Gaia reference frame amid quasar variability and proper motion patterns in the data

Gaia's very accurate astrometric measurements will allow the International Celestial Reference Frame (ICRF) to be improved by a few orders of magnitude in the optical. Several sets of quasars are used to define a kinematically stable non-rotating reference frame with the barycentre of the Solar System as its origin. Gaia will also observe a large number of galaxies which could obtain accurate positions and proper motions although they are not point-like. The optical stability of the quasars is critical and we investigate how accurately the reference frame can be recovered. Various proper motion patterns are also present in the data, the best known is caused by the acceleration of the Solar System Barycentre, presumably, towards the Galactic centre. We review some other less-well-known effects that are not part of standard astrometric models. We model quasars and galaxies using realistic sky distributions, magnitudes and redshifts. Position variability is introduced using a Markov chain model. The reference frame is determined using the algorithm developed for the Gaia mission which also determines the acceleration of the Solar System. We also test a method to measure the velocity of the Solar System barycentre in a cosmological frame. We simulate the recovery of the reference frame and the acceleration of the Solar System and conclude that they are not significantly disturbed in the presence of quasar variability which is statistically averaged. However, the effect of a non-uniform sky distribution of the quasars can result in a correlation between the reference frame and acceleration which degrades the solution. Our results suggest that an attempt should be made to astrometrically determine the redshift dependent apparent drift of galaxies due to our velocity relative to the CMB, which in principle could allow the determination of the Hubble parameter.Comment: published in A&A, revised version (v2) (Abstract is same as v1 as the character limit is 1920, see the pdf for v2

Electroweak Constraints from Atomic Parity Violation and Neutrino Scattering

Precision electroweak physics can provide fertile ground for uncovering new physics beyond the Standard Model (SM). One area in which new physics can appear is in so-called "oblique corrections", i.e., next-to-leading order expansions of bosonic propagators corresponding to vacuum polarization. One may parametrize their effects in terms of quantities $S$ and $T$ that discriminate between conservation and non-conservation of isospin. This provides a means of comparing the relative contributions of precision electroweak experiments to constraints on new physics. Given the prevalence of strongly $T$-sensitive experiments, there is an acute need for further constraints on $S$, such as provided by atomic parity-violating experiments on heavy atoms. We evaluate constraints on $S$ arising from recently improved calculations in the Cs atom. We show that the top quark mass $m_t$ provides stringent constraints on $S$ within the context of the Standard Model. We also consider the potential contributions of next-generation neutrino scattering experiments to improved $(S,T)$ constraints.Comment: 10 pages, 4 figures, final corrected version to be published in Physical Review

The Phenomenology of Impairment

This written research document supports The Phenomenology of Impairment, a mixed media installation consisting of three hundred and sixty-five self-portraits. This work was a part of a phenomenological exercise that combined both art making and writing. Using the five central concepts of the phenomenological method, I was able to uncover a level of meaning in my work that transcended my original intentions. The creation of the work became an enterprise in self-discovery and, through descriptions of my lived experiences and relationships, I was able to uncover the essence of those experiences and relationships.  M.F.A

A nanofabricated, monolithic, path-separated electron interferometer

We report a self-aligned, monolithic electron interferometer, consisting of two 45 nm thick silicon layers separated by 20 $\mu$m. This interferometer was fabricated from a single crystal silicon cantilever on a transmission electron microscope grid by gallium focused ion-beam milling. Using this interferometer, we demonstrate beam path-separation, and obtain interference fringes in a Mach-Zehnder geometry, in an unmodified 200 kV transmission electron microscope. The fringes have a period of 0.32 nm, which corresponds to the $\left[\bar{1}\bar{1}1\right]$ lattice planes of silicon, and a maximum contrast of 15 %. This design can potentially be scaled to millimeter-scale, and used in electron holography. It can also be applied to perform fundamental physics experiments, such as interaction-free measurement with electrons.Comment: 21 pages (including supplementary info), 8 figures; Corrected typos, added references for introduction and conclusion, changed ordering of paragraphs of Discussion, results unchange

Space missions to comets

The broad impact of a cometary mission is assessed with particular emphasis on scientific interest in a fly-by mission to Halley's comet and a rendezvous with Tempel 2. Scientific results, speculations, and future plans are discussed