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The First Half of the Universe (roughly, 2022 - )

On one level, this is the golden age of cosmology: over the last thirty years, cosmology has turned into a precision science through ever more precise observations of the Cosmic Microwave Background. On the other hand, there are large aspects of the history of the universe that we know almost nothing about. One example is the era between the end of inflation (that could be as early as 10^-33 seconds) and the time of Big Bang Nucleosynthesis (around 1 second); there are essentially no observational constraints on what happened in the universe between these two epochs. As this epoch can cover thirty orders of magnitude in time, on a log scale it corresponds to around half of the evolution history of the universe. Much of my current work (May 2024) lies in thinking about this era and the ways the dynamics of string theory moduli can come to dominate the universe in this era; the ultimate goal is to find observable legacies that this dynamics can lead to.

Holography, Moduli Stabilisation and the Swampland (roughly, 2018 - 2022)

The `Swampland' research programs refers to understanding the type of low-energy theories that can consistently arise in quantum gravity: are there theories that simply cannot arise within any theory of quantum gravity? I pioneered the study of existing moduli stabilisation scenarios through a holographic lens, revealing interesting structure that was opaque from the normal perspective. In particular, CFTs dual to scale-separated scenarios such as LVS or DGKT seem rather special and unique: the low-lying conformal dimensions take fixed values (independent of fluxes) and are integers for the case of DGKT. This hints that, if they exists, such dual CFTs have a rather special structure.

ALPS (roughly, 2014 - 2018)

ALPs stands - not for the mountains next to CERN - but instead for axion-like particles. These are generalisations of the QCD axion and are characteristed by an axionic coupling to electromagnetism, but not to the strong force. This lack of coupling to the strong force means that - unlike the QCD axion - there is no relation between the mass and coupling of the particle. However, the coupling to electromagnetism is responsible for the defining feature of ALP phenomenology - the ability for ALP-photon conversion in the presence of a coherent background magnetic field.

As conversion is a quantum-mechanical effect, the conversion probability depends on the square of the coherence length. It turns out that the best sources for conversion are not lab sources (1 T magnetic field, 10m coherence length) but rather astrophysical magnetic fields such as arise in galaxies or clusters of galaxies (0.1 nanoTesla magnetic field, kiloparsec coherence length).

There is a unique and powerful synergy between axion-like particles and X-ray astronomy and I took a lead in developing the relationship between these two subjects. I worked on searches for, and constraints on, ALPs using modulations in the spectra of X-ray point sources (such as AGNs) passing through galaxy clusters. I also worked on an explanation for the long-standing galaxy cluster soft excess through conversion into photons of a diffuse Cosmic Axion Background, and models for the 3.5 keV line involving dark matter decay into axions and then conversion to photons within the cluster magnetic field.

Dark Radiation (roughly, 2012 - 2014)

My research on ALPs evolved out of work on dark radiation and how one would detect it. Dark radiation refers to a component of the universe that is dark and relativistic (dark matter is dark and non-relativistic). Dark radiation is a topic with good theoretical and experimental motivation. In string theory, reheating generally occurs through the decay of the last modulus. These fields are gravitationally coupled, and so moduli are expected to decay to both visible and hidden sectors with comparable branching ratios. Decays to relativistic, weakly interacting hidden sectors constitute dark radiation, and so the existence of dark radiation can be fairly said to be a generic property of string compactifications.

My work focused both on the origin of dark radiation and more particularly on how you can detect it. Dark radiation in the form of axions naturally arises from modulus decays, and today would be expected to have energies in the extreme ultraviolet or soft X-ray wavebands. Axions can convert to photons in magnetic fields, and together with David Marsh I have proposed that this axion-photon conversion could be responsible for the soft X-ray excess observed in the spectrum of galaxy clusters.

Cosmology (on and off, 2006 - 2012)

The world started not with a whimper but a bang. We know that at very early times the energy density of the universe was much higher than it is today. We only actually know that the energy densities corresponded to temperatures around a few MeV, which is when nucleosynthesis occurred. However if we are allowed to extrapolate further back in time, the energy densities would grow larger and larger and eventually reach Planckian magnitudes. The early universe may then be sensitive to physics at the string scale.

I have worked on developing string models of inflation and on analysing the cosmological implications of the light moduli fields that are present in string models. Just as for supersymmetry breaking, understanding the dynamics of the moduli is the most important element in building cosmological models in UV-complete theories. One problem I find particulalrly interesting is that of building consistent supersymmetric cosmologies. Most string inflationary models tend to be incompatible with low scale supersymmetry, and most models of low scale supersymmetry suffer from the cosmological moduli problem. Personally, I find the cosmological moduli problem the most pressing issue facing low-scale supersymmetry. One promising idea I have worked on recently is to use no-scale supergravity and the LARGE volume models to push the gravitino (and moduli) masses up while keeping the soft terms small.

Supersymmetry Breaking (roughly, 2005 - 2010)

By the Coleman-Mandula theorem, supersymmetry is the only space-time symmetry left to be discovered. String theory also tells us that supersymmetry is an apparently necessary component of consistent ultraviolet physics. Furthermore, supersymmetry broken at the TeV scale stabilises the Higgs potential, removing quadratic divergences and addressing the biggest outstanding problem of the Standard Model.

I spent quite a lot of time studying the structure of soft terms generated by string models of susy breaking, particularly those arising in the LARGE volume scenario. The LVS has a special no-scale structure, which causes many cancellations in this computation and implies soft terms are much smaller than one would naively expect. This work has involved the whole train of calculating the soft terms, evolving them to TeV scales and studying the resulting soft mass spectrum.

I retain an interest in this area, but am not planning to work on it again directly unless there is a discovery of SUSY at the LHC, which looks ever less likely.

Model Building (roughly, 2008 - 2011)

If string theory describes this world, then there is a concrete configuration of branes and/or bundles in an extra-dimensional space that describes the Standard Model or one of its extensions. Such explicit constructions can serve as examples to show that the Standard Model can be embedded in a UV complete structure, and in principle can also give insight into the detailed structure of the Standard Model couplings.

My work on this area has involved what are called `local models'. These are brane constructions where the Standard Model degrees of freedom are localised in a small region of the extra dimensions. This allows the extra dimensional volume to be taken to be large while keeping the Standard Model couplings finite. They also ease the perturbativity constraints of global models such as the heterotic string, where there is a tension between the observed Standard Model couplings and keeping the world-sheet and space-time expansions under control. Furthermore, in the context of LARGE volume, local models are a necessity!

Moduli Stabilisation (roughly, 2005 - 2010)

String theory needs ten dimensions, and so requires six extra small compactified dimensions. In a Kaluza-Klein decomposition, the geometric deformations of the extra dimensions appear as scalar fields in four dimensions. These moduli fields control the couplings and mass scales of the Standard Model, and require stabilising.

My biggest contribution in this area is the co-discovery of the LARGE volume method of moduli stabilisation. Through a combination of perturbative alpha' corrections and non-perturbative superpotential corrections, this stabilises the Calabi-Yau at an exponentially large volume, breaking supersymmetry at a hierarchically low scale. Using alpha' corrections to stabilise at exponentially large volumes sounds paradoxical at first (if some string corrections are important why aren't they all?). The key to their importance is the no-scale structure of the tree level potential, which implies alpha' corrections are the leading perturbative corrections to the scalar potential.

Last edited by Joseph Conlon on 8th May 2024.