The Zwicky Transient Facility (ZTF) published a collection of 3628 high-resolution data sets of Type Ia supernovae that were captured by the unique instrument between March 2018 and December 2020. Supernovae of this type are cosmic explosions that are used for the measurements of distances in the universe. The unprecedentedly extensive data set of this phenomenon was published by the ZTF Working Group, of which DESY is a member, in a special edition of the journal Astronomy and Astrophysics. “The publication reveals new insights about the nature of Type Ia supernovae and their use as a cosmological candle, ringing in an age of high-precision supernovae cosmology,” rejoices Marek Kowalski, lead scientist at DESY and a professor at the Humbolt University in Berlin, and whose working group contributes to the experiment.

Type Ia supernovae are bombastic explosions of white dwarf stars that are at the end of their lifespan. Within about two weeks, each of these events reaches a peak luminosity the equivalent of 10 million stars as bright as our sun. The unusual consistency of the luminosity of the various explosions allows for them to be an ideal measurement reference for researchers in order to determine distances in the universe, in that their light streams can be compared. The further away an object, the weaker the light; the closer, the brighter. The accelerated expansion of the universe, whose discovery was awarded the 2011 Nobel Prize, was discovered through around 100 such supernovae in the 1990s. Since then, cosmologists have been searching for the reason for this expansion. The most popular explanation postulates the existence of a Dark Energy of an as of yet unexplained origin.

The Zwicky Transient Facility, named for the astrophysicist Fritz Zwicky, contains a camera with more than 500 million pixels and is able to capture a particularly large visual angle in pictures at its highest resolution. It was specially developed for the purpose of following the highest possible number of star explosions and other short-lived, energetic cosmic events with high precision. It “sees” events that are up to a million times weaker than the weakest stellar bodies that are still visible with the naked eye, and because of this sensitivity, it can pick up traces of supernovae in a range of 1.5 billion light years away from the Earth. The data taken in the measuring time from March 2018 to December 2020 enable the researchers to envision Dark Energy as being more complex as originally considered. The data bring the science closer as well to answering the question of what exactly takes place when a supernova occurs, and whether the Type Ia supernovae really are so similar as previously thought. In total, the ZTF cosmological working group has published 21 articles that deal with these 3628 Type Ia supernovae, articles that are compiled in the special edition of Astronomy & Astrophysics.

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

“One of the key findings of these studies is that Type Ia supernovae vary as a function of their environment, more so than previously thought. This could change the way we measure the expansion history of the universe and could have important consequences for the current divergence in the standard model of cosmology,” says Kowalski.

“We are providing the community with thousands of density-sampled and well-calibrated Type Ia supernovae light curves and spectra. In a next step, we will combine the ZTF data with more distant supernovae. This will allow us to track the expansion rate of the universe over the last 10 billion years and gain new insights into the nature of dark energy,”summarizes Jakob Nordin, head of the Berlin ZTF Cosmology Group.