Family dispute: do Type IIP supernova siblings agree on their distance?
A brief summary of the scientific motivation, the sibling-supernova test, and the main conclusions of this first-author paper.
Key result: Type II supernova distances derived with the tailored Expanding Photosphere Method are internally consistent at the level of a few percent. Independent supernovae exploding in the same host galaxy yield statistically compatible distances, providing a direct validation of the method for precision cosmology.
Motivation
In the era of the Hubble tension, it is no longer enough for a distance indicator to produce small error bars; those uncertainties must also be trustworthy. Type II supernovae analysed with the tailored Expanding Photosphere Method, or tailored EPM, have been shown to yield precise distances, but a key question remains: are these measurements also internally consistent when applied to different explosions with different physical properties?
The tailored EPM framework combines photometric measurements of the supernova flux with expansion velocities inferred from spectroscopy to estimate the angular size of the photosphere and, from that, the distance. Since this analysis relies on radiative-transfer modelling and assumptions about the supernova atmosphere, it is important to test whether the method gives the same answer when applied to different Type IIP supernovae residing at exactly the same distance.
Method
A particularly clean way to perform this test is to use supernova siblings: multiple supernovae that exploded in the same host galaxy. Whatever their individual explosion properties may be, they must share the same true distance. They therefore provide a stringent test of the internal consistency of any supernova-based distance estimator.
We searched the literature for sibling Type IIP supernovae with data of sufficient quality for tailored EPM analysis. After applying strict selection criteria, we identified four suitable pairs. Each supernova was then analysed independently, and the derived distances were compared within each host galaxy.
Figure 1. Example host galaxies containing multiple Type II supernovae. These sibling systems provide a direct and intuitive test of whether supernova-based distance measurements remain consistent across different explosions.
Main result
The comparison showed that the tailored EPM distances derived for sibling supernovae are generally consistent within their uncertainties. Despite differences in explosion parameters and observational coverage, the independently obtained distances agree at the level of a few percent within each galaxy.
Figure 2. Comparison of tailored EPM-derived distances for supernova siblings in the same host galaxies. The close agreement between independent measurements demonstrates the internal consistency of the method.
This result is important because it shows that the method is not only precise, but also internally consistent. Residual differences could be understood in terms of known sources of uncertainty, such as observational noise, limited temporal coverage, or modelling assumptions. At least within the four pairs analyzed, different Type IIP supernovae do not appear to introduce large hidden systematics when treated within the same framework.
The conclusion strengthens the case for Type II supernovae as an independent cosmological probe. They offer a complementary route to measuring extragalactic distances, one that does not rely on the same assumptions as more traditional rungs of the cosmic distance ladder. Demonstrating internal consistency is therefore a crucial step toward establishing their usefulness for precision cosmology.