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  If your life sometimes seems directionless,you might legitimately blame the universe.


  According to the key tenets of modern physics,the cosmos is“isotropic”at multi-billion-light-year scales—meaning it should have the same look and behavior in every direction.Ever since the big bang nearly 14 billion years ago,the universe ought to have expanded identically everywhere.And that expectation matches what astronomers see when they observe the smooth uniformity of the big bang's all-sky afterglow:the cosmic microwave background(CMB).Now,however,an x-ray survey of distances to galaxy clusters across the heavens suggests some are significantly closer or farther away than isotropy would predict.This finding could be a sign that the universe is actually“anisotropic”—expanding faster in some regions than it does in others.With apologies to anyone seeking a cosmic excuse for personal woes,maybe the universe is not so directionless after all.


  This possible evidence for anisotropy comes from an international team led by astronomer Konstantinos Migkas of the University of Bonn in Germany.And it relies on new or archival data on nearly 850 galaxy clusters seen by NASA's Chandra X-ray Observatory,the European Space Agency's XMM-Newton satellite and Japan's AdvancedSatellite for Cosmology and Astrophysics.

  這種各向異性的可能證據是由德國波恩大學的天文學家康斯坦丁諾斯·米格卡斯(Konstantinos Migkas)領導的國際團隊所提出的。這一證據立足于美國宇航局錢德拉X射線天文臺、歐洲航天局的XMM-牛頓衛星和日本先進宇宙學與天體物理學衛星所觀測到的近850個星系團的最新數據與檔案數據。

  The study,which appeared in the April edition of Astronomy&Astrophysics,treats each cluster a bit like a lighthouse—gauging their distances by how bright or dim each one appears.By measuring the kinds and amounts of x-rays emitted by the hot,rarefied gas suffusing a given cluster,the team could determine that gas's temperature.Doing so allowed the researchers to estimate the cluster's x-ray luminosity—and therefore its distance.Next,they calculated each cluster's luminosity via a separate technique that relied,in part,on preexisting determinations of the universe's expansion rate.Comparing the two independent cluster luminosity values allowed Migkas and his colleagues to probe potential deviations in the universe's rate of expansion across the entire sky,revealing two regions where clusters were some 30 percent brighter or fainter(and thus potentially closer or farther away)than expected.


  “We managed to pinpoint a region that seems to expand slower than the rest of the universe and one that seems to expand faster,”Migkas says.“There are many studies with optical supernovae and with infrared galaxies that have detected similar anisotropies toward the same directions as well.And there are also many studies with similar data sets that do not show any anisotropies!Therefore,the situation is still vague.We do not argue to know the origin of the anisotropies,only that they are there.”

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  An Astonishing,Depressing Anisotropy


  An anisotropic universe would shake the pillars of physics,demanding major revisions to current thinking about cosmic evolution.“If[the universe's growth]is indeed different in different directions,that brings a whole new wrinkle into a cosmological assumption about homogeneity of the expansion over sufficiently large regions of space,”says Megan Donahue,a Michigan State University astrophysicist who was not involved in the study.A lopsided expansion“would be astonishing and depressing,”she adds,because it would suggest our understanding of the universe's large-scale structure and evolution is profoundly—perhaps permanently—incomplete.

  宇宙的各向異性將動搖物理學的基礎,并會對當前關于宇宙演化的觀點提出重大修改的要求。未參與此項研究的密歇根州立大學天體物理學家梅根·多納休(Megan Donahue)說:“如果[宇宙的增長]確實在不同方向上有所不同,那么這將給整個宇宙學界關于宇宙在足夠大的空間區域內均勻膨脹這一假說帶來一個新的棘手問題。”她補充道,不均勻的膨脹“將是出乎意料而又令人沮喪的”,因為這表明我們對宇宙的宏觀結構和演化的理解或許永遠都會是極其片面的。

  To explain such a thing—and to reconcile it with the nearly perfect isotropy seen in the CMB—cosmologists could turn to dark energy,the mysterious force driving an acceleration to the universe's growth.Perhaps,somewhere in the intervening eons between the CMB's picture of the“early”universe and the“late”one of the past several billion years,dark energy's effects became stronger in some select parts of the cosmos,creating a lopsided expansion.


  "It would be remarkable if dark energy were found to have different strengths in different parts of the universe,”said study co-author Thomas Reiprich of the University of Bonn in a recent statement.“However,much more evidence would be needed to rule out other explanations and make a convincing case.”

  “如果發現暗能量在宇宙的不同區域具有不同的強度,那將是一項引人注目的發現,”該項研究的共同作者,波恩大學的托馬斯·里普里希(Thomas Reiprich)在最近的一份聲明中說,“但是,需要更多的證據來排除其它可能的解釋,并使其具有更高的可信度。”

  Alternatively,the universe might not be lopsided at all:the aberrant galaxy clusters could be caught up in a“bulk flow,”pulled out of place by the gravitational grip of even bigger and more distant clusters,a bit like boats swept along in a river's swift currents.But most cosmologists have not expected bulk flows to occur across the extremely large scales probed by the study,which made measurements out to roughly five billion light-years.


  “It could very well be a bulk flow,”Migkas says.“Nevertheless,this would be very important as well,simply because most studies do not take that into account!Any existing bulk flows could heavily affect our results and measurements if people do not correct for these motions appropriately.”
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  Cosmic Blind Spots


  The most obvious explanation,of course,would be that the apparent asymmetries in cluster spacing are because of flaws in the data or their analysis.Yet that scenario could still demand updates to scientists'understanding of how errors creep into their best reckonings of cosmic distances.


  “Studies using clusters as probes of cosmology have been giving screwy results for a while,”says Adam Riess,an astronomer at Johns Hopkins University,who is unaffiliated with Migkas's team,citing recent analyses by other researchers that highlight inconsistencies between cluster-based work and other measurement techniques.Such inconsistencies suggest correlations between a galaxy cluster's x-ray temperature and its luminosity are not as clear-cut as researchers would prefer.Furthermore,Riess says,there are other potential problems to deal with right here in the Milky Way:namely,our galaxy's gas-and-dust-filled disk,which obscures astronomers'views of the wider cosmos in various vexing ways.It may not be coincidental,he says,that the region of greatest apparent cosmic anisotropy identified by Migkas and his colleagues borders the place where the Milky Way's x-ray-absorbing gas and dust are thickest.“They are claiming the weird direction of the universe is right in our blind spot,”Riess adds.“That seems suspicious!”

  “使用星團作為宇宙學探針的研究導致錯誤結論已有一段時日。”與米格卡斯團隊無利益相關的約翰·霍普金斯大學的天文學家亞當·里斯(Adam Riess)說道。他的判斷是根據其他研究者最近針對基于星團所作的研究與其它測量技術之間的不一致性分析而得出的。這種不一致性表明星團的X射線溫度與其光度之間并沒有研究者所希望的明確相關性。此外,里斯說,在銀河系中還有其它潛在的問題需要解決:即我們這個充滿氣體與塵埃的磁盤狀星系,以各種令人費解的方式限制了天文學家對比銀河系更廣闊的宇宙空間的看法。這可能并非巧合,他表示,米格卡斯等人所確定的宇宙表現出最明顯各向異性現象的區域標志著銀河系能夠吸收X射線的氣體和塵埃最稠密的地方。“他們聲稱宇宙表現詭異的方向剛好就在我們的盲區中,”里斯補充說:“這似乎令人懷疑!”

  David Spergel,a cosmologist at Princeton University and the Flatiron Institute in New York City,also suspects faults in the cluster-based measurements—in part because so many other techniques provide fundamentally conflicting results.“This is a paper that is very important if[it is]true but very unlikely to be true,”he says.“We have many much more accurate tests of anisotropy based on observations of[the CMB]and of large-scale structure.These observations are simpler,cleaner and have been reproduced in multiple different ways.”Anisotropies of the scale suggested by the new study,he says,would lead to fluctuations in the CMB that were 1,000 times brighter than what astronomers have observed.

  普林斯頓大學和紐約Flatiron研究所的宇宙學家戴維·斯珀格爾(David Spergel)也懷疑基于星團的測量結果存在錯誤,部分原因是許多其它技術給出了截然不同的結果。“如果結果是正確的,那么這是一篇非常重要的論文,但很可能并非如此。”他說,“基于[宇宙微波背景]和宏觀結構的觀測,我們有許多更準確的各向異性測試方法。這些觀察結果更簡單、清晰,并且能通過多種不同方式再現。”他表示,這項新研究提出的各向異性的規模將導致宇宙微波背景產生波動,導致其亮度比天文學家所觀察到的高出1000倍。

  Even so,Migkas and his colleagues argue that decisively ruling against—or for—a lopsided universe requires additional,more comprehensive probes of large-scale cosmic structure.They are now looking for further hints of galaxy-cluster anisotropy within maps of the CMB and seeking to validate their x-ray-based cluster studies with complementary infrared observations.Conclusive results could ultimately come from new space telescopes—such as eROSITA,a German-Russian x-ray observatory,or the European Space Agency's upcoming Euclid mission—that will perform deeper and broader surveys of clusters across the entire sky.


  “Generally,we believe that more and more people should look into the isotropy of the universe—finding new methods and tools to do so—considering the enormous significance this has for standard cosmology,”Migkas says.“It would be great if we knew,once and for all,if the late universe looks isotropic or not.”



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