ASTUnderstanding cosmological probes: constraining the progenitors and diversity of Type Ia supernovae

One of the fundamental questions that exists about our Universe is what is this 'dark energy' that we now know makes up greater than 70% of the mass-energy of the Universe? The discovery of this mysterious quantity at the end of the last century from measurements of distant exploding stars was completely unexpected. Until then, it was assumed that the expansion of the Universe would be slowing down due to gravitational attraction but instead it was found to be accelerating, invoking the presence of 'dark energy' that is acting against gravity to push the Universe apart! There are many theoretical predictions for the origin of dark energy (Einstein's 'cosmological constant'? new exotic physics? alterations to general relativity?) but current measurements cannot yet distinguish between them.

The most successful and mature probe of the origin of dark energy to date is measurements of distant exploding stars (supernovae). The 2011 Nobel Prize in Physics was awarded to two groups who made measurements of distant supernovae and discovered the accelerating Universe. Current and future state-of-the-art telescopes and surveys (e.g. Dark Energy Survey, the European Space Agency mission, Euclid and the Large Synoptic Survey Telescope) are being designed to constrain theories of dark energy using larger supernova samples. However, to distinguish between these theories of dark energy, it is also very important to understand the supernovae themselves. This is the aim of my Ernest Rutherford research proposal.

I will do this using observations of Type Ia supernovae, which are the luminous stellar deaths of accreting white dwarfs (small, dense stars) in binary systems. I will investigate, 'is there more than one way to make a Type Ia supernovae?', 'how do they explode?' and 'how do the properties of the galaxies within which they explode affect their properties?'

I will analyse data coming from many telescopes, including one of the world's largest telescopes, the European Southern Observatory's Very Large Telescope (VLT) in Chile, to understand if there is more than one way to explode a star and produce a Type Ia supernova. I will use advanced analysis techniques on these data, in collaboration with the UK's leading supernova theorists to uncover how exactly these stars explode. This will allow us to refine current and future supernova samples to make improved dark-energy measurements.

I will also investigate the properties of the distant galaxies in which the supernovae explode. Type Ia supernovae can occur in many different types of galaxies, from spiral galaxies like our Milky Way to small dwarf galaxies that are barely visible with even the most powerful telescopes. I will investigate how the properties of the supernovae such as brightness, the presence of different atomic elements and the energy released depend on the properties of the galaxy within which they explode. I have previously shown that the properties of Type Ia supernovae change depending on the amount of heavy elements present in their host galaxies. Now it is important to understand the cause of this diversity and how it might vary throughout the Universe. If Type Ia supernovae look very different at different distances in the Universe this would affect the dark-energy measurements made with them.

As an Ernest Rutherford fellow, I will be in the unique position to progress from past measurements of dark energy, by significantly improving our understanding of SN Ia physics and cosmology. I will build on my extensive experience in this field and help future surveys reveal what is this mysterious quantity that makes up greater than 70% of our Universe.