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Different Hubble constants
What are the astrophysical implications
of different Hubble constants by Planck https://arxiv.org/abs/1502.01589 Hubble constant 67.8 +/- .9 km/s/Mpc and by H0LiCOW collaboration http://shsuyu.github.io/H0LiCOW/site/ Hubble constant 71.9 +2.4 -3.0 km/s/Mpc ? Richard D Saam |
#2
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Different Hubble constants
In article , "Richard D.
Saam" writes: What are the astrophysical implications of different Hubble constants by Planck https://arxiv.org/abs/1502.01589 Hubble constant 67.8 +/- .9 km/s/Mpc and by H0LiCOW collaboration http://shsuyu.github.io/H0LiCOW/site/ Hubble constant 71.9 +2.4 -3.0 km/s/Mpc ? First, progress. For decades, people worried about a factor of two or more between different measurements of the Hubble constant. Now, we are talking about 5 per cent. Also, any "tension" assumes that the error bars are correct. We used to have 100+/-10 and 50+/-7 or whatever. (In these cases, not only were the error bars too large, but the measurements themselves were wrong.) If you have one measurement, you know the value. If you have two different measurements, you don't. :-) So we need a third, independent, accurate measurement of comparable precision. Much has been written about this. Just google "tension Hubble constant Planck" (without the quotes). Assuming that the tension is real, there are various scenarios which could account for it, most of which don't call the Big Bang into question. While it is good to explore these, especially given the history of the Hubble constant (first estimates were around 600), it is probably more productive to wait until the tension has been confirmed or ruled out. [[Mod. note -- An old mariner's saying (which supports Phillip Helbig's point): "Never go to sea with two chronometers: take either one or three". -- jt]] |
#3
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Different Hubble constants
On 1/28/17 11:44 AM, Phillip Helbig (undress to reply) wrote:
In article , "Richard D. Saam" writes: What are the astrophysical implications of different Hubble constants by Planck https://arxiv.org/abs/1502.01589 Hubble constant 67.8 +/- .9 km/s/Mpc and by H0LiCOW collaboration http://shsuyu.github.io/H0LiCOW/site/ Hubble constant 71.9 +2.4 -3.0 km/s/Mpc ? First, progress. For decades, people worried about a factor of two or more between different measurements of the Hubble constant. Now, we are talking about 5 per cent. Also, any "tension" assumes that the error bars are correct. We used to have 100+/-10 and 50+/-7 or whatever. (In these cases, not only were the error bars too large, but the measurements themselves were wrong.) If you have one measurement, you know the value. If you have two different measurements, you don't. So we need a third, independent, accurate measurement of comparable precision. Cepheids and Supernovae data provide fourth and fifth independent measurements. I did not properly present the H0LiCOW collaboration data: "Since the value measured from the SH0ES project using Cepheids and Supernovae is completely independent of the H0LiCOW value, both can be combined into a single measurement of the Hubble Constant in the Local Universe. This new value of H0 = 72.5 +/- 1.4 km/s/Mpc is 3.1 sigma higher than the most recent measurement of Planck, where H0 = 67.8 +/- 0.9 km/s/Mpc." So the question should be framed as much more significant H0 difference (3.1 sigma): What are the astrophysical implications of different Hubble constants by Planck https://arxiv.org/abs/1502.01589 Hubble constant 67.8 +/- .9 km/s/Mpc and by H0LiCOW collaboration and SH0ES project http://shsuyu.github.io/H0LiCOW/site/ Hubble constant 72.5 +/- 1.4 km/s/Mpc ? Much has been written about this. Just google "tension Hubble constant Planck" (without the quotes). Yes, much discussion about dark energy dark matter densities. Assuming that the tension is real, there are various scenarios which could account for it For discussion, point out one. most of which don't call the Big Bang into question. Agreed While it is good to explore these, especially given the history of the Hubble constant (first estimates were around 600), The measured value has been around 75 +/- 10 km/s/Mpc for the last 20 years. it is probably more productive to wait until the tension has been confirmed or ruled out. It would appear timing for discussion is here. [[Mod. note -- An old mariner's saying (which supports Phillip Helbig's point): "Never go to sea with two chronometers: take either one or three". -- jt]] As an old Navy navigator before GPS, we had the luxury of NIST WWV Fort Collins, Colorado -Greenwich Mean Time. A 3 star (with 120 degree Hour Angle separation) fix was the standard. Daylight sun and moon 2 LOP fixes were problematic. The interfering parameters were clouds and a non stable observing platform for the sextant due to high sea state for surface ships and air turbulence for aircraft resulting in positions a few nautical miles in breadth which were adequate in the vast ocean expanse. (present ~cm GPS positions would have been deemed unnecessarily accurate) RDS |
#4
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Different Hubble constants
In article , "Richard D.
Saam" writes: If you have one measurement, you know the value. If you have two different measurements, you don't. So we need a third, independent, accurate measurement of comparable precision. Cepheids and Supernovae data provide fourth and fifth independent measurements. I did not properly present the H0LiCOW collaboration data: "Since the value measured from the SH0ES project using Cepheids and Supernovae is completely independent of the H0LiCOW value, both can be combined into a single measurement of the Hubble Constant in the Local Universe. This new value of H0 = 72.5 +/- 1.4 km/s/Mpc is 3.1 sigma higher than the most recent measurement of Planck, where H0 = 67.8 +/- 0.9 km/s/Mpc." So the question should be framed as much more significant H0 difference (3.1 sigma): What are the astrophysical implications of different Hubble constants by Planck https://arxiv.org/abs/1502.01589 Hubble constant 67.8 +/- .9 km/s/Mpc and by H0LiCOW collaboration and SH0ES project http://shsuyu.github.io/H0LiCOW/site/ Hubble constant 72.5 +/- 1.4 km/s/Mpc ? The main point is that the PLANCK measurement involves much higher redshifts than the other three, which are, by comparison, "local". Cepheids and supernovae are just different types of standard candles. H0LiCOW measures the Hubble constant via gravitational-lens time delays, and as such is independent (not based on the distance-ladder approach), but still in roughly the same redshift range. The fact that all of the non-Planck approaches agree is really amazing. The real question is whether there is any real tension between Planck and "local" measurements. Of course, all tension goes away if the error bars have been underestimated. Since the local and PLANCK approaches don't agree, at least one MUST have error bars which are underestimated, or there is some systematic effect. Again, considering the history of the Hubble constant, it probably makes sense to wait and see if the tension is real before trying to come up with something which would make the Hubble constant depend SLIGHTLY on the redshift range in which it is measured. WMAP was also a CMB satellite and it measured a higher value for the Hubble constant. We shouldn't just forget that, especially since PLANCK is more recent and perhaps all systematic effects are not yet understood (there are other indications that there might be systematic effects in the PLANCK data). |
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Different Hubble constants
In article ,
"Phillip Helbig (undress to reply)" writes: The real question is whether there is any real tension between Planck and "local" measurements. I agree that the most likely reason is some small systematic error. it probably makes sense to wait and see if the tension is real before trying to come up with something which would make the Hubble constant depend SLIGHTLY on the redshift range in which it is measured. While I also agree that coming up with possible explanations isn't worth a major effort, I think a bit of speculation might not be a bad idea. It might suggest independent avenues of research to see whether there might be evidence for some hypothesis or other. I'm not enough of an expert to have any real idea what might cause discrepancies between the local and CMB values of the Hubble constant. I am _guessing_ a "hiccup" in the expansion history might do it, but I have no idea what might cause such a hiccup nor how one might test that. Any other ideas? -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
#6
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Different Hubble constants
In article , Steve Willner
writes: The real question is whether there is any real tension between Planck and "local" measurements. I agree that the most likely reason is some small systematic error. Especially since there are some other indications that there might be systematic errors in one of the PLANCK frequency bands. While I also agree that coming up with possible explanations isn't worth a major effort, I think a bit of speculation might not be a bad idea. It might suggest independent avenues of research to see whether there might be evidence for some hypothesis or other. I'm not enough of an expert to have any real idea what might cause discrepancies between the local and CMB values of the Hubble constant. I am _guessing_ a "hiccup" in the expansion history might do it, but I have no idea what might cause such a hiccup nor how one might test that. Any other ideas? The most common idea is an appreciable departure from homogeneity. Of course, the universe is not homogeneous, but it is to a good approximation. However, if we were near (otherwise there would be a dependence on direction, which is not observed) the centre of a region with a different density than the universal average, it could affect the Hubble constant measured within that region, compared to the value measured on a larger scale. On the one hand over- or underdensities can affect various distance measures. On the other hand, the density contrast might be large enough that it affects the rate of expansion (so-called backreaction). In the first case, the Hubble constant is not really different; rather, the model to calculate the observed brightness knowing the absolute brightness and the redshift is too simple. In the second case, it is really different. |
#7
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Different Hubble constants
On 03/02/2017 23:06, Steve Willner wrote:
In article , "Phillip Helbig (undress to reply)" writes: The real question is whether there is any real tension between Planck and "local" measurements. I agree that the most likely reason is some small systematic error. Indeed. The speed of light in vacuum determination as a function of time with error bars is a salutary lesson in this regard. I remember the graph from an introductory relativity textbook in our library. The gist of the problem was that a very famous experimentalist made an error in the sign of the correction for imperfect vacuum and everyone afterwards made exactly the same mistake. It was only when a new method with even greater precision and radically different systematic errors came onstream that the problem was uncovered. BTW Anyone recognise the book title from this description? it probably makes sense to wait and see if the tension is real before trying to come up with something which would make the Hubble constant depend SLIGHTLY on the redshift range in which it is measured. While I also agree that coming up with possible explanations isn't worth a major effort, I think a bit of speculation might not be a bad idea. It might suggest independent avenues of research to see whether there might be evidence for some hypothesis or other. I'm not enough of an expert to have any real idea what might cause discrepancies between the local and CMB values of the Hubble constant. I am _guessing_ a "hiccup" in the expansion history might do it, but I have no idea what might cause such a hiccup nor how one might test that. Any other ideas? If the universe is accelerating with time due to dark energy then the Hubble constant might be expected to vary slightly with increasing Z. But I suspect by amounts much smaller than the experimental error bars. -- Regards, Martin Brown |
#8
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Different Hubble constants
In article , Martin Brown
writes: On 03/02/2017 23:06, Steve Willner wrote: In article , "Phillip Helbig (undress to reply)" = writes: The real question is whether there is any real tension between Planck and "local" measurements. I agree that the most likely reason is some small systematic error. Indeed. The speed of light in vacuum determination as a function of time= with error bars is a salutary lesson in this regard. I remember the graph from an introductory relativity textbook in our library. The gist of the problem was that a very famous experimentalist made an error in the sign of the correction for imperfect vacuum and everyone afterwards made exactly the same mistake. It was only when a new method with even greater precision and radically different systematic errors came onstream that the problem was uncovered. OK. I only parsed it correctly on second reading. This is an example where similar "tension" was resolved when a systematic error was corrected for. (You don't mean that a variable speed of light in vacuum could be the cause of the current tension with regard to the Hubble constant.) it probably makes sense to wait and see if the tension is real before trying to come up with something which would make the Hubble constant depend SLIGHTLY on the redshift range in which it is measured. While I also agree that coming up with possible explanations isn't worth a major effort, I think a bit of speculation might not be a bad idea. It might suggest independent avenues of research to see whether there might be evidence for some hypothesis or other. I'm not enough of an expert to have any real idea what might cause discrepancies between the local and CMB values of the Hubble constant. I am _guessing_ a "hiccup" in the expansion history might do it, but I have no idea what might cause such a hiccup nor how one might test that. Any other ideas? If the universe is accelerating with time due to dark energy then the Hubble constant might be expected to vary slightly with increasing Z. But I suspect by amounts much smaller than the experimental error bars. This is a red herring. In general, the Hubble constant is not constant in time. (It is called the Hubble constant because it is a constant like "a" in the equation y = ax +b, not because it is constant in time.) It varies quite dramatically. At the big bang, it is infinite. It decreases at first, then can increase again, in some universes (such as hours) asymptotically approaching a constant value. This is well known and is always taken into account. The tension in measurements we are talking about is tension in measurements of H_0, which is the current value. Even if the objects involved are at high redshift, it is still the current value which is measured. (Even at low redshift, the Hubble constant is not "directly" measured.) |
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