Zhi-Guo Liu

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.

A Galileon Design of Slow Expansion

We show a model of the slow expansion, in which the scale invariant spectrum of curvature perturbation is adiabatically induced by its increasing mode, by applying a generalized Galileon field. In this model, initially epsilon << -1, which then is rapidly increasing, during this period the universe is slowly expanding. There is not the ghost instability, the perturbation theory is healthy. When epsilon sim -1, the slow expansion phase ends, and the available energy of field can be released and the universe reheats. This scenario might be a viable design of the early universe.

CMB anomalies from an inflationary model in string theory

Recent Planck measurements show some CMB anomalies on large angular scales, which confirms the early observations by WMAP. We show that an inflationary model, in which before the slow-roll inflation the Universe is in a superinflationary phase, can generate a large-scale cutoff in the primordial power spectrum, which may account for not only the power suppression on large angular scales, but also a large dipole power asymmetry in the CMB. We discuss an implementation of our model in string theory.

Amplification of curvature perturbations in cyclic cosmology

We analytically and numerically show that through the cycles with nonsingular bounce, the amplitude of curvature perturbation on a large scale will be amplified and the power spectrum will redden. In some sense, this amplification will eventually destroy the homogeneity of the background, which will lead to the ultimate end of cycles of the global universe. We argue that for the model with increasing cycles, it might be possible that a fissiparous multiverse will emerge after one or several cycles, in which the cycles will continue only at corresponding local regions.

Phantom inflation in little rip

Abstract We study the phantom inflation in little rip cosmology, in which the current acceleration is driven by the field with the parameter of state w − 1 , but since w tends to −1 asymptotically, the rip singularity occurs only at infinite time. In this scenario, before the rip singularity is arrived, the universe is in an inflationary regime. We numerically calculate the spectrum of primordial perturbation generated during this period and find that the results may be consistent with observations. This implies that if the reheating happens again, the current acceleration might be just a start of phantom inflation responsible for the upcoming observational universe.

Preinflationary genesis with CMB B-mode polarization

Recent B-mode polarization observation seems to imply the tensor tilt n(T) greater than or similar to 1 at large angular scale if the primordial signal is dominated. We show that for a primordial universe, which is in a slowly expanding genesis phase before the slow-roll inflation, the primordial tensor spectrum will get a large-scale cutoff, i.e., n(T) greater than or similar to 1 at large scales while n(T) similar or equal to 0 at small scale. We find that this inflationary scenario not only may be consistent with the observation, but also predicts a large-scale anomaly in BB power spectrum, i.e., due to the large suppression of tensor perturbation amplitude we will hardly see the reionization bump at low-l, which may be falsified by the Planck polarization data.

A Galileon design of slow expansion: Emergent universe

How to apprehend the beginning of the hot “big bang” model has been still a significant issue. The inflationary scenario [1],[2],[3] is the current paradigm of the early universe, which not only homogenizes the universe but provides the scale invariant primordial perturbation responsible for the observable universe [4]. However, the universe still requires a beginning, since in inflationary scenario any backward null or timelike geodesic has a finite affine length [5]. The idea of the emergent universe is interesting [6],[7], in this scenario the universe originates from a static state in the infinite past. In emergent universe scenario, it seems that there might be not a beginning, since the affine length of backward null or timelike geodesic is classical infinite. However, for the model, in which the initial static state is constructed by applying a positive curvature, the static universe will inevitably collapse quantum mechanically [8],[9]. In Refs.[6],[7], after the universe emerges from the static state, the inflation will be required to set the initial conditions of “big bang” model, i.e.the homogenizing and the scale invariant primordial perturbation. However, when the universe emerges, or begins to deviate from static state, it might be slowly expanding. In Ref.[10], it has been for the first time observed that the slow expansion might adiabatically generate the scale invariant curvature perturbation, see [11] for that induced by the entropy perturbation. Thus it might be imaginable that in emergent universe scenario, the initial state of “big bang” evolution could be set during this slowly expanding period. During the slow expansion, ǫ < 0 is required [10],[12]. Thus in Ref.[10], the phantom has been applied for a phenomenological studying. Recently, the cosmological application of Galileon, [13], or its nontrivial generalization [14],[15],[16], has acquired increasing attentions, e.g. see [14],[17] for dark energy, [15],[18] for inflation, [19] for curvaton and [20] for bouncing universe, in which ǫ < 0 can be implemented stably, there is not the ghost instability. The models of the emergent universe scenario can be realized with Galileon, e.g.[21], and also [22]. However, in Refs.[21],[23], the adiabatic perturbation is not scale invariant, thus the obtaining of the scale invariant curvature perturbation has to appeal to the conversion of the perturbations of other light scalar fields, i.e. the mechanism similar to that in Refs.[24],[25]. However, we showed that actually the scale invariant curvature perturbation may be adiabatically generated [22]. Here, with Ref.[5], we will make the relevant scenario clearer. Here, we will parameterize the evolutions of the slow expansion, which emerges from a static state in infinite past, into different classes, and clarify how the adiabatical perturbation generated could be scale invariant for these parameterizations. When the slow expansion phase ends, the universe reheats and the evolution of “big bang” model begins. We have showed in Ref.[22] that one of these parameterizations can be realized by applying a generalized Galileon. Here, we will show that the other parameterization can be also realized similarly, and the results may be consistent with the observations.

Scalar perturbations through cycles

We analytically and numerically investigate the evolutions of the scalar perturbations through the cycles with nonsingular bounce. It is found that the amplitude of the curvature perturbation on large scale will be amplified cycle by cycle, and the isocurvature perturbations also obtain an amplification, but the rate of its amplification is slower than that of curvature perturbation, unless its coupling to the metric perturbation is not negligible.

Phantom inflation with a steplike potential

The phantom inflation predicts a slightly blue spectrum of tensor perturbation, which might be tested in coming observations. In normal inflation models, the introduction of step in its potential generally results in an oscillation in the primordial power spectrum of curvature perturbation. We will check whether there is the similar case in the phantom inflation with steplike potential. We find that for same potentials, the oscillation of the spectrum of phantom inflation is nearly same with that of normal inflation, the difference between them is the tilt of power spectrum.

Galilean islands in eternally inflating background

We show that the observational universe may emerge classically from a de Sitter background with low energy scale. We find, after calculating the curvature perturbation, that the resulting scenario is actually a style of the eternal inflation scenario, in which some regions will go through the slowly expanding Galilean genesis phase with the rapidly increasing energy density and become island universes, while other regions are still eternally inflating, which will make the room for more island universes to emerge.