Scientific Series

An astrophysical black hole is completely described with just two parameters: its mass and its dimensionless spin. A few dozen black holes have mass estimates, but until recently none had a reliable spin estimate. The first spins have now been measured for black holes in X-ray binaries.

Suppose we are given two probability distributions on some N-element set. How many samples do we need to test whether the two distributions are close or far from each other in the L_1 norm? This problem known as Statistical Difference has been extensively studied during the last years in the field of property testing. I will describe quantum algorithms for Statistical Difference problem that provide a polynomial speed up in terms of the query complexity compared to the known classical lower bounds. Specifically, I will assume that each distribution can be generated by querying an oracle function on a random uniformly distributed input string. It will be shown that testing whether distributions are orthogonal requires approximately N^{1/2} queries classically and approximately N^{1/3} queries quantumly. Testing whether distributions are close requires approximately N^{2/3} queries classically and O(N^{1/2}) queries quantumly. This is a joint work with Aram Harrow (University of Bristol) and Avinatan Hassidim (The Hebrew University).

Astronomers have discovered many candidate black holes in the universe and have studied their properties in ever-increasing detail. Over the last decade, a few groups have developed observational tests for the presence of event horizons in candidate black holes. The talk will discuss one of these tests, which indicates that the supermassive black hole at the center of our Galaxy must have a horizon.

I will discuss the possibility that a 'Wilson line' degree of freedom can play the role of an inflaton in a warped flux compactification, in the context of the DBI inflationary scenario. I will show how warped DBI Wilson line inflation offers an attractive alternative to ordinary (position field) DBI inflation, inasmuch as observational and theoretical constraints get considerably relaxed. Thus, besides the large non-Gaussianities produced in DBI scenarios, Wilson lines allow for an observable amount of gravitational waves, within consistent approximations.

Over the last two years, the Compact Muon Solenoid (CMS) detector has been installed in the tunnel of the Large Hadron Collider (LHC) at CERN and commissioned to its full functionality. The CMS detector successfully collected beam halo and beam dump data, while the beams were circulating in the LHC in September 2008. After the LHC incident, the commissioning of CMS continued with a one month campaign of continuous cosmic rays data taking at nominal magnetic field. This allowed further tuning of the detector, consolidation of its operation and characterization of its performances. In this talk, the status of the CMS detector and its performance, in view of LHC collisions in 2009, will be described.

We derive geometric correlation functions in the new spinfoam model with coherent states techniques, making connection with quantum Regge calculus and perturbative quantum gravity. In particular we recover the expected scaling with distance for all components of the propagator. We expect the same technique to be well-suited for other spinfoam models.

We have only scratched the surface of the potential for using large-scale structure (LSS) as a probe of fundamental physics/cosmology, i.e., quantitatively, we have only measured a small fraction of a percent of the accessible LSS information. Future measurements will probe dark energy, inflation, dark matter properties, neutrino masses, modifications of gravity, etc. with unprecedented precision. I will discuss three probes of LSS: the traditional galaxy redshift survey, the Lyman-alpha forest (LyaF), and the new idea of 21 cm intensity mapping; and two future experiments that cover these probes: SDSS-III/BOSS (galaxies and LyaF) and the proposed CHIME (21 cm). I will discuss recent theoretical/phenomenological developments that promise to greatly enhance the power of LSS surveys, related to the connection between bias, redshift-space distortions, and non-Gaussianity.

A standard canonical quantization of general relativity yields a time-independent Schroedinger equation whose solutions are static wavefunctions on configuration space. Naively this is in contradiction with the real world where things do change. Broadly speaking, the problem how to reconcile a theory which contains no concept of time with a changing world is called 'the problem of time'. In this seminar we shall study this problem using a reformulation of Newtonian mechanics due to Jacobi (Jacobi's timeless mechanics) which allows one to study the problem of time without all the technical difficulties present in quantized general relativity. We show explicitly that Jacobi's timeless mechanics is a straightforward counterexample to the claim that all first class constraints generate gauge transformations, i.e. physically indistinguishable states. The implications of this is unclear. By making use of deBroglie-Bohm trajectories we derive a necessary and sufficient condition for a time-dependent Schroedinger equation to emerge for subsystems. The importance of 'strong' entanglement between subsystem and environment for the emergence of time is stressed.

Braneworlds are a fascinating way of hiding extra dimensions by confining ourselves to live on a brane. One particular model (Randall-Sundrum) has a link to string theory via living in anti de Sitter space. I'll describe how the ads/cft correspondence has been used to claim that a braneworld black hole would tell us how Hawking radiation back reacts on spacetime, thus solving one of the outstanding problems of quantum gravity - the ultimate fate of an evaporating black hole. I'll review evidence for this conjecture, ending with some recent work that shows it may be problematic.

Motivated by recent mathematical developements in non-commutative Donaldson-Thomas theory, we construct a new statistical mechanicalmodel of crystal melting to count BPS bound states of D0 and D2 branes on a single D6 brane wrapping an arbitrary toric Calabi-Yau threefold. We also discuss the wall crossing phenomena, which are crucial for the proper understanding of the relation between the crystal melting and the topological string theory.

The world will start to run out of cheap, conventionally produced oil much sooner than most people expect, possibly within the next decade. This talk will discuss the reasoning that leads to that conclusion and the likely consequences if it is correct. It may be possible, with considerable difficulty to substitute other fossil fuels for the missing oil, but if we do that we may do irreparable damage to the Earth’s climate. And even then we would start to run out of all fossil fuels, including coal, probably within this century. Can civilization survive if that happens? We will consider the possibilities.