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宇宙膨胀的速度究竟有多快?

更新时间:2017-2-22 10:24:50 来源:纽约时报中文网 作者:佚名

Cosmos Controversy: The Universe Is Expanding, but How Fast?
宇宙膨胀的速度究竟有多快?

There is a crisis brewing in the cosmos, or perhaps in the community of cosmologists. The universe seems to be expanding too fast, some astronomers say.

在宇宙中——或许是在宇宙学家群体中,一场危机正在酝酿。一些天文学家称,宇宙似乎膨胀得太快了。

Recent measurements of the distances and velocities of faraway galaxies do not agree with a hard-won “standard model” of the cosmos that has prevailed for the past two decades.

最近对遥远星系的距离和速度的测量结果,与宇宙的“标准模型”不符。这个费尽周折得出的模型,在过去20年里一直占据主流地位。

The latest result shows a 9 percent discrepancy in the value of a long-sought number called the Hubble constant, which describes how fast the universe is expanding. But in a measure of how precise cosmologists think their science has become, this small mismatch has fostered a debate about just how well we know the cosmos.

最新结果显示,一项长期寻找的数据的值出现了9%的偏差。这项数据叫哈勃常数,用于描述宇宙膨胀的速度。但小小的偏差引发了一场争论,从中可以看出宇宙学家认为自己研究的这门学科已经达到了何等精确的程度。这场争论关乎我们究竟有多了解宇宙。

A small discrepancy in the value of a long-sought number has fostered a debate about just how well we know the cosmos.
过去20年里,一直占据主流地位、用以衡量宇宙膨胀速度的“标准模型”似乎出现了偏差。但现在,这个模型的精确度似乎提高了,而宇宙大概又陷入了麻烦。

“If it is real, we will learn new physics,” said Wendy Freedman of the University of Chicago, who has spent most of her career charting the size and growth of the universe.

“如果是真的,我们将学到新的物理,”芝加哥大学(University of Chicago)的温迪·弗里德曼(Wendy Freedman)说。她把职业生涯中的大部分时间都用在了用图表描述宇宙的大小和膨胀上。

The Hubble constant, named after Edwin Hubble, the Mount Wilson and Carnegie Observatories astronomer who discovered that the universe is expanding, has ever given astronomers fits. In an expanding universe, the farther something is away from you, the faster it is receding. Hubble’s constant tells by how much.

哈勃常数得名于发现了宇宙在不断膨胀的威尔逊山天文台(Mount Wilson)和卡内基科学研究院天文台(Carnegie Observatories)天文学家埃德温·哈勃(Edwin Hubble),是一个始终困扰着天文学家的东西。在一个不断膨胀的宇宙中,离我们越远的东西后退的速度越快。哈勃常数衡量的就是这种变化的幅度。

But measuring it requires divining the distances of lights in the sky — stars and even whole galaxies that we can never visit or recreate in the lab. The strategy since Hubble’s day has been to find so-called standard candles, stars or whole galaxies whose distances can be calculated by how bright they look from Earth.

但测量哈勃常数须探测天空中的光点——我们永远无法到达或在实验室里复制的恒星乃至星系——的距离。从哈勃所在的时代至今,采用的办法一直是找到所谓的标准烛光,即可通过从地球上看它们时的亮度,计算出其与地球之间的距离的恒星或整个星系。

But the calibrators themselves need to be calibrated, which has led to a rickety chain of assumptions and measurements in which small errors and disagreements — about, say, how much dust is interfering with observations — can build up to cosmic proportions. Only three decades ago, renowned astronomers could not agree on whether the universe was 10 billion or 20 billion years old. Now everybody has settled on its age as about 13.8 billion years.

但校准器本身也需要校准,这引发了一系列不牢靠的假设和测量结果。在这些测量结果中,小小的误差和不一致,比如关于灰尘在多大程度上干扰了观测结果,可能会逐渐变成巨大的差异。就在30年前,著名天文学家在宇宙的年龄是100亿年还是200亿年上还无法达成一致意见。现在,所有人都认定它的年龄约为138亿年。

Using a new generation of instruments like the Hubble Space Telescope, astronomers have steadily whittled down the uncertainty in the Hubble constant.

利用新一代工具,如哈勃太空望远镜,天文学家逐步降低了哈勃常数的不确定性。

Getting Closer

越来越近

In 2001, a team led by Freedman reported a value of 72 kilometers per second per megaparsec (about 3.3 million light years), in the galumphing units astronomers prefer. It meant that for every 3.3 million light years a galaxy was farther away from us, it was moving 72 kilometers a second faster.

2001年,弗里德曼率领的一个团队报告的哈勃常数值是72公里每秒每百万秒差距(约330万光年)。天文学家喜欢用这种冗长的单位,它的意思是一个星系与我们之间的距离每增加330万光年,它的移动速度会增加72公里每秒。

Hubble’s original estimate was much higher, at 500 in the same units of measurement.

哈勃的最初估值要高得多,为500公里每秒每百万秒差距。

Freedman’s result had an error margin that left it happily consistent with other more indirect calculations, that had gotten a slightly slower and lower value of 67 for the Hubble constant. Those were derived from studies of microwaves emitted and still lingering in the sky from the primordial Big Bang fireball.

弗里德曼的结果有一个误差范围,使它恰好与其他较间接的计算相符。那些计算得出的哈勃常数结果是67,稍微低一些、慢一些。它们来源于对最初的大爆炸火球发射的微波进行的研究。那些微波现在仍留在空中。

As a result, in recent years, astronomers have settled on a recipe for the universe that is as black and as decadent as a double dark chocolate chunk brownie. The universe consists of roughly 5 percent atomic matter by weight, 27 percent mysterious dark matter and 68 percent of the even more mysterious dark energy that is speeding up the cosmic expansion. Never mind that we do not know exactly what all this dark stuff is. Astronomers have a good theory about how it behaves, and that has allowed them to tell a plausible story about how the universe evolved from when it was a trillionth of a second old until today.

结果是,近年来天文学家在这个昏暗、颓靡得堪比双倍黑巧克力布朗尼的宇宙的组成上意见一致。按重量计算,宇宙由大约5%的原子物质、27%的神秘暗物质和68%的暗能量组成。暗能量比暗物质更神秘,正在加快宇宙的膨胀。我们不知道这些黑暗的物质究竟是什么,但这不要紧。关于它们是如何运转的,天文学家有一个精妙的理论,使他们能够讲出一个看上去合理的故事,介绍宇宙是如何从年龄只有一万亿分之一秒的时候进化到现在的。

But now the Hubble precision has gotten seemingly better, and the universe might be in trouble again.

但现在,哈勃常数的精度似乎提高了,而宇宙可能又陷入了麻烦。

Last summer a team led by Adam Riess of Johns Hopkins University and the Space Telescope Science Institute, using the Hubble Space Telescope and the giant Keck Telescope on Mauna Kea in Hawaii and supernova explosions as the ultimate distance markers, got a value of 73 plus or minus only 2.4 percent for the elusive constant.

去年夏天,供职于约翰·霍普金斯大学(Johns Hopkins University)和太空望远镜科学研究所(Space Telescope Science Institute)的亚当·里斯(Adam Riess)带领的一个团队利用了哈勃太空望远镜和夏威夷冒纳凯阿火山上的巨型望远镜凯克望远镜(Keck Telescope),并把超新星爆炸作为终极距离标记,得出的哈勃常数值为73,正负误差仅为2.4%。

That made waves because it meant that, if true, the Hubble constant as observed today was now clearly incompatible with a result of the lower slower value of 67 inferred from data obtained in 2013 by the European Planck spacecraft of relic radiation from the Big Bang. The Planck mission observations that show the universe when it was only 380,000 years old are considered the gold standard of cosmology.

这引起了轩然大波,因为它意味着,如果属实,那么今天观察到的哈勃常数,显然和2013年欧洲普朗克(Planck)航天器从大爆炸残留辐射的相关数据中推断出的67不符。后者更低,代表膨胀速度更慢。普朗克太空飞行任务的观察结果显示的是年龄仅为38万年的宇宙。它们被视作宇宙学的金本位。

Whether the standard cosmic recipe might now need to be modified — for example, to account for a new species of subatomic particles streaming through space from the Big Bang — depends on whom you talk to. Some say it is too soon to get excited about new physics sneaking through such a small discrepancy in a field noted for controversy. With more data and better understanding of statistical uncertainties, the discrepancy might disappear, they say.

标准的宇宙构成现在是否需要调整——比如,把一种从大爆炸开始就在太空中涌动的新型亚原子粒子计算在内——取决于你在跟谁讲话。有人说,在一个以争议著称的领域,对通过这么小的一个差异反映出的新物理感到兴奋还为时过早。他们认为,搜集到更多数据,并增强对统计不确定性的了解后,这种差异也许就消失了。

“No explanation I know of is less ugly than the problem,” Lawrence M. Krauss, a theorist at Arizona State University, said.

“据我所知,解释永远比问题更可怕,”亚利桑那州立大学(Arizona State University)的理论学者劳伦斯·M·克劳斯(Lawrence M. Krauss)说。

Others say this could be the beginning of something big. David Spergel, a cosmologist at Princeton University and the Simons Foundation, called the discrepancy “very intriguing,” but said he was not yet convinced that this was the signature of new physics. Michael S. Turner of the University of Chicago said, “If the discrepancy is real, this could be a disruption of the current highly successful standard model of cosmology and just what the younger generation wants — a chance for big discoveries, new insights and breakthroughs.”

还有一些人则表示这可能是某个大事件的开端。任职于普林斯顿大学(Princeton University)和西蒙斯基金会(Simons Foundation)的宇宙学者戴维·斯佩格尔(David Spergel)称这个偏差“非常有趣”,但表示他还不相信这标志着出现了新的物理。芝加哥大学(University of Chicago)的迈克尔·S·图尔纳(Michael S. Turner)说,“如果这个差异是真实存在的,这可能会扰乱当前非常成功的标准宇宙学模型,也正是年青一代想要的——一个取得大发现、新见解和突破的机会。”

Riess and his colleague Stefano Casertano got roughly the same answer of 73 later last summer, strengthening the claim for a mismatch of Hubble constants. They used early data from the European spacecraft GAIA, which is measuring the distances of more than 1 billion stars by triangulation, thus allowing astronomers to skip some of the lower rungs on the distance ladder.

去年夏天晚些时候,里斯和同事斯特凡诺·卡塞尔塔诺(Stefano Casertano)得到了大致相同的结果,73。这巩固了哈勃常数发生了改变的说法。他们使用的是欧洲盖亚(GAIA)航天器提供的早期数据。盖亚正在用三角测量的方法测量逾10亿颗恒星的距离,因而使天文学家能够跳过距离阶梯上较低的一些等级。

They calculated that the odds of this mismatch being a statistical fluke were less than 1 part in 100 — which might sound good in poker but not in physics, which requires odds of less than 1 in 1 million to cement a claim of a discovery.

他们计算出,这个差异只是统计巧合的可能性不到1%。在扑克牌中,这也许听上去不错,但在物理学中不是。后者要求可能性在100万分之一以下,才能巩固有了新发现的说法。

“I think it’s a potentially serious issue,” said Alex Filippenko, a University of California astronomer who is part of the team. “In this line of research the devil is in the details. And after getting the details right, we’re left with a major puzzle.”

“我觉得这是一个可能会比较严重的问题,”团队成员之一、加利福尼亚大学(University of California)天文学者亚力克斯·菲利片科(Alex Filippenko)说。“在这个研究领域,细节决定成败。细节没问题了,我们就只剩一个等待解决的难题了。”

What comes next?

接下来怎么办?

There is wiggle room, Riess and others say, for both the modern and the primordial results to be right, because Planck measures the Hubble constant only indirectly as one of several parameters in the standard model of the universe. Other parameters could be tweaked.

里斯和其他人均表示,现代的和原始的结果都需要进行调整,才能做到准确,因为普朗克只是间接地把哈勃常数作为标准宇宙模型多个参数中的其中一个进行测量的。其他参数也许会得到完善。

That is where new physics might come in.

这正是也许会出现新的物理的地方。

The most likely candidates to fill the gap, Riess said, might be a new form of the ghostly particles called neutrinos, already known to be abundant in the cosmos. They come in three types that can change into one another as they traverse space; some physicists have suggested there could be a fourth kind, called sterile neutrinos, that do not interact with anything at all.

里斯说,最有可能解释这种差异的,也许是一种新的幽灵粒子,叫中微子。据知,宇宙中存在大量这种粒子。它们分三种,在宇宙中穿行时可从一种变成另外一种。一些物理学家称可能还有一种幽灵粒子,叫惰性中微子,完全不与任何东西相互作用。

Their discovery could unlock new realms in particle physics and perhaps shed light, so to speak, on the quest to understand the dark matter that suffuses space and provides the gravitational scaffolding for galaxies.

他们的发现可能揭开了粒子物理学的新领域,也许还可以说让外界对认识暗物质的追求有所了解。暗物质充斥着太空,为星系提供引力脚手架。

Another possibility is that the most popular version of dark energy — known as the cosmological constant, invented by Einstein 100 years ago and then rejected as a blunder — might have to be replaced in the cosmological model by a more virulent and controversial form known as phantom energy, which could cause the universe to eventually expand so fast that even atoms would be torn apart in a Big Rip billions of years from now.

另一种可能性是,在宇宙学模型中,最受欢迎的暗能量形式——被称作宇宙学常数,是爱因斯坦在100年前提出的,后来被当成错误遭到否定——也许不得不被一种危害更大、更有争议的形式所取代。这种暗能量叫幽灵能量,可能会导致宇宙最终膨胀得太快,以至原子都会在距今数十亿年后的一场大撕裂中被撕碎。

“This is a very interesting tension,” Riess said. “This is why we play the game. We look for something not fitting.”

“这是一种非常有趣的紧张状态,”里斯说。“这也正是我们玩这个游戏的原因。我们要找的就是某种不正常的东西。”

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