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Thread: The Hubble Constant -Is it best derived from the CMB? Or by optical telescopes with distance ladders and quasars. etc.?

  1. #1 The Hubble Constant -Is it best derived from the CMB? Or by optical telescopes with distance ladders and quasars. etc.? 
    Forum Junior Double Helix's Avatar
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    A recent article in the BBC News science section (1) reminded me of the differences calculated for the Hubble Constant (2) based on the CMB vs. direct observations with optical telescopes. The value from the CMB, first obtained by ESA's Planck satellite, appears to be well accepted at around 67.4 km/s/Mpc. All reported values for the Hubble Constant "account" for the expansion rate of the universe, and this result is obtained from "early universe" techniques. Recent data from the Atacama Cosmology Telescope yields a similar value. However, data from other telescopes do not.

    From our perspective, there can be only one value, so seemingly reasonable measurements of the Hubble Constant which are of significant variance to those obtained from the CMB should draw some attention, and so they have.

    The conflicting values were calculated empirically using optical telescopes ("late universe" techniques) and are significantly different from the value obtained from the CMB. Some of these approaches depend on "calibrated distance ladders", and these results vary a little on which technique is employed. Combined, these data suggest a Hubble Constant of around 73-74 km/s/Mpc. These observations range from Cepheid variable stars, which defines one kind of distance ladder, and another approach using a distant quasar which does not rely on a distance ladder (3). Remarkably, observations of this quasar (doubly imaged with the Hubble Space Telescope and the Kecks) provides a Hubble Constant of ca. 72.5 km/s/Mpc.

    The BBC story goes on to suggest there may be a need for a significant revision in the Standard Model of Cosmology since the optical data indicate the universe is expanding faster than that obtained from the CMB. So which do we go with? The data from optical telescopes indicate a value of ca. 73-74 km/s/Mpc, approaching 10% faster than that obtained from the CMB. And this is not a trivial difference. It would seem to those who prefer direct observations that the optical data is more likely to offer the true value, and does suggest that a re-work of the Standard Model of Cosmology may be in order.

    Doubtless there are more than a few cosmologists who would not agree with any need for revisions.

    But what would their reasoning involve?

    And how would their position(s) defy the conflicting (yet largely consistent) values from optical observations?


    Hopefully, time will tell. It usually does.



    1. https://www.bbc.com/future/article/20210326-the-mystery-of-our-expanding-universe



    2. https://en.wikipedia.org/wiki/Hubble_Constant



    "H0LiCOW - IX. Cosmographic analysis of the doubly imaged quasar SDSS 1206+4332 and a new measurement of the Hubble constant"

    3. https://ui.adsabs.harvard.edu/abs/20...4726B/abstract


    Last edited by Double Helix; April 5th, 2021 at 08:24 PM.
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  3. #2  
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    Hubble is a very good telescope, it has brought an incredible amount of discoveries in space exploration. And how upset and scared everyone was when he stopped sending photographs to Earth, but, fortunately, had already resumed work. Unbelievable, since it has already turned 30 years old since its launch.
    There is now an ambitious satellite imager manufacturer that is inventing space cameras that innovate and help take research to the next level.
    I believe technology is the future.


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