Expanding Universe

Radiation and Matter in the Early Universe

早期宇宙中的辐射与物质

In the early stages of the big bang, most of the energy was in the form of radiation and that radiation was the dominant influence on the expansion of the universe. Later, with cooling from the expansion the roles of mass and radiation changed and the universe entered a mass-dominated era. Recently results suggest that we have already entered an era dominated by "dark energy" , but examination of the roles of mass and radiation are most important for understanding the early universe. Some insight into the expansion can be obtained from a Newtonian expansion model which leads to a simplified version of the Friedman equation.

在大爆炸的早期阶段，大部分的能量以辐射形式存在，而这种辐射对宇宙的膨胀起着主导作用。后来，随着宇宙的膨胀而冷却，质量和辐射的作用发生了变化，宇宙进入了以质量为主导的时期。最近的研究结果表明，我们已经进入了由‘暗能量’主导的时期，但对质量和辐射作用的考察对于理解早期宇宙最为重要。一些关于膨胀的见解可以从牛顿膨胀模型中获得，该模型导出了简化版的 Friedman 方程。

Using the dimensionless scale factor R to characterized the expansion of the universe, the effective energy densities of radiation and mass scale differently.

利用无量纲的尺度因子R来表征宇宙的膨胀，辐射能和质量的有效能量密度变化方式不同。

z parameter

这导致早期宇宙中辐射占主导地位，但后来在某个时间点过渡到物质主导的宇宙。上图左侧的密度对膨胀参数z的对数曲线示意图展示了这一过渡。

Radiation-dominated Expansion of the Universe

宇宙辐射主导的膨胀

In the early stages of the big bang, most of the energy was in the form of radiation and that radiation was the dominant influence on the expansion of the universe. Later, with cooling from the expansion the roles of mass and radiation changed and the universe entered a mass-dominated era. Recently results suggest that we have already entered an era dominated by "dark energy" , but the examination of the radiation-dominated expansion is essential for understanding the early universe. Some insight into the expansion can be obtained from a Newtonian expansion model which leads to a simplified version of the Friedman equation.

在大爆炸的早期阶段，大部分的能量以辐射形式存在，而这种辐射对宇宙的膨胀起到了主导作用。后来，随着宇宙的膨胀冷却，辐射和质量的作用发生了变化，宇宙进入了以质量为主导的时期。最近的研究结果表明，我们已经进入了由‘暗能量’主导的时期，但是对辐射主导的膨胀进行研究对于理解早期宇宙是至关重要的。一些关于膨胀的见解可以从牛顿膨胀模型中获得，该模型导出了简化版的弗里德曼方程。