# Dr M. C. David Marsh

## Career

**2015-date:**Stephen Hawking Advanced Fellow/Senior Research Associate, DAMTP, Cambridge.**2012-2015:**Postdoc, Rudolph Peierls Centre for Theoretical Physics, University of Oxford.**2012:**Ph.D., Cornell University.**2007:**M.Sc., Uppsala University.

For a more detailed CV, see my personal webpage (for the web-version) or send me an email (for the full version). My publication list can be found at inspirehep.

**Research**

My research is in theoretical high-energy physics and cosmology. I am interested in applying well-motivated ideas from particle physics and string theory to cosmology, and in confronting these ideas with cosmological and astrophysical observations. In the past, this has led me to address a broad range of problems connected by this overall theme.

**Axions and their astrophysical signatures**

Axion and axion-like particles are very common in high-energy theory and string theory, solve outstanding problems in particle physics, and have extremely interesting cosmological and astrophysical consequences.

**Axions and dark radiation: **In the past, I have considered the cosmological effects of the production of axions at the onset of big bang cosmology (reheating) and showed that well-motivated string theory models generically give rise to a Cosmic Axion Background (CAB) of very weakly coupled, relativistic axions. Such a CAB is quite hard to detect, but can be constrained by its indirect imprints on the Cosmic Microwave Background (CMB) and the nuclear abundances generated at Big Bang Nucleosynthesis (BBN). Moreover, axions can transform into photons in the presence of coherent magnetic fields, and galaxy clusters turn out to be ideal environments for such conversions to occur. This means that by looking at clusters of galaxies (at X-ray energies), we may see signals of a novel type of particles propagating straight from the “big bang” to us. Intriguingly, the predicted signal from popular string models agree very well with the long-standing unexplained excess photon flux in the soft X-ray range from clusters of galaxies.

**Galaxy clusters as axion converters:** The fact that galaxy clusters are so efficient axion/photon converters opens a new research front within axion physics, and lead to a number of additional consequences. First, I have shown that some of the strongest bounds on the axion-photon coupling arise from the observed smoothness of the thermal spectrum of the intracluster medium that fills galaxy clusters and radiate at X-ray energies. Second, if dark matter particles in a cluster decay into light axions, these axions may convert into photons and give rise to a line-signal. In fact, such a scenario provides a very compelling explanation of an unidentified emission line at 3.55 keV from clusters of galaxies, first detected in 2014.

**Inflation**

In addition to axions, compactifications of string theory give rise to a large number of scalar moduli fields in the low-energy four-dimensional theory. While these may well be too heavy and weakly coupled to produce in laboratory experiments or in nearby astrophysical environments, they crucially affect the early universe cosmology, in particular during inflation when the energy density of then universe was very large.

Moduli give rise to multiple-field effects during inflation, but unfortunately, very little is known of the properties of inflation with several interacting fields. It’s then natural to ask: are there any universal properties of inflation with many fields that are rather independent of the underlying details of the model? If so, do these properties have observable consequences?

**RMT inflation:** To address this question, I have used non-equilibrium random matrix theory (“Dyson Brownian Motion”) to construct a novel class of random potentials which is particularly well-suited to study multi-field inflation. With this method, the potential is mapped out locally along the dynamically determined inflaton trajectory, according to rules derived from random matrix theory. This method is highly computationally efficient and allows for the study of inflation with hundreds of interacting fields (an improvement from ~a few using previously existing methods).

**Universal predictions from complex inflationary physics:** Curiously, while these models are very complex and involve a large number of fields during inflation (sourcing significant superhorizon evolution of the curvature perturbation), the most important observable predictions are very simple and can be understood as arising directly from local “eigenvalue repulsion” of the squared masses of the fields — a property that is generic for any interacting model and hence leads to the expectation that these results are applicable far beyond this explicit construction. In particular, the power spectrum, the spectral index and its running all appear to quickly tend to well-defined, sharpened distributions as the number of fields is increased.

**The cosmological constant, dark energy, de Sitter vacua**

The cosmological constant problem (c.c.p.) is quite possibly the most severe problem in modern physics. Using standard techniques of quantum field theory, we can compute the energy density of the vacuum; it turns out to be rather large. The total energy density determines the expansion rate of the universe, and observations of distant supernovae, the large-scale structure of the universe (LSS) and the CMB all point to non-vanishing observed vacuum energy density; however, one that turns out to be very small. The ratio of the expected energy density to the observed energy density is somewhere between 10^53 and 10^120, depending on what physics appears at very high energy scales. This is the cosmological constant problem.

String theory has been suggested to solve the c.c.p. by virtue of having many four-dimensional vacuum solutions, between which the effective vacuum energy density differs. If the number of vacua is much larger than 10^120, we may well expect a small subset of the solutions to have an effective vacuum energy solution compatible with the observationally inferred value by chance. If these solutions are populated by some mechanism (e.g. eternal inflation), we may well live in a multiverse in which different laws of nature apply in different sub-universes. The reason for the observed value of the c.c. may then be “anthropic”: it’s typical among the values that we possibly could have observed, but atypical among the full set of universes, most of which would lead to quickly accelerating empty universes, incompatible with the basic observation that we are here to observe it.

It is important to establish whether string theory really gives rise to such a landscape; if there are additional correlations or predictions that could be derived from it; and what observations could possibly falsify it.

**Dark energy making the c.c.p. worse:** I have recently shown that there exists phenomenological models of dark energy which, if realised in nature, could decisively rule out the anthropic solution to the c.c.p. in any theory with a finite number of vacua. These models, which currently receive significant observational interest and which will be probed by upcoming LSS observations, turn out to make the cosmological constant problem much worse, and hence, can only be realised together with the anthropic solution to the c.c.p. at all if the number of vacua is extremely large (some these models will only be realised by chance at all if there are at least 10^(10^10) vacua).

Most currently known four-dimensional vacuum solutions in string theory arise from flux compactifications of the higher-dimensional theory, with the topologically most complex compactification manifolds giving rise to the largest numbers vacua. Such compactifications also have a very large number of moduli fields, but unfortunately, very little is known about complicated compactifications with many fields. I have in the past addressed this issue from multiple angles.

**The wasteland of random supergravities:** First, I have shown that in supergravities with no additional string theory structure, the vast majority of all critical points with positive vacuum energy are not metastable vacua (only a fraction of exp(- (#fields)^2) are), but unstable saddle points. Metastable de Sitter vacua are never generic in such theories, but metastability becomes more common for compactifications in which there is a large hierarchy between the supersymmetric mass scale and the scale of supersymmetry breaking.

**Moduli decoupling from approximate no-scale symmetry:** Second, I have shown that compactifications which only very weakly break the underlying “no-scale” symmetry that is common in string compactifications, metastability is much improved. This indicates that typical metastable de Sitter solutions in string theory may have significant common properties.

**Universal properties of IIB/F-theory compactifications at large complex structure:** Third, I have shown that for any flux compactification with any number of fields, there exists interesting regions of the moduli space in which all properties of the theory can be explicitly computed, with significant implications for proposed models of string inflation and the prevalence of vacua.

**Supersymmetry breaking in string theory; baryogenesis**

**In the past, I have also worked on supersymmetry breaking and so called “sequestering” in string theory, using both effective field theory and world-sheet techniques. The key result here being that even if the Standard Model of particle physics is realised locally in the compactification from branes at a singularity, the extra-dimensional geometric separation is not sufficient to guarantee that distant hidden sectors (such as stacks of branes undergoing gauging condensation) do not induce dangerous CP-violating and FCNC-inducing operators in the visible sector. Experimental particle physics constraints can then be interpreted as constraints on the topology of the compactification manifold.**

Sequestering:

Sequestering:

**Affleck-Dine baryogenesis in supergravity and string theory:** I have also considered the problem of how the matter/antimatter asymmetry of the universe arose, where I in particular showed that Affleck-Dine baryogenesis (ADB) is not possible in certain string inflation and moduli stabilisation scenarios, but may give rise to signals like the lack of power on large scales in the CMB. ADB is particularly interesting in the light of string compactifications, as these tend to give rise to a reheating temperature much too low for most other proposed mechanisms of baryogenesis.