湍流火焰自相似临界微尺度分析

自相似临界尺度表征了湍流火焰内保持自相似特性的最大尺度，是确定湍流各相同性的特征尺度．本文利用光学显微技术和计算机图像处理技术，提取并分析了湍流火焰微尺度结构的曲线度和线性回归均方差在不同放大倍率下的变化规律，提出了利用相似度偏差和临界放大倍率来确定自相似临界尺度的思路，并借助于Ｋｏｌｍｏｇｏｒｏｖ微尺度η的定义，推导出临界尺度η＇的数学关联式．     

《相对论、宇宙与时空》连载(11)——演化的宇宙

自然界存在4种相互作用(强作用、弱作用、电磁作用和万有引力作用),它们的强度相差很大.如果以强作用的强度定为1,则电磁作用为10-2,弱作用为10-14,而引力作用为10-39.引力相互作用与强、弱、电磁作用相比太微弱了,完全可以忽略不计.     

宇宙年龄问题上的疑难

宇宙年龄是大爆炸宇宙学中的一个关键性的问题．文章对这问题的缘由、历史以及现状作了阐明．重点讨论了哈勃常数的实测值大小在宇宙年龄问题中的核心作用     

多频率控制开关变换器的自相似和混频现象分析

在多频率控制开关变换器的工作原理及状态方程的基础上,建立了多频率控制断续导电模式开关变换器的离散迭代映射模型,并推导了相应的特征值方程和Lyapunov指数.根据离散迭代映射模型,对多频率控制开关变换器进行了分岔分析.结果表明:在电路参数变化时,变换器存在边界碰撞分岔行为和多周期行为;当误差界限电压较大时,变换器表现出自相似现象,而当误差界限电压较小时,变换器则表现出混频现象.采用Lyapunov指数研究了变换器的稳定性,验证了分岔分析的正确性.经过电路仿真,进一步分析了负载变化时变换器的时域波形和相轨图,并验证了离散迭代映射模型的有效性和理论分析的正确性.最后通过实验结果验证了文中的仿真结果.     

宇宙年龄问题

自适应光学技术的运用和空间望远镜修复的成功使人们可以观测到更遥远星系中的造父变星,并由此准确地定出这些星系离我们的距离、从这些距离可以推知哈勃常数Ｈ的值,从而估计出宇宙年龄。观测的初步结果表明,由此测得的宇宙年龄比由恒星的观测和理论得到的最老恒星年龄还低。这使得长期以来未获解决的宇宙年龄问题的矛盾变得更加尖锐。这一矛盾提示我们,在天体物理研究中,我们对一些非常重要的问题可能仍缺乏充分的了解．     

新的宇宙曙光

哈勃太空望远镜拍摄的原始深场照片是天文学中最具标志性的图片之一。这张图片由数量惊人的遥远星系组成,星系镶嵌在黑色的背景之上,它是哈勃在1995年12月对大熊座的一小块区域进行了一系列观测之后得到的。受这张经典照片启发,天文学家们开始计划一项新任务来研究早期宇宙——这项任务可以看到更早的宇宙,观测到宇宙大爆炸之后3亿年就已经存在的最早的星系。但要做到这一点,需要一个有史以来最大的望远镜,一个比哈勃2.4 m的镜子大得多的望远镜。答案是:下一代太空望远镜(Next Generation Space Telescope,NGST),这是一个巨大的太空望远镜,拥有6.5 m的拼接式主镜,它有望实现一系列新的发现。     

新的宇宙曙光

哈勃太空望远镜拍摄的原始深场照片是天文学中最具标志性的图片之一。这张图片由数量惊人的遥远星系组成,星系镶嵌在黑色的背景之上,它是哈勃在1995年12月对大熊座的一小块区域进行了一系列观测之后得到的。受这张经典照片启发,天文学家们开始计划一项新任务来研究早期宇宙——这项任务可以看到更早的宇宙,观测到宇宙大爆炸之后3亿年就已经存在的最早的星系。但要做到这一点,需要一个有史以来最大的望远镜,一个比哈勃2.4 m的镜子大得多的望远镜。答案是:下一代太空望远镜(Next Generation Space Telescope,NGST),这是一个巨大的太空望远镜,拥有6.5 m的拼接式主镜,它有望实现一系列新的发现。     

爱因斯坦宇宙常数和宇宙中暗能量

综述了宇宙在加速膨胀的观察证据,从爱因斯坦场方程和动力学方程出发详细分析爱因斯坦引入宇宙常数在宇宙加速膨胀中的作用,探讨宇宙常数和宇宙中暗能量的关系．     

线性啁啾光脉冲在具有纵向增益光纤中的自相似传播研究

非线性薛定谔方程是光脉冲在光纤中传播时所遵循的基本方程.本文利用分步傅里叶方法从数值结算的角度研究了具有线性啁啾的光脉冲在具有横向增益光纤中的自相似传播特性.研究表明:当群速度色散系数和光脉冲的啁啾系数符号相同时,由于在传播过程中自相位调制和群速度色散分别起主要作用,不论何种形式的光脉冲在传输过程中均会受到压缩,但随着传播距离的增加压缩的光脉冲会展宽;埃尔米特高斯输入和正弦输入在传播过程中的光强分布是对称的,拉盖尔高斯输入在传播过程中会发生能量交换而表现出明显的偏转;当群速度色散系数和光脉冲的啁啾系数符号不相同时,在传播过程中群速度色散始终起主要作用,脉冲一直展宽.这些研究结论可能为未来光纤压缩器、光纤放大器以及新型THz调制波源的制备提供新的理论基础和方法.     

线性啁啾光脉冲在具有纵向增益光纤中的自相似传播研究

非线性薛定谔方程是光脉冲在光纤中传播时所遵循的基本方程.本文利用分步傅里叶方法从数值结算的角度研究了具有线性啁啾的光脉冲在具有横向增益光纤中的自相似传播特性.研究表明:当群速度色散系数和光脉冲的啁啾系数符号相同时,由于在传播过程中自相位调制和群速度色散分别起主要作用,不论何种形式的光脉冲在传输过程中均会受到压缩,但随着传播距离的增加压缩的光脉冲会展宽;埃尔米特高斯输入和正弦输入在传播过程中的光强分布是对称的,拉盖尔高斯输入在传播过程中会发生能量交换而表现出明显的偏转;当群速度色散系数和光脉冲的啁啾系数符号不相同时,在传播过程中群速度色散始终起主要作用,脉冲一直展宽.这些研究结论可能为未来光纤压缩器、光纤放大器以及新型THz调制波源的制备提供新的理论基础和方法.     

Mechanical properties of the thermal equilibrium Friedmann–Robertson–Walker universe model

The mechanical property of the thermal-equilibrium Friedmann-Robertson-Walker （TEFRW） universe is first studied. The equation of state and the scale factor of the TEFRW universe take the forms ofw = w（a;zT） and a = a（a;zT,Ho）. For the universe consisting of the nonrelativistic matter and the dark energy, the behavior of the dark energy depends on the value of the present-day matter fraction. For the TEFRW universe consisting of N ingredients, the effective temperature is introduced. Lastly, a simple TEFRW universe model is analyzed.     

Radiating Kerr particle in Einstein universe

A generalized Kerr-NUT type metric is considered in connection with Einstein field equations corresponding to perfect fluid plus a pure radiation field. A general scheme for obtaining the exact solutions of these field equations is developed. Two physically meaningful particular cases are investigated in detail. One gives the field of a radiating Kerr particle embedded in the Einstein universe. The other solution may probably represent a deSitter-like universe pervaded by a pure radiation field.     

Non-singular model universe from a perfect fluid scalar-metric cosmology

To seek for a singularity free model universe from a perfect fluid scalar-metric cosmology, we work in the “Emergent Cosmology” (EC) paradigm which is a non-singular alternative for cosmological inflation. By using two methods including Linear Stability Theory and Effective Potential Formalism, we perform a classical analysis on the possible static solutions (that are called usually as Einstein Static Universes (ESU)in literature) in order to study EC paradigm in a FRW background. Our model contains a kinetic term of the scalar field minimally coupled to the background geometry without a potential term. The matter content of the model consists of a perfect fluid plus a cosmological constant \(\Lambda \) as a separate source. In the framework of a local dynamical system analysis, we show that in the absence or presence of \(\Lambda \), depending on some adopted values for the free parameters of the underlying cosmological model with flat and non-flat spatial geometries, one gets some static solutions which are viable under classical linear perturbations. By extending our study to a global dynamical system analysis, we show that in the presence of \(\Lambda \) with non-flat spatial geometries there is a future global de Sitter attractor in this model. Following the second method, we derive a new static solution that represents a stable ESU but this time without dependence on the free parameters of the cosmological model at hand. As a whole, our analysis suggests the possibility of graceful realization of a non-singular EC paradigm (i.e. leaving the initial static phase and entering the inflation period as the universe is evolving) through either preserving or violation of the strong energy condition.     

第四讲暴涨宇宙学的研究与进展

文章简单介绍了标准(大爆炸)宇宙模型的成功和困难，着重介绍了暴涨宇宙学的研究历史和最近的进展，并展望了今后人们可能的关注方向     

Emergent universe from scale invariant two measures theory

The dilaton-gravity sector of a linear in the scalar curvature, scale invariant Two Measures Field Theory (TMT), is explored in detail in the context of closed FRW cosmology and shown to allow stable emerging universe solutions. The model possesses scale invariance which is spontaneously broken due to the intrinsic features of the TMT dynamics. We study the transition from the emerging phase to inflation, and then to a zero cosmological constant phase. We also study the spectrum of density perturbations and the constraints that impose on the parameters of the theory.     

Cardassian universe constrained by latest observations

Several Cardassian universe models including the original, modified polytropic and exponential Cardassian models are constrained by the latest Constitution Type Ia supernova data, the position of the first acoustic peak of CMB from the five years WMAP data and the size of baryonic acoustic oscillation peak from the SDSS data. Both the spatial flat and curved universes are studied, and we also take into account the possible bulk viscosity of the matter fluid in the flat universe case.     

Group velocity of gravitational waves in an expanding universe

The group velocity of gravitational waves in a flat Friedman–Robertson–Walker universe is investigated. For plane waves with wavelength well inside the horizon, and a universe filled with an ideal fluid with the pressure to density ratio less than 1/3, the group velocity is greater than the velocity of light. As a result, a planar pulse of gravitational waves propagating through the universe during the matter/dark energy dominated era arrives to the observer with the peak shifted towards the forefront. For gravitational waves emitted by inspiralling supermassive black holes at the edge of the observable universe, the typical shift that remains after the effects of nonplanarity are suppressed is of order of 10 ps.     

Einstein pseudotensor and total energy of the universe

The total energy of the universe has been calculated assuming that it is the sum of the contributions from the matter part and gravitational part. The calculations involve the use of Einstein pseudotensor. Calculations have been carried out in some specific examples of spacetime geometries. In some cases the total energy is indeed zero confirming previous results but in other cases the total energy is nonzero. So Rosen’s idea that the pseudotensorial calculations will lead to the result that the total energy of the universe is zero, is very much model dependent.     

Bianchi type I inflationary universe in general relativity

In this paper, we have investigated Bianchi type I inflationary universe in the presence of massless scalar field with a flat potential. To get an inflationary solution, we have considered a flat region in which potential V is constant. The inflationary scenario of the model is discussed in detail.     

A neutrino universe with viscosity

A solution of the Einstein field equation corresponding to a distribution of fluid with equation of stateρ = 3p but with a nonvanishing shear viscosity is presented. The solution is spherical symmetric and the flow lines are geodetic.