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14:00 Georges Obied
Title: Topological gravity as the early phase of our universe
Abstract:
Motivated by string dualities we propose topological gravity as the early phase of our universe. The topological nature of this phase naturally leads to the explanation of many of the puzzles of early universe cosmology. A concrete realization of this scenario using Witten's four dimensional topological gravity is considered. This model leads to the power spectrum of CMB fluctuations which is controlled by the conformal anomaly coefficients $a,c$. In particular the strength of the fluctuation is controlled by $1/a$ and its tilt by $c g^2$ where $g$ is the coupling constant of topological gravity. The positivity of $c$, a consequence of unitarity, leads automatically to an IR tilt for the power spectrum. In contrast with standard inflationary models, this scenario predicts $\mathcal{O}(1)$ non-Gaussianities for four- and higher-point correlators and the absence of tensor modes in the CMB fluctuations.
14:30 Anirudh Prabhu
Title: Astrophysical Signatures of Dense Axion Clumps
Abstract:
Originally proposed as part of a solution to the strong CP problem, the axion is a promising dark matter candidate in part because known mechanisms of axion production can generate the correct abundance to account for dark matter. In certain cosmological scenarios, small-scale spikes may form in the spectrum of axion fluctuations, which subsequently collapse to form dense axion clumps. Due to the axion coupling to photons, axion clumps have a rich set of associated electromagnetic signatures, which can potentially be detected by existing and planned radio astronomy missions. I will present two such potentially detectable astrophysical signatures. The first signature pertains to the dispersive refraction of light in the presence of transverse field gradients in an axion clump. The second arises from resonant conversion of axion clumps to photons in pulsar magnetospheres.