Physics Primer: The CMB

物理入门:宇宙微波背景


The Cosmic Microwave Background (CMB) 宇宙微波背景(CMB

For a helpful quick overview of the CMB, click here.

为了快速了解CMB,请点击这里

 

The microwaves of the CMB which we detecttoday are the redshifted form of the very "first light" that radiatedin the universe. Astronomers date this "first light" to about 370,000years after the big bang. In the immediate aftermath of the big bang, theuniverse consisted of an extremely hot dense plasma of ionized gas in whichphotons (light particles) were continually scattered by the free electrons inthe plasma, trapped as in a fog, so that the universe was completely dark.Eventually the universe cooled to a temperature at which the positive ions andelectrons were able to combine to form neutral atoms (called the "epoch ofrecombination"), so that photons no longer were scattered and light couldtravel freely through space. Cosmologists accordingly refer to this moment whenthe universe became transparent as both the "epoch of last scatter"and the "first light." The expansion of space caused this emittedcosmic radiation to gradually cool off from an original temperature of 3000K totoday's 2.7K (-450 degrees F) as the radiation was increasingly redshifted tothe microwave frequency -- the CMB.

我们今天探测到的微波微波背景辐射是在宇宙中辐射的“第一束光”的红移形式。天文学家在宇宙大爆炸后约37万年的时间里,对这一“第一光”进行了测定。在宇宙大爆炸的直接后果中,宇宙由一个极其炽热稠密的电离气体等离子体构成,其中光子(光粒子)被等离子体中的自由电子连续散射,被困在雾中,因此宇宙是完全黑暗的。最终,宇宙冷却到一个温度,正离子和电子能够结合形成中性原子(称为“重组的时代”),使光子不再散射,光可以自由地穿过空间。因此,宇宙学家将这一时刻称为宇宙变得透明的时候,同时也是“最后一个散射的时代”和“第一个光”。宇宙空间的膨胀导致宇宙辐射逐渐冷却,从最初的3000K到今天的2.7K(-450华氏度),因为辐射越来越红移到微波频率——CMB

CMB Temperature

CMB温度

 So far we hav only talked about the frequencyor wavelength of the cosmic radiation / light. So what does temperature has todo with it? How can we talk about the temperature of a ray of light?

到目前为止,我们只讨论了宇宙辐射/光的频率或波长。那么温度和它有什么关系呢?我们怎么能谈论一束光的温度呢?

Recall from lesson 3 (Unit Analysis), theenergy formula we have introduced:

回顾第3课(单位分析),我们介绍的能量公式:

 E=kT

where k is called Boltzmann's constant andT is a temperature measured in Kelvins. Then, if we use Planck's constant, wecan relate the frequency to the temperature:

 其中K为玻耳兹曼常数,T是温度测量的开尔文。然后,如果我们使用普朗克常数,我们可以把频率与温度联系起来:

E=hν

hν=kT

This is a very "rough" estimateformula that is only partially correct and doesn't allow us to obtain a preciseresult. However, it does capture the physical connection between frequency andtemperature, and it also gives us a correct estimate of the scale.

这是一个非常粗略的估计公式,只是部分正确,不允许我们得到精确的结果。然而,它确实捕捉了频率和温度之间的物理联系,也给了我们对尺度的正确估计。

The CMB radiation extends over a wholespectrum, but for this problem we'll approximate it to ν=160 GHz. Also, we knowthe values for the two constants h=6×10−34 Js and k=1.3×10−23 J/K. 

CMB辐射延伸到整个频谱,但对这个问题我们将近似它ν= 160 GHz。同时,我们知道这两个常数h =6×10−34 JSK = 1.3×10−23  J / K.的概念。


The CMB radiation is not overall spatiallyuniform – the radiation coming from different points in the sky can haveslightly different frequencies, and by association, different temperatures. Ifwe were to map the differences compared to the average temperature of 2.7 K, wewould get this picture of the fluctuations:

CMB辐射在空间上不是均匀的,天空中不同点的辐射会有轻微的不同频率,并且通过不同的温度关联。如果我们将这些差异与平均温度2.7 K进行比较,我们就可以得到这幅波动图:

 

Image of full-sky map of CMB from WMAP9-year data (2012) [NASA/WMAP]

WMAP 9年数据(2012)[NASA/WMAP]的全天空地图图像

The red and blue “spots” (also calledanisotropies) are the temperature fluctuations compared to the average. Theanisotropies represent compressed and rarefacted regions in the otherwiseuniform ionized gas of the early universe. The red hot spots (compressed) areregions with particles of slightly higher velocity (yielding slightly shorterwavelength radiation) and the cold blue spots (rarefacted)are regions with particles of slightly lower velocity (yielding radiation withslightly longer wavelength). These temperature fluctuations are tiny, of theorder of a part per million.            

红色和蓝色的“斑点”(也叫各向异性)是与平均温度相比的温度波动。各向异性在早期宇宙的均匀电离气体中表示压缩和稀疏区域。红色的热点(压缩)是具有稍高速度粒子(产生稍短的波长辐射)和冷蓝色斑点(稀疏)的区域,这些区域的粒子速度略低(产生的辐射波长稍长)。这些温度波动很小,为百万分之一。

The Concordance Model of RelativisticCosmology

相对论宇宙学的一致性模型

Cosmological parameters include cosmiccurvature (the geometry of space at the largest scale – frequently labeled “thegeometry of the universe”), dark energy, ordinary (baryonic) matter, and darkmatter. Varying these parameters in numerical simulations imposes constraintson the observational data coming from the CMB, Type Ia supernovae, galaxyclusters, and from other studies.             

宇宙学参数包括宇宙曲率(在最大的尺度空间的几何经常标记的“宇宙”的几何),暗能量,普通物质(重子),和暗物质。在数值模拟中改变这些参数对来自CMBIa超新星、星系团以及其他研究的观测数据施加了限制。

Let’s take as an example the geometry ofthe universe. As you learned in Lesson 4, according to Einstein’s generaltheory of relativity, the geometry of the universe is related to the total massΩm and total energy ΩΛ in the universe.

让我们以宇宙的几何为例。正如你在第4课学到的,根据爱因斯坦的广义相对论,宇宙的几何是宇宙中总质量Ωm和总能量ΩΛ有关。            

In relativistic cosmology, there are threepossibilities for the geometry of the universe depending on the relationbetween the total energy and the total mass:

在相对论宇宙学中,根据总能量与总质量的关系,宇宙几何有三种可能性:

l        Flat geometry (zero cosmiccurvature) with ΩmΛ=1 (the universe can be eitherinfinite or finite for a flat space)

l        平面几何(零宇宙曲率)与ΩmΛ=1(宇宙可以是无限或有限的平面空间)             

l        Open geometry (negative cosmiccurvature) with ΩmΛ<1 (the universe can be eitherinfinite or finite for a negatively-curved space) 

l        开放几何(负宇宙曲率)与ΩmΛ<1(宇宙可以是无限或有限的负曲率空间)             

l        Closed geometry (positivecosmic curvature) with Ωm+ΩΛ>1 (the universe is finite for apositively-curved space) 

l        封闭几何(正宇宙曲率)与ΩmΛ>1(宇宙是有限的正曲率空间)

The curvature, matter, and energyparameters have all been constrained by independent studies – most notably bystudies of the CMB, studies of Type Ia supernovae, and studies of baryonacoustic oscillation (BAO) signals from galaxy clusters – in the past decade.These parameter constraints all converge to a particular cosmological model –the concordance model of cosmology (Lambda-CDM Model) – which is indicated bythe gray ellipses (the intersection of the orange CMB line, blue Supernovaeellipses, and the green BAO line) in Figure 3.

曲率、物质和能量参数都受到了独立研究的限制,特别是对CMB的研究,Ia型超新星的研究,以及过去10年来星系团重子声振荡信号的研究。这些参数约束都收敛到一个特定的宇宙学模型——宇宙学的一致性模型(λCDM模型),这一点由图3中的灰色椭圆(橙色CMB线、蓝超新星椭圆和绿色宝线的交叉点)表示。    

                      

Image source: Supernova Cosmology Project,Lawrence Berkeley National Laboratory

图片来源:超新星宇宙学计划,劳伦斯伯克利国家实验室

The concordance model of cosmology maintainsthat the universe is flat (observe that the gray ellipses fall along the linelabeled “Flat” in the figure), is expanding at an accelerating rate, is about13.8 billion years old, and is made up of about 68% dark energy, 27% darkmatter, and 5% ordinary (baryonic) matter.

宇宙学的一致性模型认为,宇宙是平的(观察到灰色椭圆下降沿线标记的“平”中的人物),是在加速膨胀,大约138亿岁,是由大约68%的暗能量,27%的暗物质,和5%的普通物质(重子)。

CMB Simulation using NASA's CMB Analyzer

利用美国国家航空航天局的中巴分析仪进行CMB模拟                           

Try out this "Build a Universe"tool from NASA which enables you to adjust different cosmological parametersand observe the effect on the geometry and age of the universe, and on theappearance of the CMB: click here. On the page, be sure to read theinstructions and the information provided about the angular power spectrumgraph, and about the "ingredients" of the universe and "otherproperties" of the universe that you will be adjusting, and then click on"Make full screen in new window" to use the CMB Analyzer.

试试NASA的“宇宙构造”工具,它可以让你调整不同的宇宙参数,观察宇宙几何和年龄的影响,以及CMB的外观:点击这里。在页面上,一定要阅读有关角功率谱图的说明和信息,以及关于宇宙的“成分”和“宇宙”的其他属性,然后单击“在新窗口中全屏”使用CMB分析器。