arXiv subjects

Khamphee Karwan

Publications and source records attributed to Khamphee Karwan.

26 records · Page 2Linked to original sources

arXiv2012

Reheating in 3-form inflation

We consider the 3-form field, which has been considered as a candidate for realizing inflation, coupled to a scalar field which models the relativistic matter particles produced during the reheating epoch. We have investigated the stability conditions for this theory and found that introducing such a coupling does not lead to any ghosts or Laplacian instabilities. We have also investigated the reheating temperature and the production of particles due to parametric resonances. We have found that this process is more efficient in this theory compared to the result of the standard-scalar-field inflationary scenario.

astro-ph.CO↗

arXiv2012

Stability of the 3-form field during inflation

We consider the minimally coupled 3-form field which has been considered as a candidate to realize inflation. We have studied the conditions to avoid ghosts and Laplacian instabilities and found that some classes of potentials, e.g. the Mexican-hat one, will in general be unstable. We then propose other classes of potentials which are instead free from any instability, drive a long-enough slow-roll regime followed by an oscillatory epoch, and as a consequence, can provide successful inflation. Finally, we also provide stable potentials which lead to a small enough propagation speed for the scalar perturbations, giving a possibility for these models to produce non-Gaussianities.

hep-ph↗

arXiv2011

Non-adiabatic perturbations in Ricci dark energy model

We show that the non-adiabatic perturbations between Ricci dark energy and matter can grow both on superhorizon and subhorizon scales, and these non-adiabatic perturbations on subhorizon scales can lead to instability in this dark energy model. The rapidly growing non-adiabatic modes on subhorizon scales always occur when the equation of state parameter of dark energy starts to drop towards -1 near the end of matter era, except that the parameter α of Ricci dark energy equals to 1/2. In the case where α = 1/2, the rapidly growing non-adiabatic modes disappear when the perturbations in dark energy and matter are adiabatic initially. However, an adiabaticity between dark energy and matter perturbations at early time implies a non-adiabaticity between matter and radiation, this can influence the ordinary Sachs-Wolfe (OSW) effect. Since the amount of Ricci dark energy is not small during matter domination, the integrated Sachs-Wolfe (ISW) effect is greatly modified by density perturbations of dark energy, leading to a wrong shape of CMB power spectrum. The instability in Ricci dark energy is difficult to be alleviated if the effects of coupling between baryon and photon on dark energy perturbations are included.

astro-ph.CO↗

arXiv2011

Dynamics of entropy perturbations in assisted dark energy with mixed kinetic terms

We study dynamics of entropy perturbations in the two-field assisted dark energy model. Based on the scenario of assisted dark energy, in which one scalar field is subdominant compared with the other in the early epoch, we show that the entropy perturbations in this two-field system tend to be constant on large scales in the early epoch and hence survive until the present era for a generic evolution of both fields during the radiation and matter eras. This behaviour of the entropy perturbations is preserved even when the fields are coupled via kinetic interaction. Since, for assisted dark energy, the subdominant field in the early epoch becomes dominant at late time, the entropy perturbations can significantly influence the dynamics of density perturbations in the universe. Assuming correlations between the entropy and curvature perturbations, the entropy perturbations can enhance the integrated Sachs-Wolfe (ISW) effect if the signs of the contributions from entropy perturbations and curvature perturbations are opposite after the matter era, otherwise the ISW contribution is suppressed. For canonical scalar field the effect of entropy perturbations on ISW effect is small because the initial value of the entropy perturbations estimated during inflation cannot be sufficiently large. However, in the case of k-essence, the initial value of the entropy perturbations can be large enough to affect the ISW effect to leave a significant imprint on the CMB power spectrum.

astro-ph.CO↗

arXiv2010

CMB constraints on noncommutative geometry during inflation

We investigate the primordial power spectrum of the density perturbations based on the assumption that spacetime is noncommutative in the early stage of inflation. Due to the spacetime noncommutativity, the primordial power spectrum can lose rotational invariance. Using the k-inflation model and slow-roll approximation, we show that the deviation from rotational invariance of the primordial power spectrum depends on the size of noncommutative length scale L_s but not on sound speed. We constrain the contributions from the spacetime noncommutativity to the covariance matrix for the harmonic coefficients of the CMB anisotropies using five-year WMAP CMB maps. We find that the upper bound for L_s depends on the product of sound speed and slow-roll parameter. Estimating this product using cosmological parameters from the five-year WMAP results, the upper bound for L_s is estimated to be less than 10^{-27} cm at 99.7% confidence level.

astro-ph.CO↗

arXiv2008

The Coincidence Problem and Interacting Holographic Dark Energy

We study the dynamical behaviour of the interacting holographic dark energy model whose interaction term is $Q=3H(\lam_dρ_d + \lam_cρ_c)$, where $ρ_d$ and $ρ_c$ are the energy density of dark energy and CDM respectively. To satisfy the observational constraints from SNIa, CMB shift parameter and BAO measurement, if $\lam_c = \lam_d$ or $\lam_d, \lam_c >0$, the cosmic evolution will only reach the attractor in the future and the ratio $ρ_c/ρ_d$ cannot be slowly varying at present. Since the cosmic attractor can be reached in the future even when the present values of the cosmological parameters do not satisfy the observational constraints, the coincidence problem is not really alleviated in this case. However, if $\lam_c \neq \lam_d$ and they are allowed to be negative, the ratio $ρ_c/ρ_d$ can be slowly varying at present and the cosmic attractor can be reached near the present epoch. Hence, the alleviation of the coincidence problem is attainable in this case. The alleviation of coincidence problem in this case is still attainable when confronting this model to SDSS data.

astro-ph↗

arXiv2007

A Model of Large Quintessence Isocurvature Fluctuations and Low CMB Quadrupole

Based on quintessence models with anticorrelation between the quintessence fluctuations and adiabatic perturbations in the other matter components, we show that if the quintessence potential is sufficiently steep in the initial era, i.e., at the beginning of radiation dominated epoch, the quintessence fluctuations can be large enough to sufficiently suppress the CMB power spectrum at low multipoles without excess gravitational waves. The leaping kinetic term quintessence and the crossover quintessence models, can realize such steep potential at the initial stage, and therefore can give rise to a low CMB quadrupole as required by observations.

astro-ph↗

arXiv2005

Observational constraints on the dark energy density evolution

We constrain the evolution of the dark energy density from Cosmic Microwave Background, Large Scale Structure and Supernovae Ia measurements. While Supernovae Ia are most sensitive to the equation of state $w_0$ of dark energy today, the Cosmic Microwave Background and Large Scale Structure data best constrains the dark energy evolution at earlier times. For the parametrization used in our models, we find $w_0 < -0.8$ and the dark energy fraction at very high redshift $Ω_{early} < 0.03$ at 95 per cent confidence level.

astro-ph↗