Our ZTF SNe Ia cosmology working group performed an analysis of ZTF SN Ia DR2, the SNe Ia discovered by ZTF from 2018 to 2020, the largest low-redshift sample of SNe Ia of all flavors, a work that resulted in a Special Issue at Astronomy & Astrophysics. I led the analysis of their observed properties and their relative rates, combining photometric, spectroscopic and host galaxy information to map the diversity of the thermonuclear supernova population.
A long-standing puzzle in the study of thermonuclear supernovae is the existence of so-called Super-Chandrasekhar Type Ia supernovae, whose extreme luminosities imply ejecta masses that exceed the canonical Chandrasekhar limit for a non-rotating white dwarf. These events are characterised by unusually slow light-curve evolution, high peak luminosities, and strong carbon signatures in their early-time spectra, challenging standard explosion models. Through detailed photometric and spectroscopic analyses of SN 2020esm and SN 2021zny, we proposed that events are mergers of two carbon oxygen white dwarfs, with additional studies with JWST of SN 2022pul confirming our results.
Thanks to the Kepler Space Telescope’s K2 Supernova Cosmology Experiment, one of the telescope’s final missions before it ran out of fuel, we were fortunate to observe SN 2018oh in exceptional detail, combining extremely high-cadence Kepler observations with dedicated ground-based follow-up campaigns. SN 2018oh exhibited a small bump in its light curve during the first ~5 days after explosion — an early flux excess superimposed on the radioactively powered luminosity typical of normal Type Ia supernovae. Interpretations of this behaviour range from the interaction of the supernova ejecta with a non-degenerate companion star to the detonation of a helium shell on the surface of the exploding white dwarf prior to the main explosion. Today, a variety of early light-curve bumps are routinely observed in Type Ia supernovae, highlighting the growing diversity of thermonuclear explosion pathways.
The Legacy Survey of Space and Time (LSST) of the Vera C. Rubin Observatory will usher in a new era of time-domain astronomy by repeatedly imaging the southern sky with unprecedented depth, cadence, and uniformity. By discovering and monitoring millions of supernovae and other transients over a wide range of timescales and redshifts, LSST will open vast new discovery space for rare, extreme, and previously unknown classes of variable phenomena. Its scale and homogeneity will enable population-level studies that move beyond individual events, allowing systematic mapping of the diversity and evolution of stellar explosions, accretion-driven transients, and variable sources across cosmic history. In parallel, the cosmological exploitation of LSST data is being led by the Dark Energy Science Collaboration (DESC), which will deliver precision measurements of cosmic expansion and structure growth, providing transformative constraints on dark energy and fundamental physics.
The 4-metre Multi-Object Spectroscopic Telescope (4MOST) is a next-generation wide-field spectroscopic facility designed to provide massively multiplexed follow-up of imaging surveys. Within 4MOST, the Time-Domain Extragalactic Survey (TiDES) will deliver systematic spectroscopic classification and characterisation of transient and variable sources from the Legacy Survey of Space and Time (LSST) and other surveys, which will define the discovery space for southern-hemisphere time-domain astronomy. By obtaining large, homogeneous samples of supernova and transient spectra across a wide range of luminosities and redshifts, TiDES will enable population-level studies of explosive phenomena and their host galaxies. In parallel, TiDES will provide a critical spectroscopic foundation for SNe Ia cosmology at the LSST era, supporting precision measurements of the cosmic expansion.
The Zwicky Transient Facility (ZTF) has reshaped time-domain astronomy by providing high-cadence, wide-field optical monitoring of the northern sky and unprecedented access to the dynamic Universe. Its ability to discover large numbers of transients shortly after explosion has enabled transformative studies of the earliest phases of supernovae, revealed the prevalence of early-time light-curve excesses, and uncovered new classes of rare and fast-evolving transients. Beyond stellar explosions, ZTF has dramatically expanded samples of tidal disruption events, changing-look active galactic nuclei, and electromagnetic counterparts to gravitational-wave sources, linking optical variability to compact-object physics and black-hole accretion. At the same time, ZTF is delivering the most comprehensive and well-controlled low-redshift Type Ia supernova sample to date, providing a critical foundation for precision cosmology and anchoring distance measurements used by higher-redshift surveys.
