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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?

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.


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.


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.


A small discrepancy in the value of a long-sought number has fostered a debate about just how well we know the cosmos.

“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.


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.


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


Getting Closer


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.


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


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.


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.


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


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.


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.


“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.


“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?


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.


That is where new physics might come in.


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.


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.


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.


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