The speed with which supernovae are moving away from us has alerted scientists to the existence of dark energy, one of the most important discoveries in decades. A new analysis of 1,048 of these powerful explosions hints at something even stranger, causing inexplicable changes in the rate of expansion of the universe. The astronomers who made this announcement emphasize that their work is not yet definitive – it may be an artifact of an instrumental error or deviation in sampling. However, if the model is real, it means that our understanding of physics misses something big.
Type Ia supernovae are especially valued by astronomers because of their consistency in the amount of light they emit. By comparing the apparent brightness of a supernova with our estimate of its true luminosity, we can estimate its distance. The speed at which the supernova galaxy moves away from us can be estimated by the redshift of its light. Combined, they allow us to calculate the “Hubble constant”
Attempts to measure H, however0 have encountered different values obtained by different methods. The supernovae failed to resolve the issue, perhaps because the samples used were too small or biased in some way. Dr. Maria Dainotti of the National Astronomical Observatory of Japan set out to study with the help of the so-called Pantheon sample of 1,048 supernovae.
Dainotti and co-authors conclude in The Astrophysical Journal (preprint on ArXiv.org) that this sample is consistent with H0 as it grew with the aging of the universe. If true, we will have to fundamentally rewrite physics – probably by adding some currently unknown forces – to explain this.
Dainotti’s large sample size does not fit well with the change of idea in H0 are the product of insufficient data. However, it is not yet ready to abolish the standard model of cosmological physics. The paper acknowledges that there would be a much less dramatic explanation for these results if there was a bias towards the supernovae we find. No matter how large your sample is, it will create a distorted picture if it is unrepresentative. At the beginning of the universe, the stars had less metal in them, and it is possible that type Ia supernovae from metal-poor stars are not as bright, causing us to miss some and calculate the distance to those we see.
As a result, efforts to find weaker supernovae that we can currently ignore continue, as do efforts to control the composition of host galaxies. It is possible, perhaps even more likely, that more powerful telescopes will give a better sample, which will make this discrepancy disappear.
Nevertheless, Dainotti’s data adds to the suspicion that something very big is happening in the universe that has not yet occurred to us. Given recent evidence that new physics may be needed on a very small scale, the laws of science seem far from finalizing.
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