Periodic Reporting for period 4 - USNAC (Understanding Type Ia SuperNovae for Accurate Cosmology)
Reporting period: 2022-09-01 to 2024-08-31
For 100 years, following the work of E. Hubble and G. Lemaître, we know that The Universe is expanding, and for 20 years we now know that this expansion is accelerating under the influence of a mysterious form of energy called Dark Energy (Nobel Price 2011: S. Perlmutter, A. Riess and B. Schmidt). To measure the expansion history of the Universe and thereby understand the fundamental physics of its constituents, we are using Type Ia Supernova. This astrophysical event, result of the thermonuclear explosion of a white dwarf, reaches almost always the same luminosity at its pick brightness. We are thus able to derive their distance from the observation of their flux. And by combining this distance information with the apparent radial velocity of their host galaxy, aka their redshift, we are able to reconstruct the expansion history of the universe and probe the dark energy properties.
Today, the state-of-the-art measurement of the dark energy equation of state parameters is measured at the 5% level compatible with the expected value from the standard model of cosmology where the dark energy is a simple fundamental constant in Einstein’s equation. However, the current expansion rate, aka, the Hubble Constant H0, directly measured by Type Ia Supernova is incompatible with that predicted by this standard model. This rises the question of a the existence of new fundamental physics, including a change of Einstein’s theory of gravity.
The goal of the next generation of surveys (LSST, Euclid, Roman) is to increase by at least 10 the precision in the measurements of the dark energy properties as many alternative model of cosmology deviate from the standard model at this level of precision. Solving the question of the current expansion rate is also one of the most active subject of research in the cosmological community for the last few years. In USNAC we are focusing on one of the most puzzling problem related of observational Cosmology : the fact that we still largely ignore how and why the white dwarf explode into a Type Ia Supernova and most importantly, how this ignorance is affecting the derivation of the cosmological parameters. In fact, “astrophysical biases” in Supernova-Cosmology are currently limiting further progress on the derivation of the properties of dark energy and could be the root cause of the observed discrepancy in the measurements of H0.
Today, the state-of-the-art measurement of the dark energy equation of state parameters is measured at the 5% level compatible with the expected value from the standard model of cosmology where the dark energy is a simple fundamental constant in Einstein’s equation. However, the current expansion rate, aka, the Hubble Constant H0, directly measured by Type Ia Supernova is incompatible with that predicted by this standard model. This rises the question of a the existence of new fundamental physics, including a change of Einstein’s theory of gravity.
The goal of the next generation of surveys (LSST, Euclid, Roman) is to increase by at least 10 the precision in the measurements of the dark energy properties as many alternative model of cosmology deviate from the standard model at this level of precision. Solving the question of the current expansion rate is also one of the most active subject of research in the cosmological community for the last few years. In USNAC we are focusing on one of the most puzzling problem related of observational Cosmology : the fact that we still largely ignore how and why the white dwarf explode into a Type Ia Supernova and most importantly, how this ignorance is affecting the derivation of the cosmological parameters. In fact, “astrophysical biases” in Supernova-Cosmology are currently limiting further progress on the derivation of the properties of dark energy and could be the root cause of the observed discrepancy in the measurements of H0.
We our initial work started in march 2018 by focussing on: building a model for the SN variabilities and securing SN Ia data to the second phase of the program.
For the the latter, we worked within the Zwicky Transient facility collaboration where we contributed to the definition and the management of the Cosmology program. Our own focus were to secure the Supernovae spectroscopic classification needed to ensure that we do, indeed collect these of "Type Ia" for our scientific program. We published the first fully automated spectroscopic pipeline in Rigault et al. 2019, that process all ZTF spectrocopic data since summer 2018. This pipeline where improved in Kim et al. 2022, and the exact SN flux extraction from the spectroscopic signal accounting from host galaxy contamination were presented in Lezmy et al. 2022. This code provided ~3000 Type Ia Supernovae classification for the USNAC program.
For the former, we presented in Rigault et al 2020, the first scientifically motivated connection between SN properties and that of their environment, demonstrating the importance of the progenitor age. In that publication, we did several predictions on how SN Ia should vary as a function of redshift and environments. We later tested and validated the redshift prediction in Nicolas et al. 2021, and the environmental variations in Briday et al. 2022. This major milestone formed the "SN Ia age model" now highly discussed in the literature.
In that first period we also worked on planning the next step for cosmology using ZTF SN Ia, presenting notably the first case of a pure-SN Ia test of the laws of gravity using SN Ia peculiar motion (Graziani et al. 2020).
In 2021, we started the second period of the ERC Grant: that exploiting the ZTF data. This long effort was rewarded by the publication of a special issue in Astronomy and Astrophysics of 21 papers, nearly half of which led by USNAC members (Rigault ; 2-papers, Ginolin 2-papers, Amenouche 1-paper, Smith 1-papers, Popovic 1-papers, Ruppin 1-papers). These papers were submitted right before the end of the ERC (summer 2024) and officially published early 2025. We led the release paper (Rigault et al. 2025) that provide to the community the first single instrument multi-thousand SN Ia dataset, 10x what other survey did before. Throughout these papers, we confirmed the SN Ia age-model and highlighted other important discovery, such as notably, the non-linearity of the 3 decade old brighter-slower relations in SN cosmology (Ginolin et al 2025a). We also clearly demonstrated the two-fold origin of the SN color (Popovic et al. 2025, Ginolin et al 2025b).
Impact for cosmology are discussed in these papers but actual cosmological parameters measurements are now started, as planned in the original ERC program.
For the the latter, we worked within the Zwicky Transient facility collaboration where we contributed to the definition and the management of the Cosmology program. Our own focus were to secure the Supernovae spectroscopic classification needed to ensure that we do, indeed collect these of "Type Ia" for our scientific program. We published the first fully automated spectroscopic pipeline in Rigault et al. 2019, that process all ZTF spectrocopic data since summer 2018. This pipeline where improved in Kim et al. 2022, and the exact SN flux extraction from the spectroscopic signal accounting from host galaxy contamination were presented in Lezmy et al. 2022. This code provided ~3000 Type Ia Supernovae classification for the USNAC program.
For the former, we presented in Rigault et al 2020, the first scientifically motivated connection between SN properties and that of their environment, demonstrating the importance of the progenitor age. In that publication, we did several predictions on how SN Ia should vary as a function of redshift and environments. We later tested and validated the redshift prediction in Nicolas et al. 2021, and the environmental variations in Briday et al. 2022. This major milestone formed the "SN Ia age model" now highly discussed in the literature.
In that first period we also worked on planning the next step for cosmology using ZTF SN Ia, presenting notably the first case of a pure-SN Ia test of the laws of gravity using SN Ia peculiar motion (Graziani et al. 2020).
In 2021, we started the second period of the ERC Grant: that exploiting the ZTF data. This long effort was rewarded by the publication of a special issue in Astronomy and Astrophysics of 21 papers, nearly half of which led by USNAC members (Rigault ; 2-papers, Ginolin 2-papers, Amenouche 1-paper, Smith 1-papers, Popovic 1-papers, Ruppin 1-papers). These papers were submitted right before the end of the ERC (summer 2024) and officially published early 2025. We led the release paper (Rigault et al. 2025) that provide to the community the first single instrument multi-thousand SN Ia dataset, 10x what other survey did before. Throughout these papers, we confirmed the SN Ia age-model and highlighted other important discovery, such as notably, the non-linearity of the 3 decade old brighter-slower relations in SN cosmology (Ginolin et al 2025a). We also clearly demonstrated the two-fold origin of the SN color (Popovic et al. 2025, Ginolin et al 2025b).
Impact for cosmology are discussed in these papers but actual cosmological parameters measurements are now started, as planned in the original ERC program.
The details result summary are given in "Work performed from the beginning of the project" for this last report.