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Research Highlights 2011-12

Includes fast estimation of higher-order correlators for large scale structure, electroweak symmetry breaking and the matter asymmetry of the universe, and MaGICC disks: matching observed galaxy relationships

The scientific output of the COSMOS consortium has continued with about 100 publications and arXiv papers since January 2011 in four key inter-related areas: (i) extreme universe, (ii) cosmic microwave sky, (iii) dark energy and (iv) galaxy formation. Three highlight projects are:

Fast Estimation of Higher-order Correlators for Large-scale Structure: A critical observational test of inflation is whether or not Gaussian statistics accurately describe the primordial perturbations which seeded galaxies. We have developed and implemented a fast non-Gaussian estimator for the 3D particle distribution in N-body codes, i.e. we rapidly calculate the three-point correlator or bispectrum (arXiv:1207.5678). The method relies on a separable modal expansion, implemented for Planck CMB analysis in 2D. For the first time the computational cost of bispectrum estimation becomes small relative to N-body evolution, so it can be used as a regular diagnostic with the power spectrum. We have studied the evolving gravitational and primordial dark matter bispectra for both Gaussian and non-Gaussian initial conditions (using several bispectrum shapes). In the nonlinear regime (k < 2hMpc−1), we find an excellent correlation between the measured bispectrum and a simple model based on a ‘constant’ plus the first-order gravitational bispectrum. The amplification of the constant mode in the primordial bispectrum can be understood as a gravitational bispectrum time-shift. We are incorporating effects relevant for the analysis of galaxy surveys.



Left: Dark matter distribution in a (40Mpc/h)3 section from an N-body simulation at z = 4, 0. Right: Measured bispectrum in the range 0.016h/Mpc ≤ k ≤ 2h/Mpc, reflecting the changing morphology of structures, from edges (pancakes) to equilateral (filaments/points).

Electroweak Symmetry Breaking and the Matter Asymmetry of the Universe: We are now able to simulate the whole electroweak sector of the Standard Model, in real-time and out of thermal equilibrium. We model the quantum fermions using the ensemble method and treat the bosonic scalar and non-abelian gauge fields classically (see JHEP 1107, 066 (2012) and 1111.7136). Our main interest is electroweak baryogenesis, and we test the approach by considering Standard Model baryon number violation through the chiral anomaly. In principle, we are able to compute the asymmetry between matter and antimatter generated in the early Universe, and compare it to that which is observed. In its ultimate implementation, large scale simulations are required of the order of millions of cpu hours and 10TB of memory.

MaGICC Disks: Matching Observed Galaxy Relationships: The MaGICC project (see MNRAS 424, 1275, 2012) - Making Galaxies in a Cosmological Context – has been addressing a key problem of galaxy modeling: the formation of far too many stars and in the wrong place, typically a ball of stars around a fairly ‘stubby’ star disk (unlike the extended disk of the Milky Way). By being less conservative in the energy outflows from very massive stars (i.e. dialing a higher energy release figure for exploding stars), we find simulated galaxies more closely resemble the Milky Way. This study implies that large-scale outflows are the primary driver of the dependence of disk galaxy properties on mass. Simulated galaxies form sub-components which can be assigned to a thin stellar disk, thick disk, and a low mass stellar halo via a chemical decomposition (arXiv:1206.0740). This work will impact Gaia mission science exploitation.