Cosmology & Gravitation

This series consists of talks in the areas of Cosmology, Gravitation and Particle Physics.

Seminar Series Events/Videos

Currently there are no upcoming talks in this series.

 

Tuesday Sep 09, 2008
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The so-called cosmological backreaction arises when one directly averages the Einstein equations to recover cosmology. While usually applied to avoid employing dark energy models, strictly speaking any cosmological model should be built from such an averaging procedure rather than an assumed background. We apply the Buchert formalism to Einstein-de Sitter, Lambda CDM and quintessence cosmologies, and as a first approach to the full problem, evaluate numerically the discrepancies arising from linear perturbation theory between the averaged behaviour and the assumed behaviour.

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Tuesday Jul 29, 2008
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At the end of inflation, dynamical instability can rapidly deposit the energy of homogeneous cold inflation into excitations of other fields. This process (known as preheating) essentially starts the hot big bang as we know it. I will present simulations of several preheating models using a new numerical solver DEFROST I developed. The results trace the evolution of the fields, which quickly become very inhomogeneous as the instability kicks in. Surprisingly, there appears to be a certain universality across preheating models with different decay channels.

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Tuesday Jun 17, 2008
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Observations of the Milky Way by the SPI/INTEGRAL satellite have confirmed the presence of a strong 511 KeV gamma-ray line emission from the bulge, which require an intense source of positrons in the galactic center. These observations are hard to account for by conventional astrophysical scenarios, whereas other proposals, such as light DM, face stringent constraints from the diffuse gamma-ray background. I will describe how light superconducting strings could be the source of the observed 511 KeV emission.

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Tuesday Jun 10, 2008
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In this talk I will analyse the stochastic background of gravitational waves coming from a first order phase transition in the early universe. The signal is potentially detectable by the space interferometer LISA. I will present a detailed analytical model of the gravitational wave production by the collision of broken phase bubbles, together with analytical results for the gravitational wave power spectrum. Gravitational wave production by turbulence and magnetic fields will also be briefly discussed.

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Tuesday May 13, 2008
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Recent developments in the field of Numerical Relativity have not only provided key insights of binary black hole systems but also began influencing its future role. Undoubtedly one of the most important future drivers in the near future of the field will be its role as another element within the study of spectacular astrophysical phenomena involving strongly gravitation scenarios. Connecting (yet to be observed) gravitational waves with observations within the electromagnetic spectra will be one ultimate goal of this enterprise.

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Thursday May 08, 2008
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I will review relativistic quantum theory that is based on Wigner\'s unitary representations of the Poincare group, Dirac\'s forms of dynamics, and Newton-Wigner\'s definition of the position operator. Formulas will be derived that transform particle observables between different inertial reference frames. In the absence of interactions, these formulas coincide with Lorentz transformations from special relativity. However, when interaction is turned on, some deviations appear.

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Tuesday May 06, 2008
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We study the effective field theory of inflation, i.e. the most general theory describing the fluctuations around a quasi de Sitter background, in the case of single field models. The scalar mode can be eaten by the metric by going to unitary gauge. In this gauge, the most general theory is built with the lowest dimension operators invariant under spatial diffeomorphisms, like g^{00} and K_{mu nu}, the extrinsic curvature of constant time surfaces.

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Monday May 05, 2008
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I will review an old (Greenberg and Schweber, 1958) and undeservedly forgotten idea in quantum field theory. This idea allows one to reformulate QFT as a Hamiltonian theory of physical (rather than bare) particles and their direct interactions. The dressed particle approach is scattering-equivalent to the traditional one, however it doesn\'t require renormalization and may provide a valuable tool for calculations of wave functions of bound states and time evolution.

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