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 Mar 29, 2016
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Since the Universe was filled with hydrogen atoms at early times, the

most promising method for observing the epoch of the first stars is to

measure the hyperfine line of hydrogen at a wavelength of 21 cm. Such

an observation of the "cosmic dawn" era was considered speculative

when I helped lay out the theoretical predictions a decade ago, but

there is now an enormous, promising observational effort (in which I

am involved within the Square Kilometre Array). There is also an
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Tuesday Mar 22, 2016
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I will discuss the cosmology of galileon models with a Minkowski limit and discuss whether they can account for the currently observed cosmological model. The full galileon model predicts the speed of gravitational waves to be different from that of photons. I will discuss this and compare with observations. I will then discuss a subdominant galileon model which is compatible. Finally I will discuss the shape dependence of screening in galileon models, showing that the fifth force is unscreened for planar objects.

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Tuesday Mar 15, 2016
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I will discuss phenomena associated with particle production and field excitation during inflation. In the first part of the talk I will present several

signatures that can originate from the coupling of an axion inflaton to gauge fields. In the second part I will test the robustness of the standard

implications associated with the detection of a gravity wave (GW) signal at CMB scales, by discussing what conditions can allow a visible

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Tuesday Mar 08, 2016
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Alternative theories of gravity are popular alternatives to the LCDM model because they can self-accelerate without a cosmological constant. On smaller scales, consistency with solar system tests of gravity is achieved by utilising screening mechanisms, which act to hide fifth-forces locally. This makes them difficult to distinguish from general relativity. In this talk I will describe recent work using astrophysical objects---stars, galaxies, and clusters---as new and novel probes of alternative gravity theories.

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Tuesday Feb 16, 2016

The kinetic Sunyaev-Zel'dovich effect is a direct probe of the distribution and velocity of electrons on cosmological scales. Recent progress in Cosmic Microwave Background observations allow statistical detections of this subtle effect originating from a number of different tracers populations. In my talk, I will review the observational status, highlight the consequences for astrophysics and cosmology and discuss future directions.

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Tuesday Feb 09, 2016
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The global redshifted 21-cm radiation background is expected to be a powerful probe of the re-heating and re-ionization of the intergalactic medium. However, its measurement is technically challenging: one must extract the small, frequency-dependent signal from under much brighter and spectrally smooth foregrounds. Traditional approaches to study the global signal have used single-antenna systems, where one must calibrate out frequency-dependent structure in the overall system gain, as well as remove the noise bias from auto-correlating a single amplifier output.

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Tuesday Feb 02, 2016
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The Atacama Cosmology Telescope (ACT) has been pushing our measurements of the Cosmic Microwave Background on small scales to high resolution and deeper sensitivity since 2008. While ACT stopped taking temperature-only measurements in 2010, ACTPol is now operating with polarisation-sensitive detectors. I will present some of the current ACTPol results in terms of the power spectrum constraints.

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Thursday Jan 14, 2016
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One possibility for studying reheating is to link the duration and final temperature after reheating, and its equation of state, to inflationary observables. By restricting the equation of state to lie within a broad physically allowed range, one can bracket an allowed range of $n_s$ and $r$ for models of inflation. The results are similar to, but do a little better, than requiring the length of inflation lie between 50 and 60 efolds. The added constraints can help break degeneracies between inflation models that otherwise overlap in their predictions.

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Thursday Jan 14, 2016
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According to the standard model of cosmology 96% of the matter and energy in the universe is invisible. The dark matter particles comprising the invisible material have so far not been detected in laboratory and astrophysical experiments.  The dark energy responsible for the acceleration of the universe is still a controversial issue. A modified gravity theory is presented that can potentially fit current cosmological and astrophysical data. The black holes and their shadows predicted by MOG can differ from the predictions of Einstein gravity.

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Tuesday Jan 12, 2016

Fast radio bursts (FRBs) are bright, broadband, non-repeating, millisecond flashes of unknown astronomical origin.

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