Jun-Qing Xia

Quintom Cosmology: Theoretical implications and observations

We review the paradigm of quintom cosmology. This scenario is motivated by the observational indications that the equation of state of dark energy across the cosmological constant boundary is mildly favored, although the data are still far from being conclusive. As a theoretical setup we introduce a no-go theorem existing in quintom cosmology, and based on it we discuss the conditions for the equation of state of dark energy realizing the quintom scenario. The simplest quintom model can be achieved by introducing two scalar fields with one being quintessence and the other phantom. Based on the double-field quintom model we perform a detailed analysis of dark energy perturbations and we discuss their effects on current observations. This type of scenarios usually suffer from a manifest problem due to the existence of a ghost degree of freedom, and thus we review various alternative realizations of the quintom paradigm. The developments in particle physics and string theory provide potential clues indicating that a quintom scenario may be obtained from scalar systems with higher derivative terms, as well as from nonscalar systems. Additionally, we construct a quintom realization in the framework of braneworld cosmology, where the cosmic acceleration and the phantom divide crossing result from the combined effects of the field evolution on the brane and the competition between four and five dimensional gravity. Finally, we study the outsets and fates of a universe in quintom cosmology. In a scenario with null energy condition violation one may obtain a bouncing solution at early times and therefore avoid the Big Bang singularity. Furthermore, if this occurs periodically, we obtain a realization of an oscillating universe. Lastly, we comment on several open issues in quintom cosmology and their connection to future investigations.

Perturbations of the quintom models of dark energy and the effects on observations

We study in this paper the perturbations of the quintom dark energy model and the effects of quintom perturbations on the current observations. Quintom describes a scenario of dark energy where the equation of state gets across the cosmological constant boundary w = -1 during evolution. We present a new method to show that the conventional dark energy models based on a single k-essence field and perfect fluid cannot act as the quintom due to the singularities and classical instabilities of perturbations around w= -1. One needs to add extra degrees of freedom for successful quintom model building. There are no singularities or classical instabilities in perturbations of realistic quintom models and they are potentially distinguishable from the cosmological constant. Basing on the realistic quintom models in this paper we provide one way to include the perturbations for dark energy models with parametrized equation of state across -1. Compared with those assuming no dark energy perturbations, we find that the parameter space which allows the equation of state to get across -1 will be enlarged, in general, when including the perturbations.

Probing for dynamics of dark energy and curvature of universe with latest cosmological observations

We use the newly released 182 type la supernova data combined with the third-year Wilkinson Microwave Anisotropic Probe data (WMAP3) and large scale structure (LSS) information including SDSS and 2dFGRS to constrain the dark energy equation of state (EoS) as well as the curvature of universe Omega(K). Using the full dataset of Cosmic Microwave Background (CMB) and LSS rather than the shift parameter and linear growth factor, we make a Markov Chain Monte Carlo (MCMC) global fit, while paying particular attention to the dark energy perturbation. Parameterizing the EoS as w(DE)(a) = w(0)+w(1)(1-a), we find the best fit of (w(0), w(1)) is(-1.053, 0.944) and for W-DE(a) = w(0)+w(1) sin(3/2 pi ln(a)), the best fit for (w(0), w(1)) is (- 1.614, -1.046)). We find that a flat universe is a good approximation, namely, \Omega(K)\ > 0.06 has been excluded by 2 sigma yet the inclusion of Omega(K) can affect the measurement of DE parameters owing to their correlation and the present systematic effects of SNIa measurements. (c) 2007 Elsevier B.V. All rights reserved.

CONSTRAINTS ON THE SOUND SPEED OF DYNAMICAL DARK ENERGY

In this paper we study in detail the sound speed —

CONSTRAINTS ON OSCILLATING QUINTOM FROM SUPERNOVA, MICROWAVE BACKGROUND AND GALAXY CLUSTERING

c_s^2

Observing dark energy dynamics with supernova, microwave background and galaxy clustering

, which is directly related to the classical perturbations — of dynamical dark energy (DE). We consider especially the case with an equation of state crossing the cosmological constant boundary, and show its implications for cosmic microwave background (CMB) anisotropy. With the present observational data on the CMB, the Type Ia supernova (SNIa) and galaxy clustering, we perform a global analysis to constrain the sound speed of DE, using the Markov chain Monte Carlo method. We find that the sound speed of DE is weakly constrained by current observations, and thus futuristic precision measurements of the CMB on a very large angular scale (low multipoles) are necessary.

High-redshift cosmography

We consider in this paper a simple oscillating Quintom model of dark energy which has two free parameters and an equation of state oscillating and crossing -1. For low redshifts the equation of state of this model resembles itself similar to the linearly parameterized dark energy, however differ substantially at large redshifts. We fit our model to the observational data separately from the new high redshift supernova observations from the HST/GOODS program and previous supernova, CMB and galaxy clustering. Our results show that because of the oscillating feature of our model the constraints from observations at large redshifts such as CMB become less stringent.

Model of inflationary cosmology without singularity

Observing dark energy dynamics is the most important aspect of the current dark energy research. In this paper we perform a global analysis of the constraints on the property of dark energy from the current observations. We pay particular attention to the effects of dark energy perturbations. Using the data from SNIa (157 gold sample), WMAP, and SDSS we find that the best fitting dark energy model is given by the dynamical model with the equation of state crossing -1. Nevertheless the standard {lambda}CDM model is still a good fit to the current data and evidence for dynamics is currently not very strong. We also consider the constraints with the recent released SNIa data from Supernova Legacy Survey.

Constraint on coupled dark energy models from observations

We constrain the parameters describing the kinematical state of the universe using a cosmographic approach, which is fundamental in that it requires a very minimal set of assumptions (namely to specify a metric) and does not rely on the dynamical equations for gravity. On the data side, we consider the most recent compilations of Supernovae and Gamma Ray Bursts catalogues. This allows to further extend the cosmographic fit up to z = 6.6, i.e. up to redshift for which one could start to resolve the low z degeneracy among competing cosmological models. In order to reliably control the cosmographic approach at high redshifts, we adopt the expansion in the improved parameter y = z/(1+z). This series has the great advantage to hold also for z > 1 and hence it is the appropriate tool for handling data including non-nearby distance indicators. We find that Gamma Ray Bursts, probing higher redshifts than Supernovae, have constraining power and do require (and statistically allow) a cosmographic expansion at higher order than Supernovae alone. Exploiting the set of data from Union and GRBs catalogues, we show (for the first time in a purely cosmographic approach parametrized by deceleration q0, jerk j0, snap s0) a definitively negative deceleration parameter q0 up to the 3σ confidence level. We present also forecasts for realistic data sets that are likely to be obtained in the next few years.

Clustering of submillimetre galaxies in a self-regulated baryon collapse model

In this paper, we consider a model of inflationary cosmology with inflation preceded by a bounce and study its primordial curvature perturbations. Our model gives rise to a primordial power spectrum with a feature of oscillation on large scales compared with the nearly scale-invariant spectrum generated by the traditional slow-rolling inflation model. This effect changes the cosmic microwave background temperature power spectrum and the large scale structure matter power spectrum. Further, with a detailed simulation we will show that in some detailed models this signal might be detectable in forthcoming observations, such as PLANCK and LAMOST.