arXiv subjects

W. Zhao

Publications and source records attributed to W. Zhao.

80 records · Page 5Linked to original sources

arXiv2009

Quantum Yang-Mills Condensate Dark Energy Models

We review the quantum Yang-Mills condensate (YMC) dark energy models. As the effective Yang-Mills Lagrangian is completely determined by the quantum field theory, there is no adjustable parameter in the model except the energy scale. In this model, the equation-of-state (EOS) of the YMC dark energy, $w_y > -1$ and $w_y < -1$, can both be naturally realized. By studying the evolution of various components in the model, we find that, in the early stage of the universe, dark energy tracked the evolution of the radiation, i.e. $w_y \to 1/3$. However, in the late stage, $w_y$ naturally runs to the critical state with $w_y = -1$, and the universe transits from matter-dominated into dark energy dominated stage only at recently $z \sim 0.3$. These characters are independent of the choice of the initial condition, and the cosmic coincidence problem is avoided in the models. We also find that, if the possible interaction between YMC and dust matter is considered, the late time attractor solution may exist. In this case, the EOS of YMC must evolve from $w_y>0$ into $w_y < -1$, which is slightly suggested by the observations. At the same time, the total EOS in the attractor solution is $w_{tot} = -1$, the universe being the de Sitter expansion in the late stage, and the cosmic big rip is naturally avoided. These features are all independent of the interacting forms.

astro-ph.CO↗

arXiv2009

Detecting relics of a thermal gravitational wave background in the early Universe

A thermal gravitational wave background can be produced in the early Universe if a radiation dominated epoch precedes the usual inflationary stage. This background provides a unique way to study the initial state of the Universe. We discuss the imprint of this thermal spectra of gravitons on the cosmic microwave background (CMB) power spectra, and its possible detection by CMB observations. Assuming the inflationary stage is a pure de Sitter expansion we find that, if the number of e-folds of inflation is smaller than 65, the signal of this thermal spectrum can be detected by the observations of Planck and PolarBear experiments, or the planned EPIC experiments. This bound can be even looser if inflation-like stage is the sub-exponential.

gr-qc↗

arXiv2009

Detecting Relic Gravitational Waves in the CMB: Comparison of Planck and Ground-based Experiments

We compare the detection abilities for the relic gravitational waves by two kinds of forthcoming cosmic microwave background radiation (CMB) experiments, space-based Planck satellite and the various ground-based experiments. Comparing with the ground-based experiments, Planck satellite can observe all the CMB power spectra in all the multipole range, but having much larger instrumental noises. We find that, for the uncertainty of the tensor-to-scalar ratio $Δr$, PolarBear (II) as a typical ground-based experiment can give much smaller value than Planck satellite. However, for the uncertainty of the spectral index $Δn_t$, Planck can give the similar result with PolarBear (II). If combining these two experiments, the value of $Δn_t$ can be reduced by a factor 2. For the model with $r=0.1$, the constraint $Δn_t=0.10$ is expected to be achieved, which provides an excellent opportunity to study the physics in the very early universe. We also find the observation in the largest scale ($\ell<20$) is very important for constraining the spectral index $n_t$. So it is necessary to combine the observations of the future space-based and ground-based CMB experiments to determine the relic gravitational waves.

astro-ph.CO↗

arXiv2009

Detecting relic gravitational waves in the CMB: Comparison of different methods

In this paper, we discuss the constraint on the relic gravitational waves by both temperature and polarization anisotropies power spectra of cosmic microwave background radiation. Taking into account the instrumental noises of Planck satellite, we calculate the signal-to-noise ratio $S/N$ by the simulation and the analytic approximation methods. We find that, comparing with the $BB$ channel, the value of $S/N$ is much improved in the case where all the power spectra, $TT$, $TE$, $EE$ and $BB$, are considered. If the noise power spectra of Planck satellite increase for some reasons, the value of $S/N$ in $BB$ channel is much reduced. However, in the latter case where all the power spectra of cosmic microwave background radiation are considered, the value of $S/N$ is less influenced. We also find that the free parameters $A_s$, $n_s$ and $n_t$ have little influence on the value of $S/N$ in both cases.

astro-ph.CO↗

arXiv2009

Detecting relic gravitational waves in the CMB: Optimal parameters and their constraints

The prospect of detecting relic gravitational waves (RGWs), through their imprint in the cosmic microwave background radiation, provides an excellent opportunity to study the very early Universe. In simplest viable theoretical models the RGW background is characterized by two parameters, the tensor-to-scalar ratio $r$ and the tensor spectral index $n_t$. In this paper, we analyze the potential joint constraints on these two parameters, $r$ and $n_t$, using the data from the upcoming cosmic microwave background radiation experiments. Introducing the notion of the best pivot multipole $\ell_t^*$, we find that at this pivot multipole the parameters $r$ and $n_t$ are uncorrelated, and have the smallest variances. We derive the analytical formulae for the best pivot multipole number $\ell_t^*$, and the variances of the parameters $r$ and $n_t$. We verify these analytical calculations using numerical simulation methods, and find agreement to within 20%. The analytical results provides a simple way to estimate the detection ability for the relic gravitational waves by the future observations of the cosmic microwave background radiation.

astro-ph.CO↗

arXiv2009

On the road to discovery of relic gravitational waves: The TE and BB Correlations in the cosmic microwave background radiation

The detection of primordial gravitational waves is one of the biggest challenges of the present time. The existing (Wilkinson Microwave Anisotropy Probe) observations are helpful on the road to this goal, and the forthcoming experiments (Planck) are likely to complete this mission. We show that the 5-year Wilkinson Microwave Anisotropy Probe $TE$ data contains a hint of the presence of gravitational wave contribution. In terms of the parameter $R$, which gives the ratio of contributions from gravitational waves and density perturbations to the temperature quadrupole, the best-fit model produced $R=0.24$. Because of large residual noises, the uncertainty of this determination is still large, and it easily includes the R=0 hypothesis. However, the uncertainty will be strongly reduced in the forthcoming observations which are more sensitive. We numerically simulated the Planck data and concluded that the relic gravitational waves with $R=0.24$ will be present at a better than 3$σ$ level in the $TE$ observational channel, and at a better than 2$σ$ level in the `realistic' $BB$ channel. The balloon-borne and ground-based observations may provide a healthy competition to Planck in some parts of the lower-$\ell$ spectrum.

astro-ph↗

arXiv2008

Relic Gravitational Waves And CMB Polarization In The Accelerating Universe

In this paper we briefly present our works on the relic gravitational waves (RGW) and the CMB polarization in the accelerating universe. The spectrum of RGW has been obtained, showing the influence of the dark energy. Compared with those from non-accelerating models, the shape of the spectrum is approximately similar, nevertheless, the amplitude of RGW now acquires a suppressing factor of the ratio of matter over dark energy $\propto Ω_m/Ω_Λ\sim 0.4$ over almost the whole range of frequencies. The RGW spectrum is then used as the source to calculate the spectra of CMB polarization. By a two half Gaussian function as an approximation to the visibility function during the photon decoupling, both the "electric" and "magnetic" spectra have been analytically derived, which are quite close to the numerical ones. Several physical elements that affect the spectra have been examined, such as the decoupling process, the inflation, the dark energy, the baryons, etc.

astro-ph↗

arXiv2006

Yang-Mills condensate dark energy coupled with matter and radiation

The coincidence problem is studied for the dark energy model of effective Yang-Mills condensate in a flat expanding universe during the matter-dominated stage. The YMC energy $ρ_y(t)$ is taken to represent the dark energy, which is coupled either with the matter, or with both the matter and the radiation components. The effective YM Lagrangian is completely determined by quantum field theory up to 1-loop order. It is found that under very generic initial conditions and for a variety of forms of coupling, the existence of the scaling solution during the early stages and the subsequent exit from the scaling regime are inevitable. The transition to the accelerating stage always occurs around a redshift $z\simeq (0.3\sim 0.5)$. Moreover, when the Yang-Mills condensate transfers energy into matter or into both matter and radiation, the equation of state $w_y$ of the Yang-Mills condensate can cross over -1 around $z\sim 2$, and takes on a current value $\simeq -1.1$. This is consistent with the recent preliminary observations on supernovae Ia. Therefore, the coincidence problem can be naturally solved in the effective YMC dark energy models.

gr-qc↗