Yun-Song Piao

Oscillating quintom and the recurrent universe

Current observations seem to mildly favor an evolving dark energy with the equation of state getting across -1. This form of dark energy, dubbed quintom, is studied phenomenologically in this Letter with an oscillating equation of state. We find oscillating quintom can unify the early inflation and current acceleration of the universe, leading to the oscillations of the Hubble constant and a recurring universe. Our oscillating quintom would not lead to a big crunch nor big rip. The scale factor keeps increasing from one period to another and leads naturally to a highly flat universe. The universe in this model recurs itself and we are only staying among one of the epochs, in which sense the coincidence problem is reconciled. (c) 2006 Elsevier B.V. All rights reserved.

Bouncing universe with Quintom matter

The bouncing universe provides a possible solution to the Big Bang singularity problem. In this paper we study the bouncing solution in the universe dominated by the Quintom matter with an equation of state (EoS) crossing the cosmological constant boundary. We will show explicitly the analytical and numerical bouncing solutions in three types of models for the Quintom matter with an phenomenological EoS, the two scalar fields and a scalar field with a modified Born-Infeld action.

Two-field models of dark energy with equation of state across-1

In this paper, we study the possibility of building two-field models of dark energy with equation of state across -1. Specifically we will consider two classes of models: one consists of two scalar fields (quintessence + phantom) and another includes one scalar (phantom) and one spinor field (neutrino). Our studies indicate to some extent that two-field models give rise to a simple realization of the dynamical dark energy model with the equation of state across w=-1.

Assisted tachyonic inflation

The model of inflation with a single tachyon field generates larger anisotropy and has difficulties in describing the formation of the Universe [L. Kofman and A. Linde, J. High Energy Phys. 07, 004 (2002)]. In this paper we consider a model with multitachyon fields and study the assisted inflationary solution. Our results show that this model satisfies the observations.

On perturbations of a quintom bounce

A Quintom universe with an equation-of-state crossing the cosmological constant boundary can provide a bouncing solution dubbed the Quintom Bounce and thus resolve the Big Bang singularity. In this paper, we investigate the cosmological perturbations of the Quintom Bounce both analytically and numerically. We find that the fluctuations in the dominant mode in the post-bounce expanding phase couple to the growing mode of the perturbations in the pre-bounce contracting phase.

Primordial Perturbation in Horava-Lifshitz Cosmology

Abstract Recently, Hořava has proposed a renormalizable theory of gravity with critical exponent z = 3 in the UV. This proposal might imply that the scale invariant primordial perturbation can be generated in any expansion of early universe with a ∼ t n and n > 1 / 3 , which, in this Letter, will be confirmed by solving the motion equation of perturbation mode on super sound horizon scale for any background evolution of early universe. It is found that if enough efolding number of primordial perturbation suitable for observable universe is required, then n ≳ 1 needs to be satisfied, unless the scale of UV regime is quite low. However, the possible UV completeness of HL gravity helps to relax this bound.

Obtaining the CMB anomalies with a bounce from the contracting phase to inflation

Recent Planck data show the anomalies of cosmic microwave background (CMB) fluctuations on large angular scales, which confirms the early observations by WMAP. We continue studying an inflationary model, in which before the slow-roll inflation the universe is in a contracting phase, and fit the model with the Planck data. We show that this model may generate not only the power deficit at low l, but also a large hemispherical power asymmetry in the CMB. We also discuss the implication of the result to the eternal inflation scenario.

Cosmic duality in quintom Universe

In this Letter we study the duality in two-field quintom models of dark energy. We find that an expanding Universe dominated by quintom-A field is dual to a contracting Universe with quintom-B field.

Suppressing the CMB quadrupole with a bounce from the contracting phase to inflation

Recent released WMAP data show a low value of quadrupole in the CMB temperature fluctuations, which confirms the early observations by COBE. In this paper, a scenario in which a contracting phase is followed by an inflationary phase is constructed. We calculate the perturbation spectrum and show that this scenario can provide a reasonable explanation for lower CMB anisotropies on large angular scales.

Mass-varying massive gravity

It has recently been shown that the graviton can consistently gain a constant mass without introducing the Boulware-Deser ghost. We propose a gravity model where the graviton mass is set by a scalar field and prove that this model is free of the Boulware-Deser ghost by analyzing its constraint system and showing that two constraints arise. We also initiate the study of the models cosmic background evolution and tentatively discuss possible cosmological implications of this model. In particular, we consider a simple scenario where the scalar field setting the graviton mass is identified with the inflaton and the graviton mass evolves from a high to a low energy scale, giving rise to the current cosmic acceleration.