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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.


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.


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 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.


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.


Sotiriou T.P.,University of Cambridge | Li B.,University of Cambridge | Li B.,Kavli Institute for Cosmology Cambridge | Barrow J.D.,University of Cambridge
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

We analyze the relation between teleparallelism and local Lorentz invariance. We show that generic modifications of the teleparallel equivalent to general relativity will not respect local Lorentz symmetry. We clarify the reasons for this and explain why the situation is different in general relativity. We give a prescription for constructing teleparallel equivalents for known theories. We also explicitly consider a recently proposed class of generalized teleparallel theories, called f(T) theories of gravity, and show why restoring local Lorentz symmetry in such theories cannot lead to sensible dynamics, even if one gives up teleparallelism. © 2011 American Physical Society.


Li B.,University of Cambridge | Li B.,Kavli Institute for Cosmology Cambridge | Sotiriou T.P.,University of Cambridge | Barrow J.D.,University of Cambridge
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

In this work we study the cosmology of the general f(T) gravity theory. We express the modified Einstein equations using covariant quantities, and derive the gauge-invariant perturbation equations in covariant form. We consider a specific choice of f(T), designed to explain the observed late-time accelerating cosmic expansion without including an exotic dark energy component. Our numerical solution shows that the extra degree of freedom of such f(T) gravity models generally decays as one goes to smaller scales, and consequently its effects on scales such as galaxies and galaxies clusters are small. But on large scales, this degree of freedom can produce large deviations from the standard ΛCDM scenario, leading to severe constraints on the f(T) gravity models as an explanation to the cosmic acceleration. © 2011 American Physical Society.


Li B.,University of Cambridge | Li B.,Kavli Institute for Cosmology Cambridge | Sotiriou T.P.,University of Cambridge | Barrow J.D.,University of Cambridge
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

We show that in theories of generalized teleparallel gravity, whose Lagrangians are algebraic functions of the usual teleparallel Lagrangian, the action and the field equations are not invariant under local Lorentz transformations. We also argue that these theories appear to have extra degrees of freedom with respect to general relativity. The usual teleparallel Lagrangian, which has been extensively studied and leads to a theory dynamically equivalent to general relativity, is an exception. Both of these facts appear to have been overlooked in the recent literature on f(T) gravity, but are crucial for assessing the viability of these theories as alternative explanations for the acceleration of the Universe. © 2011 American Physical Society.


Li B.,University of Cambridge | Li B.,Kavli Institute for Cosmology Cambridge | Barrow J.D.,University of Cambridge
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

We describe in detail the general methodology and numerical implementation of consistent N-body simulations for coupled-scalar-field models, including background cosmology and the generation of initial conditions (with the different couplings to different matter species taken into account). We perform fully consistent simulations for a class of coupled-scalar-field models with an inverse power-law potential and negative coupling constant, for which the chameleon mechanism does not work. We find that in such cosmological models the scalar-field potential plays a negligible role except in the background expansion, and the fifth force that is produced is proportional to gravity in magnitude, justifying the use of a rescaled gravitational constant G in some earlier N-body simulation works for similar models. We then study the effects of the scalar coupling on the nonlinear matter power spectra and compare with linear perturbation calculations to see the agreement and places where the nonlinear treatment deviates from the linear approximation. We also propose an algorithm to identify gravitationally virialized matter halos, trying to take account of the fact that the virialization itself is also modified by the scalar-field coupling. We use the algorithm to measure the mass function and study the properties of dark-matter halos. We find that the net effect of the scalar coupling helps produce more heavy halos in our simulation boxes and suppresses the inner (but not the outer) density profile of halos compared with the ΛCDM prediction, while the suppression weakens as the coupling between the scalar field and dark-matter particles increases in strength. © 2011 The American Physical Society.


Li B.,University of Cambridge | Li B.,Kavli Institute for Cosmology Cambridge
Monthly Notices of the Royal Astronomical Society | Year: 2011

We study the properties of voids in two different types of coupled scalar field theories. Due to the fifth force produced by the scalar field coupling, the matter particles feel stronger attraction amongst each other and cluster more quickly than they do in the standard ΛCDM model. Consequently, voids in the coupled scalar field theories start to develop earlier and end up bigger, which is confirmed by our numerical simulations. We find that a significantly larger portion of the whole space is underdensed in the coupled scalar field theories and there are more voids whose sizes exceed given thresholds. This is more prominent in early times because at later times the underdense regions have already been evacuated in coupled scalar field theories and there is time for the ΛCDM model to catch up. The coupled scalar field theories also predict a sharper transition between voids and high-density regions. All in all, the qualitative behaviour is different not only from the ΛCDM result, but also amongst specific coupled scalar field models, making voids a potential candidate to test alternative ideas about the cosmic structure formation. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.

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