Bjoern Penning is New Assistant Professor of Physics

Bjoern PenningBjoern Penning has joined the Physics department as a new Assistant Professor. He researches dark matter (DM) and has performed direct DM searches at the LUX-Zeplin (LZ) experiment and collider DM searches with CMS and ATLAS.

At Brandeis, he is a member of the High-Energy Physics Group. He will focus on direct dark matter searches with LZ and phenomenological dark matter research.

Penning received his Ph.D. from the University of Freiburg. Previous to his arrival at Brandeis, Penning was a Lecturer in Experimental Particle Physics at the University of Bristol.

Penning will teach Particle Physics (PHYS 107b) during the Fall 2017 semester.

Marcelle Soares-Santos Joins the Physics Department

Marcelle Soares-Santos

Marcelle Soares-Santos is joining Brandeis as an Assistant Professor in the Physics department starting in September 2017. Soares-Santos will continue her research into the nature of the accelerated expansion of the Universe.  She is also a member of the Dark Energy Survey (DES) Collaboration and the Large Synoptic Survey Telescope Dark Energy Science Collaboration (LSST/DESC).

Nature recently profiled Marcelle in “Turning point: Galactic groundbreaker. In the article, she discusses her research, career trajectory and future plans.

SciFest VII Wraps Up Summer 2017 Undergraduate Research Session

The Brandeis University Division of Science held its annual undergraduate research poster session SciFest VII on August 3, 2017, as more than one hundred student researchers presented summer’s (or last year’s) worth of independent research. We had a great audience of grad students and postdocs (many of whom were mentors), faculty, proud parents, friends, and senior administrators.

More pictures and abstract books are available at the SciFest site.

SciFest VII by numbers

Chakraborty lab provides new understanding on the physics of granular materials

By Kabir Ramola, Ph.D

In the late 1980’s Sir Sam Edwards proposed a framework for describing the large scale properties of granular materials, such as sand or salt. In this description, similar to the well-established framework of statistical mechanics, the global properties of a complex system are determined by an average over all possible microscopic configurations consistent with a given global property. This is usually attributable to the very fast dynamics of the constituent particles making up the system. The extension of such treatments to granular systems where particles are static or ‘jammed’ represents a fundamental challenge in this field. Even so, Edwards’ conjecture postulated that for given external parameters such as volume, all possible packings of a granular material are equally likely. Such a conjecture, like Boltzmann’s hypothesis in statistical mechanics, can then be used as a starting point to develop new physical theories for such materials based on statistical principles. Indeed, several frameworks have been developed assuming this conjecture to be true.

Figure 1 : Snapshot of the system studied and illustration of the associated energy landscape at different volume fractions.

A simple illustration of this conjecture would be, if one were to pour sand into a bowl, and not bias the preparation in any way, then all the trillion trillions of configurations allowed for the grains would be equally likely. Clearly such a conjecture is utterly infeasible to test experimentally.  In a recent paper that appeared in Nature Physics, we instead performed detailed numerical computations on a theoretical system of soft disks (in two dimensions) with hard internal cores. We focused on a system of 64 disks which already pushed the limits of current computational power. We found that if one fixes the density of a given system of disks, the probability of a packing occurring depends on the pressure, violating Edwards’ proposition. However, at a critical density, where particles just begin to touch or ‘jam’, this probability remarkably becomes independent of the pressure, and all configurations are indeed equally likely to occur. This jamming point is in fact very interesting in its own right since most granular materials are found at the threshold of being jammed and ‘unjammed’. To be fair to Edwards, the hypothesis was made for ‘hard’ grains in which particles are precisely at this threshold, and therefore our numerics seem to confirm the original statement. This is the first time that this statement has been out to a direct test and will no doubt lead to many interesting directions in the future.

Links to news sources describing this article:

doi: 10.1038/nphys4168
Numerical test of the Edwards conjecture shows that all packings are equally probable at jamming.
Stefano Martiniani, K. Julian Schrenk, Kabir Ramola, Bulbul Chakraborty & Daan Frenkel.
Nature Physics


Colleagues and Students Gather for Astrophysics Symposium

by Roopesh Ojha (PhD ’98)

Radio Galaxy NGC 4261. (credit: Teddy Cheung)

From June 28th through 30th, about fifty former and current students, colleagues and friends of Brandeis astrophysics Professors John Wardle and David Roberts gathered in the Physics building for a symposium titled “When Brandeis met Jansky: astrophysics and beyond.” This event was organized to celebrate their achievements in astrophysics and their impact on generations of students. Their work has established Brandeis as a major player in radio astronomy.

The symposium title refers to Karl Jansky who is credited with starting an entirely new means of studying the cosmos using radio waves. Radio astronomy arrived at Brandeis with Professor Wardle in 1972. He was joined in 1980 by Professor Roberts and together they pioneered a very powerful observational technique called Very Long Baseline Polarimetry. This involves the use of telescopes separated by thousands of kilometers to produce the sharpest images available to astronomers. Their methods allow astronomers to map the magnetic fields in and near celestial objects. With their students and colleagues, John and Dave have exploited this technique to study the magnetic fields in quasars and active galaxies, and near super massive black holes far outside our Milky Way galaxy as well as black holes closer to home.

Physics Conference Group

Professors John Wardle and David Roberts (front right) with former students and colleagues on the steps of the Abelson physics building (photo: Mike Lovett)

The reach of John and Dave’s work was reflected in the content of the presentations and the composition of the attendees, some of whom had traveled from as far afield as South Korea, India, and Europe. All major centers of radio astronomy were represented. At the conference dinner, several former students expressed their appreciation for the roles Dave and John have played as their mentors.

In their presentations, Dave and John described their current projects and highlighted the work of their undergraduates, graduate students and postdoctoral fellows, who have all gone on to successful careers in academia and industry.

The nineteen PhD theses produced by the Brandeis Radio Astronomy group

Professor Roberts has decided to retire at the end of August, though his retirement plans include a huge program of continuing research into unusual-shaped radio galaxies. These may represent galaxy mergers and the possible merger of their central black holes, and is being carried out with colleagues in India. Professor Wardle has no intention of retiring and is expanding his horizons so to speak — he is part of the Event Horizon Telescope collaboration, an international team of astronomers that is attempting to make the first image of the ‘event horizon’* of a black hole!

The symposium was organized by Teddy Cheung (PhD ’05, now at the Naval Research Laboratory) and Roopesh Ojha (PhD ’98, now at NASA, Goddard Space Flight Center), with generous help and support from the Physics Department.

* The boundary around a black hole beyond which nothing can escape.

Two Brandeis Professors Receive 2017 Simons Fellowships, part II

Spectral Flow

Spectral Flow (full caption below)

Read Part I

Two Brandeis professors have been awarded highly prestigious and competitive Simons Fellowships for 2017. Daniel Ruberman received a 2017 Simons Fellowship in Mathematics. Matthew Headrick was awarded a 2017 Simons Fellowship in Theoretical Physics. This is the second of two articles where each recipient describes their award-winning research.

Daniel Ruberman’s research asks “What is the large-scale structure of our world?” Einstein’s unification of physical space and time tells us that the universe is fundamentally 4-dimensional. Paradoxically, the large-scale structure, or topology, of 4-dimensional spaces, is much less understood than the topology in other dimensions. Surfaces (2-dimensional spaces) are completely classified, and the study of 3-dimensional spaces is largely dominated by geometry. In contrast, problems about spaces of dimension greater than 4 are translated, using the technique called surgery theory, into the abstract questions of algebra.

Ruberman will work on several projects studying the large-scale topology of 4-dimensional spaces. His work combines geometric techniques with the study of partial differential equations arising in physics. One major project, with Nikolai Saveliev (Miami) is to test a prediction of the high-dimensional surgery theory, that there should be `exotic’ manifolds that resemble a product of a circle and a 3-dimensional sphere. The proposed method, which would show that this prediction is incorrect, is to compare numerical invariants derived from the solutions to the Yang-Mills and Seiberg-Witten equations, by embedding both in a more complicated master equation. The study of the Seiberg-Witten invariants is complicated by their instability with respect to varying geometric parameters in the theory. A key step in their analysis is the introduction of the notion of end-periodic spectral flow, which compensates for that instability, as illustrated below.

Other projects for the year will apply techniques from 4-dimensional topology to classical problems of combinatorics and geometry about configurations of lines in projective space. In recent years, combinatorial methods have been used to decide if a specified incidence relation between certain objects (“lines”) and other objects (“points”) can be realized by actual points and lines in a projective plane. For the real and complex fields, one can weaken the condition to look for topologically embedded lines (circles in the real case, spheres in the complex case) that meet according to a specified incidence relation. Ruberman’s work with Laura Starkston (Stanford) gives new topological restrictions on the realization of configurations of spheres in the complex projective plane.

Caption: Solutions to the Seiberg-Witten equations of quantum field theory provide topological information about 4-dimensional spaces. However, the set of solutions, or moduli space, can undergo a phase transition as a parameter T is varied, making those solutions hard to count. This figure illustrates a key calculation: the phase transition is equal to the end-periodic spectral flow, a new concept introduced in work of Mrowka-Ruberman-Saveliev. In the figure, the spectral set, illustrated by the red curves, evolves with the parameter T. Every time the spectral set crosses the cylinder, the moduli space changes, gaining or losing points according to the direction of the crossing.

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