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Zhao G.-B.,University of Portsmouth | Li B.,University of Cambridge | Li B.,Kavli Institute for Cosmology Cambridge | Koyama K.,University of Portsmouth
Physical Review Letters | Year: 2011

In this Letter, we investigate the environmental dependence of dark matter halos in theories which attempt to explain the accelerated expansion of the Universe by modifying general relativity (GR). Using high-resolution N-body simulations in f(R) gravity models which recover GR in dense environments by virtue of the chameleon mechanism, we find a significant difference, which depends on the environment, between the lensing and dynamical masses of dark matter halos. This environmental dependence of the halo properties can be used as a smoking gun to test GR observationally. © 2011 American Physical Society.


Bonvin C.,Kavli Institute for Cosmology Cambridge | Bonvin C.,University of Cambridge | Durrer R.,University of Geneva | Maartens R.,University of the Western Cape | Maartens R.,University of Portsmouth
Physical Review Letters | Year: 2014

If the B-mode signal in the cosmic microwave background polarization seen by the BICEP2 experiment is confirmed, it has dramatic implications for models of inflation. The result is also in tension with Planck limits on standard inflationary models. It is, therefore, important to investigate whether this signal can arise from alternative sources. If so, this could lessen the pressure on inflationary models and the tension with Planck data. We investigate whether vector and tensor modes from primordial magnetic fields can explain the signal. We find that, in principle, magnetic fields generated during inflation can indeed produce the required B mode, for a suitable range of energy scales of inflation. In this case, the primordial gravitational wave amplitude is negligible, so that there is no tension with Planck and no problems posed for current inflationary models. However, the simplest magnetic model is in tension with Planck limits on non-Gaussianity in the trispectrum. It may be possible to fine tune the magnetogenesis model so that this non-Gaussianity is suppressed. Alternatively, a weaker magnetic field can pass the non-Gaussianity constraints and allow the primordial tensor mode to be reduced to r≃0.09, thus removing the tension with Planck data and alleviating the problems with simple inflationary models. © 2014 American Physical Society.


Li B.,University of Cambridge | Li B.,Kavli Institute for Cosmology Cambridge | Zhao H.,University of St. Andrews
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2010

In this paper we present the results of N-body simulations with a scalar field coupled differently to cold dark matter (CDM) and baryons. The scalar field potential and coupling function are chosen such that the scalar field acquires a heavy mass in regions with high CDM density and thus behaves like a chameleon. We focus on how the existence of the scalar field affects the formation of nonlinear large-scale structures, and how the different couplings of the scalar field to baryons and CDM particles lead to different distributions and evolutions for these two matter species, both on large scales and inside virialized halos. As expected, the baryon-CDM segregation increases in regions where the fifth force is strong, and little segregation in dense regions. We also introduce an approximation method to identify the virialized halos in coupled scalar field models which takes into account the scalar field coupling and which is easy to implement numerically. It is found that the chameleon nature of the scalar field makes the internal density profiles of halos dependent on the environment in a very nontrivial way. © 2010 The American Physical Society.


Zhao G.-B.,University of Portsmouth | Li B.,University of Cambridge | Li B.,Kavli Institute for Cosmology Cambridge | Koyama K.,University of Portsmouth
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

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 nonlinear 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 and Schmidt, and extend the resolution up to k∼20h/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 nonlinear scales. © 2011 American Physical Society.


Bonvin C.,Kavli Institute for Cosmology Cambridge | Bonvin C.,University of Cambridge | Durrer R.,University of Geneva | Durrer R.,CEA Saclay Nuclear Research Center
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

In this paper we compute the quantity which is truly measured in a large galaxy survey. We take into account the effects coming from the fact that we actually observe galaxy redshifts and sky positions and not true spatial positions. Our calculations are done within linear perturbation theory for both the metric and the source velocities but they can be used for nonlinear matter power spectra. We shall see that the complications due to the fact that we only observe on our background light cone, and that we do not truly know the distance of the observed galaxy but only its redshift, not only cause an additional difficulty, but provide even more a new opportunity for future galaxy surveys. © 2011 American Physical Society.

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