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Greatest Mysteries: Where is the Rest of the Universe?

 Scientists trying to create a detailed inventory of all the matter and energy in the cosmos run into a curious problem--the vast majority of it is missing.


 "I call it the dark side of the universe," said Michael Turner, a cosmologist at the University of Chicago, referring to the great mysteries of dark matter and dark energy.


 In fact, only 4 percent of the matter and energy in the universe has been found. The other 96 percent remains elusive, but scientists are looking in the farthest reaches of space and deepest depths of Earth to solve the two dark riddles.


 Missing matter


 Einstein's famous equation "E=mc^2" describes energy and matter (or mass) as one and the same--maps of the cosmos refer to the energy-matter combination as energy density, for short. The problem with detecting dark matter, thought to make up 22 percent of the universe's mass/energy pie, is that light doesn't interact with it.


 But it does exhibit the tug of gravity.


 Initial evidence for the mysterious matter was discovered 75 years ago when astrophysicists noticed an anomaly in a jumble of galaxies: The galactic cluster had hundreds of times more gravitational pull than it should have, far outweighing its visible mass of stars.


 "We can predict the motions of the sun and planets very accurately, but when we measure distant things we see anomalies," said Scott Dodelson, an astrophysicist at Fermi National Accelerator Laboratory in Illinois. "Dark matter is currently the best possible solution, even though we've never seen any of it."


 Another hallmark of dark matter is gravitational lensing, similar to the effect of light passing through a piece of polished glass. Massive objects like the sun can bend light, but colossal clouds of dark matter create "bubbles" in the cosmos that magnify, distort and duplicate the light of galaxies or stars behind them.


 Gravitational lensing recently exposed evidence of the unseen mass in the Bullet cluster as well as in a ring around a cluster of colliding galaxies called ZwCl0024+1652.



 Particle hunt


 In spite of the ghostly evidence, pieces of dark matter have yet to be pinned down by researchers. "Until we actually discover particles, we're not home yet," Dodelson said. 尽管存在幽灵般的证据,但组成暗物质的材料究竟是什么,还有待研究人员去搞清楚。多戴尔森指出:“直到真正发现了组成暗物质的粒子,我们才算是搞彻底了。”
 Particle physicists have detected neutrinos, which are extremely lightweight particles that pour out of the sun and hardly interact into ordinary matter, but Turner said they make up an extremely small fraction of dark matter in the universe. 粒子物理学家已经发现了中微子。中微子是从太阳中发出来的极其轻微的粒子,它们几乎无法进入任何普通物质。然而,特纳说中微子仅仅构成了宇宙中暗物质的极少一部分。
 "We arrested one of the members of the gang, but not the leader of the gang," Turner said of neutrinos. He thinks the leader is actually a WIMP: a weakly interactive, massive particle. Unfortunately, WIMPS are just a theory so far.  提到中微子的时候,特纳说:“我们捕捉到这群粒子中的一种,但这并是其中最主要的一种。”他认为暗物质中最主要的粒子实际上是弱作用重粒子。遗憾的是,到目前为止弱作用重粒子仅仅是理论的产物。
 The thinking goes that WIMPs are very heavy, yet like neutrinos they rarely bump into matter to produce a detectable signal. But the idea that WIMPS--such as theoretical axion or neutralino particles--can bump into visible matter at all gives scientists hope. 科学家们通常认为,弱作用重粒子非常沉重,然而就象中微子一样不会进入物质而产生可以探测到的信号。但是,意识到有些弱作用重粒子,如:理论上的轴子和中性伴随子,可能会进入可见物质,使科学家们有了希望。
 "This is a story that may soon be at its end," Turner said, noting that the Cryogenic Dark Matter Search in the Soudan mine of Minnesota and other experiments below the ground should be sensitive enough to detect a WIMP. 特纳注意到明尼苏达州苏丹矿山里的低温暗物质搜寻计划及其他的地下试验非常精确灵敏,应该可以探测到弱作用重粒子,他说:“情况可能很快就会有结果的。”
 The anti-gravity 反引力
 Perhaps the biggest mystery of all is dark matter's big cousin, dark energy. 也许最高深莫测之谜当属暗物质的近亲——暗能量了。
 The invisible force is thought to be a large-scale "anti-gravity," pushing apart galactic clusters and causing the unexplainable, accelerating expansion of the universe. Turner thinks dark energy is the biggest mystery of them all--and quite literally, since physicists predict that it makes up 74 percent of energy density in the universe. 这种不可见的力量被认为是大范围的“反引力”,它将星系团撕裂开来,导致宇宙莫名其妙地加速膨胀。特纳认为,暗能量是所有谜团当中最高深莫测之谜。确实如此,因为科学家们预测暗能量在宇宙能量密度中占有74%的份额。
 "So far, the greatest achievement with dark energy is giving it a name," Turner said of the elusive force. "We are really at the very beginning of this puzzle."  有关这种难以捉摸的力量,特纳指出:“到目前为止,在暗能量方面所取得的最了不起的成就是为它取了个名字。在解决这个难题方面,我们确实尚处于最初始的阶段。”
 Turner described dark energy as "really weird stuff," best thought of as an elastic, repulsive gravity that can't be broken down into particles. "We know what it does, but we don't know what it is," Turner said. 特纳将暗能量描述成“真正的怪物”,它最容易使人想到一种不可分解为粒子的、富有弹性的排斥力。特纳说:“我们知道暗能量所起的作用,却不知道它到底是什么。”
 While astrophysicists look deep into space to gather more details about dark energy's effects, Turner noted that theoretical physicists are focusing on explaining how the force actually works. And at this point, he joked, any physicist's explanation for dark energy is probably good enough to consider. 尽管天体物理学家们把视线指向太空的深邃之处,以获取有关暗能量的更多详细情况,特纳却注意到理论物理学家们在集中精力解释暗能量究竟是如何起作用的。对此,特纳开玩笑说,大概所有物理学家们对暗能量的解释都很妙,均值得考虑。
 "We're at this very early stage, at the crime scene of dark energy's existence, if you will," Turner said. "It's a highly creative period, and now is the time for ideas." 特纳指出:“我们的研究还处于最初始的阶段,而我们也就在暗能量的包围圈之中。这也是一个极具创造性的阶段,现在正是百家争鸣的时期。”
来自:翟波  > 宇宙探秘
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