Journal Club: 9/11/10

We will meet as usual at 11:30am in B1.19 (after coffee).  This week Kei Yamamoto and Baojiu Li will, respectively, be leading the discussion on:

Cosmological magnetic fields from inflation in extended electromagnetism
Jose Beltran Jimenez, Antonio L. Maroto
arXiv:1010.3960

N-body Simulations for f(R) Gravity using a Self-adaptive Particle-Mesh Code
Gong-Bo Zhao, Baojiu Li and Kazuya Koyama
arXiv:1011.1257


Full Details of this Week's Papers:
Cosmological magnetic fields from inflation in extended electromagnetism
Jose Beltran Jimenez, Antonio L. Maroto
arXiv:1010.3960

In this work we consider an extended electromagnetic theory in which the scalar state which is usually eliminated by means of the Lorenz condition is allowed to propagate. This state has been shown to generate a small cosmological constant in the context of standard inflationary cosmology. Here we show that the usual Lorenz gauge-breaking term now plays the role of an effective electromagnetic current. Such a current is generated during inflation from quantum fluctuations and gives rise to a stochastic effective charge density distribution. Due to the high electric conductivity of the cosmic plasma after inflation, the electric charge density generates currents which give rise to both vorticity and magnetic fields on sub-Hubble scales. Present upper limits on vorticity coming from temperature anisotropies of the CMB are translated into lower limits on the present value of cosmic magnetic fields. We find that, for a nearly scale invariant vorticity spectrum, magnetic fields $B_{\lambda}> 10^{-12}$ G are typically generated with coherence lengths ranging from sub-galactic scales up to the present Hubble radius. Those fields could act as seeds for a galactic dynamo or even account for observations just by collapse and differential rotation of the protogalactic cloud. 

N-body Simulations for f(R) Gravity using a Self-adaptive Particle-Mesh Code
Gong-Bo Zhao, Baojiu Li and Kazuya Koyama
arXiv:1011.1257

We perform high resolution N-body simulations for f(R) gravity based on a self-adaptive particle- mesh code MLAPM. The Chameleon mechanism that recovers General Relativity on small scales is fully taken into account by self-consistently solving the non-linear equation for the scalar field. We independently confirm the previous simulation results, including the matter power spectrum, halo mass function and density profiles, obtained by Oyaizu et al. (Phys.Rev.D 78, 123524, 2008) and Schmidt et al. (Phys.Rev.D 79, 083518, 2009), and extend the resolution up to k~20 h/Mpc for the measurement of the matter power spectrum. Based on our simulation results, we discuss how the Chameleon mechanism affects the clustering of dark matter and halos on full non-linear scales.