Scientific Series

How many interacting quantum (field) theories of four-dimensional geometry are there which have General Relativity as their classical limit? Some of us still harbour hopes that a quantum theory of gravity is "reasonably unique", i.e. characterized by a finite number of free parameters. One framework in which such universality may manifest itself is that of "Quantum Gravity from Causal Dynamical Triangulations (CDT)". I will summarize the rationale behind this nonperturbative formulation and CDT's main achievements in trying to explain the micro- and macro-structure of spacetime from first principles. This includes the remarkable property of "dynamical reduction" of the spacetime dimension from four to two at the Planck scale.

A new force mediated by a new vector boson with mass in the MeV to GeV range and with very weak coupling to ordinary matter appears naturally in many theoretical models and could also explain a variety of observed anomalies. Such anomalies include the discrepancy between the predicted and the experimentally observed value for the muon anomalous magnetic moment, and recent cosmic-ray data that can be explained by dark matter interacting through this force with ordinary matter. This talk will review the motivation for such a force and present a broad array of probes of this physics. These probes include high-luminosity e+e- colliders, such as BaBar and BELLE, whose existing data sets may contain thousands of spectacular events; new high-intensity fixed-target experiments at electron accelerators such as Jefferson Laboratory; and indirect astrophysical probes such as gamma-ray observations of Milky-Way dwarf satellite galaxies, which constitute some of the least luminous and most dark matter dominated galaxies known.

ABJM theory is a world-volume theory for an arbitrary number of M2-branes. One of the unique features of ABJM theory is its characteristic scaling behaviour, exhibited for example by the free energy and correlation functions of chiral primary operators. In more detail, ABJM theory has a holographic dual where thermodynamics at strong coupling is determined by a system of black M2-branes. The zero-coupling (black-body radiation) free energy disagrees with the strong coupling result. Even the scaling in the 't Hooft coupling is different (strongly suppressed at strong coupling). It is therefore important to check that the weak and strong coupling results converge as loop corrections are taken into account. The leading order computation indeed confirms that the first correction goes in the right direction.

Gamma-ray production by dark matter annihilation is one of the most universal indirect dark matter signals. In order to avoid intensive astrophysical background, one can study the gamma-rays away from the Galactic plane. The problems is that the dark matter annihilation signal at high latitudes is smooth and most probably subdominant to Galactic and extragalactic fluxes. I will discuss the use of spherical harmonics decomposition as a tool to distinguish a large scale small amplitude dark matter signal from astrophysical fluxes. The sensitivity of this method for currently available Fermi data is similar to the signal from thermal WIMP dark matter annihilation into W+W- or b-bbar.

Recently rediscovered results in the theory of partial differential equations show that for free fields, the properties of the field in an arbitrarily small volume of space, traced through eternity, determine completely the field everywhere at all times. Over finite times, the field is determined in the entire region spanned by the intersection of the future null cone of the earliest event and the past null cone of the latest event. Thus this paradigm of classical field theory exhibits a fascinating form of nonlocality. I'll discuss this result and what it tells us about the possibility of constructing a classical, nonlocal theory which accommodates all the phenomena we observe.

The long awaited discovery of the double radio pulsar system, PSR J0737-3039A/B, surpassed most expectations, both theoretical and observational, as a tool to probe general relativity, stellar evolution and pulsar theories. The Double Pulsar provides a unique and the most complete and clean test of theories of gravity in a regime sensitive to possible strong-gravitational self-field effects. All six post-Keplerian parameters have been measured (including the measurement of the relativistic spin precession), some parameters to a precision of 10^{-4}.

The resonant tunneling phenomenon is well understood in quantum mechanics. I argue why a similar phenomenon must be present in quantum field theory. Using the functional Schr\"odinger method I show how resonant tunneling through multiple barriers takes place in quantum field theory with a single scalar field. I also show how this phenomenon in scalar quantum field theory can lead to an exponential enhancement of the single-barrier tunneling rate. My analysis is carried out in the thin-wall approximation. I discuss a possible explanation of the fast nucleation of the B phase of superfluid Helium-3 as an application.

While understanding the evolution of galaxies is one of the major themes of contemporary astronomy, most empirical studies focus only on the evolution of distribution functions (e.g., the luminosity function), effectively treating galaxies in isolation. The new generation of large imaging and spectroscopic surveys make it possible to measure the clustering of galaxies with different physical properties as a function of redshift, providing complementary information to traditional distribution function studies. This approach is especially powerful, because most theoretical models of galaxy evolution are based on the underlying distribution of dark matter halos and they make strong predictions for clustering evolution. To link galaxies to dark matter halos from the observed galaxy clustering, I will introduce the halo occupation distribution (HOD), which characterizes the relation between galaxies and dark matter halos by the probability distribution that a halo of virial mass M contains N galaxies of a given type, together with the spatial and velocity distributions of galaxies within halos. I will present HOD modeling results for galaxy clustering measured in several surveys, including the SDSS (z~0), the DEEP2 (z~1), and the NOAO Deep Wide-Field Surveys (0<z<1). I will demonstrate that, by linking galaxies to dark matter halos, HOD modeling of galaxy clustering opens a new direction in studying galaxy evolution (and cosmology).

Quantum Mechanics has been shown to provide a rigorous foundation for Statistical Mechanics. Concentration of measure, or typicality, is the main tool to construct a purely quantum derivation for the methods of Statistical Mechanics. From this point of view statistical ensembles are effective description for isolated quantum systems, since typically a random pure state of the system will have properties similar to those of the ensemble. Nevertheless, it is often argued that most of the states of the Hilbert space are not relevant for realistic systems. This talk will address this issue, presenting recent results on the emergence of typicality in the context of matrix product states, a set of physically and computationally relevant states associated to many-body Hamiltonians.

Cosmic strings, generic in brane inflationary models, may be detected by the current generation of gravitational wave detectors. An important source of gravitational wave emission is from isolated events on the string called cusps and kinks. I first review cosmic strings, discussing their effective action and motion, and showing how cusps and kinks arise dynamically. I then show how allowing for the motion of the strings in extra dimensions gives a potentially significant reduction in signal strength, and comment on current LIGO bounds.

I will discuss a powerful way to examine the nature of dark energy using a measurement of the growth of galaxy clusters over cosmic time. A novel technique that uses the Cosmic Microwave Background as a backlight allows the detection of galaxy clusters out to the time of their first formation. Using this technique, I will present the first constraints on cosmological parameters obtained with the Atacama Cosmology Telescope, as well as exciting prospects for the future.

I discuss a new scenario called Hylogenesis (hylo=matter) that explains the baryon and dark matter densities of Universe in a unified way. Early universe dynamics generate the baryon asymmetry and an equal antibaryon asymmetry in a GeV-scale hidden sector. The hidden antibaryons are dark matter. Our model has a striking signature that dark matter can annihilate baryons, mimicking nucleon decay. I discuss the effective nucleon decay rates and implications for existing nucleon decay searches.