加拿大太空總署體認到這是讓大家對太空計畫產生興趣的絕佳機會,於是趁我在外太空,製作了一百多支短片。這種教育性質的推廣工作也是太空人職責的一部分,而我特別熱愛。在拍片之前,我已經花了二十年時間在許多小鎮的鎮民會堂、小學與扶輪社演講,推廣太空計畫,基本上是哪裡有需要就去。二○一○年,我還創立了一個計畫叫「與太空人的午餐對談」(On the Lunch Pad),利用我吃午餐的時間,用Skype軟體與學校的孩子對談。
根據廣義相對論的計算,一旦有重力波經過,不同脈衝星訊號之間的相關性與脈衝星在天球上的夾角會滿足一條特定的曲線,稱為 HD 曲線(Hellings-Downs curve)。
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科學家以兩顆脈衝星為一組觀測單位,藉由觀測多組脈衝星的訊號、計算它們之間的相關性,再比較這些數據是否符合 HD 曲線,就能夠進一步推斷低頻重力波是否存在。值得一提的是,由於重力波訊號非常微弱,用來作為陣列的脈衝星必須有非常穩定的計時條件,因此一般會選擇自轉週期在毫秒(ms)級別的毫秒脈衝星作為觀測對象。
NANOGrav 在今年 6 月發布的觀測結果就是利用位於波多黎各的阿雷西博天文台(Arecibo Observatory,已於 2020 年因結構老舊而退役)、美國的綠堤望遠鏡(Robert C. Byrd Green Bank Telescope)和甚大天線陣(Very Large Array, VLA)觀測 68 顆毫秒脈衝星。
他們分析了長達 15 年的觀測數據後,發現這些脈衝星訊號的相關性與 HD 曲線相當吻合,證實了低頻重力波確實存在於我們的宇宙中。
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