arXiv subjects

Qing-Guo Huang

Publications and source records attributed to Qing-Guo Huang.

At least 163 records · Page 9Linked to original sources

arXiv2009

Fitting the Constitution SNIa Data with Redshift Binned Parameterization Method

In this work, we explore the cosmological consequences of the recently released Constitution sample of 397 Type Ia supernovae (SNIa). By revisiting the Chevallier-Polarski-Linder (CPL) parameterization, we find that, for fitting the Constitution set alone, the behavior of dark energy (DE) significantly deviate from the cosmological constant $Λ$, where the equation of state (EOS) $w$ and the energy density $ρ_Λ$ of DE will rapidly decrease along with the increase of redshift $z$. Inspired by this clue, we separate the redshifts into different bins, and discuss the models of a constant $w$ or a constant $ρ_Λ$ in each bin, respectively. It is found that for fitting the Constitution set alone, $w$ and $ρ_Λ$ will also rapidly decrease along with the increase of $z$, which is consistent with the result of CPL model. Moreover, a step function model in which $ρ_Λ$ rapidly decreases at redshift $z\sim0.331$ presents a significant improvement ($Δχ^{2}=-4.361$) over the CPL parameterization, and performs better than other DE models. We also plot the error bars of DE density of this model, and find that this model deviates from the cosmological constant $Λ$ at 68.3% confidence level (CL); this may arise from some biasing systematic errors in the handling of SNIa data, or more interestingly from the nature of DE itself. In addition, for models with same number of redshift bins, a piecewise constant $ρ_Λ$ model always performs better than a piecewise constant $w$ model; this shows the advantage of using $ρ_Λ$, instead of $w$, to probe the variation of DE.

astro-ph.CO↗

arXiv2009

A geometric description of the non-Gaussianity generated at the end of multi-field inflation

In this paper we mainly focus on the curvature perturbation generated at the end of multi-field inflation, such as the multi-brid inflation. Since the curvature perturbation is produced on the super-horizon scale, the bispectrum and trispectrum have a local shape. The size of bispectrum is measured by $f_{NL}$ and the trispectrum is characterized by two parameters $τ_{NL}$ and $g_{NL}$. For simplicity, the trajectory of inflaton is assumed to be a straight line in the field space and then the entropic perturbations do not contribute to the curvature perturbation during inflation. As long as the background inflaton path is not orthogonal to the hyper-surface for inflation to end, the entropic perturbation can make a contribution to the curvature perturbation at the end of inflation and a large local-type non-Gaussiantiy is expected. An interesting thing is that the non-Gaussianity parameters are completely determined by the geometric properties of the hyper-surface of the end of inflation. For example, $f_{NL}$ is proportional to the curvature of the curve on this hyper-surface along the adiabatic direction and $g_{NL}$ is related to the change of the curvature radius per unit arc-length of this curve. Both $f_{NL}$ and $g_{NL}$ can be positive or negative respectively, but $τ_{NL}$ must be positive and not less than $({6\over 5}f_{NL})^2$.

hep-th↗

arXiv2009

The Trispectrum in the Multi-brid Inflation

The trispectrum is at least as important as the bispectrum and its size can be characterized by two parameters $τ_{NL}$ and $g_{NL}$. In this short paper, we focus on the Multi-brid inflation, in particular the two-brid inflation model in arXiv.0805.0974, and find that $τ_{NL}$ is always positive and roughly equals to $({6\over 5}f_{NL})^2$ for the low scale inflation, but $g_{NL}$ can be negative or positive and its order of magnitude can be the same as that of $τ_{NL}$ or even larger

hep-th↗

arXiv2009

The Curvature Perturbation in the Axion-type Curvaton Model

We study the axion-type curvaton model, with emphasis on the large field regime where analytic results are very difficult to obtain. We evaluate the tensor-scalar ratio $r$ using WMAP normalization and the non-linearity parameters $f_{NL}$ and $g_{NL}$ by solving the equations numerically using the $δN$ formalism. We compare them with results for the curvaton with quadratic potential. We find that $r$ is much smaller for the axion-type case compared to the result from the quadratic potential, with the difference increasingly more pronounced at larger field values. $g_{NL}$ is found to be positive, unlike the quadratic case where it is negative, and the amplitude of $g_{NL}$ is much larger. Moreover, there is a nearly linear scaling between $g_{NL}$ and $f_{NL}$, with small deviation from linearity at large field values. The slope between $g_{NL}$ and $f_{NL}$ depends on the parameters characterizing the axion-type curvaton model. We further consider a mixed scenario where both the inflaton and the curvaton contribute to the primordial power spectrum and the non-Gaussianity parameters are found to be much larger than those in the case with quadratic potential.

astro-ph.CO↗

arXiv2008

Curvaton with Polynomial Potential

In general a weakly self-interacting curvaton field is expected and the curvaton potential takes the polynomial form. The curvaton potential can be dominated by the self-interaction term during the period of inflation if the curvaton field stays at a large vacuum expectation value. We use the $δ{\cal N}$ formalism to calculate the primordial curvature perturbation in the various possible scenarios which make the curvaton model much richer.

hep-th↗

arXiv2008

Large Non-Gaussianity Implication for Curvaton Scenario

We argue that the typical energy density of a light scalar field should not be less than $H^4$ in the inflationary Universe. This requirement implies that the non-Gaussianity parameter $f_{NL}$ is typically bounded by the tensor-scalar ratio $r$ from above, namely $f_{NL}\lesssim 518\cdot r^{1\over 4}$. If $f_{NL}=10^2$, inflation occurred around the GUT scale.

hep-th↗

arXiv2008

N-vaton

In general there are a large number of light scalar fields in the theories going beyond standard model, such as string theory, and some of them can be taken as the candidates of curvatons. For simplicity, we assume all of curvatons have the same decay rate and suddenly decay into radiation at the same time. In order to distinguish this scenario from the more general case, we call it "N-vaton". We use $δ{\cal N}$ formalism to calculate the primordial power spectrum and bispectrum in N-vaton model and investigate various bounds on the non-Gaussianity parameter $f_{NL}$. A red tilted primordial power spectrum and a large value of $f_{NL}$ can be naturally obtained if the curvature perturbation generated by inflaton also makes a significant contribution to the primordial power spectrum. As a realistic N-vaton model, we suppose that the axions in the KKLT compactifications of Type IIB string theory are taken as curvatons and a rich phenomenology is obtained.

hep-th↗

arXiv2008

Curvaton Dynamics and the Non-Linearity Parameters in Curvaton Model

We investigate the curvaton dynamics and the non-linearity parameters in curvaton model with potential slightly deviating from the quadratic form in detail. The non-linearity parameter $g_{NL}$ will show up due to the curvaton self-interaction. We also point out that the leading order of non-quadratic term in the curvaton potential can be negative, for example in the axion-type curvaton model. If a large positive $g_{NL}$ is detected, the axion-type curvaton model will be preferred.

hep-th↗

arXiv2008

Spectral Index in Curvaton Scenario

A red tilted primordial power spectrum is preferred by WMAP five-year data and a large positive local-type non-Gaussianity $f_{NL}$ might be observed as well. In this short note we find that a red tilted and large non-Gaussian primordial power spectrum cannot be naturally obtained in curvaton model, because $f_{NL}$ is related to the initial condition of inflation.

hep-th↗

arXiv2008

The Cosmological Constant Problem and Inflation in the String Landscape

An earlier paper points out that a quantum treatment of the string landscape is necessary. It suggests that the wavefunction of the universe is mobile in the landscape until the universe reaches a meta-stable site with its cosmological constant $Λ_0$ smaller than the critical value $Λ_c$, where $Λ_c$ is estimated to be exponentially small compared to the Planck scale. Since this site has an exponentially long lifetime, it may well be today's universe. We investigate specific scenarios based on this quantum diffusion property of the cosmic landscape and find a plausible scenario for the early universe. In the last fast tunneling to the $Λ_0$ ($<Λ_c$) site in this scenario, all energies are stored in the nucleation bubble walls, which are released to radiation only after bubble collisions and thermalization. So the $Λ_0$ site is chosen even if $Λ_0$ plus radiation is larger than $Λ_c$, as long as the radiation does not destabilize the $Λ_0$ vacuum. A consequence is that inflation must happen before this last fast tunneling, so the inflationary scenario that emerges naturally is extended brane inflation, where the brane motion includes a combination of rolling, fast tunnelings, slow-roll, hopping and percolation in the landscape. We point out that, in the brane world, radiation during nucleosynthesis are mostly on the standard model branes (brane radiation, as opposed to radiation in the bulk). This distinction may lead to interesting dynamics. We consider this paper as a road map for future investigations.

hep-th↗

arXiv2007

Weak Gravity Conjecture for the Effective Field Theories with N Species

We conjecture an intrinsic UV cutoff for the validity of the effective field theory with a large number of species coupled to gravity. In four dimensions such a UV cutoff takes the form $Λ=\sqrt{λ/ N}M_p$ for $N$ scalar fields with the same potential $λϕ_i^4$, $i=1,...,N$. This conjecture implies that the assisted chaotic inflation or N-flation might be in the swampland, not in the landscape. Similarly a UV cutoff $Λ=gM_p/\sqrt{N}$ is conjectured for the U(1) gauge theory with $N$ species.

hep-th↗

arXiv2007

Theoretic Limits on the Equation of State Parameter of Quintessence

The value of scalar field coupled to gravity should be less than the Planck scale in the consistent theory of quantum gravity. It provides a theoretic constraint on the equation of state parameter for the quintessence. In some cases our theoretic constraints are more stringent than the constraints from the present experiments.

astro-ph↗

arXiv2007

Constraints on the spectral index for the inflation models in string landscape

We conjecture that the inflation models with trans-Planckian excursions in the field space should be in the swampland. We check this conjecture in a few examples and investigate the constraints on the spectral index for the slow-roll inflation model in string landscape where the variation of inflaton during the period of inflation is less than the Planck scale $M_p$. A red primordial power spectrum with a lower bound on the spectral index is preferred. Both the tensor-scalar ratio and the running can be ignored.

hep-th↗

arXiv2007

Eternal Chaotic Inflation is Prohibited by Weak Gravity Conjecture

We investigate whether the eternal chaotic inflation can be achieved when the weak gravity conjecture is taken into account. We show that even the assisted chaotic inflation with potential $λϕ^4$ or $m^2ϕ^2$ can not be eternal. The effective field theory description for the inflaton field breaks down before inflation reaches the eternal regime. We also find that the total number of e-folds is still bounded by the inflationary entropy for the assisted inflation.

hep-th↗

arXiv2007

Slow-roll reconstruction for running spectral index

We not only reconstruct the slow-roll parameters for fit to the running spectral index from WMAP three-year data in the usual slow-roll inflation model and noncommutative inflation model, but also investigate the evolution of these slow-roll parameters. Requiring slow-roll inflation lasts more than 20 e-folds after CMB scales leave the horizon suggests that the potential at the last stage of inflation takes the form $V(ϕ)=V_0(1+{η_c\over 2}{(ϕ-ϕ_c)^2\over M_p^2})$, where $η_c$ is a constant. A natural mechanism to end inflation at $ϕ=ϕ_c$ is hybrid type inflation.

astro-ph↗

arXiv2007

Weak gravity conjecture constraints on inflation

We consider the gravitational correction to the coupling of the scalar fields. Weak gravity conjecture says that the gravitational correction to the running of scalar coupling should be less than the contribution from scalar fields. For instance, a new scale $Λ=λ_4^{1/2}M_p$ sets a UV cutoff on the validity of the effective $λ_4 ϕ^4$ theory. Furthermore, this conjecture implies a possible constraint on the inflation model, e.g. the chaotic inflation model might be in the swampland.

hep-th↗

arXiv2007

Simplified Chain Inflation

We propose a simplified chain inflation model and calculate the primordial power spectra of the scalar and tensor fluctuations. The spectral index and the tensor-scalar ratio are respectively 0.972 and 0.089 which are consistent with present cosmological observations.

hep-th↗

arXiv2007

Observational Consequences of Quantum Cosmology

Our universe is born of a tunnelling from nothing in quantum cosmology. Nothing here can be interpreted as a state with zero entropy. As a reliable modification of the Hartle-Hawking wave function of the universe, the improved Hartle-Hawking wave function proposed by Firouzjahi, Sarangi and Tye gives many interesting observational consequences which we explore in this paper. Fruitful observations are obtained for chaotic inflation, including a detectable spatial curvature and a negligible tunnelling probability for eternal chaotic inflation. And we find that the tensor-scalar ratio and the spatial curvature for brane inflation type models should be neglected.

hep-th↗