Scientific Series

The relation between loop quantum gravity (LQG) and ordinary quantum field theory (QFT) on a fixed background spacetime still bears many obstacles. When looking at LQG and ordinary QFT from a mathematical perspective it turns out that the two frameworks are rather different: Although LQG is a true continuum theory its Hilbert space is defined in terms of certain embedded graphs which are labeled by irreducible representations of SU(2). The natural arena for ordinary QFT, on the other hand, is a Fock space which strongly uses the metric properties of the underlying continuum spacetime. In this talk I will review this issue and show how one can use Born--Oppenheimer methods to further progress towards an understanding of (matter) quantum field theories from first principles.

Of all four forces only the weak interaction has experimentally exhibited parity violation. At the same time observations suggest that general relativity may require modification to account for dark matter and dark energy. Could it be that this modification involves gravitational parity violation? Many of the dominant approaches to quantum gravity, such as string theory and loop quantum gravity, point to an effective parity violating extension to general relativity known as Chern-Simons General Relativity (CSGR). In this colloquium I will discuss the uniqueness and phenomenological implications of parity violation in CSGR. In particular, I will discuss how CSGR can work together with inflation to generate the cosmic baryon asymmetry in a most economical fashion, through gravitational waves; and its predictions for upcoming CMB polarization experiments. I will also discuss current predictions of CSGR on binary pulsars, neutron stars and prospects for the LISA/LIGO gravitational wave detectors. While we focus on a specific theory for concreteness, some of the results presented in this colloquium can be seen as model independent.

Thanks to the ongoing Planck mission, a new window will be opened on the properties of the primordial density field, the cosmological parameters, and the physics of reionization. Much of Planck's new leverage on these quantities will come from temperature measurements at small angular scales and from polarization measurements. These both depend on the details of cosmological hydrogen recombination; use of the CMB as a probe of energies greater than 10^16 GeV compels us to get the ~eV scale atomic physics right. One question that remains is how high in hydrogen principle quantum number we have to go to make sufficiently accurate predictions for Planck. Using sparse matrix methods to beat computational difficulties, I have modeled the influence of very high (up to and including n=200) excitation states of atomic hydrogen on the recombination history of the primordial plasma, resolving all angular momentum sub-states separately and including, for the first time, the effect of hydrogen quadrupole transitions. I will review the basic physics, explain the resulting plasma properties, discuss recombination histories, and close by discussing the effects on CMB observables.

For generic field theories at finite temperature, a power-law falloff of correlation functions of conserved currents at long times is a prediction of non-linear hydrodynamics. We demonstrate, through a one-loop computation in Einstein gravity in Anti de Sitter space, that this effect is reproduced by the dynamics of black hole horizons. The result is in agreement with the gauge-gravity correspondence.

Dualities appear in nearly all disciplines of physics and play a central role in statistical mechanics and field theory. I will discuss in a pedagogical way our recent findings motivated by a quest for a simple unifying framework for the detection and treatment of dualities. I will explain how classical and quantum dualities, as well as duality relations that appear only in a sector of certain theories (i.e. emergent dualities), can be unveiled, and systematically established. Our method relies on the use of morphisms of the "bond algebra" of a quantum Hamiltonian. Dualities are characterized as unitary mappings implementing such morphisms, whose even powers become symmetries of the quantum problem. Dual variables (non-local mappings between the elementary degrees of freedom of the theory) which were guessed in the past can be derived in our formalism. New self-dualities for four-dimensional Abelian gauge field theories will be discussed.

Is there a theory yet to be discovered that underlies quantum theory and explains its structure? If there is such a theory, one of the features it will have to explain is the central role of complex numbers as probability amplitudes. In this talk I explore the physical meaning of the statement “probability amplitudes are complex” by comparing ordinary complex-vector- space quantum theory with the real-vector-space theory having the same basic structure. Specifically, I discuss three questions that bring out qualitative differences between the two theories: (i) Is information about a preparation expressed optimally in the outcomes of a measurement? (ii) Are multipartite states locally accessible? (iii) Is entanglement “monogamous”?

In his brilliant article "Against 'Measurement'", John Bell famously argued that the word has had such a damaging effect on the discussion, that it should now be banned altogether in quantum mechanics. But in the beginning was the word, and the word is still with us. Indeed, David Mermin responded In Praise of Measurement that within the field of quantum computer science the concept of measurement is precisely defined, unproblematic, and forms the foundation of the entire subject, a verdict reaffirmed by the development of measurement-based quantum computation. Bell's arguments deserve a more direct response: I shall try to give one.

Two possible explanations for the type SNe Ia supernovae observations are a nonlinear, underdense void embedded in a matter dominated Einstein-de Sitter spacetime or dark energy in the ?CDM model. Both of these alternatives are faced with Copernican fine-tuning problems. A case is made for the void scenario that avoids introducing undetected dark energy.

Non-relativistic versions of the AdS/CFT conjecture have recently been investigated in some detail. These have primarily been in the context of the Schrodinger symmetry group. Here we talk of a study based on a different non-relativistic conformal symmetry: one obtained by a parametric contraction of the relativistic conformal group. The resulting Galilean conformal symmetry has the same number of generators as the relativistic symmetry group and thus is different from the Schrodinger group (which has fewer). One of the interesting features of the Galilean Conformal Algebra is that it admits an extension to an infinite dimensional symmetry algebra (which can potentially be dynamically realised). The latter contains a Virasoro-Kac-Moody subalgebra. We comment on realisations of this extended symmetry in a boundary field theory. We also propose a somewhat unusual geometric structure for the bulk gravity dual to any realisation of this symmetry. This involves taking a Newton-Cartan like limit of Einstein's equations in anti de Sitter space which singles out an $AdS_2$ comprising of the time and radial direction. The infinite dimensional Virasoro extension is identified with the asymptotic isometries of this $AdS_2$.

We explore a new scenario explaining mass origin of standard model (SM) particles without a Higgs boson. In this framework SM W, Z gauge bosons and fermions are composites getting masses from confinement of substructure at IR (conformal symmetry breaking). Therefore here SM electroweak gauge symmetry and its breaking are IR emergent phenomena. Using AdS/CFT we build a calculable warped 5D model. Realistic mass spectrum and good fit to electroweak precision data (S, T parameters) can be obtained. Furthermore the composite nature of W,Z may offer novel solution for WW scattering unitarization and predicts deviation from SM which can lead to distinctive signatures at the LHC.

How sure are you that spacetime is continuous? One approach to quantum gravity, causal set theory, models spacetime as a discrete structure: a causal set. This talk begins with a brief introduction to causal sets, then describes a new approach to modelling a quantum scalar field on a causal set. We obtain the Feynman propagator for the field by a novel procedure starting with the Pauli-Jordan commutation function. The candidate Feynman propagator is shown to agree with the continuum result. This model opens the door to physical predictions for scalar matter on a causal set.

It has been conjectured that higher-dimensional rotating black holes become unstable at a sufficiently large value of the rotation, and that new black holes with pinched horizons appear at the threshold of the instability. We search numerically, and find, the stationary axisymmetric perturbations of Myers-Perry black holes with a single spin that mark the onset of the instability and the appearance of the new black hole phases. We also find new ultraspinning Gregory-Laflamme instabilities of rotating black strings and branes.