我们能从宇宙的“婴儿照”中学到什么？

如果你看看自己小时候的照片，也许能从中看出今天的你的一些影子——眼神里某种特别的神情，或者未来鼻子、耳朵的雏形。同样，科学家们也在研究宇宙的“婴儿照”，寻找它如何成长为今天我们所知宇宙的线索。这张婴儿照就是cosmic microwave background (CMB)，一种弥漫在整个宇宙各个方向的微弱辉光。

多年来，多台望远镜都承担过观测宇宙婴儿照的任务。其中之一是PIPER，也就是Primordial Inflation Polarization Explorer，它搭乘气球飞行在大气层边缘，寻找宇宙微波背景中细微的图案。

This image shows an oval that represents the whole sky if you folded it out onto a flat piece of paper. The oval is dotted with many regions ranging from blue, green, yellow, and red in color. While the image looks chaotic, the blue regions are nearly always surrounded by green. The yellow are embedded in the green, and red embedded in the yellow. These show temperature differences in the early universe, which ultimately lead us to see where matter clumped together to form the structures we see today.

这张由九年WMAP数据绘制的婴儿宇宙全天精细图像，揭示了137.7亿年前的温度涨落（以颜色差异表示），它们对应着后来成长为星系的种子。NASA/WMAP科学团队

宇宙微波背景很冷，非常非常冷。平均温度约为华氏零下455度。它形成于大爆炸之后38万年，科学家认为大爆炸发生在约138亿年前。

Before the CMB, the universe was a fog of hot, dense plasma. (By hot, we’re talking about 500 million degrees F.) That’s so hot that atoms couldn’t exist yet – there was just a soup of electrons and protons. Electrons are great at deflecting light. So, any light that existed in the first few hundred thousand years after the big bang couldn’t travel very far before bouncing off electrons, similar to the way a car’s headlights get diffused in fog.

Against a background looking like a fog, a number of red and green particles appear to dance with blue blobs. Then, the red and green particles combine into grey particles, and the blue blobs zoom off. The camera follows one blob as the background turns from a fog, to black, and finally to a starry sky.

Just 370,000 years after the universe began, all that existed was a hot plasma similar to a candle flame. Protons and electrons (red and green balls) were bouncing around scattering the light. Particles of light, called photons (blue), couldn’t go far without colliding with an electron. As the universe cooled, the protons and electrons could pair up, forming hydrogen atoms, and the light was free to travel.

NASA/JPL

After the big bang, the universe started expanding rapidly in all directions. This expansion is still happening today. As the universe continued to expand, it cooled. By the time the universe reached its 380,000th birthday, it had cooled enough that electrons and protons could combine into hydrogen atoms for the first time. (Scientists call this era recombination.) Hydrogen atoms don’t deflect light nearly as well as loose electrons and the fog lifted. Light could now travel long distances across the universe.

This animation shows some characteristics of microwave light and where it is seen in a city like Seattle. A honeybee cleans its antennae in a box on the left side, representing the physical size of microwave wavelengths. In a circle on the rights is an image of Seattle silhouetted by an orange sky, which emanates microwaves. There are also small circles of orange throughout the city, showing where people use them, especially in cooking.

This animation depicts the microwave segment of the electromagnetic spectrum.

NASA’s Goddard Space Flight Center/Conceptual Image Lab

The light we see in the CMB comes from the recombination era. As it traveled across the universe, through the formation of stars and galaxies, it lost energy. Now we observe it in the microwave part of the electromagnetic spectrum, which is less energetic than visible light and therefore invisible to our eyes. The first baby photo of the CMB – really, a map of the sky in microwaves – came from our Cosmic Background Explorer, which operated from 1989 to 1993.

This animated GIF opens on a galaxy in the background upon which a pie chart is drawn. A radius of the circular chart sweeps around counterclockwise, the first 5% of the circle represents visible matter at 5% of the known matter of the universe. The next 27% is dark matter. The final 68% is dark energy.

这个动态饼图展示了宇宙三个已知组成部分的数值：普通物质（5%）、暗物质（27%）和暗能量（68%）。NASA’s Goddard Space Flight Center

我们为什么对宇宙的“婴儿照片”如此感兴趣？嗯，它帮助我们了解了很多关于今天周围宇宙结构的知识。例如，Wilkinson Microwave Anisotropy Probe 制作了一张宇宙微波背景的详细地图，并帮助我们了解到宇宙是由 68 percent dark energy, 27 percent dark matter and just 5 percent normal matter 组成的——也就是你和恒星都是由这种东西构成的。

This animation opens on a bright explosion which fills the screen with white, which then fades back to a point at the center of the screen. As the point fades, galaxies rush from the center of the image off the screen, as though passing by the viewer.

天文学家认为宇宙大约在138亿年前诞生，从一个极其致密且无比炽热的状态迅速膨胀，正如这段动画所示。NASA’s Goddard Space Flight Center/Conceptual Image Lab

就在 big bang 之后，我们相当确定宇宙非常小。真的非常小。我们今天看到的一切都被塞进了一个比质子还小的东西里。如果宇宙一开始就那么小，那么它就会遵循量子力学的规律。量子力学允许各种奇怪的事情发生。物质和能量可以从未来“借”来，然后又碰撞回虚无。接着宇宙暴胀发生了，宇宙突然膨胀了一万亿万亿倍。

This illustration shows the difference between two types of polarization, which is a measure of the organization of light. The top of the image says “Polarization.” On the left is “E-Mode.” At the beginning of the animation, there is a circle of lines that forms a kind of 8-pointed star similar to an asterisk, but with the middle of the star missing. A second copy of this figure “folds” out to the right. Then the second figure slides back to the position of the original one, exactly covering the original. The two figures, the original and the “folded out” one are the same. On the right side of the figure is “B-Mode” polarization. The starting figure here shows a circle of eight lines, this time each one is at an angle from an imaginary central circle, kind of like a pinwheel that opens up clockwise. A second copy of this figure folds out to the left, making a second copy that is like a pinwheel that opens counterclockwise. When the second copy slides back onto the original, the lines make x’s on the top, right, bottom, and left, and plus signs between each of the x’s. This shows that the original figure and its folded-out version are not the same.

宇宙微波背景（CMB）是一种微弱的辉光，弥漫在整个宇宙的各个方向。当科学家仔细观察时，他们能在宇宙微波背景的光中看到被称为E模和B模的独特图案。B模图案在镜子中反射时会改变数值，而E模图案则不会。NASA’s Goddard Space Flight Center

所有这些混沌创造了一片引力波的海洋。（这些被称为“原初”引力波，来源与 gravitational waves you may have heard about from merging neutron stars 和黑洞不同。）原初引力波的信号有点像白噪声，而合并的死恒星发出的信号则像一声可以穿透噪声的口哨。

这些引力波充满了婴儿宇宙，并在宇宙微波背景的光中创造了独特的图案，称为B模偏振。这些图案具有手性，这意味着尽管它们是彼此的镜像，但并不对称——就像试图把左手手套戴在右手上一样。它们与另一种称为E模的偏振不同，E模是对称的，呼应了宇宙中物质的分布。

这就是PIPER登场的地方。PIPER的两台望远镜位于一个热水浴缸大小的 liquid helium 容器中，温度约为华氏零下452度。它观测85%的天空，并且极其 sensitive，因此它帮助我们更多地

This infographic shows many factoids about the PIPER (Primordial Inflation Polarization Explorer) balloon mission. At the top, there is part of an image of the cosmic microwave background with the text, “The cosmic microwave background (CMB) is heat left over from the big bang and is the oldest light we can see in the universe.” Just below that, near a circular, bright “explosion” is the text, “Scientists are studying the CMB for proof that the universe expanded rapidly immediately after the big bang.” Next is an image of two circles of yellow lines that look like 8-pointed pinwheels that would swirls in opposite directions with text, “This process, called inflation, should have created gravitational waves. If so, light from the CMB should contain a distinct twisting polarization pattern called B-mode.” There is a cut-out line drawing of the PIPER payload with the silhouette of a person standing next to it, showing that it is about two persons’ high. The text says, “PIPER is a microwave telescope flown on a high-altitude balloon. The instrument has twin telescopes in a hot-tub-sized container of liquid helium.” Next there is text with icicles below it that reads, “Liquid helium keeps the instrument at -452 degrees Fahrenheit (-269 Celsius).” Near a map of the United States with a telescope by Maryland and a lightbulb near California is the text, “The detectors on PIPER could spot a 60-watt incandescent light bulb in California from PIPER's base of operations at NASA's Goddard Space Flight Center in Maryland.” A silhouette of a hippopotamus accompanies the text, “PIPER's total weight, including instrument, balloon and ballast, is about 8,000 pounds (3,600 kilograms), as much as a full-grown male hippopotamus.” An illustration of the fully inflated balloon appears with the text, “Width of fully inflated balloon: 430 feet (130 meters).” Underneath the balloon are three houses. The text nearby explains, “The balloon expands to approximately 40 million cubic feet (1.1 million cubic meters) when it reaches maximum altitude. It's made from polyethylene film comparable in thickness to ordinary plastic sandwich wrap.” A drawing of Australia, Texas and New Mexico is accompanied by the text, “Flights are planned from Ft. Sumner, New Mexico, Palestine, Texas and Alice Springs, Australia. Overnight flights from these three locations will allow PIPER to cover 85 percent of the sky.” And finally, a commercial airplane near the bottom has text, “PIPER's maximum altitude is about 120,000 feet (36,000 meters). That's more than three times the cruising altitude of a commercial airplane.”

The Primordial Inflation Polarization Explorer (PIPER) is a NASA scientific balloon mission that flies to the edge of Earth’s atmosphere to study twisty patterns of light in the universe’s “baby picture.” This infographic highlights some facts about PIPER’s instruments, capabilities, and goals.

NASA’s Goddard Space Flight Center