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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 nov 05, 2013

A central problem in
galaxy formation is to understand why star formation is so inefficient. Within
individual galaxies, gas is converted into stars at a rate two orders of
magnitude slower than unimpeded gravitational collapse predicts, a fact
embodied in the low normalization of the observed Kennicutt-Schmidt (K-S)
relationship between star formation rate surface density and gas surface
density. Star formation in galaxies is also globally inefficient in the sense

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Jeudi oct 31, 2013

In this talk I will give an
introduction to some of my research into modified gravity over the last three
years. I will begin by describing my implementation of chameleon models into
supersymmetry and discuss some of the new features and cosmology that arise
in this formalism. I will then change direction and talk about my work
using astrophysical effects as novel probes of modified gravity theories
and present some new results on modified gravity stellar oscillation theory.

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Mardi oct 29, 2013

of gravitational lensing in the Cosmic Microwave Background (CMB)
directly probe the projected distribution of dark matter out to high
redshifts. The CMB lensing maps thus encode a wealth of information about both
fundamental physics (e.g., dark energy and neutrino properties) and
high-redshift astrophysics. I will illustrate this by first reviewing
measurements of CMB lensing with the Atacama Cosmology Telescope, discussing
both CMB lensing auto-correlations and cross-correlations with quasars,

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Mardi oct 22, 2013

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

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

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

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

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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