Why supernovae are important for understanding the Universe

Type Ia supernovae are the explosive deaths of white dwarf stars. Since they tend to explode with almost the same intrinsic brightness, astronomers use them as “standard candles”: by comparing their known true brightness with their apparent brightness from Earth, scientists can measure cosmic distances.

This technique was key to discovering that the expansion of the Universe is accelerating, a phenomenon attributed to dark energy, one of the biggest mysteries of modern physics. However, there is a catch: not all Type Ia supernovae are exactly the same.

The problem: supernovae are affected by their environments

Over the last two decades, astronomers have discovered that the brightness of these supernovae depends slightly on the galaxies in which they explode. For example, supernovae in the most massive or oldest galaxies tend to look slightly different from those in smaller or younger galaxies.

To date, these effects had been corrected using simple, approximate adjustments, which could limit how precisely we can measure the distances to these supernovae.

A unified solution: comprehensive models

The new study tackles this problem by modelling everything at once: supernova explosions, the galaxies that host them, the dust that dims and reddens their light, the frequency with which supernovae occur over cosmic time, and even the expansion of the Universe itself.

Instead of analysing each piece separately, the team built a single, self-consistent model that links all these elements physically and statistically.

“A powerful way of modelling the Universe is to simulate it ab initio in the computer using Bayesian inference,” says Raúl Jiménez (ICREA-ICCUB), co-author of the study. “This provides a way to vary all possible parameters at the same time to predict what Universe we live in. Furthermore, by having this capacity, one can look into possible ‘unknown unknown’ systematics to understand their effect. The impact of these systematics in our inference is arguably the most important missing ingredient in current approaches to model the Universe.”

Artificial intelligence and cosmology

To make this ambitious approach computationally feasible, the team used a modern set of techniques known as simulation-based inference.

In simple terms, the method works like this: first, scientists simulate many possible universes using physical models; next, a neural network (a type of artificial intelligence) learns how the simulated data relate to the underlying physical parameters, and finally the trained system can infer these parameters directly from real observations.

This allows the analysis of tens of thousands of supernovae at once, something that would be impossible with traditional methods.

A key result: precise distances without spectroscopy

One of the most important results is that the method can estimate galaxy distances (redshifts) very accurately using only images.

Redshift measures how much a galaxy’s light is stretched as the Universe expands. It shows how far away and how long ago we are seeing it.

The new approach achieves precision comparable to spectroscopic measurements, but without the need for spectra. This is crucial, because future sky surveys will discover millions of supernova candidates, but only a small fraction can realistically be studied with spectroscopy.

Preparation for the Rubin Observatory era

The Vera C. Rubin Observatory, currently under construction in Chile, will soon begin a 10-year sky survey. It will detect an unprecedented number of supernovae, approximately 99% of which will be observed only photometrically, that is, via images in different colours.

The CIGaRS framework is precisely designed for this scenario

“Unlike other frameworks, which require analytic simplifications, our no-compromise end-to-end simulation-based inference approach is uniquely capable of extracting the full cosmological and astrophysical information from the Rubin Observatory’s hard-earned data, while avoiding the pitfalls of selection and modelling biases.” says Konstantin Karchev (ICCUB-SISSA Trieste), lead author of the study.

Beyond cosmology: discovering how stars explode

In addition to improving measurements of dark energy, the study also sheds light on how and when Type Ia supernovae form. By reconstructing how supernovae occurrence rates depend on the ages of the stars in galaxies, the model helps to address long-standing questions about their progenitor systems.

The results show that the combination of physics-based modelling with artificial intelligence can overcome key limitations in current cosmological analyses. According to the authors, this approach could improve cosmological constraints by up to a factor of four, compared with traditional methods, which rely solely on a small subset of spectroscopically observed supernovae.

With the Rubin Observatory set to transform astronomy in the coming years, methods such as CIGaRS ensure it will be ready to fully understand the data and the Universe they reveal.