arXiv subjects

Sourish Dutta

Publications and source records attributed to Sourish Dutta.

35 records · Page 2Linked to original sources

arXiv2010

Thermal Relic Abundances of Particles with Velocity-Dependent Interactions

We reexamine the evolution of thermal relic particle abundances for the case where the interaction rate depends on the particle velocities. For the case of Sommerfeld enhancement, we show that the standard analytic approximation, modified in a straightforward way, provides an estimate of the relic particle abundance that is accurate to within 10% (in comparison to less than 1% error for the non-Sommerfeld-enhanced case). We examine the effect of kinetic decoupling on relic particle abundances when the interaction rate depends on the velocity. For the case of pure p-wave annihilation, the effect of kinetic decoupling is an increase in the relic abundance, but the effect is negligible when the kinetic decoupling temperature is much less than the chemical decoupling temperature. For the case of Sommerfeld-enhanced s-wave annihilations, after kinetic decoupling occurs, annihilations continue to change the particle abundance down to arbitrarily low temperatures, until either matter domination begins or the Sommerfeld effect cuts off. We derive analytic approximations to give the final relic particle abundances for both of these cases.

astro-ph.CO↗

arXiv2009

Observational constraints on Horava-Lifshitz cosmology

We use observational data from Type Ia Supernovae (SNIa), Baryon Acoustic Oscillations (BAO), and Cosmic Microwave Background (CMB), along with requirements of Big Bang Nucleosynthesis (BBN), to constrain the cosmological scenarios governed by Horava-Lifshitz gravity. We consider both the detailed and non-detailed balance versions of the gravitational sector, and we include the matter and radiation sectors. We conclude that the detailed-balance scenario cannot be ruled out from the observational point of view, however the corresponding likelihood contours impose tight constraints on the involved parameters. The scenario beyond detailed balance is compatible with observational data, and we present the corresponding stringent constraints and contour-plots of the parameters. Although this analysis indicates that Horava-Lifshitz cosmology can be compatible with observations, it does not enlighten the discussion about its possible conceptual and theoretical problems.

hep-th↗

arXiv2009

Unparticle dark energy

We examine a dark energy model where a scalar unparticle degree of freedom plays the role of quintessence. In particular, we study a model where the unparticle degree of freedom has a standard kinetic term and a simple mass potential, the evolution is slowly rolling and the field value is of the order of the unparticle energy scale ($λ_u$). We study how the evolution of $w$ depends on the parameters $B$ (a function of the unparticle scaling dimension $d_u$), the initial value of the field $ϕ_i$ (or equivalently, $λ_u$) and the present matter density $Ω_{m0}$. We use observational data from Type Ia supernovae, BAO and CMB to constrain the model parameters and find that these models are not ruled out by the observational data. From a theoretical point of view, an unparticle dark energy model is very attractive, since unparticles (being bound states of fundamental fermions) are protected from radiative corrections. Further, coupling of unparticles to the standard model fields can be arbitrarily suppressed by raising the fundamental energy scale $M_F$, making the unparticle dark energy model free of most of the problems that plague conventional scalar field quintessence models.

astro-ph.CO↗

arXiv2009

Slow-roll k-essence

We derive slow-roll conditions for thawing k-essence with a separable Lagrangian $p(X,ϕ)=F(X)V(ϕ)$. We examine the evolution of the equation of state parameter, $w$, as a function of the scale factor $a$, for the case where $w$ is close to -1. We find two distinct cases, corresponding to $X \approx 0$ and $F_X \approx 0$, respectively. For the case where $X\approx0$ the evolution of $ϕ$ and hence $w$ is described by only two parameters, and $w(a)$ is model-independent and coincides with similar behavior seen in thawing quintessence models. This result also extends to non-separable Lagrangians where $X\approx0$. For the case $F_X \approx 0$, an expression is derived for $w(a)$, but this expression depends on the potential $V(ϕ)$, so there is no model-independent limiting behavior. For the $X \approx 0$ case, we derive observational constraints on the two parameters of the model, $w_0$ (the present-day value of $w$), and the $K$, which parametrizes the curvature of the potential. We find that the observations sharply constrain $w_0$ to be close to -1, but provide very poor constraints on $K$.

astro-ph.CO↗

arXiv2009

New Parametrization for the Scale Dependent Growth Function in General Relativity

We demonstrate the scale dependence of the growth function of cosmological perturbations in dark energy models based on General Relativity. This scale dependence is more prominent on cosmological scales of $100h^{-1}Mpc$ or larger. We derive a new scale dependent parametrization which generalizes the well known Newtonian approximation result $f_0(a)\equiv \frac{d\ln δ_0}{d\ln a}=\om(a)^γ$ ($γ={6/11}$ for \lcdm) which is a good approximation on scales less than $50h^{-1}Mpc$. Our generalized parametrization is of the form $f(a)=\frac{f_0(a)}{1+ξ(a,k)}$ where $ξ(a,k)=\frac{3 H_{0}^{2} \omm}{a k^2}$. We demonstrate that this parametrization fits the exact result of a full general relativistic evaluation of the growth function up to horizon scales for both \lcdm and dynamical dark energy. In contrast, the scale independent parametrization does not provide a good fit on scales beyond 5% of the horizon scale ($k\simeq 0.01 h^{-1}Mpc$).

astro-ph.CO↗

arXiv2009

Dark Energy from a Phantom Field Near a Local Potential Minimum

We examine dark energy models in which a phantom field $ϕ$ is rolling near a local minimum of its potential $V(ϕ)$.We require that $(1/V)(dV/dϕ) \ll 1$, but $(1/V)(d^2 V/dϕ^2)$ can be large. Using techniques developed in the context of hilltop quintessence, we derive a general expression for $w$ as a function of the scale factor, and as in the hilltop case, we find that the dynamics of the field depend on the value of $(1/V)(d^2 V/dϕ^2)$ near the mimimum. Our general result gives a value for $w$ that is within 1% of the true (numerically-derived) value for all of the particular cases examined. Our expression for $w(a)$ reduces to the previously-derived phantom slow-roll result of Sen and Scherrer in the limit where the potential is flat, $(1/V)(dV/dϕ) \ll 1$.

astro-ph.CO↗

arXiv2009

Dark radiation as a signature of dark energy

We propose a simple dark energy model with the following properties: the model predicts a late-time dark radiation component that is not ruled out by current observational data, but which produces a distinctive time-dependent equation of state w(z) for z < 3. The dark energy field can be coupled strongly enough to Standard Model particles to be detected in colliders, and the model requires only modest additional particle content and little or no fine-tuning other than a new energy scale of order milli-electron volts.

astro-ph.CO↗

arXiv2009

Island Cosmology

If the observed dark energy is a cosmological constant, the canonical state of the universe is de Sitter spacetime. In such a spacetime, quantum fluctuations that violate the null energy condition will create islands of matter that, if large enough, may resemble our observable universe. Phenomenological approaches to calculating density fluctuations yield a scale invariant spectrum with suitable amplitude. With time, the island of matter that is our observable universe, re-enters the cosmological constant sea.

gr-qc↗

arXiv2009

Dark energy from a quintessence (phantom) field rolling near potential minimum (maximum)

We examine dark energy models in which a quintessence or a phantom field, $ϕ$, rolls near the vicinity of a local minimum or maximum, respectively, of its potential $V(ϕ)$. Under the approximation that $(1/V)(dV/dϕ) \ll 1$, [although $(1/V)(d^2 V/dϕ^2)$ can be large], we derive a general expression for the equation of state parameter $w$ as a function of the scale factor for these models. The dynamics of the field depends on the value of $(1/V)(d^2 V/dϕ^2)$ near the extremum, which describes the potential curvature. For quintessence models, when $(1/V)(d^2 V/dϕ^2)<3/4$ at the potential minimum, the equation of state parameter $w(a)$ evolves monotonically, while for $(1/V)(d^2 V/dϕ^2)>3/4$, $w(a)$ has oscillatory behavior. For phantom fields, the dividing line between these two types of behavior is at $(1/V)(d^2 V/dϕ^2) = -3/4$. Our analytical expressions agree within 1% with the exact (numerically-derived) behavior, for all of the particular cases examined, for both quintessence and phantom fields. We present observational constraints on these models.

astro-ph.CO↗

arXiv2009

On the dangers of using the growth equation on large scales in the Newtonian gauge

We examine the accuracy of the growth equation $\ddotδ + 2H\dotδ - 4πGρδ= 0$, which is ubiquitous in the cosmological literature, in the context of the Newtonian gauge. By comparing the growth predicted by this equation to a numerical solution of the linearized Einstein equations in the $Λ$CDM scenario, we show that while this equation is a reliable approximation on small scales ($k\gtrsim $h Mpc$^{-1}$), it can be disastrously inaccurate ($\sim 10^4% $) on larger scales in this gauge. We propose a modified version of the growth equation for the Newtonian gauge, which while preserving the simplicity of the original equation, provides considerably more accurate results. We examine the implications of the failure of the growth equation on a few recent studies, aimed at discriminating general relativity from modified gravity, which use this equation as a starting point. We show that while the results of these studies are valid on small scales, they are not reliable on large scales or high redshifts, if one works in the Newtonian gauge. Finally, we discuss the growth equation in the synchronous gauge and show that the corrections to the Poisson equation are exactly equivalent to the difference between the overdensities in the synchronous and Newtonian gauges.

astro-ph↗

arXiv2008

Hilltop Quintessence

We examine hilltop quintessence models, in which the scalar field is rolling near a local maximum in the potential, and w is close to -1. We first derive a general equation for the evolution of the scalar field in the limit where w is close to -1. We solve this equation for the case of hilltop quintessence to derive w as a function of the scale factor; these solutions depend on the curvature of the potential near its maximum. Our general result is in excellent agreement (delta w < 0.5%) with all of the particular cases examined. It works particularly well (delta w < 0.1%) for the pseudo-Nambu-Goldstone Boson potential. Our expression for w(a) reduces to the previously-derived slow-roll result of Sen and Scherrer in the limit where the curvature goes to zero. Except for this limiting case, w(a) is poorly fit by linear evolution in a.

astro-ph↗

arXiv2008

A new perspective on the relation between dark energy perturbations and the late-time ISW effect

The effect of quintessence perturbations on the ISW effect is studied for a mixed dynamical scalar field dark energy (DDE) and pressureless perfect fluid dark matter. A new and general methodology is developed to track the growth of the perturbations, which uses only the equation of state (EoS) parameter $w_{\rm DDE} (z) \equiv p_{\rm DDE}/ρ_{\rm DDE}$ of the scalar field DDE, and the initial values of the the relative entropy perturbation (between the matter and DDE) and the intrinsic entropy perturbation of the scalar field DDE as inputs. We also derive a relation between the rest frame sound speed $\hat{c}_{s,{\rm DDE}}^2$ of an arbitrary DDE component and its EoS $w_{\rm DDE} (z)$. We show that the ISW signal differs from that expected in a $Λ$CDM cosmology by as much as +20% to -80% for parameterizations of $w_{\rm DDE}$ consistent with SNIa data, and about $\pm$ 20% for parameterizations of $w_{\rm DDE}$ consistent with SNIa+CMB+BAO data, at 95% confidence. Our results indicate that, at least in principle, the ISW effect can be used to phenomenologically distinguish a cosmological constant from DDE.

astro-ph↗

arXiv2008

Evolution of Oscillating Scalar Fields as Dark Energy

Oscillating scalar fields, with an oscillation frequency much greater than the expansion rate, have been proposed as models for dark energy. We examine these models, with particular emphasis on the evolution of the ratio of the oscillation frequency to the expansion rate. We show that this ratio always increases with time if the dark energy density declines less rapidly than the background energy density. This allows us to classify oscillating dark energy models in terms of the epoch at which the oscillation frequency exceeds the expansion rate, which is effectively the time at which rapid oscillations begin. There are three basic types of behavior: early oscillation models, in which oscillations begin during the matter-dominated era, late oscillation models, in which oscillations begin after scalar-field domination, and non-oscillating models. We examine a representative set of models (those with power-law potentials) and determine the parameter range giving acceptable agreement with the supernova observations. We show that a subset of all three classes of models can be consistent with the observational data.

astro-ph↗

arXiv2008

Creating kinks from particles

We study the creation of solitons from particles, using the $λϕ^4$ model as a prototype. We consider the scattering of small, identical, wave pulses, that are equivalent to a sequence of particles, and find that kink-antikink pairs are created for a large region in parameter space. We also find that scattering at {\it low} velocities is favorable for creating solitons that have large energy compared to the mass of a particle.

hep-th↗

arXiv2007

Voids of dark energy

We investigate the clustering properties of a dynamical dark energy component. In a cosmic mix of a pressureless fluid and a light scalar field, we follow the linear evolution of spherical matter perturbations. We find that the scalar field tends to form underdensities in response to the gravitationally collapsing matter. We thoroughly investigate these voids for a variety of initial conditions, explain the physics behind their formation and consider possible observational implications. Detection of dark energy voids will clearly rule out the cosmological constant as the main source of the present acceleration.

gr-qc↗

arXiv2005

A Classical Treatment of Island Cosmology

Computing the perturbation spectrum in the recently proposed Island Cosmology remains an open problem. In this paper we present a classical computation of the perturbations generated in this scenario by assuming that the NEC-violating field behaves as a classical phantom field. Using an exactly-solvable potential, we show that the model generates a scale-invariant spectrum of scalar perturbations, as well as a scale-invariant spectrum of gravitational waves. The scalar perturbations can have sufficient amplitude to seed cosmological structure, while the gravitational waves have a vastly diminished amplitude.

astro-ph↗

arXiv2005

Islands in the Lambda-sea: An alternative cosmological model

We propose an alternate cosmological model in which our observable universe is an island in a cosmological constant sea. Initially the universe is filled with cosmological constant of the currently observed value but is otherwise empty. In this eternal or semi-eternal de Sitter spacetime, we show that local quantum fluctuations (upheavals) can violate the null energy condition and create islands of matter. The perturbation spectra of quantum fields other than that responsible for the upheaval, are shown to be scale invariant. With further cosmic evolution the island disappears and the local universe returns to its initial cosmological constant dominated state.

astro-ph↗