Night-time long exposure of the open Samuel Oschin Telescope dome at Palomar Observatory in California. Credit: Palomar/ Caltech.

A unique dataset of Type Ia Supernovae being released today could change how cosmologists measure the expansion history of the Universe.

Dr Mathew Smith and Dr Georgios Dimitriadis from Lancaster University are both members of the Zwicky Transient Facility (ZTF),  a wide-field sky astronomical survey using a new camera attached to the Samuel Oschin Telescope at Palomar Observatory in California.

Type Ia Supernovae are the dramatic explosions of white dwarf stars at the ends of their lives.

Cosmologists use them to probe distances across the universe by comparing their fluxes, as further objects appear dimmer.

The ZTF cosmology science working group is publishing twenty-one articles studying these 3628 Type Ia Supernovae, forming a Special Issue in Astronomy & Astrophysics. 

Lancaster astrophysicist Dr Mathew Smith, co-leader of the ZTF SN Ia DR2 release, said: “This release provides a game-changing dataset for supernova cosmology.

“It opens the door to new discoveries about both the expansion of the universe and the fundamental physics of supernovae.”

Rare

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.

In only two and a half years, ZTF has doubled the number available Type Ia Supernovae for cosmology acquired for the last 30 years to almost three thousand.

Head of the ZTF Cosmology Science working group Dr Mickael Rigault from the Institut des deux Infinis de Lyon (CNRS/ Claude Bernard University) said: “For the past five years, a group of thirty experts from around the world have collected, compiled, assembled, and analysed these 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.”

Detect

The ZTF camera, installed on the 48-inch Schmidt telescope at Palomar Observatory, scans the entire northern sky daily in three optical bands, reaching a depth of 20.5 magnitude – 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.

Professor Kate Maguire from Trinity College Dublin, a co-author of the study, said: “Thanks to ZTF’s unique ability to scan the sky rapidly and deeply, we have captured multiple supernovae within days – or even hours – of explosion, providing novel constraints on how they end their lives.”

The acceleration of the expansion of the Universe, awarded by the Nobel prize in 2011, was discovered in the late 1990s using approximately a hundred of these Supernovae.

Since then, cosmologists are investigating the reason for this acceleration caused by the dark energy that plays the role of an anti-gravity force across the Universe.

Co-author Professor Ariel Goobar, director of the Oskar Klein Centre in Stockholm, one of the founding institutions of ZTF, and also member of the team that discovered the accelerated expansion of the Universe in 1998 said: “Ultimately, the aim is to address one of our time’s biggest questions in fundamental physics and cosmology, namely what is most of the Universe made of?

“For that we need the ZTF supernova data.”

One of the key outcomes of these studies is that Type Ia Supernovae intrinsically vary as a function of their host environment, more so than expected before, and the correction mechanism assumed so far has to be revisited.

This could change how we measure the expansion history of the Universe and may have important consequences for current deviation observed in the standard model of cosmology.

Rigault said: “With this large and homogeneous dataset, we can explore Type Ia supernovae with an unprecedented level of precision and accuracy.

“This is a crucial step toward honing the use of Type Ia Supernovae in cosmology and assess if current deviations in cosmology are due to new fundamental physics or unknown problem in the way we derive distances.”