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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.

 

Mardi oct 22, 2013
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We
will explore the role that conformal symmetries may play in cosmology. First,
we will discuss  the symmetries underlying the statistics of the
primordial perturbations which seeded the temperature anisotropies of the
Cosmic Microwave Background. I will show how symmetry considerations lead us to
three broad classes of theories to explain these perturbations: single-field
inflation, multi-field inflation, and the conformal mechanism. We will discuss
the symmetries in each case and derive their model-independent consequences.

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Mardi oct 15, 2013

Loop quantum
cosmology (LQC) proposes a quantization for homogeneous cosmologies which
success in solving the classical singularity problem. Realistic scenarios call
for the consideration of inhomogeneities. Focusing on the simplest inhomogenous
cosmological model, the Gowdy model with three-torus spatial topology
and linearly polarized gravitational waves, I'll describe an approach to treat
inhomogeneities in the framework of loop quantum cosmology. This is a hybrid

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Jeudi oct 10, 2013
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Mardi oct 08, 2013
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Local-type primordial non-Gaussianity couples
statistics of the curvature perturbation \zeta on vastly different physical
scales. Because of this coupling, statistics (i.e. the polyspectra) of \zeta in
our Hubble volume may not be representative of those in the larger universe -- that
is, they may be biased. The bias depends on the local background value of
\zeta, which includes contributions from all modes with wavelength k ~ and is therefore enhanced if the entire post-inflationary patch is large

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Mardi sep 24, 2013
Speaker(s): 

I'll discuss a
number of insights into the process of nonlinear structure formation which come
from the study of random walks crossing a suitably chosen barrier.  These
derive from a number of new results about walks with correlated steps, and
include a unified framework for the peaks and excursion set frameworks for
estimating halo abundances, evolution and clustering, as well as nonlinear,
nonlocal and stochastic halo bias, all of which matter for the next generation
of large scale structure datasets.

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Mardi sep 17, 2013
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ΛCDM has become the standard cosmological model because its
predictions agree so well with observations of the cosmic microwave background
and the large-scale structure of the universe. However ΛCDM has faced
challenges on smaller scales. Some of these challenges, including the “angular
momentum catastrophe" and the absence of density cusps in the centers of
small galaxies, may be overcome with improvements in simulation resolution and
feedback. Recent simulations appear to form realistic galaxies in agreement

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Mardi sep 03, 2013
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I will review recent developments in our theoretical understanding of the abundance and clustering of dark matter haloes. In the first part of this talk, I will discuss a toy model based on the statistics of peaks  of Gaussian random field (Bardeen et al 1986) and show how the clustering properties of such a point set can be easily derived from a generalised local bias expansion. In the second part, I will explain how  this peak formalism relates to the excursion set approach and present parameter-free predictions for the  mass function and bias of dark matter halos.

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Mardi juil 30, 2013
Speaker(s): 

We
employ the effective field theory approach for multi-field inflation which is a
generalization of Weinberg's work. In this method the first correction terms in
addition to standard terms in the Lagrangian have been considered. These terms
contain up to the fourth derivative of the fields including the scalar field
and the metric. The results show the possible shapes of the interaction terms
resulting eventually in non-Gaussianity in a general formalism. In addition
generally the speed of sound is different but almost unity. Since in this

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Mardi mai 14, 2013
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Screened Scalar-Tensor gravity such as chameleon and symmetron theories allow order one deviations from General Relativity on large scales whilst satisfying all local solar-system constraints. A lot of recent work has therefore focused on searching for observational signatures of these

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Mardi mai 07, 2013
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Ultra-light axions (m_a
I will also present preliminary results of constraints to this model using up-to-date cosmological observations, which verify the above picture. The parameter space is interesting to explore due to a strongly mass dependent covariance matrix, motivating comparisons between Metropolis-Hastings and nested sampling. Finally I discuss fine-tuning and naturalness in these models.
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