The researchers conducted tests inside a sophisticated vacuum chamber. An aluminum sphere (center), helped suppress dark energy fields called “chameleon fields.”
除了组成我们身体和我们日常生活中遇见的物体的原子，宇宙还包含了暗物质和暗能量。而后者导致星系各自相互远离，占据宇宙中绝大部分的质能。
1998年，人们发现了暗能量，从那时起，人们就不断做出假设来解释这个物质——其中之一就是暗能量能产生力量，这种力量只有在密度非常低的空间比如在星系之间的区域才能被测量到。
UCLA物理系和天文系的助教保罗·汉密尔顿再造了一个低密度的空间去准确地测量这股力量。他的发现进一步揭示了暗能量与普通物质之间的强烈作用。这个发现见于《科学》杂志在线版的8月21日这一期。
汉密尔顿研究的重点是探寻暗能量场的具体类型。暗能量场又被称为“变色龙场”，它展示了一股力量，其强度取决于它周边环境的密度。这股力量如果真被证实存在的话，它就是除我们现在已知的四种元素：重力，电磁，原子间的强作用力和弱作用力之外所谓的“第五种力”。
但事实上，这第五种力从未在实验室里检测到。这使物理学家们提出一个理论：当“变色龙场“处于高密度的空间中，如地球的大气层，它就会剧烈地收缩，最终让人们无从测量。
Paul Hamilton (foreground), now a UCLA professor, in the lab with his UC Berkeley colleagues.
变色龙场是由宾夕法尼亚大学物理学家，该篇科学论文合著者贾斯汀·库利首次提出的假设。但是直到2014年英国物理学家克莱尔·伯雷奇和他的同事才得出方案在实验室里利用原子来检测它们的存在。
那时，汉密尔顿还只是加州大学伯克利分校Holger Müller实验室的博士后研究员，他的团队就已经开始调查变色龙场：他们独立设计了一个实验，利用原子测量小股力量。
汉密尔顿解释道，探测变色龙场的力量需要复制一个真空的空间，因为当它们靠近质量时，场就会隐藏起来。因此，物理学家们建造了一个真空室，大概一个足球大小。室内的压力是我们平常吸入的空气的万亿分之一。研究员往里面注入一种软金属铯，从而探测该股力量。
汉密尔顿说：“原子是最好的探测粒子，它们几乎不占重量，而且体积特别小。”
他们还将一个弹珠大小的铝制球体放进真空室，利用这个真空的物体挤压变色龙场，使研究人员能够顺利地测量小的力量。原子将被冷却到绝对零度以上的一个温度的百万分之十，这样能保证原子静止不动方便科学家们开展实验。
汉密尔顿和他的团队通过向真空室闪烁近红外激光，测量铯原子在重力和有可能存在的另一种力量的作用下如何加速的来收集数据。
汉密尔顿说：“我们用光波来测量原子的加速。”
这种测量要进行两次：一次是在铝制球体离原子最近的时候，一次是在铝制球体与原子离得最远的时候。根据科学理论，变色龙场会使原子产生不同的速度，这主要取决于球体与原子的距离。
研究人员发现当他们改变铝制球体的位置时，铯原子的速度并没有改变。因此，研究人员进一步了解了变色龙场与普通物质之间强烈的相互作用，不过汉密尔顿将继续使用冷却的原子调查暗能量理论。他的下一个实验的目标是探测造成力量随时间变化的暗能量的其他可能的形式。
这个研究团队的其他成员：缪勒，博士后研究员菲利普·哈斯林格，研究生马特·杰夫以及本科生奎恩·西蒙，都是来自加州大学伯克利分校。
该研究由大卫和露希尔·帕卡德基金会、DPRPA、国家科学基金、NASA和奥地利科学基金提供资金赞助。
The researchers conducted tests inside a sophisticated vacuum chamber. An aluminum sphere (center), helped suppress dark energy fields called “chameleon fields.”
Besides the atoms that make up our bodies and all of the objects we encounter in everyday life, the universe also contains mysterious dark matter and dark energy. The latter, which causes galaxies to accelerate away from one another, constitutes the majority of the universe’s energy and mass.
Ever since dark energy was discovered in 1998, scientists have been proposing theories to explain it — one is that dark energy produces a force that can be measured only where space has a very low density, like the regions between galaxies.
Paul Hamilton, a UCLA assistant professor of physics and astronomy, reproduced the low-density conditions of space to precisely measure this force. His findings, which helped to reveal how strongly dark energy interacts with normal matter, appear Aug. 21 in the online edition of the journal Science.
Hamilton’s research focuses on the search for specific types of dark energy fields known as “chameleon fields,” which exhibit a force whose strength depends on the density of their surrounding environment. This force, if it were proven to exist, would be an example of a so-called “fifth force” beyond the four known forces of gravity, electromagnetism, and the strong and weak forces acting within atoms.
But this fifth force has never been detected in laboratory experiments, which prompted physicists to propose that when chameleon fields are in dense regions of space — for example, the Earth’s atmosphere — they shrink so dramatically that they become immeasurable.
Chameleon fields were first hypothesized in 2004 by Justin Khoury, a University of Pennsylvania physicist and co-author of the Science paper, but it wasn’t until 2014 that English physicist Clare Burrage and colleagues proposed a methodology for testing their existence in a laboratory using atoms.
At the time, Hamilton was a postdoctoral researcher in the UC Berkeley laboratory of Holger Müller. His team already had a head start on investigating chameleon fields: They had independently developed an experiment using atoms to measure small forces.
Detecting the force of chameleon fields requires replicating the vacuum of space, Hamilton explained, because when they are near mass, the fields essentially hide. So the physicists built a vacuum chamber, roughly the size of a soccer ball, in which the pressure was one-trillionth that of the atmosphere we normally breathe. The researchers inserted atoms of cesium, a soft metal, into the vacuum chamber to detect forces.
“Atoms are the perfect test particles; they don’t weigh very much and they’re very small,” Hamilton said.
They also added to the vacuum chamber an aluminum sphere roughly the size of a marble, which functioned as a dense object to suppress the chameleon fields and allow the researchers to measure small forces. The atoms were then cooled to within 10 one-millionths of a degree above absolute zero, in order to keep them still enough for the scientists to perform the experiment.
Hamilton and his team collected data by shining a near-infrared laser into the vacuum chamber and measuring how the cesium atoms accelerated due to gravity and, potentially, another force.
“We used a light wave as a ruler to measure the acceleration of atoms,” Hamilton said.
This measurement was performed twice: once when the aluminum sphere was close to the atoms and once when it was farther away. According to scientific theory, chameleon fields would cause the atoms to accelerate differently depending on how far away the sphere was.
The researchers found no difference in the acceleration of the cesium atoms when they changed the location of the aluminum sphere. As a result, the researchers now have a better understanding of how strongly chameleon fields can interact with normal matter, but Hamilton will continue to use cold atoms to investigate theories of dark energy. His next experiment will aim to detect other possible forms of dark energy that cause forces that change with time.
The study’s co-authors were Müller, postdoctoral researcher Philipp Haslinger, graduate student Matt Jaffe and undergraduate Quinn Simmons, all of UC Berkeley.
The research was supported by the David and Lucile Packard Foundation, DARPA, the National Science Foundation, NASA and the Austrian Science Fund.
