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Thread: Hubble tension

  1. #1
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    Hubble tension

    Want is the current state of play with this?
    If it is a problem with current models or measurement methods were should we be looking?
    Are we really at the point of looking for new physics to explain inconsistent measured values for the Hubble constant?
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    There was some new article about this in the mainstream press, but it didn't say anything new. I suspect that a definitive answer to this might be 50 to 100 years away, when we might have astrometry missions like Gaia (but more precise) on trips to nearby stars, and we can get accurate parallax measurements to quasars and galaxies closer than z=2.0. At that point direct measurements will enable the constructing of detailed models eliminating most hand-waving explanations, and confirming a few. It doesn't seem likely that new physics will be required, but if it is, we'll need better measurements than we can get today.
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    I was just looking at papers on, the 5th force, an extra type of dark energy that all looked pretty desperate.
    They don't even know what correction they are trying to achieve.

    I've always been a fan of ATM cosmologies anyway.

    Your time scales are a bit depressing.
    I was thinking a small telescope capable of measuring parallaxes of bright objects, with a solar powered ion thruster could be fired around the moon & then close to the Sun.
    If the payload was absolute minimum & jettisoned empty propellant tanks as it went, it might be able achieve 100 AU in maybe 12 years or less possibly?
    Maybe the large solar panels could power a simple instrument, burst transmitter on starlight?
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    Quote Originally Posted by PetTastic View Post
    I was just looking at papers on, the 5th force, an extra type of dark energy that all looked pretty desperate.
    They don't even know what correction they are trying to achieve.

    I've always been a fan of ATM cosmologies anyway.

    Your time scales are a bit depressing.
    I was thinking a small telescope capable of measuring parallaxes of bright objects, with a solar powered ion thruster could be fired around the moon & then close to the Sun.
    If the payload was absolute minimum & jettisoned empty propellant tanks as it went, it might be able achieve 100 AU in maybe 12 years or less possibly?
    Maybe the large solar panels could power a simple instrument, burst transmitter on starlight?
    Solar panels don't work well out past Jupiter. You'd want a RTG. One light year 63,000 AU, so going 100 AU out isn't much of anything compared to the distances between stars. You'd also have the issue where your ship needs to move, perhaps in an orbit to collect parallaxes and an orbit out at 100 AU is ridiculously long.
    Solfe

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    For parallaxes I think you only need images of the target in background and some foreground reference stars taken from different positions.
    So use telescopes on Earth or better another spacecraft a 100 AU in other direction give you a 100 or 200 AU baseline that is increasing for as long as the vehicle can keep transmitting.

    Probes like New Horizons needed a relatively large power supply to actively track and photo very dark objects flashing past at high speed.
    I was imagining once the acceleration phase is over long streamers of non structural thin-film solar cells could be released as it would not matter much what direction they face.
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    If you want to keep discussing an imagined long baseline parallax mission, please start a thread in Space Exploration. This thread is about the topic of tension over the Hubble Constant.
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    How sure are we the problem is real?
    There seems a wide range of Sigma numbers going around?
    Are we near sigma 5?

    I don't understands the details of some of these measurement techniques but is there a trend between angular size based methods & luminosity methods?
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    Quote Originally Posted by PetTastic View Post
    How sure are we the problem is real?
    Depends what you mean by problem. I think the consensus is that two of the main methods for determining the value of the Hubble constant give similar but not overlapping ranges of values. The probability of the difference being due to measurement uncertainty is small. What we don't know is where the difference is coming from - whether it is something up with the distance ladder for nearby Cephids, whether there is something wrong with our early universe models or whether there has been some kind of change between then and now. There are plenty of possibilities - new physics is one of them but it is one of several things that could be going on.

    Quote Originally Posted by PetTastic View Post
    There seems a wide range of Sigma numbers going around?
    Well, one per experiment or model generally. Each value has its own unique errors/biases which leads to a different spread of possible values for it.

    Quote Originally Posted by PetTastic View Post
    Are we near sigma 5?
    You'd need a meta-analysis of the results to be able to test the hypothesis that there is a discrepancy and I have not seen one (although I'll bet there is one out there if you search). What is usually reported is that we are seeing the derived values of the Hubble Constant are very unlikely to be the same for the two main methods. Interestingly last year another method has been tried that gives values bang in the middle of the main two.

    Quote Originally Posted by PetTastic View Post
    I don't understands the details of some of these measurement techniques but is there a trend between angular size based methods & luminosity methods?
    Seems to be correlated to distance. Values derived from techniques most sensitive to nearby measurements diverge from those from the early universe. The angular size methods are mostly early universe, the luminosity methods are mostly nearby.

    This article has a nice summary and breaks things down into near and far: https://www.skyandtelescope.com/astr...bble-constant/

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    Quote Originally Posted by PetTastic View Post
    How sure are we the problem is real?
    There seems a wide range of Sigma numbers going around?
    Are we near sigma 5?

    I don't understands the details of some of these measurement techniques but is there a trend between angular size based methods & luminosity methods?
    We don't know that the problem is real, but though it is within the outer error bars, it looks like there is some difference between H0 measured looking at galaxies over the last billion years, and H0 as derived from the CMBR. The distances measured more recently depend on the distance ladder, and some assumptions about variations in local density. Skipping the distance ladder and going straight to trigonometric parallax will determine the accuracy of the distance ladder, AND tell us how much variation there is from the (presumed linear) redshift equals distance rules we depend on. I don't think there is a huge amount of wiggle room in the methods we use, but there is some, and I don't think other methods will resolve it beyond dispute.
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    Quote Originally Posted by Shaula View Post

    This article has a nice summary and breaks things down into near and far: https://www.skyandtelescope.com/astr...bble-constant/
    Got a bit lost in the above.
    When looking a Cepheid periods does standard cosmology assume time dilation is directly proportional to redshift of spectral lines or is the relationship more complicated?
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    Quote Originally Posted by PetTastic View Post
    Got a bit lost in the above.
    When looking a Cepheid periods does standard cosmology assume time dilation is directly proportional to redshift of spectral lines or is the relationship more complicated?
    It assumes the standard velocity based time dilation. With velocity proportional to redshift. But these are nearby measurements so the time dilation effect is tiny compared to the uncertainty in the luminosity-period relationship. Some of the very furthest Cephid data is from Centarus A with a recessional velocity of 550 km/s. Which is a time dilation factor of 0.999998, much smaller than the uncertainty from other sources. And bear in mind this is an extreme, most of the data is from closer by.

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    At the other end of the distance ladder there seems a lot of tension over the idea that the CMBR is being modified by emissions from more local material.
    Evidence for the interstellar/intergalactic material seems to be slowly building from multiple space probes and observations.
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    Quote Originally Posted by PetTastic View Post
    At the other end of the distance ladder there seems a lot of tension over the idea that the CMBR is being modified by emissions from more local material.
    The only "tension" I've seen in this area is related to the complete lack of spectroscopic or other observational evidence for significant and uncorrected for modification of the CMBR.

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    Quote Originally Posted by Shaula View Post
    The only "tension" I've seen in this area is related to the complete lack of spectroscopic or other observational evidence for significant and uncorrected for modification of the CMBR.
    I would like to see how the CMB calculates the Hubble constant at 13, 12.5, 12, 11.5, 11, 10.5, 10 etc light years from here and the candles method every 500 million light years from here as well.
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    Quote Originally Posted by Copernicus View Post
    I would like to see how the CMB calculates the Hubble constant at 13, 12.5, 12, 11.5, 11, 10.5, 10 etc light years from here and the candles method every 500 million light years from here as well.
    Would you like a unicorn to go with that?

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    Quote Originally Posted by Shaula View Post
    Would you like a unicorn to go with that?
    I just don't think the Hubble constant appears constant at different distances. It may appear constant for awhile, but I think it will show a curve eventually.
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    Quote Originally Posted by Copernicus View Post
    I just don't think the Hubble constant appears constant at different distances. It may appear constant for awhile, but I think it will show a curve eventually.
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    Quote Originally Posted by Swift View Post
    Watch what you post in Q&A.
    Thanks!
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    I don't know which reference this article is referring to, but it may have some relevance.

    https://www.forbes.com/sites/startsw.../#19fe448d5380

    The Expanding Universe Might Not Depend On How You Measure It, But When
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    Quote Originally Posted by Copernicus View Post
    I would like to see how the CMB calculates the Hubble constant at 13, 12.5, 12, 11.5, 11, 10.5, 10 etc light years from here and the candles method every 500 million light years from here as well.
    The CMB was emitted 13.4 billion years ago and has been essentially unchanged since then. The changes have been a redshift to microwaves and some shifting due to passing through super voids and clusters. The cosmological modeling of the CMB giving Hubble constant is about the statistics of fluctuations that were established before the CMB was emitted. There will be no point in sending spacecraft light-years way and waiting thousands of years for results.

    The "candles method" is done for candles "every 500 million light years from here" - that is Hubble's law!

    Also the Hubble constant, H0, by definition is a local measurement. H0 is the value of the Hubble parameter (which does vary with time) at t=0, the current time.

    ETA: You cite The Expanding Universe Might Not Depend On How You Measure It, But When by Ethan Siegel. His article describes Testing low-redshift cosmic acceleration with large-scale structure by Nadathur, et.al.
    The paper gets H0 as 72.3 from BAO+voids ("galaxies and quasars that cluster in the nearby, late-time Universe, with no other measurements or assumptions factored in") at z < 2 and 69.0 when adding Lyman-alpha BAO at z = 2.34 ("However, adding in galaxies and quasars that cluster in the ultra-distant, early-time Universe drags the value way back down") gives H0 as 69.0.
    This is a probable example of how neglecting a factor can change the measurement of the Hubble constant. Previous analysis of large-scale structure gave 67.6, adding in voids raises this by 2.1%. The paper also says that if you look at only relatively nearby data then you get a higher rate of expansion than using the full data set.
    Last edited by Reality Check; 2020-Feb-26 at 09:00 PM.

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    Modelling of the CMBR and the surface of last scattering is highly dependant on the conditions at the end of cosmological inflation.
    One of the key inputs into the modelling of cosmological inflation is the Hubble parameter.
    Inflation models have been selected and tuned to give a CMBR that matches observation.

    How high is the risk of feedback in this process?
    Interesting number in interesting number out?
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    Quote Originally Posted by PetTastic View Post
    Modelling of the CMBR and the surface of last scattering is highly dependant on the conditions at the end of cosmological inflation.
    One of the key inputs into the modelling of cosmological inflation is the Hubble parameter.
    Inflation models have been selected and tuned to give a CMBR that matches observation.
    The inflationary epoch ended about 10-32 seconds and its effect on the CMBR emitted over 300,000 years later was the primordial fluctuations that grew into the CMBR fluctuations. Cosmologists select the kind of primordial fluctuations at the end of the inflationary epoch to use in their model.

    There is no danger of "feedback" because the Hubble constant is an output, not an input. For example Planck 2018 results. VI. Cosmological parameters. The analysis of the CMBR gives certain parameters. These are plugged into the Lambda-CDM model and the Hubble constant pops out. Note "Our baseline assumption is the CDM model with purely adiabatic scalar primordial perturbations with a power-law spectrum." in the Theoretical model section.

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    New measurements appear to add to the confusion, rather than clarifying: https://www.quantamagazine.org/new-w...isis-20200226/

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    Once again, the cosmos is what it is and does what it does, and it does not care if its attributes and the difficulties in measuring them create tension in the minds of us mortal human beings. Let's look at the bright side. It creates plenty of work for physicists and cosmologists.

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    This paper, https://lambda.gsfc.nasa.gov/educati...hubb_const.cfm , has distance ladder and inverse distance ladder. What does that mean?
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    Quote Originally Posted by Copernicus View Post
    This paper, https://lambda.gsfc.nasa.gov/educati...hubb_const.cfm , has distance ladder and inverse distance ladder. What does that mean?
    The 'inverse distance ladder' is just the main CMBR method we have been talking about. A study of the angular power spectrum of the CMBR plus at least one other parameter.

    Edit to add - I am not 100% sure why they call it an inverse distance ladder but I guess it is because instead of starting here and working out they start at the surface of last scattering and work forwards.
    Last edited by Shaula; 2020-Mar-01 at 08:57 AM.

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    Quote Originally Posted by Shaula View Post
    The 'inverse distance ladder' is just the main CMBR method we have been talking about. A study of the angular power spectrum of the CMBR plus at least one other parameter.

    Edit to add - I am not 100% sure why they call it an inverse distance ladder but I guess it is because instead of starting here and working out they start at the surface of last scattering and work forwards.
    That's exactly right.

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    Speaking of Hubble tension....


    https://arxiv.org/abs/2003.08387

    Interacting radiation after Planck and its implications for the Hubble Tension
    Nikita Blinov, Gustavo Marques-Tavares
    (Submitted on 17 Mar 2020)

    Standard cosmology predicts that prior to matter-radiation equality about 41% of the energy density was in free-streaming neutrinos. In many beyond Standard Model scenarios, however, the amount and free-streaming nature of this component is modified. For example, this occurs in models with new neutrino self-interactions or an additional dark sector with interacting light particles. We consider several extensions of the standard cosmology that include a non-free-streaming radiation component as motivated by such particle physics models and use the final Planck data release to constrain them. This release contains significant improvements in the polarization likelihood which plays an important role in distinguishing free-streaming from interacting radiation species. Fixing the total amount of energy in radiation to match the expectation from standard neutrino decoupling we find that the fraction of free-streaming radiation must be f fs >0.8 at 95% CL (combining temperature, polarization and baryon acoustic oscillation data). Allowing for arbitrary contributions of free-streaming and interacting radiation, the effective number of new non-free-streaming degrees of freedom is constrained to be N fld <0.6 at 95% CL. Cosmologies with additional radiation are also known to ease the discrepancy between the local measurement and CMB inference of the current expansion rate H0. We show that including a non-free-streaming radiation component allows for a larger amount of total energy density in radiation, leading to a mild improvement of the fit to cosmological data compared to previously discussed models with only a free-streaming component.
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    Quote Originally Posted by Roger E. Moore View Post
    Speaking of Hubble tension....


    https://arxiv.org/abs/2003.08387

    Interacting radiation after Planck and its implications for the Hubble Tension
    Nikita Blinov, Gustavo Marques-Tavares
    (Submitted on 17 Mar 2020)

    Standard cosmology predicts that prior to matter-radiation equality about 41% of the energy density was in free-streaming neutrinos...
    I don't claim to understand most of the above.
    Is this paper trying to adjust the early radiation side of "matter-radiation equality" in the opposite direction to papers about the formation of primordial supermassive black-holes?


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