Over the next three years, a new processing pipeline will combine data from two major sky surveys—the ground-based Legacy Survey of Space and Time (LSST) and the space-based Euclid telescope—to track roughly 700 exploding stars in both optical and near-infrared light. This matters because Type Ia supernovae are standard candles used to measure cosmic distances and probe dark energy, the mysterious force driving the universe’s expansion. But a persistent puzzle—the “mass step,” where supernovae in high-mass galaxies appear intrinsically brighter than those in low-mass galaxies—introduces a systematic error that could masquerade as a dark energy signal. The near-infrared offers a dust-free view, so comparing optical and infrared brightnesses should reveal whether dust or progenitor physics causes the step. If successful, this project will produce a sample of supernovae ten times larger than any existing optical-infrared dataset. That will allow researchers to pin down the mass step’s origin, sharpening dark energy measurements. This is fundamental science: it refines our understanding of cosmic expansion without immediate practical applications, but past work on supernova cosmology has underpinned Nobel Prize-winning discoveries about the accelerating universe.
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This proposal aims to harness forthcoming optical and near-infrared (NIR) surveys to answer the most pressing questions within Type Ia Supernovae (SN Ia) cosmology. SNe Ia serve as crucial cosmological probes and can provide strong constraints on the properties of Dark Energy. However, one of the most critical limiting factors for current SN Ia cosmological measurements is our incomplete understanding of the SN Ia “mass step,” i.e., the observed difference in the intrinsic brightness of SNe Ia exploding in high-mass galaxies and low-mass galaxies. The origin of this feature could be attributed either to dust (different dust extinction in high and low-mass galaxies) or to progenitor physics (different progenitor scenarios in high and low-mass galaxies). In both cases, since the fraction of SNe Ia in high and low-mass galaxies evolves with redshift, any unaccounted-for brightness step could be mistakenly attributed to Dark Energy. The most promising way to unveil the origin of the mass step is to compare SNe Ia brightnesses in the optical and the near-infrared (NIR), as the NIR offers a unique "dust-free" window into the SN Ia population. In the next 12 months, two major optical and NIR imaging surveys will start taking data and, in their first months of observations, will discover and follow up more SNe than we have discovered in the last 20 years. These are the ground-based optical Legacy Survey of Space and Time (LSST) on the Vera Rubin Observatory and the space-based NIR survey on the Euclid Space Telescope (Euclid). The first goal of this Small Award is to develop the processing pipeline to obtain and calibrate Euclid’s NIR SN data. This will require adapting available difference imagining algorithms (usually optimised for optical ground-based facilities) and testing their accuracy in recovering SN fluxes using a “fake injection” approach. The second goal of the proposed project is to combine Euclid’s NIR SN data with the LSST optical data obtained from SNe exploding in the Euclid/LSST overlapping fields. According to our forecasts, by the beginning of the third year of this award, we will have a sample of approximately 700 SNe Ia with high-quality optical/NIR data (ten times larger than currently available optical/NIR SN samples). We will analyse this unique sample to extract preliminary constraints on the SN Ia mass step in the optical and NIR and determine whether observations confirm that dust extinction is the origin of this brightness difference.
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