# 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.

## A robust constraint on cosmic textures from the cosmic microwave background

Tuesday Dec 11, 2012
Speaker(s):

Fluctuations in the cosmic
microwave background (CMB) contain information which has been pivotal in
establishing the current cosmological model. These data can also be used to
test well-motivated additions to this model, such as
cosmic textures. Textures are a type of topological defect that
can be produced during a cosmological phase transition in the early universe,
and which leave characteristic hot and cold spots in the CMB. We apply Bayesian
methods to carry out an optimal test of the texture hypothesis, using

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## Nonlinear Extensions for the Inflationary Power Spectra

Tuesday Dec 11, 2012
Speaker(s):

We expound several principles in an attempt to clarify
the debate over infrared loop corrections to the primordial scalar and tensor
power spectra from inflation. Among other things we note that existing
proposals for nonlinear extensions of the scalar fluctuation field $\zeta$
introduce new ultraviolet divergences which no one understands how to
renormalize. Loop corrections and higher correlators of these putative
observables would also be enhanced by inverse powers of the slow roll parameter

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## CMB as a Probe of New Physics: The Story of Cosmic Birefringence

Thursday Dec 06, 2012
Speaker(s):

Cosmological
birefringence is a postulated rotation of the linear polarization of photons
that arises due to a Chern-Simons coupling of a new scalar field to
electromagnetism. In particular, it appears as a generic feature of simple
quintessence models for Dark Energy, and therefore, should it be detected,
could provide insight into the microphysics of cosmic acceleration. Prior work
has sought this rotation, assuming the rotation angle to be uniform across the
sky, by looking for the parity-violating TB and EB correlations in the CMB

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## Mass and Tension (momentum) Sum Rules in AdS (dS)

Tuesday Nov 27, 2012
Speaker(s):

The stress-energy tensor in a conformal field theory has
zero trace.

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## Holography for inflation using conformal perturbation theory.

Thursday Nov 22, 2012
Speaker(s):

Holographic cosmology maps cosmological time evolution to
the inverse RG ﬂow of a dual three-dimensional QFT. In cases where this RG ﬂow
connects two closely separated ﬁxed points, QFT correlators may be calculated
perturbatively in terms of the conformal ﬁeld theory associated with one of the
ﬁxed points, even when the dual QFT is at strong  coupling.
Realising slow-roll inﬂation in these terms, we show how to derive
standard slow-roll inﬂationary power spectra and non-Gaussianities through

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## An exact solution of the Dirac equation with CP violation.

Tuesday Nov 20, 2012
Speaker(s):

After a brief overview of electroweak baryogenesis,  I will show how to construct a solution of
the Dirac equation for a CP violating kink wall. This solution nicely reduces
to the known solution for a CP violating thin (step) wall.  The novel solution can be helpful for studies
of baryogenesis sources at strong first order phase transitions, which is
relevant for electroweak scale baryogenesis studies.

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## Solid of Inflation: An alternative symmetry breaking pattern for an EFT of inflation

Thursday Nov 15, 2012
Speaker(s):

In this talk I will
discuss a cosmological model where primordial inflation is driven by a `solid',
defined as a system of three derivatively coupled scalar fields obeying certain
symmetries and spontaneously breaking a certain subgroup of these. The symmetry
breaking pattern differs drastically from that of standard inflationary models:
time translations are unbroken. This prevents our model from fitting into the
standard effective field theory description of adiabatic perturbations. Consequently,

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## Recent progress on de Sitter S-matrix

Tuesday Nov 13, 2012
Speaker(s):

The de Sitter S-matrix provides a gauge-invariant and
field redefinition-invariant window into de Sitter QFTs and may provide a
crucial entry in any dS/CFT dictionary. In this talk I will summarize recent
progress on developing the S-matrix for theories with gauge fields and
perturbative gravity. Nonrenormalization theorems, hints of supersymmetry, and
perturbative stability will be discussed.

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## Primordial Magnetic Fields & Non-Gaussianity

Tuesday Nov 06, 2012
Speaker(s):

There are indications of a lower bound on magnetic fields
in the intergalactic media. Since magnetic fields on such large scales are
difficult to generate in the late universe, this might point to inflationary
magnetogenesis as the origin of intergalactic magnetic fields. If the magnetic
fields are generated during inflation, they are naturally correlated with the
inflaton perturbations in a general class of models. This leads to a
consistency relation between the power spectrum of primordial magnetic fields

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## Kicking Chameleons: Early Universe Challenges for Chameleon Gravity

Thursday Oct 25, 2012
Speaker(s):

Chameleon gravity is
a scalar-tensor theory that mimics general relativity in the Solar System. The
scalar degree of freedom is hidden in high-density environments because the
effective mass of the chameleon scalar depends on the trace of the
stress-energy tensor.  In the early Universe, when the trace of the
stress-energy tensor is nearly zero, the chameleon is very light and Hubble
friction prevents it from reaching its potential minimum.  Whenever a
particle species becomes non-relativistic, however, the trace of the

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