Luminosity Evolution in Type 1a Supernovae?

Figure 1 of Kang et al.

During this afternoon’s very exciting Meeting of the Faculty of Science and Engineering at Maynooth University I suddenly remembered a paper I became aware of over Christmas but then forgot about. There’s an article here describing the paper that makes some pretty strong claims, which was what alerted me to it. The actual paper, by Kang et al., which has apparently been refereed and accepted for publication by the Astrophysical Journal, can be found on the arXiv here. The abstract reads:

The most direct and strongest evidence for the presence of dark energy is provided by the measurement of galaxy distances using type Ia supernovae (SNe Ia). This result is based on the assumption that the corrected brightness of SN Ia through the empirical standardization would not evolve with look-back time. Recent studies have shown, however, that the standardized brightness of SN Ia is correlated with host morphology, host mass, and local star formation rate, suggesting a possible correlation with stellar population property. In order to understand the origin of these correlations, we have continued our spectroscopic observations to cover most of the reported nearby early-type host galaxies. From high-quality (signal-to-noise ratio ~175) spectra, we obtained the most direct and reliable estimates of population age and metallicity for these host galaxies. We find a significant correlation between SN luminosity (after the standardization) and stellar population age at a 99.5% confidence level. As such, this is the most direct and stringent test ever made for the luminosity evolution of SN Ia. Based on this result, we further show that the previously reported correlations with host morphology, host mass, and local star formation rate are most likely originated from the difference in population age. This indicates that the light-curve fitters used by the SNe Ia community are not quite capable of correcting for the population age effect, which would inevitably cause a serious systematic bias with look-back time. Notably, taken at face values, a significant fraction of the Hubble residual used in the discovery of the dark energy appears to be affected by the luminosity evolution. We argue, therefore, that this systematic bias must be considered in detail in SN cosmology before proceeding to the details of the dark energy.

Of course evidence for significant luminosity evolution of Type Ia supernovae would throw a big spanner in the works involved in using these objects to probe cosmology (specifically dark energy), but having skimmed the paper I’m a bit skeptical about the results, largely because they seem to use only a very small number of supernovae to reach their conclusions and I’m not convinced about selection effects. I have an open mind, though, so I’d be very interested to hear through the comments box the views of any experts in this field.


3 Responses to “Luminosity Evolution in Type 1a Supernovae?”

  1. Is it true that dark energy is firmly established if we completely threw out the supernovae data? (And also without adopting strong assumptions like flatness)

    I thought the various cosmological probes carved out orthogonal slices through parameter space so that we are left with a highly constrained set of parameters. If we erase the supernova contours will we still have a posterior on Omega_Lambda that is concentrated above zero? Can you point us to any papers/plots?

  2. Peter and others: still waiting for comments on

  3. It has been obvious for decades that an evolution of the zeropoint of SN brightness vs decline rate could mimic the effect of an accelerating Universe – see astro-ph/0201196 for example. But this excuse for the SNe data is no longer interesting. Back in 2003 the CMB data allowed any model along a geometric degeneracy line in the Omega_M,Omega_Lambda plane from the flat, accelerating LambdaCDM model with H~70 all the way to a super Sandage model with no dark energy, Omega_tot ~ 1.3, and H~30. But the current CMB data alone cut off the degeneracy and require enough dark energy to make the Universe accelerate. The combination of the BAO data with CMB data favors a flat accelerating model. As Phillip Helbig said, flat LambdaCDM is called the concordance model because it fits all the data, so no one dataset is essential.

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