Scientific Series

One of the leading hypotheses for dark matter is that it consists of bosonic particles with masses below the eV scale, such as axions, moduli and dark photons. Unlike spin-0 particles, spin-1 particles do not have a misalignment mechanism to produce the desired abundance of dark matter, and population of light dark photon dark matter has been an open question in cosmology. I will present a novel mechanism to produce light spin-1 dark matter in cosmology. The dark matter energy density is initially stored in an axion-like field which is misaligned from its minimum during inflation. This energy density is efficiently transferred to dark photons using tachyonic particle production. This mechanism opens up extensive parameter space for dark photon dark matter.

The appearance of scale invariance and diverging response functions in many-body systems is inseparably linked to the presence of a critical point and spontaneous symmetry breaking. In thermal equilibrium critical points mostly correspond to isolated spots in parameter space, which require rather strong fine tuning of e.g., the temperature of magnetic fields, in order to be reached. Pushing systems away from thermal equilibrium, e.g., by exposing them to external drive fields or dissipation, can give rise to more unconventional forms of criticality. External driving and dissipation may even turn critical points robust and attractive, imposing scale invariance for a large variety of external parameters, a phenomenon known as self-organized criticality (SOC).

I will discuss the manifestation of SOC in driven dissipative cold atom ensembles, for which a direct mapping of their microscopic parameters to an SOC field theory exists. This yields an opportunity to study SOC, typically associated with hard-to-control, large scale non-equilibrium setups, such as earthquakes, solar flares, or neural networks, under very controlled conditions. In addition, I will briefly explain the theoretical challenges for a quantum theory of self-organized criticality and point out current steps towards their realization.

Statistical evidence has previously suggested that the Galactic Center GeV Excess (GCE) originates largely from point sources, and not from annihilating dark matter. In this talk, I will discuss the impact of unmodeled source populations on identifying the true origin of the GCE. In a proof-of-principle example with simulated data, I will demonstrate that unmodeled sources in the Fermi Bubbles can lead to a dark matter signal being misattributed to point sources. Furthermore, I will show there is striking behavior consistent with a mismodeling effect in the real Fermi data, finding that large artificial injected dark matter signals are completely misattributed to point sources. Consequently, I will conclude that dark matter may provide a dominant contribution to the GCE after all, and discuss future directions.

It is generally believed that modification of general relativity inevitably introduce extra physical degree(s) of freedom.
In this talk I argue that this is not the case by constructing modified gravity theories with two local physical degrees of freedom. After classifying such theories into two types, I show explicit examples and discuss their cosmology and phenomenology.

It was during his stay in Prague when Einstein started in earnest to develop his ideas about general relativity. I will evoke those days in 1911 and 1912, discuss Einstein's papers on gravitation from that period, emphasize which new concepts and ideas he introduced. I also want to show you how the main themes that preoccupied him then, the principle of equivalence, bending of light, gravitational redshift and frame dragging effects are still alive in contemporary relativity.

Atomic hydrogen gas clouds originating from the Galactic Center offer a novel way to test dark matter phenomenology. By exploiting the inefficient gas cooling rates at low temperatures, bounds for various interactions between dark and baryonic matter can be set. We demonstrate this new method and present limits for a number of dark matter models including ultra-light dark photons and super-heavy candidates.

Fracton order is a new kind of phase of matter which is similar to topological order, except its excitations have mobility constraints. The excitations are bound to various n-dimensional surfaces with exotic fusion rules that determine how excitations on intersecting surfaces can combine.

This talk will focus on one of the simplest fracton models: the X-cube model. I will explain how this fracton model can be thought of as starting from a 3D toric code and stacks of 2D toric code layers, and then condensing certain excitations on the 2D layers. This is the first TQFT picture of the X-cube model as a 3D order with non-invertible 2D "defects". When we zoom out so that the layers appear close together, I'll show that the model can be described by what I call a foliated field theory. I define a foliated field theory to be a field theory that couples to a foliation field, which describes the geometry of the foliating layers (in analogy to a metic which describes the geometry of spacetime). In this case, the foliated field theory takes the form of a 3+1D BF theory which is coupled to stacks of 2+1 BF theories, with stacking structure described by the foliation field.

This talk is based on arXiv:1812.01613 and another forthcoming work.

New physics in the neutrino sector might be necessary to address anomalies between different neutrino oscillation experiments. Intriguingly, it also offers a possible solution to the discrepant cosmological measurements of H_0. 
We show here that delaying the onset of neutrino free-streaming until close to the epoch of matter-radiation equality can naturally accommodate a larger value for the Hubble constant, while not degrading the fit to the cosmic microwave background (CMB) damping tail. We achieve this by introducing neutrino self-interactions in the presence of a non-vanishing sum of neutrino masses. This "strongly interacting" neutrino cosmology prefers a 3+1 neutrino scenario, which has interesting implications for particle model-building and neutrino oscillation anomalies. Due to their impact on the evolution of the gravitational potential at early times, self-interacting neutrinos and their subsequent decoupling leave a tell-tale structure on the matter power spectrum. 
Our analysis shows that it is possible to find radically different cosmological models that nonetheless provide excellent fits to the data, hence providing an impetus to thoroughly explore alternate cosmological scenarios.

Modified gravity theories typically feature numerous additional parameters and functions as compared to general relativity, which are unmotivated by observations and challenging to meaningfully constrain. We instead propose a new theory of gravity with the startling property of having *fewer* degrees of freedom than general relativity with a cosmological constant, by invoking a duality property within a first-order formulation that supports torsion. In this theory the cosmological constant becomes a space-time variable without introduction of kinetic terms, but its behaviour is tied to that of matter. I will describe the main properties of the theory, including its implications for cosmology. While there are strong indications that the simplest incarnation of the theory is not observationally viable, the theory can be used as a starting point for various extensions which appear more promising.

Based on arXiv:1905.10380 and arXiv:1905.10382, with Alexander, Cortês, Magueijo, Sims, and Smolin.

I shall  analyze three specific general-relativistic  problems in which gravitomagnetism plays important role: the dragging of  magnetic fields  around  rotating black holes, dragging inside a collapsing slowly rotating spherical shell of dust, compared with the dragging by rotating gravitational waves (CQG 34, 205006 (2017), Phys. Rev. D 85 124003, (2012) etc). I shall also briefly show how "instantaneous Machian gauges“ can be useful in the cosmological perturbation theory (Phys. Rev. D 76, 063501 (2007)).