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Understanding Type Ia SuperNovae for Accurate Cosmology

Periodic Reporting for period 2 - USNAC (Understanding Type Ia SuperNovae for Accurate Cosmology)

Reporting period: 2019-09-01 to 2021-02-28

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.
We started our work in march 2018 by focussing on the Hubble constant problem. In Rigault et al 2018 confirmed a former claim that Type Ia Supernovae arising from young environments are fainter that those from old ones. This confirms that indeed (1) it exists two Type Ia Supernovae populations, one with young white dwarf progenitor and one with old ones, and that (2) each have slightly different physics. This echoed several papers at the dawn of modern Supernova Cosmology in the mid-2000 that already showed that young and old populations were necessary to explain their observed rates. It is expected that, because two different Supernovae sample are needed to derive H0, one of which strongly favour young environment, this “age-bias” could be the root cause of the observed disagreement. We are currently working on this topic.
To further investigate the astrophysical biases, we are also modelling how the Type Ia Supernovae property would evolve as a function of cosmic time the age-bias is real and is a dominated factor. In the submitted Nicolas, Rigault et al. 2020 we indeed show that our primary model indeed follows very well the observation and is the most simple explanation for the measured evolutions. We also published in Kim et al. 2019 and Kang et al. 2020 public data studies in light of the aforementioned astrophysical biases in cosmology.
But to complete our objective of solving the astrophysical biases issue in Supernovae Cosmology more data are necessary in order to disentangle complexe and highly covariant effect. This improved statistics is provided by the Zwicky Transient Facility Survey the USNAC group is a member of. Thanks to its extremely large camera and its massive telescope field of view, we are observing all the Northern hemisphere every day acquiring all Type Ia Supernova within a distance of 1.5 billions of light-years. In two years, we are collected 5 times more such Supernovae than what was acquired in the last decade. In that project, on top of our active participation to the Cosmology working group, we have built the spectroscopic pipeline making it the first fully automated spectrograph in the world. Thanks that this unique facility, we are responsable to the classification of more than half of all the transients discovered in the world. Photometric calibration is ongoing and we first supernovae release is soon to be expected.
In that context, we have presented in Graziani et al. 2019 and in Graziani, Rigault et al. 2020 how large nearby supernova data could provide a novel and independent approach to test the laws of gravity. This very promising avenue would shade new light in how the Universe’s light scale structures are formed.
Now that the ZTF dataset is arriving we should be able to test subtle variations within the Supernovae population. We will notably focus on the relative influence of progenitor age and interstellar dust in the observed color variabilities. In addition, we will study how various host property tracers shade different light in the Supernova astrophysical variabilities. These tracers variations are confusing as disagreement between analyses are difficult to interpret as actual physical differences or just that we are not comparing apple to apple. Finally we are going to integrate of supernovae property modelling in the state of the art machinery , called SNANA, that is currently used in all cosmological analyses. The goal would be to have a consistent end-to-end structure to accurately understand the impact of our results on cosmology and on how to account for them in the future. We will also continue exploring the study of the current growth rate of structure with Type Ia Supernova as it is an independent test of the fundamental properties of the Universe.