arXiv subjects

Chung-Chi Lee

Publications and source records attributed to Chung-Chi Lee.

At least 37 records · Page 2Linked to original sources

arXiv2019

Cosmological evolutions in Tsujikawa model of $f(R)$ Gravity

We concentrate on the cosmological properties in the Tsujikawa model (TM) of viable $f(R)$ gravity with the dynamical background evolution and linear perturbation theory by using the CosmoMC package. We study the constraints of the cosmological variables along with the model parameter from the current observational data. In particular, we show that the matter density fluctuation is suppressed and the constraint of the neutrino mass sum is relaxed in the TM, which are similar to other viable $f(R)$ models. In addition, we discuss the parameters of the deceleration and equation of state for dark energy in the TM and compare them with those in the $Λ$CDM model.

gr-qc↗

arXiv2019

Matter Power Spectra in Viable $f(R)$ Gravity Models with Dynamical Background

We study the matter power spectra in the viable $f(R)$ gravity models with the dynamical background evolution and linear perturbation theory by using the CosmoMC package. We show that these viable $f(R)$ models generally shorten the age of the universe and suppress the matter density fluctuation. We examine the allowed ranges of the model parameters and the constraints of the cosmological variables from the current observational data, and find that the dynamical evolution of $ρ_{DE}(z)$ plays an important role to constrain the neutrino masses.

gr-qc↗

arXiv2018

Observational constraints on running vacuum model

We investigate the power spectra of the CMB temperature and matter density in the running vacuum model (RVM) with the time-dependent cosmological constant of $Λ= 3 νH^2 + Λ_0$, where $H$ is the Hubble parameter. In this model, dark energy decreases in time and decays to both matter and radiation. By using the Markov chain Monte Carlo method, we constrain the model parameter $ν$ as well as the cosmological observables. Explicitly, we obtain $ν\leq 1.54 \times 10^{-4}$ (68\% confidence level) in the RVM with the best-fit $χ^2_{\mathrm{RVM}} = 13968.8$, which is slightly smaller than $χ^2_{Λ\mathrm{CDM}} = 13969.8$ in the $Λ$CDM model of $ν=0$.

astro-ph.CO↗

arXiv2018

Cosmological perturbation and matter power spectrum in bimetric massive gravity

We discuss the linear perturbation equations with the synchronous gauge in a minimal scenario of the bimetric massive gravity theory. We find that the matter density perturbation and matter power spectrum are suppressed. We also examine the ghost and stability problems and show that the allowed deviation of this gravitational theory from the cosmological constant is constrained to be smaller than $\mathcal{O}(10^{-2})$ by the large scale structure observational data.

gr-qc↗

arXiv2017

Constraints on running vacuum model with $H(z)$ and $f σ_8$

We examine the running vacuum model with $Λ(H) = 3 νH^2 + Λ_0$, where $ν$ is the model parameter and $Λ_0$ is the cosmological constant. From the data of the cosmic microwave background radiation, weak lensing and baryon acoustic oscillation along with the time dependent Hubble parameter $H(z)$ and weighted linear growth $f (z)σ_8(z)$ measurements, we find that $ν=(1.37^{+0.72}_{-0.95})\times 10^{-4}$ with the best fitted $χ^2$ value slightly smaller than that in the $Λ$CDM model.

astro-ph.CO↗

arXiv2017

Observational constraints on successful model of quintessential Inflation

We study quintessential inflation using a generalized exponential potential $V(ϕ)\propto exp(-λϕ^n/Mpl^n), n>1$, the model admits slow-roll inflation at early times and leads to close-to-scaling behaviour in the post inflationary era with an exit to dark energy at late times. We present detailed investigations of the inflationary stage in the light of the Planck 2015 results, study post-inflationary dynamics and analytically confirm the existence of an approximately scaling solution. Additionally, assuming that standard massive neutrinos are non-minimally coupled, makes the field $ϕ$ dominant once again at late times giving rise to present accelerated expansion of the Universe. We derive observational constraints on the field and time-dependent neutrino masses. In particular, for $n=6 (8)$, the parameter $λ$ is constrained to be,$\log λ> -7.29 (-11.7)$; the model produces the spectral index of the power spectrum of primordial scalar (matter density) perturbations as $ n_s = 0.959 \pm 0.001 (0.961 \pm 0.001)$ and tiny tensor-to-scalar ratio, $r<1.72 \times 10^{-2} (2.32 \times 10^{-2})$ respectively. Consequently, the upper bound on possible values of the sum of neutrino masses $Σm_ν \lesssim 2.5$ eV significantly enhances compared to that in the standard $Λ$CDM model.

gr-qc↗

arXiv2017

Probing gravitational non-minimal coupling with dark energy surveys

We investigate observational constraints on a specific one-parameter extension to the minimal quintessence model, where the quintessence field acquires a quadratic coupling to the scalar curvature through a coupling constant $ξ$. The value of $ξ$ is highly suppressed in typical tracker models if the late-time cosmic acceleration is driven at some field values near the Planck scale. We test $ξ$ in a second class of models in which the field value today becomes a free model parameter. We use the combined data from type-Ia supernovae, cosmic microwave background, baryon acoustic oscillations and matter power spectrum, to weak lensing measurements and find a best-fit value $ξ> 0.289$ where $ξ= 0$ is excluded outside the 95 per cent confidence region. The effective gravitational constant $G_{\rm eff}$ subject to the hint of a non-zero $ξ$ is constrained to $-0.003 < 1- G_{\rm eff}/G < 0.033$ at the same confidence level on cosmological scales, and can be narrowed down to $1- G_{\rm eff}/G < 2.2 \times 10^{-5}$ when combining with Solar System tests.

astro-ph.CO↗

arXiv2016

Matter density perturbation and power spectrum in running vacuum model

We investigate the matter density perturbation $δ_m$ and power spectrum $P(k)$ in the running vacuum model (RVM) with the cosmological constant being a function of the Hubble parameter, given by $Λ= Λ_0 + 6 σH H_0+ 3νH^2$, in which the linear and quadratic terms of $H$ would originate from the QCD vacuum condensation and cosmological renormalization group, respectively. Taking the dark energy perturbation into consideration, we derive the evolution equation for $δ_m$ and find a specific scale $d_{cr}=2 π/k_{cr}$, which divides the evolution of the universe into the sub and super-interaction regimes, corresponding to $k \ll k_{cr}$ and $k \gg k_{cr}$, respectively. For the former, the evolution of $δ_m$ has the same behavior as that in the $Λ$CDM model, while for the latter, the growth of $δ_m$ is frozen (greatly enhanced) when $ν+ σ>(<)0$ due to the couplings between radiation, matter and dark energy. It is clear that the observational data rule out the cases with $ν<0$ and $ν+ σ<0$, while the allowed window for the model parameters is extremely narrow with $ν, |σ| \lesssim \mathcal{O}(10^{-7})$.

astro-ph.CO↗

arXiv2016

Running cosmological constant with observational tests

We investigate the running cosmological constant model with dark energy linearly proportional to the Hubble parameter, $Λ= σH + Λ_0$, in which the $Λ$CDM limit is recovered by taking $σ=0$. We derive the linear perturbation equations of gravity under the Friedmann-Lemaïtre-Robertson-Walker cosmology, and show the power spectra of the CMB temperature and matter density distribution. By using the Markov chain Monte Carlo method, we fit the model to the current observational data and find that $σH_0/ Λ_0 \lesssim 2.63 \times 10^{-2}$ and $6.74 \times 10^{-2}$ for $Λ(t)$ coupled to matter and radiation-matter, respectively, along with constraints on other cosmological parameters.

astro-ph.CO↗

arXiv2016

Deflation of the cosmological constant associated with inflation and dark energy

In order to solve the fine-tuning problem of the cosmological constant, we propose a simple model with the vacuum energy non-minimally coupled to the inflaton field. In this model, the vacuum energy decays to the inflaton during pre-inflation and inflation eras, so that the cosmological constant effectively deflates from the Planck mass scale to a much smaller one after inflation and plays the role of dark energy in the late-time of the universe. We show that our deflationary scenario is applicable to arbitrary slow-roll inflation models. We also take two specific inflation potentials to illustrate our results.

gr-qc↗

arXiv2016

Observational constraints on varying neutrino-mass cosmology

We consider generic models of quintessence and we investigate the influence of massive neutrino matter with field-dependent masses on the matter power spectrum. In case of minimally coupled neutrino matter, we examine the effect in tracker models with inverse power-law and double exponential potentials. We present detailed investigations for the scaling field with a steep exponential potential, non-minimally coupled to massive neutrino matter, and we derive constraints on field-dependent neutrino masses from the observational data.

astro-ph.CO↗

arXiv2015

Constraints on the Combined Models with $R^{2-q}$ Inflation and Viable $f(R)$ Dark Energy

We investigate the observational constraints on the modified gravity, which combines the $R^{2-q}$ inflation with the power-law (exponential) type of the viable $f(R)$ dark energy models. We discuss the difference between the combined model and $R^{2-q}$ gravity in the inflationary epoch and obtain the constraints on the deviation power $q$ as well as the parameters in $f(R)$ by using the CosmoMC package. The allowed ranges of the spectral index and tensor-to-scalar ratio from the Planck data are highly restricted, resulting in $q < 2.66 \times 10^{-2}$ and $2.17 \times 10^{-2}$ for the power-law and exponential types of $f(R)$ gravity, respectively.

astro-ph.CO↗

arXiv2015

Constraints on the exponential $f(R)$ model from latest Hubble parameter measurements

We investigate the viable exponential $f(R)$ gravity in the metric formalism with $f(R)=-βR_s (1-e^{-R/R_s})$. The latest sample of the Hubble parameter measurements with 23 data points is used to place bounds on this $f(R)$ model. A joint analysis is also performed with the luminosity distances of Type Ia supernovae and baryon acoustic oscillations in the clustering of galaxies, and the shift parameters from the cosmic microwave background measurements, which leads to $0.240<Ω_m^0<0.296$ and $β>1.47$ at 1$σ$ confidence level. The evolutions of the deceleration parameter $q(z)$ and the effective equations of state $ω_{de}^{eff}(z)$ and $ω_{tot}^{eff}(z)$ are displayed. By taking the best-fit parameters as prior values, we work out the transition redshift (deceleration/acceleration) $z_T$ to be about 0.77. It turns out that the recent observations are still unable to distinguish the background dynamics in the $Λ$CDM and exponential $f(R)$ models.

astro-ph.CO↗

arXiv2014

Matter Power Spectra in Viable $f(R)$ Gravity Models with Massive Neutrinos

We investigate the matter power spectra in the power law and exponential types of viable $f(R)$ theories along with massive neutrinos. The enhancement of the matter power spectrum is found to be a generic feature in these models. In particular, we show that in the former type, such as the Starobinsky model, the spectrum is magnified much larger than the latter one, such as the exponential model. A greater scale of the total neutrino mass, $Σm_ν$, is allowed in the viable $f(R)$ models than that in the $Λ$CDM one. We obtain the constraints on the neutrino masses by using the CosmoMC package with the modified MGCAMB. Explicitly, we get $Σm_ν < 0.451 \ (0.214)\ \mathrm{eV}$ at 95% C.L. in the Starobinsky (exponential) model, while the corresponding one for the $Λ$CDM model is $Σm_ν < 0.200\ \mathrm{eV}$. Furthermore, by treating the effective number of neutrino species $N_{\mathrm{eff}}$ as a free parameter along with $Σm_ν$, we find that $N_{\mathrm{eff}} = 3.78^{+0.64}_{-0.84} (3.47^{+0.74}_{-0.60})$ and $Σm_ν = 0.533^{+0.254}_{-0.411}$ ($< 0.386) \ \mathrm{eV}$ at 95% C.L. in the Starobinsky (exponential) model.

astro-ph.CO↗

arXiv2013

Singularity Problem in Teleparallel Dark Energy Models

We study future singularity in teleparallel dark energy models, particularly its behavior and its (non)occurrence in the observationally viable models. For the models with a general self-potential of the scalar field, we point out that both at early times and in the future near the singularity the behavior of dark energy can be described by the analytic solutions of the scalar field we obtained for the model with no self-potential. As to the (non)occurrence in the viable models, we consider a natural binding-type self-potential, the quadratic potential, when fitting observational data, and illustrate the constraining region up to the $3σ$ confidence level as well as the region where a singularity will occur. As a result, the singularity region is outside the $3σ$ constraint. Thus, although the future singularity problem potentially exists in teleparallel dark energy models, the observationally viable models may not suffer this problem.

astro-ph.CO↗

arXiv2013

Einstein Static Universe in Exponential $f(T)$ Gravity

We analyze the stability of the Einstein static closed and open universe in two types of exponential $f(T)$ gravity theories. We show that the stable solutions exist in these two models. In particular, we find that large regions of parameter space in equation of state $w=p/ρ$ for the stable universe are allowed in the $f(T)$ theories.

gr-qc↗

arXiv2013

Asymptotic Cosmological Behavior of Scalar-Torsion Mode in Poincare Gauge Theory

We study the cosmological effect of the simple scalar-torsion ($0^+$) mode in Poincaré gauge theory of gravity. We find that for the non-constant (affine) curvature case, the early evolution of the torsion density $ρ_T$ has a radiation-like asymptotic behavior of $a^{-4}$ with $a$ representing the scale factor, along with the stable point of the torsion pressure ($P_T$) and density ratio $P_T/ρ_T\rightarrow 1/3$ in the high redshift regime $(z \gg 0)$, which is different from the previous result in the literature. We use the Laurent expansion to resolve the solution. We also illustrate our result by the execution of numerical computations.

astro-ph.CO↗

arXiv2012

Teleparallel Dark Energy with Purely Non-minimal Coupling to Gravity

We propose the simplest model of teleparallel dark energy with purely a non-minimal coupling to gravity but no self-potential, a single model possessing various interesting features: simplicity, self-potential-free, the guaranteed late-time cosmic acceleration driven by the non-minimal coupling to gravity, tracker behavior of the dark energy equation of state at earlier times, a crossing of the phantom divide at a late time, and the existence of a finite-time future singularity. We find the analytic solutions of the dark-energy scalar field respectively in the radiation, matter, and dark energy dominated eras, thereby revealing the above features. We further illustrate possible cosmic evolution patterns and present the observational constraint of this model obtained by numerical analysis and data fitting.

astro-ph.CO↗