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Search Results: 1 - 10 of 10584 matches for " Patrick Greene "
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Inflationary Reheating and Fermions
Patrick B. Greene
Physics , 1999, DOI: 10.1063/1.59436
Abstract: Coherent oscillations of the inflaton field at the end of inflation can parametrically excite fermions in much the same way that bosons are created in preheating. Although Pauli-blocking prohibits the occupation number of created fermions from growing exponentially, fermion production occurs in a manner significantly different from the expectations of simple perturbation theory. Here, I discuss the nature of fermion production after inflation and possible applications including the efficient transfer of inflaton energy and the production of super-massive fermions during fermionic preheating.
Preheating of Fermions
Patrick B. Greene,Lev Kofman
Physics , 1998, DOI: 10.1016/S0370-2693(99)00020-9
Abstract: In inflationary cosmology, the particles constituting the Universe are created after inflation in the process of reheating due to their interaction with the oscillating inflaton field. In the bosonic sector, the leading channel of particle production is the non-perturbative regime of parametric resonance, preheating, during which bosons are created exponentially fast. Pauli blocking prohibits the unbounded creation of fermions. For this reason, it has been silently assumed that the creation of fermions can be treated with perturbation theory for the decay of individual inflatons. We consider the production of fermions interacting with the coherently oscillating inflatons. We find that the actual particle production occurs in a regime of the parametric excitation of fermions, leading to preheating of fermions. Fermion preheating differs significantly from the perturbative expectation. It turns out that the number density of fermions varies periodically with time. The total number of fermions quickly saturates to an average value within a broad range of momenta $\propto q^{1/4}$, where $q$ is the usual resonance parameter. The resonant excitation of fermions may affect the transfer inflaton energy, estimations of the reheating temperature, and the abundance of superheavy fermions and gravitinos. Back in the bosonic sector, outside of the parametric resonance bands there is an additional effect of parametric excitation of bosons with bounded occupation number in the momentum range $\propto q^{1/4}$.
On the Theory of Fermionic Preheating
Patrick B. Greene,Lev Kofman
Physics , 2000, DOI: 10.1103/PhysRevD.62.123516
Abstract: In inflationary cosmology, the particles constituting the Universe are created after inflation due to their interaction with moving inflaton field(s) in the process of preheating. In the fermionic sector, the leading channel is out-of equilibrium particle production in the non-perturbative regime of parametric excitation, which respects Pauli blocking but differs significantly from the perturbative expectation. We develop theory of fermionic preheating coupling to the inflaton, without and with expansion of the universe, for light and massive fermions, to calculate analytically the occupation number of created fermions, focusing on their spectra and time evolution. In the case of large resonant parameter $q$ we extend for rermions the method of successive parabolic scattering, earlier developed for bosonic preheating. In an expanding universe parametric excitation of fermions is stochastic. Created fermions very quickly, within tens of inflaton oscillations, fill up a sphere of radius $\simeq q^{1/4}$ in monetum space. We extend our formalism to the production of superheavy fermions and to `instant' fermion creation.
Spinors, Inflation, and Non-Singular Cyclic Cosmologies
C. Armendariz-Picon,Patrick B. Greene
Physics , 2003, DOI: 10.1023/A:1025783118888
Abstract: We consider toy cosmological models in which a classical, homogeneous, spinor field provides a dominant or sub-dominant contribution to the energy-momentum tensor of a flat Friedmann-Robertson-Walker universe. We find that, if such a field were to exist, appropriate choices of the spinor self-interaction would generate a rich variety of behaviors, quite different from their widely studied scalar field counterparts. We first discuss solutions that incorporate a stage of cosmic inflation and estimate the primordial spectrum of density perturbations seeded during such a stage. Inflation driven by a spinor field turns out to be unappealing as it leads to a blue spectrum of perturbations and requires considerable fine-tuning of parameters. We next find that, for simple, quartic spinor self-interactions, non-singular cyclic cosmologies exist with reasonable parameter choices. These solutions might eventually be incorporated into a successful past- and future-eternal cosmological model free of singularities. In an Appendix, we discuss the classical treatment of spinors and argue that certain quantum systems might be approximated in terms of such fields.
Hippocampal Anatomy Supports the Use of Context in Object Recognition: A Computational Model
Patrick Greene,Mike Howard,Rajan Bhattacharyya,Jean-Marc Fellous
Computational Intelligence and Neuroscience , 2013, DOI: 10.1155/2013/294878
Abstract: The human hippocampus receives distinct signals via the lateral entorhinal cortex, typically associated with object features, and the medial entorhinal cortex, associated with spatial or contextual information. The existence of these distinct types of information calls for some means by which they can be managed in an appropriate way, by integrating them or keeping them separate as required to improve recognition. We hypothesize that several anatomical features of the hippocampus, including differentiation in connectivity between the superior/inferior blades of DG and the distal/proximal regions of CA3 and CA1, work together to play this information managing role. We construct a set of neural network models with these features and compare their recognition performance when given noisy or partial versions of contexts and their associated objects. We found that the anterior and posterior regions of the hippocampus naturally require different ratios of object and context input for optimal performance, due to the greater number of objects versus contexts. Additionally, we found that having separate processing regions in DG significantly aided recognition in situations where object inputs were degraded. However, split processing in both DG and CA3 resulted in performance tradeoffs, though the actual hippocampus may have ways of mitigating such losses. 1. Introduction We make sense of the world by comparing our immediate sensations with memories of similar situations. A very basic type of situation is an encounter with objects in a context. For example, objects such as a salt shaker, a glass, and a sink are expected in a kitchen. Even if these objects are encountered in an office, they suggest a kitchen-like function to the area (e.g., it is a kitchenette—not a work cubicle). In other words, the objects evoke the context in which they have been experienced in the past, and the context evokes objects that have been experienced there. The hippocampus, which is essential for the storage and retrieval of memories, is likely to play a central role in this associational process. In rats, the hippocampus is oriented along a dorsal-ventral axis, while in primates this axis becomes an anterior-posterior axis. In both species, signals reach the hippocampus via the entorhinal cortex (EC layers II and III), which can be divided into lateral and medial portions (denoted LEC and MEC, resp.). Both the LEC and MEC can be further subdivided into caudolateral and rostromedial bands, with the caudolateral bands projecting mainly to the posterior half of the hippocampus and the
Sine-Gordon Parametric Resonance
Patrick B. Greene,Lev Kofman,Alexei A. Starobinsky
Physics , 1998, DOI: 10.1016/S0550-3213(99)00018-8
Abstract: We consider the instability of fluctuations in an oscillating scalar field which obeys the Sine-Gordon equation. We present simple closed-form analytic solutions describing the parametric resonance in the Sine-Gordon model. The structure of the resonance differs from that obtained with the Mathieu equation which is usually derived with the small angle approximation to the equation for fluctuations. The results are applied to axion cosmology, where the oscillations of the classical axion field, with a Sine-Gordon self-interaction potential, constitute the cold dark matter of the universe. When the axion misalignment angle at the QCD epoch, $\theta_0$, is small, the parametric resonance of the axion fluctuations is not significant. However, in regions of larger $\theta_0$ where axion miniclusters would form, the resonance may be important. As a result, axion miniclusters may disintegrate into finer, denser clumps. We also apply the theory of Sine-Gordon parametric resonance to reheating in the Natural Inflation scenario. The decay of the inflaton field due to the self-interaction alone is ineffective, but a coupling to other bosons can lead to preheating in the broad resonance regime. Together with the preheating of fermions, this can alter the reheating scenario for Natural Inflation.
Everpresent Lambda
Maqbool Ahmed,Scott Dodelson,Patrick B. Greene,Rafael Sorkin
Physics , 2002, DOI: 10.1103/PhysRevD.69.103523
Abstract: A variety of observations indicate that the universe is dominated by dark energy with negative pressure, one possibility for which is a cosmological constant. If the dark energy is a cosmological constant, a fundamental question is: Why has it become relevant at so late an epoch, making today the only time in the history of the universe at which the cosmological constant is of order the ambient density. We explore an answer to this question drawing on ideas from unimodular gravity, which predicts fluctuations in the cosmological constant, and causal set theory, which predicts the magnitude of these fluctuations. The resulting ansatz yields a fluctuating cosmological ``constant'' which is always of order the ambient density.
Structure of Resonance in Preheating after Inflation
Patrick Greene,Lev Kofman,Andrei Linde,Alexei Starobinsky
Physics , 1997, DOI: 10.1103/PhysRevD.56.6175
Abstract: We consider preheating in the theory $1/4 \lambda \phi^4 + 1/2 g^2\phi^2\chi^2 $, where the classical oscillating inflaton field $\phi$ decays into $\chi$-particles and $\phi$-particles. The parametric resonance which leads to particle production in this conformally invariant theory is described by the Lame equation. It significantly differs from the resonance in the theory with a quadratic potential. The structure of the resonance depends in a rather nontrivial way on the parameter $g^2/\lambda$. We construct the stability/instability chart in this theory for arbitrary $g^2/\lambda$. We give simple analytic solutions describing the resonance in the limiting cases $g^2/\lambda\ll 1$ and $g^2/\lambda \gg 1$, and in the theory with $g^2=3\lambda$, and with $g^2 =\lambda$. From the point of view of parametric resonance for $\chi$, the theories with $g^2=3\lambda$ and with $g^2 =\lambda$ have the same structure, respectively, as the theory $1/4 \lambda \phi^4$, and the theory $\lambda /(4 N) (\phi^2_i)^2$ of an N-component scalar field $\phi_i$ in the limit $N \to \infty$. We show that in some of the conformally invariant theories such as the simplest model $1/4 \lambda\phi^4$, the resonance can be terminated by the backreaction of produced particles long before $<\chi^2>$ or $<\phi^2 >$ become of the order $\phi^2$. We analyze the changes in the theory of reheating in this model which appear if the inflaton field has a small mass.
Dynamics of Symmetry Breaking and Tachyonic Preheating
Gary Felder,Juan Garcia-Bellido,Patrick B. Greene,Lev Kofman,Andrei Linde,Igor Tkachev
Physics , 2000, DOI: 10.1103/PhysRevLett.87.011601
Abstract: We reconsider the old problem of the dynamics of spontaneous symmetry breaking using 3d lattice simulations, and develop a theory of tachyonic preheating, which occurs due to the spinodal instability of the scalar field. Tachyonic preheating is so efficient that symmetry breaking typically completes within a single oscillation of the field distribution as it rolls towards the minimum of its effective potential. As an application of this theory we consider preheating in the hybrid inflation scenario, including SUSY-motivated F-term and D-term inflationary models. We show that preheating in hybrid inflation is typically tachyonic and the stage of oscillations of a homogeneous component of the scalar fields driving inflation ends after a single oscillation. Our results may also be relevant for the theory of the formation of disoriented chiral condensates in heavy ion collisions.
Antimicrobial proteins and polypeptides in pulmonary innate defence
Mark P Rogan, Patrick Geraghty, Catherine M Greene, Shane J O'Neill, Clifford C Taggart, Noel G McElvaney
Respiratory Research , 2006, DOI: 10.1186/1465-9921-7-29
Abstract: The host response to bacterial infection of the airways is dependent on both innate (non-antibody-mediated) and adaptive (antibody-mediated) immune systems. The acquired immune system is primarily cellular in composition relying on the actions of B and T cells that are prolonged in activation and duration. However, the innate immune response is more immediate and depends on the activity of phagocytic cells such as macrophages and neutrophils and in the expression of a number of proteins and peptides, some of which are secreted by the respiratory tract epithelium and phagocytic cells. The rapidity of the innate immune system provides effective host defense against a vast array of microbes in a manner that is independent of prior exposure to the invading pathogen [1].Unlike any of the other vital organs, the lung is exposed daily to a large amount of pathogens present in air and is potentially vulnerable to infection and inflammation. For optimal gas exchange, the lung has a vast surface area (150 m2), a very thin delicate epithelium and extensive blood flow. Inherent in this structure is an enormous immunological burden. The 11,000–15,000 liters of air inhaled daily contain a myriad of pathogens, pollutants and allergens. In the normal lung, many inhaled microbes are trapped in the mucus layer coating the nasal epithelium and upper respiratory tract. Once trapped, they can be transported by ciliary motion to the pharynx and swallowed. For organisms that evade mucociliary clearance, further protective immune mechanisms act locally to facilitate clearance of inhaled pathogens and to modulate inflammatory responses.Organisms that reach the alveolar compartment are deposited in the epithelial lining fluid (ELF), a thin aqueous film containing pulmonary surfactant that lines the gas-exchanging surface of the pulmonary epithelium. Whenever this deposition occurs, the invader and the host initiate a series of complex offensive and defensive strategies. Sensing of the physio
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