Wolung Lee

Quantum noise and large-scale cosmic microwave background anisotropy

We propose a new source for the cosmological density perturbation which is passive fluctuations of the inflaton driven dynamically by a coloured quantum noise as a result of its coupling to other massive quantum fields. The created fluctuations grow with time during inflation before horizon-crossing. However, the larger-scale modes cross the horizon earlier, thus resulting in a suppression of their density perturbation as compared with those on small scales. By using current observed CMB data to constrain the parameters introduced, we find that a significant contribution from the noise-driven perturbation to the density perturbation is still allowed. It in turn gives rise to a suppression of the large-scale CMB anisotropy that may be relevant to the observed low quadrupole in the WMAP CMB anisotropy data. We also briefly discuss the implications to the energy scale of inflation and the spectral index and the non-Gaussianity of the density perturbation.

Trapping effects on inflation

We develop a Lagrangian approach based on the influence functional method so as to derive self-consistently the Langevin equation for the inflaton field in the presence of trapping points along the inflaton trajectory. The Langevin equation exhibits the backreaction and the fluctuation-dissipation relation of the trapping. The fluctuation is induced by a multiplicative colored noise that can be identified as the the particle number density fluctuations and the dissipation is a new effect that may play a role in the trapping with a strong coupling. In the weak coupling regime, we calculate the power spectrum of the noise-driven inflaton fluctuations for a single trapping point and studied its variation with the trapping location. We also consider a case with closely spaced trapping points and find that the resulting power spectrum is blue.

Constraining evolution of quintessence with CMB and SNIa data

The equation of state of the hypothetical dark energy component, which constitutes about two thirds of the critical density of the universe, may be very different from that of a cosmological constant. Employing a phenomenological model, we investigate semi-analytically the constraints imposed on the scalar quintessence by supernovae observations, and by the acoustic scale extracted from recent CMB data. We show that a universe with a quintessence-dominated phase in the dark age is consistent with the current observational constraints.

Inflationary dilaton-axion magnetogenesis

We discuss the generation of primordial magnetic fields during inflation in the dilaton-axion electromagnetism, in which the dilaton and axion dynamics are introduced in terms of two time dependent functions of the cosmic scale factor,

Numerical study of pseudoscalar inflation with an axion-gauge field coupling

I(a) F^2/4

Production of high stellar-mass primordial black holes in trapped inflation

and

Primordial black holes and associated gravitational waves in axion monodromy inflation

J(a) F\tilde{F}/4

PRIMORDIAL MAGNETIC FIELDS BY COSMIC INFLATION

, respectively, where

Off-equilibrium dynamics of the primordial perturbations in the inflationary universe: The O(N) model

F

Limit on the time variation of the fine structure constant driven by quintessence

is the electromagnetic field strength and