All Results

Cosmological Evolution of Interacting Phantom Energy with Dark Matter

Zong-Kuan Guo; Rong-Gen Cai; Yuan-Zhong ZhangSubjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

We investigate the cosmological evolution of an interacting phantom energy model in which the phantom field has interaction with the dark matter. We discuss the existence and stability of scaling solutions for two types of specific interactions. One is motivated by the conformal transformation in string theory and the other is motivated by analogy with dissipation. In the former case, there exist no scaling solutions. In the latter case, there exist stable scaling solutions, which may give a phenomenological solution of the coincidence problem. Furthermore, the universe either accelerates forever or ends with a singularity, which is determined by not only the model parameters but also the initial velocity of the phantom field. |

Cosm ologicalScaling Solutions and C ross-couplingExponentialPotential

Zong-Kuan Guo; Yun-Song Piao; Rong-Gen Cai; Yuan-Zhong ZhangSubjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

We present a phase-space analysis of cosmology containing multiple scalar elds with a positive or negative cross-coupling exponential potential. We show that there exist power law kinetic-potential-scaling solutions for a sciently at positive potential or for a steep negative potential.The former is the unique late-ti e attractor, but it is difficult to yield assisted ination.Thelaterisneverstablein an expanding universe.M oreover,fora steep negative potentialthere existsa kinetic-dom inated regim e in which each solution isa latetime attractor. In the presence ofordinary m atterthese scaling solutionswith a negative cross-coupling potentialare found unstable. W e briey discuss the physicalconsequences of these results. |

Inflationary Attractor from Tachyonic Matter

Zong-Kuan Guo; Yun-Song Piao; Rong-Gen Cai; Yuan-Zhong ZhangSubjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

We study the complete evolution of a flat and homogeneous universe dominated by tachyonic matter. We demonstrate the attractor behaviour of the tachyonic inflation using the Hamilton-Jacobi formalism. We else obtain analytical approximations to the trajectories of the tachyon field in different regions. The numerical calculation shows that an initial non-vanishing momentum does not prevent the onset of inflation. The slow-rolling solution is an attractor. |

Gravitational Waves from Oscillons with Cuspy Potentials

Jing Liu ; Zong-Kuan Guo; Rong-Gen Cai; Gary ShiuSubjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

We study the production of gravitational waves during oscillations of the inflaton around the minimum of a cuspy potential after inflation. We find that a cusp in the potential can trigger copious oscillon formation, which sources a characteristic energy spectrum of gravitational waves with double peaks. The discovery of such a double-peak spectrum could test the underlying inflationary physics. |

The Gravitational-Wave Physics

Rong-Gen Cai; Zhoujian Cao; Zong-Kuan Guo; Shao-Jiang Wang; Tao YangSubjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

The direct detection of gravitational wave by Laser Interferometer Gravitational-Wave Observatory indicates the coming of the era of gravitational-wave astronomy and gravitational-wave cosmology. It is expected that more and more gravitational-wave events will be detected by currently existing and planned gravitational-wave detectors. The gravitational waves open a new window to explore the Universe and various mysteries will be disclosed through the gravitational-wave detection, combined with other cosmological probes. The gravitational-wave physics is not only related to gravitation theory, but also is closely tied to fundamental physics, cosmology and astrophysics. In this review article, three kinds of sources of gravitational waves and relevant physics will be discussed, namely gravitational waves produced during the inflation and preheating phases of the Universe,the gravitational waves produced during the first-order phase transition as the Universe cools down and the gravitational waves from the three phases: inspiral, merger and ringdown of a compact binary system, respectively. We will also discuss the gravitational waves as a standard siren to explore the evolution of the Universe. |

Dodging the cosmic curvature toprobe the constancy of the speed oflight

Rong-Gen Cai ; Zong-Kuan Guo; Tao YangSubjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

We develop a new model-independent method to probe the constancy of the speed of light c. In our method, the degeneracy between the cosmic curvature and the speed of light can be eliminated, which makes the test more natural and general. Combining the independent observations of Hubble parameter H(z) and luminosity distance dL(z), we use the model-independent smoothing technique, Gaussian processes, to reconstruct them and then detect variation of the speed of light. We find no signal of deviation from the present value of the speed of light c0. Moreover, to demonstrate the improvement in probing the constancy of the speed of light from future experiments, we produce a series of simulated data. The Dark Energy Survey will be able to detect #1;c/c0 #24; 1% at #24; 1.5σ confidencelevel and #1;c/c0 #24; 2% at #24; 3σ confidence level. If the errors are reduced to one-tenth of the expected DES ones, it can detect a #1;c/c0 #24; 0.1% variation at #24; 2σ confidence level. |

Null test of the cosmic curvature using H(z) and supernovae data

Rong-Gen Cai; Zong-Kuan Guo; Tao YangSubjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

We introduce a model-independent approach to the null test of the cosmic curvature which is geometrically related to the Hubble parameter H(z) and luminosity distance dL(z). Combining the independent observations of H(z) and dL(z), we use the model-independent smoothing technique,Gaussian processes, to reconstruct them and determine the cosmic curvature (0)K in the null test relation. The null test is totally geometrical and does not assume any cosmological model. We show that the cosmic curvature (0)K = 0 is consistent with current observational data sets, falling within the 1#27; limit. To demonstrate the effect on the precision of the null test, we produce a series of simulated data of the models with different (0) K . Future observations in better quality can provide a greater improvement to constrain or refute the flat universe with (0)K = 0. |

Principal component analysis of the reionization history from Planck 2015 data

Wei-Ming Dai; Zong-Kuan Guo; Rong-Gen CaiSubjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

The simple assumption of an instantaneous reionization of the Universe may bias estimates of cosmological parameters. In this paper a model-independent principal component method for the reionization history is applied to give constraints on the cosmological parameters from recent Planck 2015 data. We find that the Universe are not completely reionized at redshifts z #21; 8.5 at 95% CL. Both the reionization optical depth and the matter fluctuation amplitude are higher than but consistent with those obtained in the standard instantaneous reionization scheme. The high estimated value of the matter fluctuation amplitude strengthens the tension between Planck CMB observations and some astrophysical data, such as cluster counts and weak lensing. The tension can significantly be relieved if the neutrino masses are allowed to vary. Thanks to a high scalar spectral index, the low-scale spontaneously broken SUSY inflationary model can fit the data well, which is marginally disfavored at 95% CL in the Planck analysis. |

Higgs in ation in Gauss-Bonnet braneworld

Rong-Gen Cai; Zong-Kuan Guo; Shao-Jiang WangSubjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

The measured masses of the Higgs boson and top quark indicate that the e#11;ective potential of the standard model either develops an unstable electroweak vacuum or stands stable all the way up to the Planck scale. In the latter case in which the top quark mass is about 2#27; below its present central value, the Higgs boson can be the inaton with the help of a large nonminimal coupling to curvature in four dimensions. We propose a scenario in which the Higgs boson can be the in aton in a#12;ve-dimensional Gauss-Bonnet braneworld model to solve both the unitarity and stability problems which usually plague Higgs ination. We #12;nd that in order for Higgs ination to happen successfully in the Gauss-Bonnet regime, the extra dimension scale must appear roughly in the range between the TeV scale and the instability scale of standard model. At the tree level, our model can give rise to a naturally small nonminimal coupling #24; #24; O(1) for the Higgs quartic coupling #21; #24; O(0:1) if the extra dimension scale lies at the TeV scale. At the loop level, the in ationary predictions at the treelevel are preserved. Our model can be confronted with future experiments and observations fromboth particle physics and cosmology. |

Reconstructing the interaction between dark energy and dark matter using Gaussian processes

Tao Yang ; Zong-Kuan Guo; Rong-Gen CaiSubjects: Physics >> General Physics: Statistical and Quantum Mechanics, Quantum Information, etc.

We present a nonparametric approach to reconstruct the interaction between dark energy and dark matter directly from SNIa Union 2.1 data using Gaussian processes, which is a fully Bayesian approach for smoothing data. In this method, once the equation of state (w) of dark energy is specified, the interaction can be reconstructed as a function of redshift. For the decaying vacuum energy case with w = ?1, the reconstructed interaction is consistent with the standard #3;CDM model, namely, there is no evidence for the interaction. This also holds for the constant w cases from ?0.9 to ?1.1 and for the Chevallier-Polarski-Linder (CPL) parametrization case. If the equation of state deviates obviously from ?1, the reconstructed interaction exists at 95% confidence level. This shows the degeneracy between the interaction and the equation of state of dark energy when they get constraints from the observational data. |