AGN dust emission as a standard candle in the LSST era

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What is the universe made of and how did it come to be as we see it today? Modern cosmology evolved out of generations of philosophers and scientists who tried to answer these two questions. According to our current understanding the universe as we know it started out with a "Big Bang" 13.8 billion years ago and has been expanding ever since. While it was extremely hot initially, the expansion cooled the matter, and gravity worked its way to form stars and galaxies as we see them today.While this has been the standard picture since at least 1964, marked by the discovery of the Big Bang afterglow, we have come to realise that the two dominant factors of the universe are completely unknown: About 85% of the matter is invisible "dark matter" and not formed of the atoms or elementary particles we know of. With the help of this huge "extra mass" we may expect that gravity eventually slows down the expansion that started during the Big Bang. However, over the last 20 years strong evidence has been found that an unknown force, the "dark energy", started pulling the universe apart, leading to an ever increasing rate of expansion. This dark energy accounts for almost 70% of the energy in the universe.So, what is behind this mysterious dark sector? Despite being observationally different phenomena, both dark matter and dark energy challenge our foundation of physics. Could it be that some of our best tested theories, like general relativity, need to be reconsidered on cosmological scales?This project aims at facilitating cosmology with a new tool. Our most precise measurements of the cosmological model come from the young universe and need to be extrapolated to the present. Yet, measurements in the local, present day universe show signs of tension with these extrapolations. It is unclear as of yet if this tension is a fact of unknown problems with the current cosmological probes or if they are pointing towards new physics that will help us understanding the dark universe.Most direct cosmological probes rely on the measurement of distances. For this, "standard candles" are invoked where the degree of dimming with distance relates to the expansion of the universe while the light was travelling to us. The currently most favoured candles are a special type of supernova explosions; however they need a complicated set of calibration to make them absolute probes of the local cosmological parameters. Here a new type of standard candles will be established that can be calibrated against themselves and potentially solve the question where the tension in cosmological parameters between the different methods come from: new physics or unknown errors. These new standard candles are supermassive black holes in the centre of galaxies that swallow matter from its surrounding. As the matter is being accreted, it lights up and part of this radiation comes from a generic, standardised region, making it a standard candle. We will use computer simulations and new sets of observations to exploit this new tool for cosmology. We will also make use of the new data to address fundamental questions related to the accretion process onto supermassive black holes.