arXiv subjects

Xinmin Zhang

Publications and source records attributed to Xinmin Zhang.

At least 109 records · Page 6Linked to original sources

arXiv2009

Testing Oscillating Primordial Spectrum and Oscillating Dark Energy with Astronomical Observations

In this paper we revisit the issue of determining the oscillating primordial scalar power spectrum and oscillating equation of state of dark energy from the astronomical observations. By performing a global analysis with the Markov Chain Monte Carlo method, we find that the current observations from five-year WMAP and SDSS-LRG matter power spectrum, as well as the "union" supernovae sample, constrain the oscillating index of primordial spectrum and oscillating equation of state of dark energy with the amplitude less than $|n_{\rm amp}|<0.116$ and $|w_{\rm amp}|<0.232$ at 95% confidence level, respectively. This result shows that the oscillatory structures on the primordial scalar spectrum and the equation of state of dark energy are still allowed by the current data. Furthermore, we point out that these kinds of modulation effects will be detectable (or gotten a stronger constraint) in the near future astronomical observations, such as the PLANCK satellite, LAMOST telescope and the currently ongoing supernovae projects SNLS.

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arXiv2008

Constraining Cosmological Parameters with Observational Data Including Weak Lensing Effects

In this paper, we study the cosmological implications of the 100 square degree Weak Lensing survey (the CFHTLS-Wide, RCS, VIRMOS-DESCART and GaBoDS surveys). We combine these weak lensing data with the cosmic microwave background (CMB) measurements from the WMAP5, BOOMERanG, CBI, VSA, ACBAR, the SDSS LRG matter power spectrum and the Type Ia Supernoave (SNIa) data with the "Union" compilation (307 sample), using the Markov Chain Monte Carlo method to determine the cosmological parameters. Our results show that the ΛCDM model remains a good fit to all of these data. For the dynamical dark energy model with time evolving EoS parameterized as w_{\DE}(a) = w_0 + w_a (1-a), we find that the best-fit model implying the mildly preference of Quintom model whose EoS gets across the cosmological constant boundary during evolution. Regarding the total neutrino mass limit, we obtain the upper limit, \sum m_ν< 0.471 eV (95% C.L.) within the framework of the flat ΛCDM model. Due to the obvious degeneracies between the neutrino mass and the EoS of dark energy model, this upper limit will be relaxed by a factor of 2 in the framework of dynamical dark energy models. For the constraints on the inflation parameters, we find that the upper limit on the ratio of the tensor to scalar is r<0.35 (95% C.L.) and the inflationary models with the slope n_s\geq1 are excluded at more than 2 σconfidence level. In this paper we pay particular attention to the contribution from the weak lensing data and find that the current weak lensing data do improve the constraints on matter density Ω_m, σ_8, \sum{m_ν}, and the EoS of dark energy.

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arXiv2008

PAMELA data and leptonically decaying dark matter

Recently PAMELA released their first results on the positron and antiproton ratios. Stimulated by the new data, we studied the cosmic ray propagation models and calculated the secondary positron and antiproton spectra. The low energy positron ratio can be consistent with data in the convection propagation model. Above $\sim 10$ GeV PAMELA data shows a clear excess on the positron ratio. However, the secondary antiproton is roughly consistent with data. The positron excess may be a direct evidence of dark matter annihilation or decay. We compare the positron and anti-proton spectra with data by assuming dark matter annihilates or decays into different final states. The PAMELA data actually excludes quark pairs being the main final states, disfavors gauge boson final states. Only in the case of leptonic final states the positron and anti-proton spectra can be explained simultaneously. We also compare the decaying and annihilating dark matter scenarios to account for the PAMELA results and prefer to the decaying dark matter. Finally we consider a decaying neutralino dark matter model in the frame of supersymmetry with R-parity violation. The PAMELA data is well fitted with neutralino mass $600\sim 2000$ GeV and life time $\sim 10^{26}$ seconds. We also demonstrate that neutralino with mass around 2TeV can fit PAMELA and ATIC data simultaneously.

hep-ph↗

arXiv2008

Cosmological CPT violating effect on CMB polarization

A dark energy scalar (or a function of the Ricci scalar) coupled with the derivative to the matter fields will violate the $CPT$ symmetry during the expansion of the Universe. This type of cosmological $CPT$ violation helps to generate the baryon number asymmetry and gives rise to the rotation of the photon polarization which can be measured in the astrophysical and cosmological observations, especially the experiments of the cosmic microwave background radiation. In this paper, we derive the rotation angle in a fully general relativistic way and present the rotation formulas used for the cosmic microwave background data analysis. Our formulas include the corrections from the spatial fluctuations of the scalar field. We also estimate the magnitude of these corrections in a class of dynamical dark energy models for quintessential baryo/leptogenesis.

astro-ph↗

arXiv2008

A Nonsingular Cosmology with a Scale-Invariant Spectrum of Cosmological Perturbations from Lee-Wick Theory

We study the cosmology of a Lee-Wick type scalar field theory. First, we consider homogeneous and isotropic background solutions and find that they are nonsingular, leading to cosmological bounces. Next, we analyze the spectrum of cosmological perturbations which result from this model. Unless either the potential of the Lee-Wick theory or the initial conditions are finely tuned, it is impossible to obtain background solutions which have a sufficiently long period of inflation after the bounce. More interestingly, however, we find that in the generic non-inflationary bouncing cosmology, perturbations created from quantum vacuum fluctuations in the contracting phase have the correct form to lead to a scale-invariant spectrum of metric inhomogeneities in the expanding phase. Since the background is non-singular, the evolution of the fluctuations is defined unambiguously through the bounce. We also analyze the evolution of fluctuations which emerge from thermal initial conditions in the contracting phase. The spectrum of gravitational waves stemming for quantum vacuum fluctuations in the contracting phase is also scale-invariant, and the tensor to scalar ratio is not suppressed.

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arXiv2008

Probing for cosmological parameters with LAMOST measurement

In this paper we study the sensitivity of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) project to the determination of cosmological parameters, employing the Monte Carlo Markov Chains (MCMC) method. For comparison, we first analyze the constraints on cosmological parameters from current observational data, including WMAP, SDSS and SN Ia. We then simulate the 3D matter power spectrum data expected from LAMOST, together with the simulated CMB data for PLANCK and the SN Ia from 5-year Supernovae Legacy Survey (SNLS). With the simulated data, we investigate the future improvement on cosmological parameter constraints, emphasizing the role of LAMOST. Our results show the potential of LAMOST in probing for the cosmological parameters, especially in constraining the equation-of-state (EoS) of the dark energy and the neutrino mass.

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arXiv2008

Determining Cosmological Parameters with Latest Observational Data

In this paper, we combine the latest observational data, including the WMAP five-year data (WMAP5), BOOMERanG, CBI, VSA, ACBAR, as well as the Baryon Acoustic Oscillations (BAO) and Type Ia Supernoave (SN) "Union" compilation (307 sample) to determine the cosmological parameters. Our results show that the $Λ$CDM model remains a good fit to the current data. In a flat universe, we obtain the tight limit on the constant EoS of dark energy as, $w=-0.977\pm0.056$ ($1 σ$). For the dynamical dark energy models with time evolving EoS, we find that the best-fit values are $w_0=-1.08$ and $w_1=0.368$, implying the preference of Quintom model whose EoS gets across the cosmological constant boundary. For the curvature of universe, our results give $-0.012<Ω_k<0.009$ (95% C.L.) when fixing $w_{\DE}=-1$. When considering the dynamics of dark energy, the flat universe is still a good fit to the current data. Regarding the neutrino mass limit, we obtain the upper limits, $\sum m_ν<0.533$ eV (95% C.L.) within the framework of the flat $Λ$CDM model. When adding the SDSS Lyman-$α$ forest power spectrum data, the constraint on $\sum m_ν$ can be significantly improved, $\sum m_ν<0.161$ eV (95% C.L.). Assuming that the primordial fluctuations are adiabatic with a power law spectrum, within the $Λ$CDM model, we find that the upper limit on the ratio of the tensor to scalar is $r<0.200$ (95% C.L.) and the inflationary models with the slope $n_s\geq1$ are excluded at more than $2 σ$ confidence level. However, in the framework of dynamical dark energy models, the allowed region in the parameter space of ($n_s$,$r$) is enlarged significantly. Finally, we find no evidence for the large running of the spectral index. (Abridged)

astro-ph↗

arXiv2008

Evolution of Metric Perturbations in Quintom Bounce model

We in the paper study the metric perturbations generated in a bouncing universe driven by the Quintom matter. Firstly, we review the background evolution of Quintom Bounce and the power spectrum of scalar perturbations. Secondly, we study the non-Gaussianity of curvature perturbations and then calculate the tensor perturbations of the model.

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arXiv2008

On using the WMAP distance priors in constraining the time evolving equation of state of dark energy

Recently, the WMAP group has published their five-year data and considered the constraints on the time evolving equation of state of dark energy for the first time from the WMAP distance information. In this paper, we study the effectiveness of the usage of these distance information and find that these compressed CMB information can give similar constraints on dark energy parameters compared with the full CMB power spectrum if dark energy perturbations are included, however, once incorrectly neglecting the dark energy perturbations, the difference of the results are sizable.

astro-ph↗

arXiv2008

A Model Of Inflationary Cosmology Without Singularity

In this letter, we propose a model of inflationary cosmology with a bounce preceded 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. We will show this effect changes the Cosmic Microwave Background (CMB) temperature power spectrum and the Large Scale Structure (LSS) matter power spectrum. And further with a detailed simulation we will point out this signal is detectable to the forthcoming observations, such as PLANCK and LAMOST.

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arXiv2008

Oscillating universe with quintom matter

In this paper, we study the possibility of building a model of the oscillating universe with quintom matter in the framework of 4-dimensional Friedmann-Robertson-Walker background. Taking the two-scalar-field quintom model as an example, we find in the model parameter space there are five different types of solutions which correspond to: (I) a cyclic universe with the minimal and maximal values of the scale factor remaining the same in every cycle, (II) an oscillating universe with its minimal and maximal values of the scale factor increasing cycle by cycle, (III) an oscillating universe with its minimal and maximal values of the scale factor decreasing cycle by cycle, (IV) an oscillating universe with its scale factor always increasing, and (V) an oscillating universe with its scale factor always decreasing.

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arXiv2008

High-resolution temporal constraints on the dynamics of dark energy

We use the recent type Ia supernova, cosmic microwave background and large-scale structure data to shed light on the temporal evolution of the dark energy equation of state $w(z)$ out to redshift one. We constrain the most flexible parametrization of dark energy to date, and include the dark energy perturbations consistently throughout. Interpreting our results via the principal component analysis, we find no significant evidence for dynamical dark energy: the cosmological constant model is consistent with data everywhere between redshift zero and one at 95% C.L.

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arXiv2008

Testing CPT Symmetry with CMB Measurements: Update after WMAP5

In this Letter we update our previous result on the test of CPT symmetry with Cosmic Microwave Background (CMB) measurements. A CPT violating interaction in the photon sector ${\cal L}_{cs} \sim p_μA_ν\tilde F^{μν}$ gives rise to a rotation of the polarization vectors of the propagating CMB photons. Recently the WMAP group used the newly released polarization data of WMAP5 to measure this rotation angle $Δα$ and obtained $Δα=-1.7\pm2.1$ deg ($1σ$). However, in their analysis the BOOMERanG 2003 data is not included. Here we revisit this issue by combining the full data of WMAP5 and BOOMERanG 2003 angular power spectra for the measurement of this rotation angle $Δα$ and find that $Δα=-2.6\pm1.9$ deg at a 68% confidence level.

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arXiv2008

On Perturbations of 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.

hep-th↗

arXiv2008

Testing CPT Symmetry with CMB Measurements

In this paper we study the possibility of testing \emph{CPT} symmetry with Cosmic Microwave Background (CMB) measurements. Working with an effective lagrangian of the photon with \emph{CPT} violation ${\cal L} \sim p_μA_ν\tilde F^{μν}$ which causes the polarization vectors of the propagating CMB photons rotated, we determine the rotation angle $Δα$ using the BOOMERanG 2003 and the WMAP3 angular power spectra. In this analysis we have included the newly released $TC$ and $GC$ ($l<450$) information of WMAP3 and found $Δα=-6.2\pm3.8$ deg at $1σ$ confidence level. This result increases slightly the significance for the \emph{CPT} violation obtained in our previous paper (Feng \emph{et al.}, 2006) $Δα=-6.0 \pm 4.0$ deg (1$σ$). Furthermore we examine the constraint on the rotation angle with the simulated Planck data. Our results show that the Planck will be sensitive to $Δα$ at the level of 0.057 deg and able to test the \emph{CPT} symmetry with a higher precision.

hep-ph↗

arXiv2008

Constraining Slow-Roll Inflation in the Presence of Dynamical Dark Energy

In this paper we perform a global analysis of the constraints on the inflationary parameters in the presence of dynamical dark energy models from the current observations, including the three-year Wilkinson Microwave Anisotropy Probe (WMAP3) data, Boomerang-2K2, CBI, VSA, ACBAR, SDSS LRG, 2dFGRS and ESSENCE (192 sample). We use the analytic description of the inflationary power spectra in terms of the Horizon-flow parameters $\{ε_i\}$. With the first order approximation in the slow-roll expansion, we find that the constraints on the Horizon-flow parameters are $ε_1<0.014 (95% C.L.)$ and $ε_2=0.034\pm0.024 (1σ)$ in the $Λ$CDM model. In the framework of dynamical dark energy models, the constraints become obviously weak, $ε_1<0.022 (95% C.L.)$ and $ε_2=-0.006\pm0.039 (1σ)$, and the inflation models with a "blue" tilt, which are excluded about $2σ$ in the $Λ$CDM model, are allowed now. With the second order approximation, the constraints on the Horizon-flow parameters are significantly relaxed further. If considering the non-zero $ε_3$, the large running of the scalar spectral index is found for the $Λ$CDM model, as well as the dynamical dark energy models.

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arXiv2008

Overcoming the Circular Problem for γ-ray Bursts in Cosmological Global Fitting Analysis

Due to the lack of low redshift long Gamma-Ray Bursts (GRBs), the circular problem has been a severe obstacle for using GRBs as cosmological candles. In this paper, we present a new method to deal with such a problem in MCMC global fitting analysis. Assuming that a certain type of correlations between different observables exists in a subsample of GRBs, for the parameters involved in the correlation relation, we treat them as free parameters and determine them simultaneously with cosmological parameters through MCMC analysis on GRB data together with other observational data. Then the circular problem is naturally eliminated in this procedure. We take the Ghirlanda relation as an example while keeping in mind the debate about its physical validity. Together with SNe Ia, WMAP and SDSS data, we include 27 GRBs with the reported Ghirlanda relation in our study, and perform MCMC global fitting. We consider the $Λ$CDM model and dynamical dark energy models. In each case, in addition to the constraints on the relevant cosmological parameters, we obtain the best fit values as well as the distributions of the correlation parameters $A$ and $C$. We find that the observational data sets other than GRBs can affect $A$ and $C$ considerably through their degeneracies with the cosmological parameters. The results on $A$ and $C$ for different cosmological models are in well agreement within $1σ$ range. The best fit value of $A$ in all models being analyzed is $A\sim 1.53$ with $σ\sim 0.08$. For $C$, we have the best value in the range of $0.94-0.98$ with $σ\sim 0.1$. It is also noted that the distributions of $A$ and $C$ are generally broader than the priors used in many studies in literature. (Abriged)

astro-ph↗

arXiv2008

Constraints on the Sound Speed of Dynamical Dark Energy

In this paper we study the sound speed - $c_s^2$, which is directly related to the classical perturbations - of the dynamical dark energy (DE), especially with an equation of state crossing the cosmological constant boundary in details and show its implications on Cosmic Microwave Background (CMB) Anisotropy. With the present observational data of CMB, Type Ia Supernova (SNIa) and galaxy clustering, we perform a global analysis to constrain the sound speed of dark energy, using the Markov Chain Monte Carlo method. We find that the sound speed of dark energy is weakly constrained by current observations thus the futuristic precision measurements of CMB on a very large angular scale (low multipoles) are necessary.

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