# Supernova calibration

1. Aug 28, 2012

### Chronos

A new paper on supernova 1a distance calibration 'The luminosity of supernovae of type Ia from TRGB distances and the value of H_0' - http://arxiv.org/abs/1208.5054 - brings into question the value of Ho. The authors derive a value of H0 = 63.7, considerably below the present accepted value of 73.8 derived from WMAP 7. This value is consistent with that derived by Beutler, et al [67.0] based on the 6dF survey - re: http://arxiv.org/abs/1106.3366. A lower value for Ho, of course, has far reaching implications. The fallout should be interesting.

2. Aug 29, 2012

### Chalnoth

I wouldn't trust a result of $H_0$ measured from supernovae as far as I could throw it.

First, $H_0$ is perfectly degenerate with the intrinsic brightness of the supernovae. All you need to do is slightly underestimate how bright supernovae are, and you'll get a lower inferred expansion rate.

Second, the curvature parameter is also highly (though not perfectly) degenerate with both $H_0$ and the intrinsic brightness. So if the universe has a little bit of spatial curvature, then it could easily cause us to get $H_0$ wrong.

There are some good observations for determining the value of $H_0$ to high accuracy, but I would not ever trust a study that relied on supernova measurements for this purpose.

As for this particular study, it looks like they attempt to obtain an accurate estimate for the brightness of six supernovae, and then extrapolate that brightness to 62 other supernovae. This seems, to me, to be a fool's errand. Individual supernovae vary a good amount in brightness. It's very, very easy for me to see these six supernovae just happening to being less bright than most supernovae by a few percent, which would screw up the whole measurement.

3. Aug 29, 2012

### Chronos

My read is the paper was raising doubts on Cepheid luminosity models, which appears well motivated in light of the earlier paper by Beutler. I do, however, share your distrust in assuming consistent intrinsic luminosity of SN1a's.

4. Aug 29, 2012

### Chalnoth

Well, the thing is, you can estimate $H_0$ without using the Cepheid luminosity at all. The combination of WMAP, BAO, and supernova data, for example, constrains $H_0$ to a high degree without any such assumptions.