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Sivanandam N.,African Institute for Mathematical Sciences
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

The matching of our epoch of existence with the approximate equality of the dark energy and dark matter densities is an apparent further fine-tuning, beyond the already troubling 120 orders of magnitude that separate dark energy from the Planck scale. In this paper I will argue that this coincidence is not a fine-tuning problem, but instead an artifact of anthropic selection. Rather than assuming observations are equally likely in all epochs, one should insist that measurements of a quantity be typical amongst all such measurements. As a consequence, particular observations will reflect the epoch in which they are most easily made. In the specific case of cosmology, most measurements of dark energy and dark matter will be done during an epoch when large numbers of linear modes are available to observers, so we should not be surprised to be living at such a time. This idea is made precise in a particular model for the probability distribution for (ΩmΩΛ, ΩΛΩm), where it is shown that if p(r)∼[N(r)]b [where N(r) is the number of linear modes, and b is some arbitrary positive power], the probability that r is greater than its observed value of 0.4 is close to 1. Thus the cosmological coincidence is no longer problematic. © 2013 American Physical Society. Source


Khosravi N.,African Institute for Mathematical Sciences
Journal of Cosmology and Astroparticle Physics | Year: 2012

We generalise Weinberg's effective field theory approach to multiple-field inflation. In addition to standard terms in the Lagrangian we consider terms containing up to the fourth derivative of the scalar fields and the metric. The results illustrate the possible shapes of the interactions which will yield non-Gaussianity. Generally we find that the speed of sound differs from, but is close to unity, however large non-Gaussianities are possible in the multi-field case. The non-Gaussianity of the adiabatic mode and the entropy mode are correlated in shape and amplitude with the amount of the non-Gaussianity depending on the curvature of the classical field path in phase-space. We emphasize that in general the time derivative of adiabatic and entropy perturbations do not invariant due to the shift symmetry. However we find two specific combinations of them are invariant under such a symmetry and these combinations should be employed to construct an effective field theory of multi-field inflation. © 2012 IOP Publishing Ltd and Sissa Medialab srl. Source


Khosravi N.,African Institute for Mathematical Sciences | Sepangi H.R.,Shahid Beheshti University | Shahidi S.,Shahid Beheshti University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

We study the cosmological perturbations of the new bimetric gravity proposed by Hassan and Rosen as a representation of massive gravity. The mass term in the model, in addition to ensuring ghost freedom for both metrics, causes the two scale factors to mix at the cosmological level, and this affects the cosmological perturbation of the model. We find two combinations corresponding to the entropy and adiabatic perturbations of the theory. In this sense we show that the adiabatic perturbations could be a source for the entropy perturbations. So in addition to the adiabatic perturbations, entropy perturbations can also be present in this theory. We also show that the adiabatic perturbations are not constant at the superhorizon scales, implying that the theory could not be used to describe the inflationary epoch, even if it can impose some corrections to the standard inflationary scenarios. © 2012 American Physical Society. Source


Owerre S.A.,Perimeter Institute for Theoretical Physics | Owerre S.A.,African Institute for Mathematical Sciences
Physical Review B - Condensed Matter and Materials Physics | Year: 2016

Motivated by recent quantum Monte Carlo (QMC) simulations of the quantum Kagome ice model by Juan Carrasquilla et al., [Nat. Commun., 6, 7421 (2015)2041-172310.1038/ncomms8421], we study the ground-state properties of this model on the triangular lattice. In the presence of a magnetic field h, the Hamiltonian possesses competing interactions between a Z2-invariant easy-axis ferromagnetic interaction J±± and a frustrated Ising term Jz. As in the U(1)-invariant model, we obtain four classical distinctive phases, however, the classical phases in the Z2-invariant model are different. They are as follows: a fully polarized (FP) ferromagnet for large h, an easy-axis canted ferromagnet (CFM) with broken Z2 symmetry for small h and dominant J±±, a ferrosolid phase with broken translational and Z2 symmetries for small h and dominant Jz, and two lobes with m=(Sz)=±1/6 for small h and dominant Jz. We show that quantum fluctuations are suppressed in this model, hence the large-S expansion gives an accurate picture of the ground-state properties. When quantum fluctuations are introduced, we show that the ferrosolid state is the ground state in the dominant Ising limit at zero magnetic field. It remains robust for Jz→∞. With nonzero magnetic field the classical lobes acquire a finite magnetic susceptibility with no Sz order. We present the trends of the ground-state energy and the magnetizations. We also present a detail analysis of the CFM. © 2016 American Physical Society. Source


Bellini E.,University of Heidelberg | Sawicki I.,African Institute for Mathematical Sciences
Journal of Cosmology and Astroparticle Physics | Year: 2014

We present a turnkey solution, ready for implementation in numerical codes, for the study of linear structure formation in general scalar-tensor models involving a single universally coupled scalar field. We show that the totality of cosmological information on the gravitational sector can be compressed - without any redundancy - into five independent and arbitrary functions of time only and one constant. These describe physical properties of the universe: the observable background expansion history, fractional matter density today, and four functions of time describing the properties of the dark energy. We show that two of those dark-energy property functions control the existence of anisotropic stress, the other two - dark-energy clustering, both of which are can be scale-dependent. All these properties can in principle be measured, but no information on the underlying theory of acceleration beyond this can be obtained. We present a translation between popular models of late-time acceleration (e.g. perfect fluids, f(R), kinetic gravity braiding, galileons), as well as the effective field theory framework, and our formulation. In this way, implementing this formulation numerically would give a single tool which could consistently test the majority of models of late-time acceleration heretofore proposed. © 2014 IOP Publishing Ltd and Sissa Medialab srl . Source

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