Searching for Axion Dark Matter from Cosmology to Condensed Matter

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Axions are a class of theoretical particles that could explain the mystery of Dark Matter (DM). The axion mass is tiny, between one trillionth the mass of the proton and one billion-trillion-trillionth. Searching for axions is like tuning a radio where you don't know the frequency of the station you want or even if it is long wave or FM. To find the axion we need lots of different types of "radios" (resonances) to cover the wide range of masses. My research looks at the lightest and heaviest masses, using resonances in cosmology and in the lab. The results of these searches could identify the nature of DM. The statistical methods allow the results to be interpreted as constraints on two specific theories: the "quantum chromodynamics" (QCD) axion, and the string theory landscape. The Lightest Axions in Cosmological and Astrophysical Data. Very light axions affect the expansion rate and growth of structure in the Universe. Data from the Planck satellite cosmic microwave background (CMB) limits these axions to be at most a few percent of the DM. String theory, however, predicts that the abundance is even smaller, so we must push further in data analysis and precision modelling. Using the same techniques developed to search for axions in the CMB, I will search for evidence of axions in other data. I will use gravitational lensing of galaxies, upcoming high precision CMB data, and radio astronomy maps of hydrogen gas distribution. Using other astrophysical observables we can extend the search to slightly heavier axions. I will search for evidence of the axion waves disturbing the motion of stars in galaxies (the popular "fuzzy" DM model). Finally, axions can affect how rapidly black holes spin. Gravitational wave detections give precise measurements of black hole masses and spins and can thus be used to search for evidence of axions. Direct detection of the heaviest DM axions in the lab. Heavier axions were predicted in the 1970's to solve a problem about the symmetry of the strong nuclear force (known as QCD). There are many experiments, from the USA to South Korea, searching for this QCD axion, because it interacts with electric and magnetic fields. I have proposed a method to search for the QCD axion in a range of frequencies no current experiment can reach, and I want to bring the global renaissance in axion experiments to the UK."Topological Resonant Axion Detection" (TOORAD) works by the axion exciting an electric field inside a material called a topological insulator when it is exposed to a magnetic field. Manufacturing the material to be magnetic itself, the electric field excites a magnetic resonance, similar to NMR in hospitals. The magnetic resonance then causes the material to emit photons. If the emitted photons can be detected then the material can be used as an axion detector. I propose to grow an international collaboration to study this and eventually perform the experiment in a partner laboratory.Astrophysical test of the QCD axion model. When the QCD axion is produced during the big bang the dynamics of the production cause it to form dense clumps with about the mass of a large asteroid: "miniclusters". It is possible to search for these lumps of DM, but it is also very hard to predict exactly how massive they are and how many of them there are. I will develop the theoretical models necessary to answer this question. With this information, I will search for evidence of miniclusters in two ways. Firstly, they can be found with gravitational lensing in the same way planets around distant stars are found. Secondly, they show up as bursts of radiation when the miniclusters collide with magnetic fields e.g. of neutron stars. Understanding the minicluster masses and abundances is also very important to precisely predict the signal of axions in labs on Earth.