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# 澳洲代写|天文学代写Astronomy代写|ASP2011

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## 澳洲代写|天文学代写Astronomy代写|Finite or Infinite Space

Although concordance between the standard model and observed nuclear abundances limits baryon density to well below the critical value needed for closure of a Friedmann universe, the question of finite or infinite space remains observationally undecided owing to other complications. In principle, it should be possible to determine the curvature constant $k$ by direct measurement of the deviation from Hubble’s simple linear relationship between velocity and distance. Unfortunately, substantial deviation is not expected, until sources at distances of the order of $1 / H$ are studied, and galaxies are too faint to have their spectra measured adequately at such distances by present technology. Furthermore, observing galaxies at such distances implies seeing them at earlier times, and estimates of their distance could be subject to systematic errors due to unknown evolution of galactic luminosity.

Since the discovery of quasars by Maarten Schmidt at the Palomar Observatory in 1963, cosmologists have hoped that these most distant observed objects could be used to extend the Hubble relationship to the nonlinear regime and decide the sign of the curvature. Quasars are now known with spectral features up to $6.0$ times their terrestrial wavelengths, corresponding to Doppler recession velocities up to $96 \%$ of the speed of light, placing them at substantial fractions of the distance from us to the horizon of the observable universe. However, uncertainties in estimating very large distances in the universe due to insufficient understanding of the evolution of galaxies, not to mention the structure of quasars, have prevented unambiguous determination of the sign of the curvature. In fact, Hubble’s law is still used to estimate distances to the quasars, rather than they being used to determine both distance and redshift and thus test their relationship. The question of whether space is finite or infinite remains unresolved by observation at this time.

## 澳洲代写|天文学代写Astronomy代写|Eternal Expansion or an End to Time

In the Friedmann cosmologies of the GIK, a finite space implies an end to proper time in the future, but this is not required in some nonstandard models. Assuming the Robertson-Walker form of the metric for a homogeneous and isotropic space-time, it is convenient to discuss the future evolution of any such expanding universe in terms of a dimensionless deceleration parameter, defined as:
$$q=-R\left(d^{2} R / d t^{2}\right) /(d R / d t)^{2}$$
Throughout most of its history, the dynamics of the universe have been dominated by matter in which the average energy density is very much greater than the pressure. Neglecting the pressure of nongravitational fields, space will reverse its expansion and collapse in finite proper time if and only if $q>\frac{1}{2}$. From the Friedmann equation for the Hubble parameter, it is easy to show that $q=\Omega / 2$ in a space-time described by the GTR with zero cosmological constant. In nonstandard models, the deceleration parameter depends on the density, the cosmological constant, and the Hubble parameter in more complicated ways. For some choices of cosmological constant, it is even possible to have an accelerating universe $(q<0)$ with a positive density. However, a cosmological constant whose magnitude substantially exceeded the critical energy density would produce detectable local effects that are not observed. Thus, we can conclude that a sufficiently large density must imply an end to time.

## 澳洲代写|天文学代写ASTRONOMY代写|ETERNAL EXPANSION OR AN END TO TIME

q=−R(d2R/d吨2)/(dR/d吨)2

## Matlab代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。