「科學種子科技英語」為上海青少年科學社特色周刊板塊，旨在通過雙語形式翻譯國內外最新科學技術科普文章，帶領青少年讀者們了解全球最前沿的科研成果概況，普及科學知識、傳播科學思維、領悟科學奧秘。2021年該板塊重新推出，特邀上海青少年科學社優秀學生會員組成科普翻譯團，通過同學們自己的視角，解讀科學背後的故事，帶領更多的小夥伴學科學、愛科學，共讀最前沿、最有趣的科普資訊。
It is well known that the expansion of the universe is accelerating due to a mysterious dark energy. Within galaxies, stars also experience an acceleration, though this is due to some combination of dark matter and the stellar density. In a new study to be published in Astrophysical Journal Letters, researchers have now obtained the first direct measurement of the average acceleration taking place within our home galaxy, the Milky Way.
眾所周知，宇宙正因一種神秘的暗能量而加速膨脹。在星系內部，恆星也在經歷加速的過程，儘管這歸因於暗物質和恆星密度的某種結合。在一項將在《天體物理學雜誌通訊》上發表的研究中，科學家們已經獲得了對我們所在的星系--銀河系--內部平均加速度的首次直接測量結果。
Led by Sukanya Chakrabarti at the Institute for Advanced Study with collaborators from Rochester Institute of Technology, University of Rochester, and University of Wisconsin-Milwaukee, the team used pulsar data to clock the radial and vertical accelerations of stars within and outside of the galactic plane. Based on these new high-precision measurements and the known amount of visible matter in the galaxy, researchers were then able to calculate the Milky Way's dark matter density without making the usual assumption that the galaxy is in a steady-state.
這項研究由高等研究所的蘇坎亞·查克拉巴爾蒂帶領，與來自羅切斯特理工學院、羅切斯特大學、和威斯康星大學密爾沃基分校的合作者共同完成，他們利用脈衝星的數據來測定銀河平面內外恆星的徑向與垂直方向的加速度。基於這些新的高精度測量以及星系中可見物質的已知量，研究人員無需做出星系處在穩定態的的慣用假設，便能夠計算出銀河系的暗物質密度。
"Our analysis not only gives us the first measurement of the tiny accelerations experienced by stars in the galaxy, but also opens up the possibility of extending this work to understand the nature of dark matter, and ultimately dark energy on larger scales," stated Chakrabarti, the paper's lead author and a current Member and IBM Einstein Fellow at the Institute for Advanced Study.
「我們的分析不僅讓我們得到了對銀河系中恆星經歷的細微加速度的首次測量結果，而且還擴展了這項工作的範圍，為理解暗物質本質，以及最終在更大尺度上理解暗能量提供了可能性。」本研究的主導作者、高等研究所現任成員、IBM愛因斯坦研究員查克拉巴爾蒂表示。
Stars hurtle through the galaxy at hundreds of kilometers per second, yet this study indicates that the change in their velocities is occurring at a literal snail's pace -- a few centimeters per second, which is about the same speed as a crawling baby. To detect this subtle motion the research team relied on the ultraprecise time-keeping ability of pulsars that are widely distributed throughout the galactic plane and halo -- a diffuse spherical region that surrounds the galaxy.
恆星以每秒幾百公裡的速度在銀河系中飛馳而過，但這項研究指出，它們的速度變化卻是以蝸牛般的速度發生的--每秒幾釐米，這與爬行的嬰兒的速度差不多。為了探測如此微妙的運動，研究團隊依靠脈衝星的超精密計時能力，這種脈衝星廣泛分布在整個銀河系平面和銀暈（包圍銀河系的一個瀰漫的球狀區域）中。
"By exploiting the unique properties of pulsars, we were able to measure very small accelerations in the Galaxy. Our work opens a new window in galactic dynamics," said co-author Philip Chang of the University of Wisconsin-Milwaukee.
「通過挖掘脈衝星獨特的性質，我們得以測量銀河系中非常微小的加速度。我們的工作為星系動力學打開了一扇嶄新的窗戶。」威斯康星大學密爾沃基分校的合著者菲利普·昌說道。
Extending outwards approximately 300,000 light years from the galactic center, the halo may provide important hints to understanding dark matter, which accounts for about 90 percent of the galaxy's mass and is highly concentrated above and below the star-dense galactic plane. Stellar motion in this particular region -- a primary focus of this study -- can be influenced by dark matter. Utilizing the local density measurements obtained through this study, researchers will now have a better idea of how and where to look for dark matter.
銀暈從銀河系中心向外延伸將近30萬光年，可能為暗物質的理解提供重要線索。暗物質約佔星系質量的90%，高度集中在恆星密布的星系平面上下。這一特定區域中的恆星運動--本研究的一個主要焦點--會受到暗物質的影響。現在，研究人員利用通過本項研究得到的局部密度測量結果，將會對如何尋找暗物質以及在哪裡尋找暗物質有更清楚的想法。
While previous studies assumed a state of galactic equilibrium to calculate average mass density, this research is based on the natural, non-equilibrium state of the galaxy. One might analogize this to the difference between the surface of a pond before and after a stone is tossed in. By accounting for the "ripples" the team was able to obtain a more accurate picture of reality. Though in this case, rather than stones, the Milky Way is influenced by a turbulent history of galactic mergers and continues to be perturbed by external dwarf galaxies like the Small and Large Magellanic Clouds. As a result, stars do not have flat orbits and tend to follow a path similar to that of a warped vinyl record, crossing above and below the galactic plane. One of the key factors that enabled this direct observational approach was the use of pulsar data compiled from international collaborations, including NANOGrav (North American Nanohertz Observatory for Gravitational Waves) that has obtained data from the Green Bank and Arecibo telescopes.
不同於先前的研究對平均質量密度進行測量的設想是銀河系處於平衡態，該研究的設想是自然的、非平衡狀態下的銀河系。這就可以類比為扔進石頭前後池塘表面的差異。通過對「漣漪」的解釋，該研究團隊更準確地描繪了現實。不過在這種情況下，不同於石頭，銀河系受到了過程動蕩的星系合併的的影響，並將持續受到外部矮星系（如大小麥哲倫雲）的幹擾。因此，恆星沒有平整的軌道，而是傾向於跟隨一個類似於扭曲的黑膠唱片的軌道，向上向下穿過銀河平面。促成這種直接的觀測方法的關鍵因素之一是利用國際合作彙編的脈衝星數據，包括從綠岸射電天文望遠鏡和阿雷西博望遠鏡中獲取數據的北美納米赫茲引力波觀測站。
This landmark paper expands upon the work of Jan H. Oort (1932); John Bahcall (1984); Kuijken & Gilmore (1989); Holmberg & Flynn (2000); Jo Bovy & Scott Tremaine (2012) to calculate the average mass density in the galactic plane (Oort limit) and local dark matter density. IAS scholars including Oort, Bahcall, Bovy, Tremaine, and Chakrabarti have played an important role in advancing this area of research.
這樣一份裡程碑式的論文在簡·H·奧爾特（1932）、約翰·巴赫恰勒（1984）、奎肯和吉爾摩（1989）、霍姆伯格和弗林（2000）、喬·博維和斯科特·特裡梅因（2012）的工作基礎上有了拓展，對局部暗物質密度和星系平面中平均質量密度（奧爾特極限）進行了計算。包括奧爾特、巴赫恰勒、博維、特裡梅因、和查克拉巴爾蒂的高等研究所學者為這一領域研究的進展起到了重要的作用。
"For centuries astronomers have measured the positions and speeds of stars, but these provide only a snapshot of the complex dynamical behavior of the Milky Way galaxy," stated Scott Tremaine, Professor Emeritus at the Institute for Advanced Study. "The accelerations measured by Chakrabarti and her collaborators are directly caused by the gravitational forces from the matter in the galaxy, both visible and dark, and thereby provide a new and promising window on the distribution and the composition of the matter in the galaxy and the universe."
「幾世紀以來，天文學家一直在測量恆星的位置和速率，但這些只是銀河星系複雜動態行為的一個簡單快照。」高等研究所的榮休教授斯科特·特裡梅因表示。「查克拉巴爾蒂和她的合作者測量出來的加速度由星系內可見物質以及暗物質的引力直接導致，因此為星系和宇宙中物質的分布與組成的研究提供了有前途的新方向。」
This particular paper will enable a wide variety of future studies. Accurate measurements of accelerations will also soon be possible using the complementary radial velocity method that Chakrabarti developed earlier this year, which measures the change in the velocity of stars with high precision. This work will also enable more detailed simulations of the Milky Way, improve constraints on general relativity, and provide clues in the search for dark matter. Extensions of this method may ultimately allow us to directly measure the cosmic acceleration as well.
這篇論文將有助於未來的多種研究。對加速度的精確測量也將很快因查克拉巴爾蒂在今年早些時候開發出來的互補徑向速度法而得以實現，此方法對恆星速度的變化進行了高精度的測量。這項工作也會實現對銀河系更細緻的模擬，改善對廣義相對論的限制，並為尋找暗物質提供線索。這一方法的延伸將會最終讓我們能夠直接測量宇宙加速度。
While a direct picture of our home galaxy -- similar to the ones of Earth taken by the Apollo astronauts -- is not yet possible, this study has provided essential new details to help envision the dynamic organization of the galaxy from within.
儘管像阿波羅太空人拍攝地球那樣直接拍攝我們的銀河系還不可能，但這項研究已經在幫助我們從內部設想星系的動態結構上提供了重要的新細節。
Story Source:
Materials provided by Institute for Advanced Study. Note: Content may be edited for style and length.
pulsar 脈衝星
stellar density 恆星密度
radial acceleration 徑向加速度
halo 光環
galactic dynamics 星系動力學
dwarf galaxy 矮星系
the Small and Large Magellanic Clouds 大小麥哲倫星雲
general relativity 廣義相對論
cosmic acceleration 宇宙加速度
譯者：付萬嘉，上海外國語大學附屬外國語學校學生
（上海青少年科學社高級會員、科普翻譯團成員）
上海青少年科學社科普翻譯團是由一群熱愛科學、有志於開展科普推廣的學生志願者組成，團隊成員均為上海青少年科學社學生會員。翻譯團旨在通過同學們自己的視角，了解科學家們的研究成果、解讀科學背後的故事，帶領更多的小夥伴學科學愛科學，共讀最前沿、最有趣的科普資訊。翻譯團所有公開作品均用以公益學習，不做任何商業推廣。如有意向與翻譯團成員共同閱讀文獻、提高英語能力、交流學習心得，歡迎加入我們！