宇宙失踪质量的一半被找到：不是暗物质

网易科技讯10月10日消息，据《新科学家》杂志报道，科学家终于发现星系与星系之间起到连接作用的物质。这次发现意义重大，因为这是我们第一次发现了占宇宙中大约一半的正常物质，而之前对恒星、星系和太空中其他明亮物体的观测存在的疑虑得到了解释。

计算机模拟呈现出一大块“宇宙网”，从这张图中我们可以看到纠缠的丝状物将宇宙的星系连接在一起,而这种纠缠状物就是由重子组成的。

重子是由三夸克组成的亚原子粒子。在现代粒子物理学的标准模型理论中，重子这一名词是指由三个夸克（或者三个反夸克组成反重子）组成的复合粒子。在这理论中它是强子的一类。值得注意的是，因为重子属于复合粒子，所以不是基本粒子。最常见的重子有组成日常物质原子核的质子和中子，与反质子、反中子合称为核子。此前天文学家发现了许多缕高温、呈散射状的“气体”，正是这些“气体”将宇宙的星系连接在一起，但是他们并不知道这些“气体”中有什么物质，而现在的发现解决了天文学家的疑惑。

因为这些呈丝状的“气体”温度虽高还不够高，因此不会释放出太多的能量，所以用X射线望远镜很难观测到这些物质。但是研究人员通过一种被称为“运动学SZ（sunyaev-zel'dovich）效应”的现象证实了这些物质的存在，这种效应描述了从大爆炸中遗留下来的光穿过热气体时状态。

你或许听说过暗物质的搜寻，所谓暗物质，是一种被认为在宇宙中弥漫的神秘物质，而我们可以通过引力来间接观测到这种神秘物质产生的影响。做个形象的解释，比如说根据目前所观察到的，某处有1个单位的普通物质，但是我们这次计算机模拟的宇宙模型却观测到了2个单位的普通物质，因此这多出的一倍“消失的物质”就是研究发现的关键。

两个独立的研究小组发现的“消失的物质”——由称为重子的粒子构成的，并不是暗物质。连接星系的丝状物弥散气体就是由重子组成的。法国空间天体物理学研究所Hideki Tanimura的团队堆叠了260000个双星系数据，英国爱丁堡大学的Anna de Graaff的研究小组使用超过一百万双星系数据。两队发现了确凿的证据证明星系之间的气体细丝。Tanimura的团队发现气体细丝几乎是预测的正常宇宙物质的三倍密度，Graaff的团队发现是六倍正常宇宙物质密度——证实了这些区域的气体足够浓密形成细丝。

“显然我们两个团队的观测结果存在差异，这一点我们在观测前就预想到了，因为我们观测的距离不同，所以造成了结果上的不同，”Tanimura说。“如果克服这个因素，我们的观测结果会和另一小组非常一致。”

两个团队都从斯隆数字巡天项目选择了双星系进行研究，双星系被认为是由重子链连接。他们堆叠两星系区域之间的普朗克信号，使得微弱的重子链可探测到。

“每个人都知道它的存在，但现在，我们两个不同的团队明确的发现了这种物质，”马萨诸塞州哈佛史密森天体物理学中心的拉尔夫·克拉夫特说道，“重子的观测很好地证明了我们关于我们关于星系如何形成以及宇宙的历史等许多观点都是正确的。”

2015年，普朗克卫星在可观测宇宙微波辐射背景地图。因为星系之间气体如此弥散，他们造成的暗斑点太弱以至于在普朗克地图上不能直接看到。

以下为《新科学家》杂志原文：

Half the universe’s missing matter has just been finally found

Discoveries seem to back up many of our ideas about how the universe got its large-scale structure

Andrey Kravtsov (The University of Chicago) and Anatoly Klypin (New Mexico State University). Visualisation by Andrey Kravtsov

By Leah Crane

The missing links between galaxies have finally been found. This is the first detection of the roughly half of the normal matter in our universe – protons, neutrons and electrons – unaccounted for by previous observations of stars, galaxies and other bright objects in space.

You have probably heard about the hunt for dark matter, a mysterious substance thought to permeate the universe, the effects of which we can see through its gravitational pull. But our models of the universe also say there should be about twice as much ordinary matter out there, compared with what we have observed so far.

Two separate teams found the missing matter – made of particles called baryons rather than dark matter – linking galaxies together through filaments of hot, diffuse gas.

“The missing baryon problem is solved,” says Hideki Tanimura at the Institute of Space Astrophysics in Orsay, France, leader of one of the groups. The other team was led by Anna de Graaff at the University of Edinburgh, UK.

Because the gas is so tenuous and not quite hot enough for X-ray telescopes to pick up, nobody had been able to see it before.

“There’s no sweet spot – no sweet instrument that we’ve invented yet that can directly observe this gas,” says Richard Ellis at University College London. “It’s been purely speculation until now.”

So the two groups had to find another way to definitively show that these threads of gas are really there.

Both teams took advantage of a phenomenon called the Sunyaev-Zel’dovich effect that occurs when light left over from the big bang passes through hot gas. As the light travels, some of it scatters off the electrons in the gas, leaving a dim patch in the cosmic microwave background – our snapshot of the remnants from the birth of the cosmos.

Stack ‘em up

In 2015, the Planck satellite created a map of this effect throughout the observable universe. Because the tendrils of gas between galaxies are so diffuse, the dim blotches they cause are far too slight to be seen directly on Planck’s map.

Both teams selected pairs of galaxies from the Sloan Digital Sky Survey that were expected to be connected by a strand of baryons. They stacked the Planck signals for the areas between the galaxies, making the individually faint strands detectable en masse.

Tanimura’s team stacked data on 260,000 pairs of galaxies, and de Graaff’s group used over a million pairs. Both teams found definitive evidence of gas filaments between the galaxies. Tanimura’s group found they were almost three times denser than the mean for normal matter in the universe, and de Graaf’s group found they were six times denser – confirmation that the gas in these areas is dense enough to form filaments.

“We expect some differences because we are looking at filaments at different distances,” says Tanimura. “If this factor is included, our findings are very consistent with the other group.”

Finally finding the extra baryons that have been predicted by decades of simulations validates some of our assumptions about the universe.

“Everybody sort of knows that it has to be there, but this is the first time that somebody – two different groups, no less – has come up with a definitive detection,” says Ralph Kraft at the Harvard-Smithsonian Center for Astrophysics in Massachusetts.

“This goes a long way toward showing that many of our ideas of how galaxies form and how structures form over the history of the universe are pretty much correct,” he says.

Journal references: arXiv, 1709.05024 and 1709.10378v1