引力如何弯曲光线

Gravity is obviously pretty important. It holds your feet down to Earth so you don’t fly away into space, and (equally important) it keeps your ice cream from floating right out of your cone! We’ve learned a lot about gravity over the past few hundred years, but one of the strangest things we’ve discovered is that most of the gravity in the universe comes from an invisible source called “dark matter.” While our telescopes can’t directly see dark matter, they can help us figure out more about it thanks to a phenomenon called gravitational lensing.

引力的“局势”

任何有质量的物体都会对所有其他有质量的物体产生引力。这是因为质量会弯曲时空（spacetime），也就是宇宙的基本结构。美洲驼、甜甜圈，甚至回形针这类东西都会弯曲时空，但由于它们质量不大，所以只弯曲一点点。行星、恒星和黑洞这类更大的物体则会大幅弯曲时空，因为它们质量大得多，不过由于它们很紧凑，这种弯曲只发生在它们附近的局部区域。它们造成的时空扭曲会影响从附近经过的物体的运动方式。

This animated GIF shows how light is bent by massive objects that create dents in space-time. The first scene shows a white grid on a black background, which represents space-time. In the center is a star shown as a glowing yellow ball. The star creates a dent in the space-time grid, so the lines of the grid are curved under and around it. A line of yellow light comes down from the top of the image and arcs around the star. Then the scene changes to show the Hubble Space Telescope, a silver cylindrical object, in the bottom right of the image, pointed up toward the left. In the upper left is a fuzzy white blob with yellow lines moving toward Hubble. Between them, in the center, is a cluster of galaxies, depicted as several different-sized orange and white specs of light. As the yellow lines approach the galaxy cluster, they bend around it, ending up pointing toward the telescope.

这段动画展示了光线绕过一个正在弯曲时空的物体时的路径

Huge clusters of galaxies are so massive that their gravity produces some fairly bizarre effects. When light passes close to a massive object, space-time is so warped that it curves the path the light must follow. Light that would normally go through the galaxy cluster bends around it, producing intensified – and sometimes multiple – images of the source. This process, called gravitational lensing, turns galaxy clusters into gigantic, intergalactic magnifying glasses that give us a glimpse of cosmic objects that would normally be too distant and faint for even our biggest telescopes to see.

This animated GIF shows how light is bent around massive objects as it travels from the source to our telescopes. It opens with a frame showing a detector plane on the left, an unseen mass of dark matter in the center, and a cluster of galaxies on the right. White lines representing the light start at the galaxy cluster on the right and are bent around the dark matter to land on the detector plane. The camera shifts to the left to reveal what the detector plane (or a telescope) would see. It reveals the cluster of galaxies, but with the galaxies near the center appearing as arcs because of the light bending around the dark matter.

One way to investigate dark matter is by using gravitational lensing. This animation shows light passing through a galaxy cluster, with its path bent by the gravity of the cluster. The amount of bending helps scientists learn about the dark matter present.

NASA’s Goddard Space Flight Center Conceptual Image Lab

Hubble “Sees” Dark Matter

Let’s recap – mass warps space-time. The more mass, the stronger the warp, and the bigger its gravitational lensing effects. In fact, by studying “lensed” objects, we can map out the quantity and location of the unseen matter causing the distortion!

Thanks to gravitational lensing, scientists have measured the total mass of many galaxy clusters, which revealed that all the matter they can see isn’t enough to create the warping effects they observe. There’s more gravitational pull than there is visible stuff to do the pulling – a lot more! Scientists have given the invisible stuff that accounts for this difference the name “dark matter.” It’s invisible to our eyes and telescopes, but it can’t hide its gravity!

我们看到的和我们知道一定存在的东西之间的不匹配可能看起来很奇怪，但并不难想象。你知道人不可能悬浮在半空中，那么如果你看到一个人似乎正在这样做呢？你会立刻知道一定有钢丝吊着他，即使你看不见那些钢丝。

This slideshow of images from the Hubble Space Telescope starts with an image of galaxy cluster SDSS J1038+4849, which looks like it has a smiley face. There are two bright orange “eyes,” a white blob for a nose, and the “smile” is formed by arcs caused by gravitational lensing. The second, fourth, and fifth images show Abell 370, Abell 2218, and SDSS J1004+4112, which are galaxy clusters. Sprinkled throughout are galaxies that appear smeared or stretched due to a phenomenon called gravitational lensing. This effect can help scientists map the presence of dark matter in the universe. The third image shows extreme gravitational lensing of LRG 3-757. In the image there is a glowing orange blob that is a luminous red galaxy. Surrounding the galaxy is the image of an even-more distant galaxy which has been distorted into a nearly complete circle.

这个幻灯片展示了哈勃太空望远镜拍摄的五张具有强引力透镜星系的图像。来自这些星系的光线路径由于它们强大的引力而绕着大质量天体弯曲，从而产生了这些扭曲的图像。NASA、ESA、哈勃、J. Lotz、HFF 团队、Andrew Fruchter、ERO 团队、K. Sharon 和 E. Ofek

我们的Hubble Space Telescope观测正在帮助揭开暗物质之谜。天文学家通过用哈勃研究引力透镜星系团，弄清了宇宙中有多少物质是“普通”物质、有多少是“暗”物质。尽管普通物质构成了从泡菜到行星的一切，但宇宙中的there’s about five times more dark matter比所有普通物质加起来还要多！

罗曼将加大搜索力度

即将到来的Nancy Grace Roman Space Telescope将把这些引力透镜观测提升到新的水平。罗曼的灵敏度足以使用同一效应的更微妙版本——称为弱引力透镜——来观察更小的暗物质团块如何扭曲遥远星系的外观。通过观测这种小尺度上的透镜效应，科学家将能够填补我们对暗物质理解中的更多空白。

Roman的视野将至少比哈勃大100倍，同时保持同样出色的图像质量。它还将更高效、成像更快，因此罗曼的透镜图将比哈勃的大近一千倍。罗曼将在第一年收集so much data，使科学家能够进行以往望远镜需要数百年才能完成的深入研究。

This animated GIF illustrates how a gravitational lens would distort our view or background galaxies. The animation opens with an image of galaxies. Some of the galaxies are small yellow, white, or red glowing circles. Others are larger fuzzy smudges with some spiral structures. As the animation plays, a gravitational lens moves from the left to the right. As it does, the images of galaxies in the background are arced and warped around the lens. A large spiral galaxy in the middle even turns into a full donut-shape as the lens passes in front of it.

这个模拟展示了一个引力透镜在背景星系场前移动。从相机和背景星系之间经过的天体由于其引力而扭曲时空。被扭曲的时空弯曲了来自背景星系的光线路径，使它们看起来扭曲且更亮。Frank Summers（STScI）

罗曼的弱引力透镜观测将使我们能够窥探更further back in time than Hubble is capable of seeing。科学家认为，宇宙底层的暗物质结构通过吸引普通物质而played a major role in the formation and evolution of galaxies。观察暗物质从早期阶段到现在的宇宙分布，将帮助科学家揭示它随时间如何演化，并可能为它未来如何继续演化提供线索。我们不知道未来会怎样，但罗曼将帮助我们找到