quantum cosmology and/or "fifth force" dark energy, although mostly at much larger lookback times than the the standard surface of last scattering. I don't understand what was meant by '"longer" (aka galactic expansion) timeframe'. Finally, "echo of the big bang" isn't really helpful in understanding the origin of the CMB photons, why they're now filling space with a thermal Planck spectrum, and why we detect small variations in that spectrum in narrow views along different directions, but like "fabric of spacetime" I guess we're kinda stuck with the expression. - -- (Open Access) Riechers, D.A., Weiss, A., Walter, F. et al. Microwave background temperature at a redshift of 6.34 from H2O absorption. Nature 602, 58–62 (2022). https://doi.org/10.1038/s41586-021-04294-5 https://doi.org/10.1038/s41586-021-04294-5

xwolfi 3y ago

https://www.sciencedaily.com/releases/2023/12/231201123626.htm https://www.sciencedaily.com/releases/2023/12/231201123626.h...

xeonmc 3y ago

Could it be that nearby galaxies are akin to small angle approximations in spacetime trajectory, but as you get really far away (e.g. CMB) the perspective distortion increases hyperbolically? I notice that if you normalize the Hubble data by their Lorentz factor you get back a constant expansion rate: https://www.desmos.com/calculator/llhnja1ocb https://www.desmos.com/calculator/llhnja1ocb

rhelz 3y ago
For real? That is a very interesting observation.
scotty79 3y ago
Probably accidental? Redshifts can be way higher than 1 that in your scaling is a magic value. https://en.wikipedia.org/wiki/List_of_the_most_distant_astronomical_objects https://en.wikipedia.org/wiki/List_of_the_most_distant_astro...

deleted 3y ago

[deleted]

lagrange77 3y ago

Interesting, thanks! Out of curiosity: Can we actually differentiate between the expansion of space itself and the drifting of objects within it in the same direction?

dmd 3y ago
You would have to come up with an explanation for such drifting to be occurring across the entire visible universe in precisely the same way, across billions and billions of light years.
- deleted 3y ago
  [deleted]
- xeonmc 3y ago
  Just a fringe speculation without any physical basis: maybe the Big Bang emits a 4-dimensional spherical shell of matter at c? With the direction in which we are traveling at c called “time”, and the other three which we're not moving through called “space”; diverging rays having spatial velocity relative to each other equal to tan(alpha) i.e. flat projection; and at any given point on the 4-sphere's 3D surface everything appear to be diverging in 3D because the emission is spherically symmetric; and trajectories sufficiently paraxial to us fits the small angle approximation but those diverging widely has much more apparent perspective distortion, with an error ratio of tan(alpha)/sin(alpha) = sec(alpha) = 1/Lorentz_Factor?
  - dmd 3y ago
    [flagged]
    - deleted 3y ago
      [deleted]
    - dvsfish 3y ago
      While its probably totally wrong, I think your tone and general attitude to people throwing around imaginative and creative ideas is extremely unproductive and frankly rude. If you want to stifle creative discussions, this is exactly how you'd do it. I found the post to be very stimulating regardless of its validity. And it's not like they're trying to spread it as gospel. There was a lot of disclaimer.

Kerbonut 3y ago

Could it be the rate of expansion is not constant?

mr_toad 3y ago
https://en.wikipedia.org/wiki/Quintessence_(physics) https://en.wikipedia.org/wiki/Quintessence_(physics)
diedyesterday 3y ago
Of course the rate of expansion is not constant; That's the whole deal with dark energy which is understood to drive accelerated expansion of space due to its negative pressure (when you make something with negative pressure larger in volume you have to do positive work on it or give it energy and that energy is dark energy). But the Hubble tension is something else: It's about measurements of H0 (the current value of the Hubble parameter) using two different methods (early universe and CMB OR recent universe and supernovae, etc.) that do not agree. The disagreement is about two estimations of the current rate of expansion called H0; Everyone agrees H(t) has been increasing (=accelerating expansion); But accelerating toward which recession velocity is the question.

bryango 3y ago

> They do _not_ resolve the Hubble tension. Thank you! They should have clarified this up front in the title or the abstract. The original article makes it sound like it has been resolved: > (title) Webb & Hubble confirm Universe’s expansion rate > (abstract) ... However, a persistent difference, called the Hubble Tension, is seen between the value of the constant measured with a wide range of independent distance indicators and its value predicted from the afterglow of the Big Bang. The NASA/ESA/CSA James Webb Space Telescope has confirmed that the Hubble Space Telescope’s keen eye was _right all along_ (emphasis mine), erasing any lingering doubt about Hubble’s measurements. This feels like bad reporting to me. A better title would be "Webb confirms Hubble's measurement of Universe’s expansion rate", but that would probably be less exciting...

naasking 3y ago

> Some sort of error in how we measure the variations in the CMB. IIRC, the prediction that comes from the CMB depends on the lambda cold dark matter model, and there are plenty of independent reasons to be skeptical of that model already. If I were betting which one's most likely to be wrong, that's where I'd put my money.