Talks

For a quantum system with a d-dimensional Hilbert space, a symmetric informationally complete measurement (SIC) can be thought of as a set of d^2 pure states all having the same overlap. Constructions of SICs for composite systems usually do not make use of the composite structure but treat the system as a whole. Indeed for some cases, one can prove that a SIC cannot have the symmetry that one naturally associates with the composite structure. In this talk I give one example showing how a SIC for three qubits can be constructed from SICs for the individual qubits. I ask whether the strategy used in this example might apply to other composite cases.

Although the observational evidence for cosmological inflation is growing, the physical mechanism behind it is still unknown. In part this is because inflation probably occurred at energy scales many orders of magnitude higher than that at man-made or astrophysical particle accelerators. So how can we learn about inflation? How does it constrain microphysical theory? One approach to answering these questions is primarily theoretical: attempting to embed inflation in fundamental theories of quantum gravity, such as string theory. Another approach is primarily observational: looking for signatures left by light fields that existed during inflation, such as isocurvature fluctuations from the QCD-axion. In this talk I discuss work on these two approaches.

The problem of the quantum backreaction in expanding spaces is an old, as yet unresolved, question. In this talk I will consider the one-loop backreaction of a massless scalar which couples to the Ricci scalar in an expanding space with constant deceleration. I will show that the infrared divergences, which generically plague the one loop stress energy, can be removed by matching onto an earlier radiation era. An insignificant backreaction occurs, unless the coupling to the Ricci scalar is negative. Similar results hold for the graviton backreaction.

Recent data from the PAMELA, Fermi/LAT and INTEGRAL/SPI experiments, among others, give evidence of excess electrons and positrons in the galaxy, which might be due to annihilation of dark matter. Models in which the dark matter transforms under a hidden nonabelian gauge symmetry can naturally account for the unusual features needed to fit these data. I will discuss generic features of such models, some of their distinctive consequences for cosmology, and new results for reconciling their predictions with the anomalous observations. Special attention will be given to the 511 keV gamma ray excess observed by INTEGRAL, and to the possibility that high energy lepton excesses get significant contributions from dark matter annihilation in subhalos rather than the main halo of our galaxy.