arXiv subjects

Swagat S. Mishra

Publications and source records attributed to Swagat S. Mishra.

23 records · Page 2Linked to original sources

arXiv2018

Initial conditions for Inflation in an FRW Universe

We examine the class of initial conditions which give rise to inflation. Our analysis is carried out for several popular models including: Higgs inflation, Starobinsky inflation, chaotic inflation, axion monodromy inflation and non-canonical inflation. In each case we determine the set of initial conditions which give rise to sufficient inflation, with at least $60$ e-foldings. A phase-space analysis has been performed for each of these models and the effect of the initial inflationary energy scale on inflation has been studied numerically. This paper discusses two scenarios of Higgs inflation: (i) the Higgs is coupled to the scalar curvature, (ii) the Higgs Lagrangian contains a non-canonical kinetic term. In both cases we find Higgs inflation to be very robust since it can arise for a large class of initial conditions. One of the central results of our analysis is that, for plateau-like potentials associated with the Higgs and Starobinsky models, inflation can be realised even for initial scalar field values which lie close to the minimum of the potential. This dispels a misconception relating to plateau potentials prevailing in the literature. We also find that inflation in all models is more robust for larger values of the initial energy scale.

gr-qc↗

arXiv2018

New tracker models of dark energy

We describe a new class of dark energy (DE) models which behave like cosmological trackers at early times. These models are based on the $α$-attractor set of potentials, originally discussed in the context of inflation. The new models allow the current acceleration of the universe to be reached from a wide class of initial conditions. Prominent examples of this class of models are the potentials $\cothφ$ and $\coshφ$. A remarkable feature of this new class of models is that they lead to large enough negative values of the equation of state at the present epoch, consistent with the observations of accelerated expansion of the universe, from a very large initial basin of attraction. They therefore avoid the fine tuning problem which afflicts many models of DE.

gr-qc↗

arXiv2018

Emulating $Λ$CDM-like expansion on the Phantom brane

In [Schmidt PRD 80 123003 (2009)], the author suggested that dynamical dark energy (DDE) propagating on the phantom brane could mimick $Λ$CDM. Schmidt went on to derive a phenomenological expression for $ρ_{\rm DE}$ which could achieve this. We demonstrate that while Schmidt's central premise is correct, the expression for $ρ_{\rm DE}$ derived in Schmidt (2009) is flawed. We derive the correct expression for $ρ_{\rm DE}$ which leads to $Λ$CDM-like expansion on the phantom brane. We also show that DDE on the brane can be associated with a Quintessence field and derive a closed form expression for its potential $V(ϕ)$. Interestingly the $α$-attractor based potential $V(ϕ) \propto \coth^2{λϕ}$ makes braneworld expansion resemble $Λ$CDM. However the two models can easily be distinguished on the basis of density perturbations which grow at different rates on the braneworld and in $Λ$CDM.

gr-qc↗

arXiv2018

Unifying Dark Matter and Dark Energy with non-Canonical Scalars

Non-canonical scalar fields with the Lagrangian ${\cal L} = X^α- V(ϕ)$, possess the attractive property that the speed of sound, $c_s^{2} = (2\,α- 1)^{-1}$, can be exceedingly small for large values of $α$. This allows a non-canonical field to cluster and behave like warm/cold dark matter on small scales. We demonstrate that simple potentials including $V = V_0\coth^2ϕ$ and a Starobinsky-type potential can unify dark matter and dark energy. Cascading dark energy, in which the potential cascades to lower values in a series of discrete steps, can also work as a unified model. In all of these models the kinetic term $X^α$ plays the role of dark matter, while the potential term $V(ϕ)$ plays the role of dark energy.

gr-qc↗

arXiv2017

Sourcing Dark Matter and Dark Energy from $α$-attractors

Recently, Kallosh and Linde have drawn attention to a new family of superconformal inflationary potentials, subsequently called $α$-attractors. The $α$-attractor family can interpolate between a large class of inflationary models. It also has an important theoretical underpinning within the framework of supergravity. We demonstrate that the $α$-attractors have an even wider appeal since they may describe dark matter and perhaps even dark energy. The dark matter associated with the $α$-attractors, which we call $α$-dark matter ($α$DM), shares many of the attractive features of fuzzy dark matter, with $V(φ) = \frac{1}{2}m^2φ^2$, while having none of its drawbacks. Like fuzzy dark matter, $α$DM can have a large Jeans length which could resolve the cusp-core and substructure problems faced by standard cold dark matter. $α$DM also has an appealing tracker property which enables it to converge to the late-time dark matter asymptote, $\langle w\rangle \simeq 0$, from a wide range of initial conditions. It thus avoids the enormous fine-tuning problems faced by the $m^2φ^2$ potential in describing dark matter.

gr-qc↗