重力理論該大修正？新模型質疑黑暗物質
Time to overhaul Newton's theory of gravitation? Galaxy cluster models cast doubt on dark matter
http://www.physorg.com/news113031879.html
October 31, 2007
近 75 年來，天文學家相信宇宙裡有大量看不見的，或「黑暗的」物質，認為它構成宇宙中 5/6 的物質。伴隨著重力（萬有引力）的傳統理論（那立足在牛頓的構想上，並且由愛因斯坦在 92 年前精練），黑暗物質幫忙解釋星系與星系團（clusters of galaxies）在最大尺度下的運動。
For almost 75 years, astronomers have believed that the Universe has a large amount of unseen or ‘dark’ matter, thought to make up about five-sixths of the matter in the cosmos. With the conventional theory of gravitation, based on Newton’s ideas and refined by Einstein 92 years ago, dark matter helps to explain the motion of galaxies, and clusters of galaxies, on the largest scales.
現在，二位在加拿大 Perimeter Institute for Theoretical Physics 的研究者建議，在遠方星系團中移動的星系，比黑暗物質的存在，更容易用修正的重力理論（Modified Gravity (MOG) theory，譯註：MOG 與 MOND 不同，前者來自於廣義相對論，後者是牛頓力學）來解釋。
Now two Canadian researchers at the Perimeter Institute for Theoretical Physics suggest that the motion of galaxies in a distant cluster is more easily explained by a Modified Gravity (MOG) theory than by the presence of dark matter.
畢業生 Joel Brownstein 與其在 Waterloo 大學的指導教授 John Moffat 在 11/21 當期的 Monthly Notices of the Royal Astronomical Society 一篇論文中發表他們的結果。
Graduate student Joel Brownstein and his supervisor Professor John Moffat of the University of Waterloo present their results in a paper in the 21 November edition of Monthly Notices of the Royal Astronomical Society.
這二位科學家分析了由哈伯太空望遠鏡、Chandra X光、Spitzer 紅外線天文台與智利的麥哲倫望遠鏡所拍攝而得的「子彈星系團（bullet cluster，http://en.wikipedia.org/wiki/Bullet_cluster）的星系影像。子彈星系團由二個合併中的星系團所組成，座落在南方星座，船底座（Carina）方向， 30 億光年遠之處。
The two scientists analysed images of the ‘Bullet Cluster’ of galaxies made using the Hubble Space Telescope, Chandra X-ray and Spitzer infrared observatories and the Magellan telescope in Chile. The Bullet Cluster consists of two merging clusters of galaxies and lies at a distance of over 3 billion light years in the direction of the southern constellation of Carina.
這些成堆的儀器設備，給予他們星系間 1 億 5 千萬度熱氣體的測繪圖，並顯現出重力透鏡（gravitational lensing，http://en.wikipedia.org/wiki/Gravitational_lensing）的效應：介於中間的天體 -- 這裡是指子彈星系團 -- 的重力，會使更遙遠星系所發出光線的路徑偏斜。
This arsenal of instrumentation gave them maps of the 150 million degree hot gas between the galaxies and show the effect of gravitational lensing, where the gravity of an intervening object – here the Bullet Cluster - deflects the path of light emitted by a more distant galaxy.
先前的研究指出子彈星系團清楚地證明黑暗物質的存在。不過當 Brownstein 與 Moffat 將所觀測到的重力透鏡與氣體的分佈和利用 MOG 理論的預測進行比較，他們發現，並沒有黑暗物質的證據存在。換言之，利用修正過的重力理論，要比將黑暗物質包含近來，能更自然地解釋這個星系團的出現。
Previous studies suggested that the Bullet Cluster clearly demonstrates the presence of dark matter. But when Brownstein and Moffat compared the observed gravitational lensing and distribution of gas with that predicted using MOG theory, they found no evidence for this. In other words, it is more natural to explain the appearance of this cluster using a revised theory of gravitation than by including dark matter.
MOG 理論從甚至連愛因斯坦都被難倒的廣義相對論中浮現，已經由 Moffat 發展了近 30 年，而現在終於產生了天文學的與宇宙論的成果。該理論已成功用來解釋超過 100 個星系當中的恆星運動，與超過 100 星系團當中的星系運動。MOG 理論或許也解釋了先鋒（Pioneer）10 號與 11 號太空探測器的異常減速（http://en.wikipedia.org/wiki/Pioneer_anomaly），這些探測器在 1970 年代初期發射升空，現在離太陽已超過 120 兆公里。
MOG theory emerges from a generalization of relativity that eluded even Einstein, has been developed by Moffat for nearly thirty years and is now yielding astronomical and cosmological results. The theory has been used to successfully explain the movement of stars in over 100 galaxies and the motion of galaxies in more than 100 clusters. MOG theory may also explain the apparent anomalous deceleration of the Pioneer 10 and 11 space probes, launched in the early 1970s and now more than 12000 million km from the Sun.
這二位科學家對於他們的發現相當熱心。Brownstein 評論，"利用 MOG 理論，在子彈星系團當中的「普通」物質就足以說明所觀測到的重力透鏡效應。及時的、更棒的觀測將使我們正在研究的這個系統，有解析度更高的照片。繼續這個尋找然後分析其他合併中的星系團，將協助我們決定是黑暗物質或是 MOG 理論，哪一個最能夠解釋宇宙的大尺度結構。"
The two physicists are enthusiastic about their findings. Brownstein comments, ‘Using Modified Gravity (MOG) theory, the ‘normal’ matter in the Bullet Cluster is enough to account for the observed gravitational lensing effect. In time, better observations will lead to higher resolution pictures of the systems we are studying. Continuing the search for and then analysing other merging clusters of galaxies will help us decide whether dark matter or MOG theory offers the best explanation for the large scale structure of the Universe.’
Moffat 教授補充，"如果現在進行中，價值幾十億美金的實驗室實驗，能成功的直接偵測到黑暗物質，那麼我將樂意見到愛因斯坦的與牛頓的重力能夠保留。然而，如果黑暗物質沒有被偵測到，而且我們必須下結論說它並不存在，那麼愛因斯坦與牛頓的重力，則必須要被修正以符合數量龐大的天文學與宇宙論資料，就如同子彈星系團那樣，那無法用其他方法來解釋。"
Professor Moffat adds, ‘If the multi-billion dollar laboratory experiments now underway succeed in directly detecting dark matter, then I will be happy to see Einsteinian and Newtonian gravity retained. However, if dark matter is not detected and we have to conclude that it does not exist, then Einstein and Newtonian gravity must be modified to fit the extensive amount of astronomical and cosmological data, such as the bullet cluster, that cannot otherwise be explained.’
(Source: Royal Astronomical Society)
※ 相關報導：
＊ Modified gravity v. dark energy
http://www.physorg.com/news95593687.html
＊ The Bullet Cluster 1E0657-558 evidence shows modified gravity in the absence of dark matter
http://www.blackwell-synergy.com/doi/abs/10.1111/j.1365-2966.2007.12275.x
J. R. Brownstein, J. W. Moffat
Monthly Notices of the Royal Astronomical Society (OnlineEarly Articles).
doi:10.1111/j.1365-2966.2007.12275.x
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