### 物理代写|宇宙学代写cosmology代考|PHYC90009

statistics-lab™ 为您的留学生涯保驾护航 在代写宇宙学cosmology方面已经树立了自己的口碑, 保证靠谱, 高质且原创的统计Statistics代写服务。我们的专家在代写宇宙学cosmology代写方面经验极为丰富，各种代写宇宙学cosmology相关的作业也就用不着说。

• Statistical Inference 统计推断
• Statistical Computing 统计计算
• (Generalized) Linear Models 广义线性模型
• Statistical Machine Learning 统计机器学习
• Longitudinal Data Analysis 纵向数据分析
• Foundations of Data Science 数据科学基础

## 物理代写|宇宙学代写cosmology代考|A Thermal History of the Universe and Primordial Nucleosynthesis

In the Friedmann-Lemaitre standard model, the universe is homogeneous and has identical characteristics everywhere. The theory however does not explain the reason for this homogeneity, which is well verified by observations. On closer inspection, it is even surprising that the CMB appears so isotropic. Whatever the direction of observation, the photons emitted by the last scattering surface all show the same temperature of $2.73 \mathrm{~K}$. However, they come from a multitude of regions that, in the strict framework of the theory, were not in causal contact at the time they emitted the CMB. Their number can be estimated at about a thousand.

Alan Guth proposed to solve this problem (Guth 1981) by assuming that the universe should undergo, shortly after the Planck era, a phase of exponential expansion called cosmic inflation, during which the scale factor $a(t)$ is multiplied by a factor $\sim e^{60}$. To generate this phenomenal acceleration, it is necessary to assume that the universe undergoes a phase transition during which its energy density $\rho$ is dominated by the potential energy of a scalar field. At the time, high-energy theoretical physicists were working on a theory for unifying the weak, strong and electromagnetic interactions. The models they arrived at, called grand unification models, naturally present all the qualities required to generate an inflationary period. The grand unified symmetry spontaneously breaks down around a cosmic time of about $10^{-35} \mathrm{~s}$ and the resulting inflation allows a single causally connected domain to become as large as the entire observable universe today. The original scenario has undergone a few modifications (Albrecht et al. 1982; Linde 1983) and now requires a dedicated scalar field called the inflaton. Its quantum fluctuations generate density perturbations that will much later transform into galaxies. Gravitational waves are also radiated during this period. They are a signature that future interferometers, such as LISA (Laser Interferometer Space Antenna), will try to identify. When inflation ends, around a cosmic age of $10^{-33}$ to $10^{-32} \mathrm{~s}$, the inflaton disintegrates and gives birth to the ylem during a reheating phase. The evolution of the universe is then described by the Friedmann-Lemaitre model.

The elementary particles sensitive to the strong interaction are called hadrons, from the Greek hadros which means strong. They are constructions of quarks bound together by gluons. The hadron family is divided into two categories. Mesons are pairs formed by a quark and an antiquark and include among them the neutral pion $\pi^0$ and the charged pion $\pi^{p m}$. Baryons are assemblies composed of three quarks, such as the proton $p$ and neutron $n$, which are themselves the constituents of atomic nuclei. Gluons carry the strong interaction and are vector bosons of zero mass with two helicity states. Their electromagnetic analogue is the photon. The standard model of high energy physics, in this case quantum chromodynamics, requires the existence of eight gluons, as well as six quarks grouped in three families of doublets. The lightest of them is composed of $u$ quarks (up) and $d$ quarks (down). It enables the proton and the neutron to be built.

In the primordial universe, the temperature is so high that quarks and gluons cease to be bound. Mesons and baryons are completely dissociated into a plasma of free quarks and gluons, called quark gluon plasma (QGP). Heavy ion collisions, achieved at CERN and studied through experimental collaborations such as ALICE, attempt to recreate QGP in the laboratory in order to study it. Yet, QGP exists freely in a natural way during the Big Bang. Nonetheless, around a cosmic age of $10 \mu$ s, a phase transition occurs in which quarks and gluons condense into hadrons. Protons, neutrons and pions then appear.

Neutrinos are very light, neutral elementary particles that experience only the weak force. They interact with each other as well as with electrons, muons and tauons ${ }^3$, with which they are associated in the family of leptons, a term derived from the Greek leptos and meaning weak. Leptons are grouped into three doublets, each consisting of a neutrino and its charged partner. In the primordial universe, neutrinos frequently interact with their environment. They are thermally coupled to the primordial plasma and share its temperature. But under the combined influence of dilution and cooling of the ylem, both resulting from the overall expansion, neutrinos have fewer and fewer collisions with other particles. Their thermodynamic coupling fades around a cosmic age of $1 \mathrm{~s}$. They then undergo a thermal freeze-out, also called kinetic decoupling, which makes them a fossilized population of particles that no longer interact with each other or with the rest of the plasma. Thermal decoupling of neutrinos will be the subject of section 1.4.

# 宇宙学代考

## 物理代写|宇宙学代写cosmology代考|A Thermal History of the Universe and Primordial Nucleosynthesis

Alan Guth 提出解决这个问题 (Guth 1981) 的方法是假设宇宙在普朗克时代之后不久会经历一个称为宇宙 憉胀的指数膨胀阶段，在此期间比例因子 $a(t)$ 乘以一个因数 $\sim e^{60}$. 为了产生这种惊人的加速度，有必要 假设宇宙经历了一个相变，在此期间它的能量密度 $\rho$ 由标量场的势能支配。当时，高能理论物理学家正在 研究统一弱相互作用、强相互作用和电磁相互作用的理论。他们得出的模型被称为大统一模型，自然而然 地呈现了产生通货膨胀时期所需的所有品质。大统一对称性在大约 10 秒的宇宙时间附近自发地破缺 $10^{-35}$ s由此产生的膨胀使得一个因果联系的领域变得和今天整个可观察的宇宙一样大。最初的场景经过 了一些修改（Albrecht 等人 1982 年；Linde 1983 年)，现在需要一个称为暴胀子的专用标量场。它的 量子涨落会产生密度扰动，这些扰动会在很久以后转变为星系。引力波也在此期间辐射。它们是末来干涉 仪 (例如 LISA (激光干涉仪空间天线) ) 将尝试识别的特征。当暴胀结束时，大约是一个宇宙时代 $10^{-33}$ 到 $10^{-32} \mathrm{~s}$ ，暴胀子在再加热阶段分解并产生 ylem。然后，弗里德曼-勒梅特模型描述了宇宙的演 化。

## 有限元方法代写

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## 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。