Physics Colloquia

Spring 2018  (Fridays 2:00 PM in SE 319) For more information contact Shianne Noel

Date Speaker Title
Jan 26 J.B. Sokoloff A Proposed Mechanism for the Difference in the Radius Dependence of Water Flow through Carbon and Boron Nitride Nanotubes
Feb 2 Prof. Zonghong Zhu Gravitational Lensing of Gravitational Waves and precision cosmology
Feb 23 Dr. Ted von Hippel  White Dwarfs, Gaia, and The Age of the Galaxy
Mar 30 Prof. Ariel Edery Spontaneous Breaking of Restricted Weyl Symmetry in Pure R^2 Gravity

Colloquium Abstracts

A Proposed Mechanism for the Difference in the Radius Dependence of Water Flow through Carbon and Boron Nitride Nanotubes

J.B. Sokoloff  (Northeastern University)

Falk, et. al., proposed a mechanism that qualitatively explains the large dependence on the tube radius of the slip length and permeability of water flowing through carbon nanotubes. Despite the fact that boron nitride nanotubes have a crystal structure that is similar that of carbon nanotubes, such large radius dependence of the slip length and permeability on tube radius is not observed in boron nitride nanotubes. The enhancement of the slip length and permeability when the radius reaches 15nm, which is much larger than the size of a water molecular and atomic spacing in the tube wall in carbon, but not boron nitride nanotubes, observed by Secchi, et. al., can be accounted for by a reduction in the contribution to the friction from electron excitations in the wall as the radius decreases, resulting from the dependence of the electron energy band gap on the tube radius.

White Dwarfs, Gaia, and The Age of the Galaxy

Dr. Ted von Hippel (Embry Riddle Aeronautical University)

What is the star-formation history of the Milky Way? How old are Galactic halo and thick disk stars? Traditional age-dating of stars relies on clusters, which only offer a limited view of these stellar populations. I will show that white dwarf stars offer a way forward. Specifically, I will show how optical and near-IR photometry, Gaia astrometry, and a Bayesian modeling approach allows us to determine precision ages (within 2%-5%) for individual white dwarfs and derive population ages.

Gravitational lensing of gravitational waves and precision cosmology  

Prof. Zonghong Zhu

The standard siren approach of gravitational wave cosmology appeals to the direct luminosity distance estimation through the waveform signals from inspiralling double compact binaries, especially those with electromagnetic counterparts providing redshifts. It is limited by the calibration uncertainties in strain amplitude and relies on the fine details of the waveform. We will show the next generation detector, e.g., the Einstein Telescope is expected to produce 10^4 −10^5 gravitational wave detections per year, 50−100 of which will be lensed. Then we report a waveform-independent strategy to achieve precise cosmography by combining the accurately measured time delays from strongly lensed gravitational wave signals with the images and redshifts observed in the electromagnetic domain. We demonstrate that just 10 such lensing systems can provide a Hubble constant uncertainty of 0.68% for a flat Lambda Cold Dark Matter universe in the era of third-generation ground-based detectors

Spontaneous Breaking of Restricted Weyl Symmetry in Pure R^2 Gravity

Prof. Ariel Edery

Recent work has shown that pure R^2 gravity (i.e. R^2 gravity with no extra R term) has a symmetry that is larger than scale symmetry and smaller than full Weyl symmetry. This has been dubbed  restricted Weyl symmetry  as it involves a Weyl transformation where the conformal factor has a constraint.  Most importantly, this symmetry is spontaneously broken when the vacuum (background spacetime) has a non-zero Ricci scalar. In this case, the theory can be shown to be equivalent to Einstein gravity with non-zero cosmological constant and a massless scalar field. The massless scalar field is identified as the Goldstone boson of the broken sector. In spontaneously broken theories, the original symmetry of the Lagrangian is realized as a shift symmetry of the Goldstone bosons. We show that this is the case also here. The unbroken R=0 sector is completely different and has no connection to Einstein gravity.