arXiv subjects

Puxun Wu

Publications and source records attributed to Puxun Wu.

At least 73 records · Page 4Linked to original sources

arXiv2020

Production of gravitational waves during preheating in the Starobinsky inflationary model

The production of GWs during preheating in the Starobinsky model with a nonminimally coupled auxiliary scalar field is studied through the lattice simulation in this paper. We find that the GW spectrum $Ω_{\rm gw}$ grows fast with the increase of the absolute value of coupling parameter $ξ$. This is because the resonant bands become broad with the increase of $|ξ|$. When $ξ<0$, $Ω_{\rm gw}$ begins to grow once the inflation ends and grows faster than the case of $ξ>0$. $Ω_{\rm gw}$ reaches the maximum at $ξ=-20$ ($ξ=42$ for the case $ξ>0$) and then decreases with slight oscillation. Furthermore we find that the GWs produced in the era of preheating satisfy the limits from the Planck and next-generation CMB experiments.

gr-qc↗

arXiv2020

Stability of the Einstein Static Universe in $4 D$ Gauss-Bonnet Gravity

By rescaling the Gauss-Bonnet (GB) coupling constant $α\rightarrow α/(D-4)$ and considering the $D \rightarrow 4$ limit, the GB gravity gives rise to nontrivial modification of general relativity in four dimensions. In this work, we investigate the realization of the emergent universe scenario in the $4 D$ GB gravity. First, we obtain the Einstein static universe filled with a perfect fluid. Then, we show that both spatially closed and open universes can be stable against both homogeneous and inhomogeneous scalar perturbations simultaneously.

gr-qc↗

arXiv2020

Scalar induced gravitational waves in inflation with gravitationally enhanced friction

We study the scalar induced gravitational wave (GW) background in inflation with gravitationally enhanced friction (GEF). The GEF mechanism, which is realized by assuming a nonminimal derivative coupling between the inflaton field and gravity, is used to amplify the small-scale curvature perturbations to generate a sizable amount of primordial black holes. We find that the GW energy spectra can reach the detectable scopes of the future GW projects, and the power spectrum of curvature perturbations has a power-law form in the vicinity of the peak. The scaling of the GW spectrum in the ultraviolet regions is two times that of the power spectrum slope, and has a lower bound. In the infrared regions, the slope of the GW spectrum can be described roughly by a log-dependent form. These features of the GW spectrum may be used to check the GEF mechanism if the scalar induced GWs are detected in the future.

astro-ph.CO↗

arXiv2020

Notes on thermodynamics of super-entropic AdS black holes

The super-entropic black hole, which possesses a noncompact horizon topology and violates the reverse isoperimetric inequality, has been found to satisfy both the thermodynamic first law and the Bekenstein-Smarr mass formula. In this paper, we first derive a new Christodoulou-Ruffini-like squared-mass formula for the four-dimensional Kerr-Newman-AdS super-entropic black hole, and then establish a set of very simple relations between thermodynamic quantities of the super-entropic Kerr-Newman-AdS$_4$ black hole and its usual counterparts. Using these relations, the thermodynamic quantities of the Kerr-Newman-AdS$_4$ super-entropic black hole can be obtained from those of the usual pro-type by taking the ultra-spinning limit properly. Then these relations are extended to the singly-rotating Kerr-AdS black holes in arbitrary dimensions and the double-rotating charged black hole in the five-dimensional minimal gauged supergravity. It can be inferred that the thermodynamic quantities of all super-entropic black holes obey similar limiting relations to those of their corresponding conventional rotating AdS black holes, and thus can be obtained by taking the ultra-spinning limit appropriately.

gr-qc↗

arXiv2019

Nonlinear preheating with nonminimally coupled scalar fields in the Starobinsky model

We study the preheating after inflation in the Starobinsky model with a nonminimally coupled scalar field $χ$. Using the lattice simulation, we analyze the rescattering between the $χ$ particles and the inflaton condensate, and the backreaction effect of the scalar metric perturbations. We find that the rescattering is an efficient mechanism promoting the growth of the $χ$ field variance. Meanwhile, copious inflaton particles can be knocked out of the inflaton condensate by rescattering. As a result, the inflaton field can become a nonnegligible gravitational wave source, even comparable with the $χ$ field in some parameter regions. For the scalar metric perturbations, which are on the sub-Hubble scale in our analysis, our results show that they have negligible effects on the evolution of scalar fields and the production of gravitational waves in the model considered in present paper.

astro-ph.CO↗

arXiv2019

Primordial Black Holes from Inflation with Nonminimal Derivative Coupling

We propose a novel enhancement mechanism of the curvature perturbations in the nonminimal derivative coupling inflation model with a coupling parameter related to the inflaton field. By considering a special form of the coupling parameter as a function of the inflaton, a period of ultra-slow-roll inflation can be realized due to the gravitationally enhanced friction, and the resulting power spectrum of the curvature perturbations has a sharp peak, which is large enough to produce the primordial black holes. Under this mechanism, we can easily obtain a sharp mass spectrum of primordial black holes around specific masses such as $\mathcal{O}(10)M_\odot$, $\mathcal{O}(10^{-5})M_\odot$, and $\mathcal{O}(10^{-12})M_\odot$, which can explain the LIGO events, the ultrashort-timescale microlensing events in OGLE data, and the most of dark matter, respectively.

astro-ph.CO↗

arXiv2019

Emergent Universe Scenario, Bouncing and Cyclic Universes in Degenerate Massive Gravity

We consider alternative inflationary cosmologies in massive gravity with degenerate reference metrics and study the feasibilities of the emergent universe scenario, bouncing and cyclic universes. We focus on the construction of the Einstein static universe, classes of exact solutions of bouncing and cyclic universes in degenerate massive gravity. We further study the stabilities of the Einstein static universe against both homogeneous and inhomogeneous scalar perturbations and give the parameters region for a stable Einstein static universe.

gr-qc↗

arXiv2018

Production of gravitational waves during preheating with nonminimal coupling

We study the preheating and the in-process production of gravitational waves (GWs) after inflation in which the inflaton is nonminimally coupled to the curvature in a self-interacting quartic potential with the method of lattice simulation. We find that the nonminimal coupling enhances the amplitude of the density spectrum of inflaton quanta, and as a result, the peak value of the GW spectrum generated during preheating is enhanced as well and can reach the limit of detection in future GW experiments. The peaks of the GW spectrum not only exhibit distinctive characteristics as compared to those of minimally coupled inflaton potentials but also imprint information on the nonminimal coupling and the parametric resonance, and thus the detection of these peaks in the future will provide us a new avenue to reveal the physics of the early universe.

gr-qc↗

arXiv2018

Inflationary dynamics and preheating of the non-minimally coupled inflaton field in the metric and Palatini formalisms

The inflationary dynamics and preheating in a model with a non-minimally coupled inflaton field in the metric and Palatini formalisms are studied in this paper. We find that in both formalisms, irrespective of the initial conditions, our Universe will evolve into a slow-roll inflationary era and then the scalar field rolls into an oscillating phase. The value of the scalar field at the end of the inflation in the Palatini formalism is always larger than that in the metric one, which becomes more and more obvious with the increase of the absolute value of the coupling parameter $|ξ|$. During the preheating, we find that the inflaton quanta are produced explosively due to the parameter resonance and the growth of inflaton quanta will be terminated by the backreaction. With the increase of $|ξ|$, the resonance bands gradually close to the zero momentum ($k=0$), and the structure of resonance changes and becomes broader and broader in the metric formalism, while, it remains to be narrow in the Palatini formalism. The energy transfer from the inflaton field to the fluctuation becomes more and more efficient with the increase of $|ξ|$, and in the metric formalism the growth of the efficiency of energy transfer is much faster than that in the Palatini formalism. Therefore, the inflation and preheating show different characteristics in different formalisms.

gr-qc↗

arXiv2018

Stability of Einstein static universe in gravity theory with a non-minimal derivative coupling

The emergent mechanism provides a possible way to resolve the big bang singularity problem by assuming that our universe originates from the Einstein static (ES) state. Thus, the existence of a stable ES solution becomes a very crucial prerequisite for the emergent scenario. In this paper, we study the stability of an ES universe in gravity theory with a non-minimal coupling between the kinetic term of a scalar field and the Einstein tensor. We find that the ES solution is stable under both scalar and tensor perturbations when the model parameters satisfy certain conditions, which indicates that the big bang singularity can be avoided successfully by the emergent mechanism in the non-minimally kinetic coupled gravity.

gr-qc↗

arXiv2018

Quantum interaction between two gravitationally polarizable objects in presence of boundaries

We investigate, in the framework of the linearized quantum gravity and the leading-order perturbation theory, the quantum correction to the classical Newtonian interaction between a pair of gravitationally polarizable objects in the presence of both Neumann and Dirichlet boundaries. We obtain general results for the interaction potential and find that the presence of a boundary always strengthens in the leading-order the interaction as compared with the case in absence of boundaries. But different boundaries yield a different degree of strengthening. In the limit when one partner of the pair is placed very close to the Neumann boundary, the interaction potential is larger when the pair is parallel with the boundary than when it is perpendicular to, which is just opposite to the case when the boundary is Dirichlet where the latter is larger than the former. In addition, we find that the pair-boundary separation dependence of the higher-order correction term is determined by the orientation of the pair with respect to boundary, with the parallel case giving a quadratic behavior and the perpendicular case a linear one.

gr-qc↗

arXiv2017

Interaction between two gravitationally polarizable objects induced by thermal bath of gravitons

The quadrupole-quadrupole interaction between a pair of gravitationally polarizable objects induced by vacuum fluctuations of the quantum linearized gravitational field is first obtained with a relatively simple method, which is then used to investigate the contribution of thermal fluctuations of a bath of gravitons to the interaction at temperature $T$. Our result shows that, in the high temperature limit, the contribution of thermal fluctuations dominates over that of vacuum fluctuations and the interaction potential behaves like $T/ r^{10} $, where $r$ is the separation between the objects, and in the low temperature limit, the contribution of thermal fluctuations is proportional to $T^{10}/r$, which only provides a small correction to the interaction induced by zero-point fluctuations.

gr-qc↗

arXiv2016

Determining $H_0$ using a model-independent method

By using type Ia supernovae (SNIa) to provide the luminosity distance (LD) directly, which depends on the value of the Hubble constant $H_0= 100 h\; {\rm km\; s^{-1}\; Mpc^{-1}}$, and the angular diameter distance from galaxy clusters or baryon acoustic oscillations (BAOs) to give the derived LD according to the distance duality relation, we propose a model-independent method to determine $h$ from the fact that different observations should give the same LD at a given redshift. Combining the Sloan Digital Sky Survey II (SDSS-II) SNIa from the MLCS2k2 light curve fit and galaxy cluster data, we find that at the $1σ$ confidence level (CL), $h=0.5867\pm0.0303$ for the sample of the elliptical $β$ model for galaxy clusters, and $h=0.6199\pm0.0293$ for that of the spherical $β$ model. The former is smaller than the values from other observations, whereas the latter is consistent with the Planck result at the $2σ$ CL and agrees very well with the value reconstructed directly from the $H(z)$ data. With the SDSS-II SNIa and BAO measurements, a tighter constraint, $h=0.6683\pm0.0221$, is obtained. For comparison, we also consider the Union 2.1 SNIa from the SALT2 light curve fitting. The results from the Union 2.1 SNIa are slightly larger than those from the SDSS-II SNIa, and the Union 2.1 SNIa + BAOs give the tightest value. We find that the values from SNIa + BAOs are quite consistent with those from the Planck and the BAOs, as well as the local measurement from Cepheids and very-low-redshift SNIa.

astro-ph.CO↗

arXiv2016

Quantum correction to classical gravitational interaction between two polarizable objects

When gravity is quantized, there inevitably exist quantum gravitational vacuum fluctuations which induce quadrupole moments in gravitationally polarizable objects and produce a quantum correction to the classical Newtonian interaction between them. Here, based upon linearized quantum gravity and the leading-order perturbation theory, we study, from a quantum field-theoretic prospect, this quantum correction between a pair of gravitationally polarizable objects treated as two-level harmonic oscillators. We find that the interaction potential behaves like $r^{-11}$ in the retarded regime and $r^{-10}$ in the near regime. Our result agrees with what was recently obtained in different approaches. Our study seems to indicate that linearized quantum gravity is robust in dealing with quantum gravitational effects at low energies.

gr-qc↗

arXiv2015

Spherical collapse in the extended quintessence cosmological models

We use the spherical collapse method to investigate the non-linear density perturbations of pressureless matter in the cosmological models with the extended quintessence as dark energy in the metric and Palatini formalisms. We find that for both formalisms, when the coupling constant is negative, the deviation from the $Λ$CDM model is the least according to the evolutionary curves of the linear density contrast $δ_{c}$ and virial overdensity $Δ_{v}$, and it is less than one percent. And this indicates that, in the extended quintessence cosmological models in which the coupling constant is negative, all quantities dependent on $δ_{c}$ or $Δ_{v}$ are essentially unaffected if the linear density contrast or the virial overdensity of the $Λ$CDM model is used as an approximation. Moreover, we find that the differences between different formalisms are very small in terms of structure formation, and thus can not be used to distinguish the metric and Palatini formalisms.

gr-qc↗

arXiv2015

The thermalization of a two-level atom in a planar dielectric system out of thermal equilibrium

We study the thermalization of an elementary quantum system modeled by a two-level atom interacting with stationary electromagnetic fields out of thermal equilibrium near a dielectric slab. The slab is held at a temperature different from that of the region where the atom is located. We find that when the slab is a nonabsorbing and nondispersive dielectric of a finite thickness $ d$, no out of thermal equilibrium effects appear as far as the thermalization of the atom is concerned, and a finite thick dielectric slab with a tiny imaginary part in the relative permittivity $\operatorname{Im} ε$ behaves like a half space dielectric substrate if $\frac{\operatorname{Im} ε}{\sqrt{\operatorname{Re}ε-1}} \frac{d}{λ_0} > 1$ is satisfied, where $λ_0$ is the transition wavelength of the atom. This condition can serve as a guide for an experimental verification, using a dielectric substrate of a finite thickness, of the effects that arise from out of thermal equilibrium fluctuations with a half-space (infinite thickness) dielectric.

quant-ph↗

arXiv2015

Emergent scenario in the Einstein-Cartan theory

We study the emergent scenario, which is proposed to avoid the big bang singularity, in the Einstein-Cartan (EC) theory with a positive cosmological constant and a perfect fluid by analyzing the existence and stability of the Einstein static (ES) solutions. We find that there is no stable ES solution for a spatially flat or open universe. However, for a spatially closed universe, the stable ES solution does exist, and in the same existence parameter regions, there also exists an unstable one. With the slow decrease of the equation of state $w$ of the perfect fluid, the stable and unstable critical points move close gradually and coincide once $w$ reaches a critical value, so that the stable critical point becomes an unstable one. As a result, if $w$ approaches a constant at $t\rightarrow -\infty$, the universe can stay at the stable ES state past eternally, and furthermore it can naturally exit from this state and evolve into an inflationary era if $w$ decreases slowly as time goes forward. Therefore, the emergent scenario that avoids the big bang singularity can be successfully implemented in the EC theory of gravity.

gr-qc↗

arXiv2014

Emergent universe in spatially flat cosmological model

The scenario of an emergent universe provides a promising resolution to the big bang singularity in universes with positive or negative spatial curvature. It however remains unclear whether the scenario can be successfully implemented in a spatially flat universe which seems to be favored by present cosmological observations. In this paper, we study the stability of Einstein static state solutions in a spatially flat Shtanov-Sahni braneworld scenario. With a negative dark radiation term included and assuming a scalar field as the only matter energy component, we find that the universe can stay at an Einstein static state past eternally and then evolve to an inflation phase naturally as the scalar field climbs up its potential slowly. In addition, we also propose a concrete potential of the scalar field that realizes this scenario.

gr-qc↗