A sample of more than 3600 Ia supernovae could change how to measure the expansion history of the Universe

Today, the most complete and advanced dataset of Type Ia supernovae is being published. This is the Zwicky Transient Facility (ZTF) survey, which includes a sample of 3,628 Type Ia supernovae collected by a single instrument between March 2018 and December 2020. These supernovae are explosions of white dwarf stars at the end of their lives. Approximately two weeks later, each individual explosion reaches a peak luminosity of 10 million sun-like stars, with remarkable consistency between different explosions. These objects are known as ‘standard candles’ in astrophysics. The dataset is published alongside a special issue of 21 articles in the journal Astronomy and Astrophysics.

The team, which has the participation of researchers from the Institute of Space Studies of Catalonia (IEEC — Institut d’Estudis Espacials de Catalunya) at the Institute of Space Sciences (ICE-CSIC), discovered a new effect that could change how the expansion history of the Universe is measured and may have important consequences for current deviation observed in the standard model of cosmology.

This release opens the era of high precision in supernovae cosmology. One of the key outcomes of the studies published is that type Ia supernovae intrinsically vary depending on their host environment, more so than expected before, and the correction mechanism assumed so far has to be revisited. 

“For the past five years, a group of thirty experts from around the world have collected, compiled, assembled, and analysed this data. We are now releasing it to the entire community. This sample is so unique in terms of size and homogeneity, that we expect it to significantly impact the field of Supernovae cosmology and to lead to many additional new discoveries in addition to results we have already published,” says Mickael Rigault, researcher at the Institut des deux Infinis de Lyon (CNRS / Claude Bernard University) and head of the ZTF Cosmology Science working group.

Cosmologists have learned to use these complex standard candles to probe distances across the Universe by comparing their fluxes, as further objects appear dimmer. The acceleration of the Universe expansion, whose discovery was awarded the Nobel prize in 2011, was first detected in the late 90s using around 100 of these supernovae. Since then, cosmologists have been investigating the reason for this acceleration caused by the dubbed dark energy that plays the role of an anti-gravity force across the Universe. 

Recent state-of-the-art type Ia supernovae datasets compile around 2000 objects gathered from many different telescopes and acquired for the last two decades. Analyses of these samples suggest that dark energy may be more complicated than a simple mathematical constant in Einstein’s equation as assumed since its first discovery. Our sparse knowledge of the exact physics responsible for the type Ia supernovae astrophysical phenomena also affects our ability to derive precision distances to probe the fundamental physics of the Universe.

“The uniformity of this dataset sets a new standard for nearby supernova observations, surpassing all those collected over the past decades. It will serve as the reference for future high-redshift supernova studies, including those from next-generation projects like the Legacy Survey of Space and Time (LSST) and the Roman Space Telescope, bringing us closer to understanding the true nature of dark energy that drives the accelerating expansion of the Universe,” explains Lluís Galbany, IEEC researcher at the ICE-CSIC, and member of the ZTF Cosmology group. Predoctoral researchers Kim Phan (IEEC, ICE-CSIC) and Alaa Alburai (ICE-CSIC) also participate in this project.

A unique instrument for a groundbreaking dataset

The ZTF camera, installed on the Samuel Oschin telescope at Palomar Observatory (United States of America), reaches a depth of 20.5 magnitude, so one million times fainter than the dimmest stars visible to the naked eye. This sensitivity allows ZTF to detect nearly all supernovae within 1.5 billion light-years of Earth. This is the first time that astrophysicists have access to such a large and homogeneous dataset. Type Ia supernovae are rare, occurring approximately once per thousand years in a typical galaxy, but ZTF’s depth and survey strategy enable researchers to detect nearly four per night.

Previous supernova samples covering this distance range contained fewer than 200 events. The published dataset increases this number by an order of magnitude, enabling much more accurate analyses, study of rare events, comparing similarities and differences between many sub-groups, etc. This breakthrough allows researchers to address fundamental questions previously hindered by limited sample sizes. 

“This is a crucial step towards honing the use of type Ia supernovae in cosmology and assess if current deviations in cosmology are due to new fundamental physics or an unknown problem in the way we derive distances,” concludes Mickael Rigault.

Press release prepared in collaboration with the Institute of Space Sciences (ICE-CSIC).

About the IEEC

The Institute of Space Studies of Catalonia (IEEC — Institut d’Estudis Espacials de Catalunya) promotes and coordinates space research and technology development in Catalonia for the benefit of society. IEEC fosters collaborations both locally and worldwide and is an efficient agent of knowledge, innovation and technology transfer. As a result of more than 25 years of high-quality research, done in collaboration with major international organisations, IEEC ranks among the best international research centres, focusing on areas such as: astrophysics, cosmology, planetary science, and Earth Observation. IEEC’s engineering division develops instrumentation for ground- and space-based projects, and has extensive experience in working with private or public organisations from the aerospace and other innovation sectors.

The IEEC is a non-profit public sector foundation that was established in February 1996. It has a Board of Trustees composed of the Generalitat de Catalunya, Universitat de Barcelona (UB), Universitat Autònoma de Barcelona (UAB), Universitat Politècnica de Catalunya · BarcelonaTech (UPC), and the Spanish Research Council (CSIC). The IEEC is also a CERCA centre.