3d Spinfoam Quantum Gravity: Matter as a Phase of the Group Field Theory

An effective field theory for matter coupled to three-dimensional quantum gravity was recently derived in the context of spinfoam models in hep-th/0512113. In this paper, we show how this relates to group field theories and generalized matrix models. In the first part, we realize that the effective field theory can be recasted as a matrix model where couplings between matrices of different sizes can occur. In a second part, we provide a family of classical solutions to the three-dimensional group field theory. By studying perturbations around these solutions, we generate the dynamics of the effective field theory. We identify a particular case which leads to the action of hep-th/0512113 for a massive field living in a flat non-commutative space-time. The most general solutions lead to field theories with non-linear redefinitions of the momentum which we propose to interpret as living on curved space-times. We conclude by discussing the possible extension to four-dimensional spinfoam models.Comment: 17 pages, revtex4, 1 figur

Relaxing Cosmological Constraints on Large Extra Dimensions

We reconsider cosmological constraints on extra dimension theories from the excess production of Kaluza-Klein gravitons. We point out that, if the normalcy temperature is above 1 GeV, then graviton states produced at this temperature will decay early enough that they do not affect the present day dark matter density, or the diffuse gamma ray background. We rederive the relevant cosmological constraints for this scenario.Comment: 17 pages, latex, revtex4; added a short discussion of other constraints, reference

A proteomic analysis of the ventral and dorsal hippocampal brain areas of serotonin knockout rats

Thesis (MScMedSc (Biomedical Sciences. Medical Physiology)--Stellenbosch University, 2008.For many centuries, scientists have engaged in a theoretical debate concerning the etiology of mood disorders, with very few ancient scholars speculating about the importance of genetic factors and affective temperaments as factors in the etiology of depression. Mood, emotion and cognition have been shown to be modulated by the serotonergic midbrain raphe system; implicated in the pathogenesis of psychiatric disorders like those of the affective spectrum. Evidence from neuroscience, genetics, and clinical investigation demonstrate that depression is a disorder of the brain. Brain imaging research is revealing that in depression, neural circuits responsible for moods, thinking, sleep, appetite, and behavior fail to function properly, and that the regulation of critical neurotransmitters is impaired. Genetics research, including studies of twins, indicates that genes play a role in depression. Vulnerability to depression appears to result from the influence of multiple genes acting together with environmental factors. Other research has shown that stressful life events, particularly in the form of loss such as the death of a close family member, may trigger major depression in susceptible individuals. Depression and anxiety have often been successfully treated by means of selective serotonin reuptake inhibitors. However, selective serotonin reuptake inhibitors do not solve all the problems inherent to the treatment of depression, for approximately 30 % of depressed patients do not respond to treatment and 20 % experience relapses whilst on treatment. Of consideration is the fact that the majority of drugs today are based on proteins, with 50 % of therapeutics on the market targeting cell membrane proteins. Up to this day the precise pathophysiology of mood disorders remains obscure, as does the neurobiology of normal mood regulation. Accordingly, there is a need for methods to identify the structural and/or signaling components which lead to changes in the brain, particularly the hippocampus, of subjects having mood disorders such as bipolar depressive disorder, chronic major depressive disorder and the like. Similarly, there is a need for the early detection, screening and diagnosis of individuals at risk for a mood disorder. As the serotonin tranpsorter is the primary target for therapeutic intervention in the treatment of numerous psychiatric disorders and considering the fact that at the structural level this protein’s function as transporter in membranes remains incompletely understood, investigating its function in psychiatric disorders are of importance . The objective of this study was to determine the role of the serotonin transporter in wild type and serotonin knockout rats, with regards to the hippocampus. Rat hippocampi were fractionated into cytosolic and membrane components, which were run and further separated in two dimensions. Firstly separation occurred by isoelectrical focusing (pI), follwed by gel iii electrophoresis (molecular weight). Gels were compared to see whether protein spots have changed between animals that have been differentially bred. Differentially expressed protein spots, as determined by PD Quest software, were excised, digested and analyzed by means of mass spectrometry. Our results indicated that metabolic, structural and cell signaling proteins were differentially expressed in both the ventral and dorsal hippocampus of the serotonin knockout rat. Futhermore, cellular stress proteins were found to be only differentially expressed in the ventral hippocampus. The majority of proteins identified in both hippocampal areas as well as both fractions, were assigned to energy metabolism. The cytosolic protein profile mirrored the pattern of the membrane protein profile. In conclusion, this proteomic study identified various protein groups that interacted with one another, thus establishing compensation for disrupted serotonin homeostasis

Observables in 3d spinfoam quantum gravity with fermions

We study expectation values of observables in three-dimensional spinfoam quantum gravity coupled to Dirac fermions. We revisit the model introduced by one of the authors and extend it to the case of massless fermionic fields. We introduce observables, analyse their symmetries and the corresponding proper gauge fixing. The Berezin integral over the fermionic fields is performed and the fermionic observables are expanded in open paths and closed loops associated to pure quantum gravity observables. We obtain the vertex amplitudes for gauge-invariant observables, while the expectation values of gauge-variant observables, such as the fermion propagator, are given by the evaluation of particular spin networks.Comment: 32 pages, many diagrams, uses psfrag

Limit on the mass of a long-lived or stable gluino

We reinterpret the generic CDF charged massive particle limit to obtain a limit on the mass of a stable or long-lived gluino. Various sources of uncertainty are examined. The $R$-hadron spectrum and scattering cross sections are modeled based on known low-energy hadron physics and the resultant uncertainties are quantified and found to be small compared to uncertainties from the scale dependence of the NLO pQCD production cross sections. The largest uncertainty in the limit comes from the unknown squark mass: when the squark -- gluino mass splitting is small, we obtain a gluino mass limit of 407 GeV, while in the limit of heavy squarks the gluino mass limit is 397 GeV. For arbitrary (degenerate) squark masses, we obtain a lower limit of 322 GeV on the gluino mass. These limits apply for any gluino lifetime longer than $\sim 30$ ns, and are the most stringent limits for such a long-lived or stable gluino.Comment: 15 pages, 5 figures, accepted for publication in JHE

Effective action and semiclassical limit of spin foam models

We define an effective action for spin foam models of quantum gravity by adapting the background field method from quantum field theory. We show that the Regge action is the leading term in the semi-classical expansion of the spin foam effective action if the vertex amplitude has the large-spin asymptotics which is proportional to an exponential function of the vertex Regge action. In the case of the known three-dimensional and four-dimensional spin foam models this amounts to modifying the vertex amplitude such that the exponential asymptotics is obtained. In particular, we show that the ELPR/FK model vertex amplitude can be modified such that the new model is finite and has the Einstein-Hilbert action as its classical limit. We also calculate the first-order and some of the second-order quantum corrections in the semi-classical expansion of the effective action.Comment: Improved presentation, 2 references added. 15 pages, no figure

Euclidean three-point function in loop and perturbative gravity

We compute the leading order of the three-point function in loop quantum gravity, using the vertex expansion of the Euclidean version of the new spin foam dynamics, in the region of gamma<1. We find results consistent with Regge calculus in the limit gamma->0 and j->infinity. We also compute the tree-level three-point function of perturbative quantum general relativity in position space, and discuss the possibility of directly comparing the two results.Comment: 16 page

The Zero Age Main Sequence of WIMP burners

We modify a stellar structure code to estimate the effect upon the main sequence of the accretion of weakly interacting dark matter onto stars and its subsequent annihilation. The effect upon the stars depends upon whether the energy generation rate from dark matter annihilation is large enough to shut off the nuclear burning in the star. Main sequence WIMP burners look much like protostars moving on the Hayashi track, although they are in principle completely stable. We make some brief comments about where such stars could be found, how they might be observed and more detailed simulations which are currently in progress. Finally we comment on whether or not it is possible to link the paradoxically young OB stars found at the galactic centre with WIMP burners.Comment: 4 pages, 3 figs. Matches published versio

Spatial Modulation Microscopy for Real-Time Imaging of Plasmonic Nanoparticles and Cells

Spatial modulation microscopy is a technique originally developed for quantitative spectroscopy of individual nano-objects. Here, a parallel implementation of the spatial modulation microscopy technique is demonstrated based on a line detector capable of demodulation at kHz frequencies. The capabilities of the imaging system are shown using an array of plasmonic nanoantennas and dendritic cells incubated with gold nanoparticles.Comment: 3 pages, 4 figure