dgruss23

2007-Jul-16, 02:31 PM

In this thread (http://www.bautforum.com/questions-answers/61643-astronomy-cosmology-science-tinkering-fiddling-cheating-4.html) the question was touched upon as to whether or not the Hubble constant (H0)could be as high as 84 km s-1 Mpc-1 rather than the currently preferred value of ~72 km s-1 Mpc-1 as determined by the Hubble Key Project (http://adsabs.harvard.edu/abs/2001ApJ...553...47F).

The purpose of this thread is to look at some of the reasons why it is still possible that the value of H0 could in fact be as large the mid 80's.

The HKP final report has been cited ~1100 times since being published in May 2001 so it is an extremely influential paper and important reason why most researchers have accepted H0=~72. This acceptance has been bolstered by the WMAP (http://arxiv.org/abs/astro-ph/0603449) results.

However, the extragalactic distance scale has numerous pieces (or rungs on the ladder) and there are a number of ways that the HKP final result could be incorrect. First it should be noted that the difference between H0=72 and H0=84 only requires a systematic 0.33 mag shift in the distance scale. For most distance indicators we're talking about a 1-2 sigma shift.

The HKP determined that H0=72 from 5 methods: The I-band Tully-Fisher relation (I-TFR -->spirals), surface brightness fluctuation method (SBF -->ellipticals - mostly), Fundamental plane (FP-->ellipticals), Type Ia Sn, Type II SN. The value of H0 was determined for each of these methods independently and then combined for a final value of H0. One of the reasons for the acceptance of their final result is that 5 methods were used.

One of the rungs underlying these distance methods is the Cepheid variable distance scale - which must be used to fix the zero point of the relations used for the 5 secondary distance indicators listed above.

The Cepheid distance scale is then one place where a systematic shift in the zero points of all 5 distance indicators could take place. Sandage has long argued for a lower value of H0 and recently recalibrated the Cepheid distance scale and concluded H0=62 (http://arxiv.org/abs/astro-ph/0603647). However, more recently van Leeuwen et al (http://arxiv.org/abs/0705.1592) showed problems with the Sandage et al Cepheid PL relation slope and also showed that the HKP Cepheid scale should be revised so that distances are closer and the value of H0 would then shift to 76.

Looking at the HKP final analysis reveals some other avenues for caution in accepting H0=72 as the final word:

One of the methods they used (the FP) actually gave a Hubble constant of 82.

Only 4 galaxies were used for the Type II SN H0 estimate and only 3 calibrators with Cepheid distances were available for calibration of the zero point.

Only 6 galaxies in 6 clusters were used for the SBF analysis - and the number of cepheid calibrators was the same size - 6.

While there were 36 Type Ia SN in the analysis, there were only 6 galaxies for calibrating the zero point.

The I-TFR distances tend to overestimate distances relative to other methods - including methods presented in their own paper for some clusters. For example, the FP distance to Abell 3574 (Table 9) is 51.6 Mpc while the I-TFR distance in Table 7 is 62.2 Mpc. The Centaurus 30 cluster I-TFR distance is 43.2 Mpc (Table 7) whereas a Cepheid distance to NGC 4603 in the same cluster is 33.3 Mpc and the SBF method from the large study of Tonry et al (2001) gives a distance of ~33 Mpc (same as the Cepheid distance). For Antlia the HKP I-TFR distance is 45.1 Mpc whereas the Tonry et al SBF distance is ~33 Mpc. Tully&Pierce (2000) (http://adsabs.harvard.edu/abs/2000ApJ...533..744T) found H0=77 from the I-band TFR, but they note that it might be more appropriate to use the maser distance to NGC 4258 to fix the zero point of the Cepheid distance scale rather than the traditionally used Large Magellanic Cloud distance. If the maser distance is used, then they would find H0=86 rather than 77. Using the maser distance would ripple through the distance indicators used by the HKP as well raising H0 above 80.

The purpose of this thread is to look at some of the reasons why it is still possible that the value of H0 could in fact be as large the mid 80's.

The HKP final report has been cited ~1100 times since being published in May 2001 so it is an extremely influential paper and important reason why most researchers have accepted H0=~72. This acceptance has been bolstered by the WMAP (http://arxiv.org/abs/astro-ph/0603449) results.

However, the extragalactic distance scale has numerous pieces (or rungs on the ladder) and there are a number of ways that the HKP final result could be incorrect. First it should be noted that the difference between H0=72 and H0=84 only requires a systematic 0.33 mag shift in the distance scale. For most distance indicators we're talking about a 1-2 sigma shift.

The HKP determined that H0=72 from 5 methods: The I-band Tully-Fisher relation (I-TFR -->spirals), surface brightness fluctuation method (SBF -->ellipticals - mostly), Fundamental plane (FP-->ellipticals), Type Ia Sn, Type II SN. The value of H0 was determined for each of these methods independently and then combined for a final value of H0. One of the reasons for the acceptance of their final result is that 5 methods were used.

One of the rungs underlying these distance methods is the Cepheid variable distance scale - which must be used to fix the zero point of the relations used for the 5 secondary distance indicators listed above.

The Cepheid distance scale is then one place where a systematic shift in the zero points of all 5 distance indicators could take place. Sandage has long argued for a lower value of H0 and recently recalibrated the Cepheid distance scale and concluded H0=62 (http://arxiv.org/abs/astro-ph/0603647). However, more recently van Leeuwen et al (http://arxiv.org/abs/0705.1592) showed problems with the Sandage et al Cepheid PL relation slope and also showed that the HKP Cepheid scale should be revised so that distances are closer and the value of H0 would then shift to 76.

Looking at the HKP final analysis reveals some other avenues for caution in accepting H0=72 as the final word:

One of the methods they used (the FP) actually gave a Hubble constant of 82.

Only 4 galaxies were used for the Type II SN H0 estimate and only 3 calibrators with Cepheid distances were available for calibration of the zero point.

Only 6 galaxies in 6 clusters were used for the SBF analysis - and the number of cepheid calibrators was the same size - 6.

While there were 36 Type Ia SN in the analysis, there were only 6 galaxies for calibrating the zero point.

The I-TFR distances tend to overestimate distances relative to other methods - including methods presented in their own paper for some clusters. For example, the FP distance to Abell 3574 (Table 9) is 51.6 Mpc while the I-TFR distance in Table 7 is 62.2 Mpc. The Centaurus 30 cluster I-TFR distance is 43.2 Mpc (Table 7) whereas a Cepheid distance to NGC 4603 in the same cluster is 33.3 Mpc and the SBF method from the large study of Tonry et al (2001) gives a distance of ~33 Mpc (same as the Cepheid distance). For Antlia the HKP I-TFR distance is 45.1 Mpc whereas the Tonry et al SBF distance is ~33 Mpc. Tully&Pierce (2000) (http://adsabs.harvard.edu/abs/2000ApJ...533..744T) found H0=77 from the I-band TFR, but they note that it might be more appropriate to use the maser distance to NGC 4258 to fix the zero point of the Cepheid distance scale rather than the traditionally used Large Magellanic Cloud distance. If the maser distance is used, then they would find H0=86 rather than 77. Using the maser distance would ripple through the distance indicators used by the HKP as well raising H0 above 80.