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 Dec 01, 2009
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The problem of the quantum backreaction in expanding spaces is an old, as yet unresolved, question. In this talk I will consider the one-loop backreaction of a massless scalar which couples to the Ricci scalar in an expanding space with constant deceleration. I will show that the infrared divergences, which generically plague the one loop stress energy, can be removed by matching onto an earlier radiation era. An insignificant backreaction occurs, unless the coupling to the Ricci scalar is negative. Similar results hold for the graviton backreaction.

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Friday Nov 27, 2009
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I will discuss the qualitative differences between the single-field and multifield cyclic universes, in particular the resulting global "phoenix" structure and its relation to dark energy. The multifield cyclic universe arises naturally from embedding the cyclic universe in supergravity and leads to distinct observational predictions regarding non-gaussian signatures in the CMB. I will present a simplified derivation of these predictions.

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Tuesday Nov 24, 2009
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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.

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Tuesday Nov 17, 2009
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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.

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Tuesday Nov 03, 2009
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Underlying the standard cosmological model is the assumption that it is possible to coarse-grain the energy density of the Universe, and that the dynamical and optical properies of space-time should be well modelled by the result. However, even if the average coarse-grained geometry does have the same dynamical properties as the fine-grained system it is intended to imitate, there are good reasons to suspect that the optical properties may be different.

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Tuesday Oct 27, 2009
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Weak gravitational lensing is a powerful probe of modifications of General Relativity on cosmological scales, since such modifications can affect both how matter produces gravitational potential wells and how photons move within these wells. I will discuss alternative theories of gravitation and how we may constrain such theories using weak lensing observables, including those that could be obtained with the balloon-borne High Altitude Lensing Observatory (HALO).

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Tuesday Sep 29, 2009
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Near the Planckian scales, quantum gravity is expected to drastically change the structure of spacetime, one feature of which may be noncomutativity of the coordinates. Based on the recent advances in
quantum field theories on such noncommutative spaces, I will consider the
fluctuations of inflaton and look for possible noncommutative corrections
in the CMB. Anisotropy and non-gaussianity are the result. The resultant
distribution is then compared with ACBAR, CBI and WMAP data to constrain
the scale of noncommutativity parameter.

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Thursday Sep 24, 2009
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I describe a number of techniques that allow for the generation of (near) scale-invariant fluctuations in the early Universe without inflation or ekpyrosis. The basic ingredient is a decaying maximal speed of propagation, for which a Universal law is found. Connections are made with k-essence, the cuscaton, and the DBI action. However the simplest realizations result from bimetric theories and deformed dispersion relations and DSR. A number of implications to theories of quantum gravity are discussed.

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