Determine whether near-ultraviolet spectroscopy reduces Type Ia supernova distance scatter

Determine whether adding rest-frame near-ultraviolet photometry and iron-group blanketing diagnostics to Type Ia supernova standardization reduces the Hubble-residual scatter from approximately 0.12 mag toward 0.08 mag and mitigates redshift-dependent population biases.

Background

The paper argues that optically similar Type Ia supernovae can differ in rest-frame near-ultraviolet flux because of differences in progenitor metallicity and iron-group-element distributions. It proposes using near-ultraviolet color and an iron-group blanketing parameter as additional standardization variables.

The anticipated reduction in distance scatter is motivated by existing optical-twin observations, but the relevant high-redshift spectroscopic evidence does not yet exist in sufficient quantity to establish the improvement empirically.

References

No existing Hubble-flow sample has enough rest-frame NUV spectra to demonstrate that reduction. The true improvement may be smaller if several unmodelled variables add independent residual scatter.

— 3.5-meter Segmented-Mirror Robotic Space Telescope Mission White Paper II. Key Scientific Mission: Wide-Field Cosmology and Galaxy Evolution  (2609.02574 - Kim et al., 2 Sep 2026) in Section 3.1.3, “Optical Twins and Near-Ultraviolet Divergence”