And that suggests understanding of cosmology is wrong

May 17th 2018

ONE of the most basic facts about the universe is that it is expanding. This observation, made by Edwin Hubble (pictured) in 1929, leads to all sorts of mind-stretching ideas. That the universe is growing implies it was smaller in the past—possibly a lot smaller. Which leads to the thought that a “Big Bang” kicked everything off. It also opens the question of whether the universe will expand for ever, or will eventually see its expansion halted and reversed by gravity, thus ending in a Big Crunch.

關(guān)于宇宙最基本的事實(shí)之一是它正在膨脹，這個(gè)由埃德溫·哈勃(如圖)在1929年所做的觀察，引發(fā)了眾多思維延伸的想法。宇宙的增長(zhǎng)意味著它在過(guò)去更小——可能小得多，這也導(dǎo)出一種思想，即“大爆炸”開(kāi)啟一切。這也引出了一個(gè)問(wèn)題：宇宙是否會(huì)永遠(yuǎn)膨脹，或者最終膨脹會(huì)被重力阻止和逆轉(zhuǎn)，一切在大坍塌中結(jié)束。

Things got stranger in 1998, when a group of astrophysicists discovered that the rate of expansion is increasing, for this finding raised another question in turn. The acceleration of the expansion was so great that it seemed something was actively pushing the universe apart. Thus was born the notion of “dark energy”—a new component of the cosmos, invoked to balance the equations.

1998年，當(dāng)一群天體物理學(xué)家發(fā)現(xiàn)擴(kuò)張的速度在增加時(shí)，事情變得更奇怪了，因?yàn)檫@一發(fā)現(xiàn)反過(guò)來(lái)又提出了另一個(gè)問(wèn)題。膨脹的加速度如此之大，以至于似乎有什么東西在主動(dòng)地把宇宙撕開(kāi)。由此誕生了“暗能量”這個(gè)概念——宇宙的一個(gè)新組成部分，用來(lái)平衡方程式。

Trying to work out what dark energy really is (or if it even exists) requires accurate measurements, particularly of the rate at which the universe is expanding. This rate is known as the Hubble constant, and there are two ways of measuring it. Unfortunately, the answers these methods come up with disagree. That is not necessarily a problem. Previous observational conflicts (for example, that the oldest stars in the universe were older than the universe itself) have gone away as measurements improved. But in this case a new set of measurements has confirmed the discrepancy. And that has got those who study astrophysics flummoxed.

想要知道暗能量到底是什么(或者它是否存在)需要精確的測(cè)量，特別是宇宙膨脹的速度。這個(gè)速率被稱為哈勃常數(shù)，有兩種測(cè)量方法。不幸的是，這兩種方法給出的答案并不一致。這不一定是個(gè)問(wèn)題。以前的觀測(cè)沖突(例如，宇宙中最古老的恒星比宇宙本身更古老)隨著測(cè)量的改進(jìn)而消失了。但針對(duì)這種情況，一組新的測(cè)量結(jié)果證實(shí)了這種差異，這讓那些研究天體物理學(xué)的人感到困惑。

The new measurements were made by a team led by Adam Riess, one of the researchers who discovered the accelerating expansion. Dr Riess works at the Space Telescope Science Institute in Baltimore. In a study just uploaded to the arXiv, a repository of papers awaiting formal approval for publication, he and his colleagues offer a new set of measurements of 50 stars of a type known as Cepheid variables.

這項(xiàng)新的測(cè)量是由研究人員亞當(dāng)·里斯領(lǐng)導(dǎo)的一個(gè)小組進(jìn)行的，正是他發(fā)現(xiàn)了加速膨脹。里斯博士在巴爾的摩的太空望遠(yuǎn)鏡科學(xué)研究所工作。在剛剛上傳到arXiv，等待正式獲準(zhǔn)出版的一疊文件的研究中，他和他的同事們提供了一套新的對(duì)50顆名為“造父變星”的恒星的測(cè)量方法。

Cepheids are important to astronomers because they are a rung on what is known as the cosmic distance ladder. A Cepheid pulsates at a frequency related to its intrinsic brightness. This makes it possible, by comparing the intrinsic brightness of such a star with its apparent brightness, as seen from Earth, to work out how far away it is relative to other Cepheids. Then, if the actual distances to some nearby Cepheids can be measured directly, those relative distances can be turned into absolute ones.

造父變星對(duì)天文學(xué)家來(lái)說(shuō)很重要，因?yàn)樗鼈兪潜环Q為宇宙距離階梯的臺(tái)階。造父變星的頻率與它的固有亮度有關(guān)。通過(guò)比較這顆恒星的固有亮度和從地球上觀察到的亮度，可以知道它與其他造父變星的距離有多遠(yuǎn)。然后，如果能直接測(cè)量到附近一些造父變星的距離，那么這些相對(duì)距離可以變成絕對(duì)距離。

This is done by observing their parallax. As Earth orbits the sun, the positions of nearby stars will seem to shift relative to those farther away, in the same way that, to a passenger on a train, trees in the middle distance appear to move with respect to far-off mountains. This means their distances can be worked out by triangulation. The result is a method that has been used since Hubble’s day to work out the distances to nearby galaxies in which individual Cepheids, which are extremely bright stars, can be detected. Then, with this rung in place, other objects, such as certain sorts of supernovae that have predictable energy outputs, can be observed in galaxies of known distance and used to extend the ladder.

這是通過(guò)觀察它們的視差來(lái)完成的。當(dāng)?shù)厍蚶@太陽(yáng)運(yùn)行時(shí)，附近恒星的位置似乎會(huì)向相對(duì)更遠(yuǎn)的地方移動(dòng)，就像在火車上的乘客所感受到的一樣，中距離的樹(shù)木似乎在遠(yuǎn)處的山脈上移動(dòng)。這意味著它們的距離可以通過(guò)三角測(cè)量得到。這一結(jié)果是自哈勃望遠(yuǎn)鏡觀測(cè)到附近星系的距離后使用的一種方法，用這種方法，非常明亮的單個(gè)造父變星可以被發(fā)現(xiàn)。在這些星系中，可以探測(cè)到單個(gè)的仙王座，它們是非常明亮的恒星。然后，通過(guò)適當(dāng)位置的臺(tái)階，其他的物體，例如某些具有可預(yù)測(cè)的能量輸出的超新星，可以在已知距離的星系中觀察到，并以此擴(kuò)展階梯。

The accuracy of the ladder, though, depends on the measurement of each rung. With this in mind, Dr Riess and his colleagues combined data from two space telescopes—Hubble, which has been in orbit since 1990, and Gaia, launched in 2013—to measure with unprecedented accuracy the distances to nearby Cepheids in the hope that this might make the cosmic-ladder-based estimate of the Hubble constant converge with one derived from observations of the Cosmic Microwave Background (CMB), a thin soup of radiation suffusing the universe that is left over from its earliest moments. It did not. Rather, it confirmed the previous estimate.

然而，梯子的精度取決于每一個(gè)臺(tái)階的測(cè)量。鑒于此,里斯博士和他的同事們結(jié)合兩個(gè)太空望遠(yuǎn)鏡的數(shù)據(jù)，它們分別是自1990年以來(lái)一直在軌道上的哈勃望遠(yuǎn)鏡，和2013年推出的蓋亞系統(tǒng)，它以前所未有的精度測(cè)量附近造父變星之間的距離，希望這可以使基于宇宙梯度估計(jì)的哈勃常數(shù)能與來(lái)自宇宙微波背景(CMB)的觀察一直（宙微波背景是彌漫在宇宙中的微弱的輻射，是最開(kāi)始的那一刻遺留下來(lái)的）。然而，事實(shí)并非如此，它證實(shí)了之前的估計(jì)。

According to the cosmic ladder, the universe is expanding at a rate of 73.24km per second per megaparsec. (In English, this means that for each additional megaparsec of distance—about 3.3m light years—the speed at which galaxies are moving away from each other rises by 73km per second.) According to the CMB method the rate is 67km per second. That suggests there really is something wrong with current understanding of the universe. Perhaps this is no more than a mismeasurement of one of the other steps on the cosmic distance ladder. But it could be quite profound. Which is good news for the employment prospects of astrophysicists.

根據(jù)宇宙階梯，宇宙以每百萬(wàn)秒73.24公里的速度膨脹。(在英語(yǔ)中，這意味著每增加百萬(wàn)秒的距離——大約330萬(wàn)光年——星系之間的移動(dòng)速度每秒增加73千米。)根據(jù)CMB的方法，速率是每秒67公里。這表明，目前對(duì)宇宙的認(rèn)識(shí)確實(shí)存在一些錯(cuò)誤。也許這只不過(guò)是對(duì)宇宙距離階梯上的另一個(gè)梯度的錯(cuò)誤測(cè)量。但這可能意義深遠(yuǎn)。這對(duì)天體物理學(xué)家的就業(yè)前景來(lái)說(shuō)是個(gè)好消息。

英文部分來(lái)自《經(jīng)濟(jì)學(xué)人》雜志