Scientific Series

After 50 years of dreaming about it, space-based microlensing observations are now underway.  A 2014 100-hr Spitzer Pilot Program generated "microlens parallaxes" for dozens of lenses, opening the prospect of measuring the Galactic distribution of planets.  This program will be expanded 8-fold in 2015.  Analogous observations by Kepler will measure the mass function of free-floating planets.

WFIRST microlensing observations will, as advertised, "complete the planetary census" but they will do an immense amount of astrophysics as well.  I discuss how microlensing's take off builds on rapid, ongoing, ground-based developments.

Does a generic quantum system necessarily thermalize? Recent developments in disordered many-body quantum systems have provided crucial insights into this long-standing question. It has been found that sufficiently disordered systems may fail to thermalize leading to a 'many-body localized' phase. In this phase, the fundamental assumption underlying equilibrium statistical mechanics, namely, the equal likelihood for all states at the same energy, breaks down. A fundamental question is: what happens as the disorder becomes weaker so that one approaches the localization-delocalization transition? For example, does the system thermalize *at* the transition? 

In this talk, I will show that very general considerations on the scaling of entanglement entropy close to the transition imply that at a continuous many-body localization transition, the system is necessarily ergodic.

Finally, I will present recent results on "eigenstate thermalization", a long standing hypothesis which posits that a single eigenstate hides within itself a thermal ensemble. In particular, I will discuss which class of operators do or do not satisfy eigenstate thermalization.

Using holography, I will describe an approach for understanding the physics of a big bang singularity by translating the problem into the language of the dual quantum field theory. Certain two-point correlators in the dual field theory are sensitive to near-singularity physics in a dramatic way, and this provides an avenue for investigating how strong quantum gravity effects in string theory might modify the classical description of the big bang.

I will talk about the physics of models in which dark matter consists of composite bound states carrying a large conserved dark “nucleon” number. The properties of sufficiently large dark nuclei may obey simple scaling laws, and this scaling can determine the number distribution of nuclei resulting from Big Bang Dark Nucleosynthesis. For plausible models of asymmetric dark matter, dark nuclei of large nucleon number, e.g. >~ 10^8, may be synthesised, with the number distribution taking one of two characteristic forms, which interestingly are broadly independent of initial conditions. A possible consequence of these scenarios is alterations to direct detection signals, which may be coherently enhanced (relative to collider signals), and could be modified by new momentum-dependent form factors. Inelastic interactions between dark matter states might also be important in astrophysical settings.

Visible matter consists mostly of hydrogen and helium, only a small fraction
of which is in stars. Until recently, the bulk of the gas in the local
universe was in fact not seen. In the largest structures, massive galaxy
clusters, the gas is seen via its x-ray emission, but in the much more
numerous groups and isolated galaxies, it has not been possible to detect
it. I will describe how, in the last year or so, the situation has changed,
with the detection of a cross-correlation between the thermal SZ effect and
lensing maps, and through the stacking of SZ images at the locations of
galaxies. These and similar techniques will tell us about the physics by
which stars and massive black holes heat and move gas in and around the
halos of galaxies and cluster, and will allow for tighter constraints on the
value of the primordial power spectrum (sigma_8).

The interplay of quantum mechanics and inter-particle interactions leads to enormously rich tapestry of quantum phases of matter. In this talk I will illustrate the unique synthesis offered by quantum entanglement on the landscape of quantum phases. I will especially discuss phases which do not show any kind of ordering even at the absolute zero temperature, two prime examples being spin liquids and quantum Hall phases. These phases are fascinating because they can exhibit extraordinary properties such as emergent fermions in a system composed only of bosons, or systems where the elementary excitations are neither fermions, nor bosons (“anyons”). I will discuss how quantum entanglement not only plays a crucial role in characterizing and diagnosing such quantum disordered phases, but in fact it also sheds light on their stability.

The Kerr metric of vacuum general relativity is expected to describe astrophysical black holes. Boson stars, on the other hand, are one of the simplest gravitating solitons, suggested as astrophysical compact objects, black holes mimickers and as dark matter candidates. Kerr black holes with scalar hair, found in [1], continuously interpolate between these two types of, per se, physically interesting solutions. I will describe the construction of these solutions and discuss theoretical, astrophysical and high energy physics aspects and challenges for Kerr black holes with scalar hair.

References:

[1] Kerr black holes with scalar hair, Carlos A. R. Herdeiro, Eugen Radu, Phys. Rev. Lett. 112 (2014) 221101; e-Print: arXiv:1403.2757

[2] A new spin on black hole hair, C. Herdeiro, E. Radu, International Journal of Modern Physics D 23 (2014) 1442014; e-Print: arXiv: 1405.3696;

[3] Construction and physical properties of Kerr black holes with scalar hair, C. Herdeiro and E. Radu, arXiv:1501.04319 [gr-qc]; Focus Issue on "Black holes and fundamental fields" to appear in Classical and Quantum Gravity

We study the separability of quantum states in bosonic system. Our main tool here is the "separability witnesses", and a connection between "separability witnesses" and a new kind of positivity of matrices--- "Power Positive Matrices" is drawn. Such connection is employed to demonstrate that multi-qubit quantum states with Dicke states being its eigenvectors is separable if and only if two related Hankel matrices are positive semidefinite. By employing this criterion, we are able to show that such state is separable if and only if it's partial transpose is non-negative, which confirms the conjecture in [Wolfe, Yelin, Phys. Rev. Lett. (2014)]. Then, we present a class of bosonic states in d⊗d system such that for general d, determine its separabilityNP-hard although verifiable conditions for separability is easily derived in case d=3,4.

We investigate through non-equilibrium molecular dynamic simulations the flow of anomalous
fluids inside rigid nanotubes. Our results reveal an anomalous increase of the overall mass flux
for nanotubes with sufficiently smaller radii. This is explained in terms of a transition from a
single-file type of flow to the movement of an ordered-like fluid as the nanotube radius increases.
The occurrence of a global minimum in the mass flux at this transition reflects the competition
between the two characteristic length scales of the core-softened potential. Moreover, by increasing
further the radius, another substantial change in the flow behavior, which becomes more evident at
low temperatures, leads to a local minimum in the overall mass flux. Microscopically, this second
transition is originated by the formation of a double-layer of flowing particles in the confined
nanotube space. These nano-fluidic features give insights about the behavior of confined isotropic
anomalous fluids.

The remnant accretion disk formed in binaries involving neutron stars and/or black holes is a source of non-relativistic ejecta. This 'disk wind' is launched on a thermal and/or viscous timescale, and can provide an amount of material comparable to that in the dynamical ejecta. I will present recent work aimed at characterizing

the properties of these winds through time-dependent radiation-hydrodynamic simulations that include the relevant physics needed to follow the ejecta composition. I will focus on the effect of black hole spin and/or hypermassive neutron star lifetime on the disk wind, and on the interaction of the wind with the dynamical ejecta. I will also discuss the implications of these results for the optical/IR signal from these events, and for the origin of r-process elements in the Galaxy.

The talk will be based on a work in progress with Stefano Kovacs
(Dublin IAS) and Yuki Sato (Wits University). In a previous work
(arxiv:1310.0016) we have shown that,
in the M-theory regime (large N with the Chern-Simon level k fixed)
of the duality between ABJM theory and M-theory on AdS4 x S7/Zk,
certain monopole operators with large R charges on the gauge theory side
correspond to spherical membranes
(which is in general in non-BPS excited states) in the pp-wave matrix
model on the dual side.

Having in mind application to
the study of three point functions of the monopole operators
from the dual side, we study the BPS instanton equation
of the pp-wave matrix model. The instanton equation describes, for example,
a process in which a single spherical membrane splits into
two spherical membranes. Under a certain
approximation which is valid when the matrix size is large,
the instanton equation can be recast
into a three dimensional Laplace equation;
a time snapshot of the membrane configuration corresponds
to an equipotential surface of the solution of the Laplace equation.
In order to study the above mentioned splitting process,
we found that one has to introduce a special boundary condition
of the Laplace equation: one prepares two copies of
three dimensional space which are connected in a manner analogous
to Riemann surfaces. We will discuss an exact solution
of the Laplace equation under this boundary condition, and 
the corresponding instanton solution, in which a membrane splits into two.