Search Talks

Recent comparison between observation and expectation could point to problems with the standard cold, non-interacting dark matter picture, one of which being how small the smallest gravitationally bound dark matter halos are. I will review the cold dark matter picture and the experimental tests. One solution to the problems comes from coupling the dark matter to neutrinos. I will describe the model building requirements of such a coupling and determine how to test this scenario.

Forthcoming 21cm intensity mapping surveys on the Square Kilometre Array (SKA) will be capable of probing unprecedentedly large volumes of the Universe. This will make it possible to detect effects beyond the matter-radiation equality peak in the power spectrum, including primordial non-Gaussianity, GR corrections, and possible signatures of modified gravity. I give an overview of the proposed SKA intensity mapping surveys, the science that they will be able to do, and some of the challenges that they face.

The study of ground spaces of local Hamiltonians is a fundamental task in condensed matter physics. In terms of computational complexity theory, a common focus in this area has been to estimate a given Hamiltonian’s ground state energy. However, from a physics perspective, it is sometimes more relevant to understand the structure of the ground space itself. In this talk, we pursue the latter direction by introducing the notion of “ground state connectivity” of local Hamiltonians. In particular, we show that determining how “connected” the ground space of a local Hamiltonian is can range from QCMA-complete to NEXP-complete. (Here, QCMA is the same as QMA, but with a classical witness.) As a result, we obtain a natural QCMA-complete problem, a task which has generally proven difficult since the conception of QCMA over a decade ago. 

Moduli fields with Planck suppressed couplings to light species are ubiquitous in string theory and supersymmetry. These scalar fields are expected to dominate the energy budget in the early universe. Their out-of-equilibrium decays can produce dark matter and baryons. Dark matter generated in this non-thermal manner typically has large annihilation rates that are strongly constrained by indirect detection. The resulting bounds on superpartner masses offer dim prospects for collider discovery of supersymmetry. We will discuss extensions of the Minimal Supersymmetric Standard Model (MSSM) that allow low scale supersymmetry accessible by direct searches, while being consistent with astrophysical and cosmological probes. The tension with indirect searches is most easily relieved by allowing the lightest MSSM superpartner to decay into new stable states that play the role of dark matter. We examine the viability of this scenario in models with light Abelian and non-Abelian hidden sectors, and asymmetric dark matter. This latter possibility has a natural connection to theories of baryogenesis.

A quantum isolated horizon can be modeled by an SU(2) Chern-Simons theory on a punctured 2-sphere. We show how a local 2-dimensional conformal symmetry arises at each puncture inducing an infinite set of new observables localized at the horizon which satisfy a Kac-Moody algebra. By means of the isolated horizon boundary conditions, we represent the gravitational fluxes degrees of freedom in terms of the zero modes of the Kac-Moody algebra defined on the boundary of a punctured disk. In this way, our construction encodes a precise notion of CFT/gravity correspondence. The higher modes in the algebra represent new non-geometric charges which can be represented in terms of free matter field degrees of freedom. When computing the CFT partition function of the system, these new states induce an extra degeneracy factor, representing the density of horizon states at a given energy level, which reproduces the Bekenstein's holographic bound for an imaginary Barbero-Immirzi parameter.

A prime challenge to our understanding of galaxy formation concerns the scarcity of dwarf galaxies compared with the numerous low-mass halos expected in the current ΛCDM paradigm. This is usually accounted for by assuming that energetic feedback from evolving stars confines dwarf galaxy formation to relatively massive halos spanning a narrow mass range. I will highlight a number of observations that may be used to test this assumption and discuss the puzzles and challenges that arise from this analysis. I will also discuss a number of challenges that ΛCDM faces on the scale of dwarf galaxies and their possible resolutions.

Pure states and pure transformations play a crucial role in most of the recent reconstructions of quantum theory. In the frameworks of general probabilistic theories, purity is defined in terms of probabilistic mixtures and bears an intuitive interpretation of ``maximal knowledge" of the state of the system or of the evolution undergone by it.  On the other hand, many quantum features do not need the probabilistic structure of the theory. For example, Schumacher and Westmoreland formulated a toy theory that only specifies which events are possible (without quantifying they probability) and still reproduces a large number of quantum features. In this talk I will provide a probability-free definition of pure states and pure transformations, which can expressed in the categorical framework of process theories developed by Abramsky and Coecke and coincides with the usual notion under standard assumptions. Building on this definition, I will present a probability-free version of the purification principle, which allows one to retrieve a large number of quantum features even in the lack of probabilistic structure. This work is part of a larger programme that aims at drawing the line between those aspects of quantum theory that can be defined solely in terms of operations in a circuit and those that rely on the subjective expectations of an agent.

Related works:  
-GC, Distinguishability and copiability of programs in general process theories, arXiv:1411.3035;  
-Categorical purification, http://www.cs.ox.ac.uk/CQM2014/programme/Giulio.pdf 
-GC, G. M. D'Ariano, and P. Perinotti, Probabilistic theories with purification, Phys. Rev. A 81, 062348 (2010)

According to the Newtonian intuition, a constant gravitational field has no physical effect on a system since it can always be redefined, and a homogeneous gradient of the gravitational field (i.e. a homogeneous gravitational force) is equivalent to an accelerated reference frame. I will show how to extend this intuition to cosmological scales; in the presence of a single clock a constant curvature perturbation and its gradient can be set to zero through a coordinate transformation. This allows one to connect the squeezed limit of an $n$-point correlation function of the curvature perturbation to an (n+1)-point correlation function in the limit in which one of the momenta is very small (the so-called squeezed limit). These consistency relations are valid from inflation to the LSS. As an example, I will use them to write down a non-perturbative relativistic relation between galaxy number over-density correlation functions.