**Title:**** ****Dependence of Type Ia supernovae luminosities on their local environment**

**Authors:** M. Roman, D. Hardin, M. Betoule, et al.

**First Author’s Institution**: Sorbonne Université, Université Paris Diderot, CNRS/IN2P3, Laboratoire de Physique Nucléaire et de Hautes Energies, LPNHE, Paris, France

**Status**: Published in Astronomy & Astrophysics [open access on Arxiv]

Type Ia supernovae (SNIa) are brilliant stellar explosions that have become very important for cosmology. They are one of the primary tools that we can use to calculate how fast the universe is expanding! This is done by calculating distances to the SNIa through comparisons of intrinsic and observed properties of the explosions. In today’s paper, the authors explore the utility of also considering the properties of the galaxies that host the supernovae. In order to better understand how this can help us, let’s start with a quick introduction into how we use SNIa right now.

**Type Ia Supernovae**

SNIa are unique for a variety of reasons. There are two different hypothesized methods for the production of SNIa: two **white dwarfs** merge and explode, or a single white dwarf accretes matter from a companion star until it reaches the **Chandrasekhar Limit**. At this point, it undergoes a runaway nuclear reaction and explodes. SNIa also happen to be standardizable candles. This means that, using what we know about how SNIa behave and correcting for it, we can determine their true luminosities (Fig. 1) – roughly a few billion times as luminous as the sun!

We can compare the brightness we observe with their true luminosities to figure out how far away SNIa are from us. By also looking at how much their light is **redshifted**, we can determine how quickly these objects are moving away from us. Coupling this distance and velocity information enables us to calculate exactly how fast the universe is expanding (spoiler: it’s accelerating!). This very calculation lead to the idea of **dark energy** and the **Nobel Prize in Physics in 2011**.

**Corrections with a Splash of Color**

Today’s paper suggests that a new correction needs to be added to our distance calculations using SNIa **light curves**. Normal corrections applied to SNIa are based on two factors: stretch, which is how fast the explosion declines in brightness; and color, which is essentially **B-V color** at peak brightness. These corrections enable us to get pretty consistent true luminosities for SNIa; the largest modern samples have scatters of only around 1% or so at peak brightness. However, the authors of today’s paper suggest that another correction for the color of the local environments of the supernovae should be included (Fig. 2). In other words, they believe that we should not just consider the supernova itself – we should also consider its surroundings.

One way to test whether this hypothesis is valid is by looking for a correlation between Hubble residuals and local color. Hubble residuals are the differences between our best fit cosmological model and the SNIa data on a distance vs. cosmological redshift plot (known as a **Hubble diagram**). If there is a correlation between these residuals and some property of SNIa or their surroundings, it means that our model lacks some information that could help fit the data better.

**Evidence of the Need for a New Correction**

Figure 3 compares Hubble residuals to the local U-V color. A statistical analysis reveals that there is evidence of a correlation between these two variables with a significance of roughly 7, meaning there is roughly a 1 in a trillion chance that this result is due to chance. This suggests that local U-V color should also be used to correct SNIa light curves, improve our model, and reduce the scatter in our sample.

The major takeaway from the paper is that we should consider both the properties of the supernova explosion and its surroundings in our cosmological analyses. With major observational projects (**LSST** and **WFIRST**) planned for the coming decades that will help us observe more SNIa than ever before, corrections like these could help us pin down what dark energy is!