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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.

  这种各向异性的可能证据是由德国波恩大学的天文学家康斯坦丁诺斯·米格卡斯(Konstantinos Migkas)领导的国际团队所提出的。这一证据立足于美国宇航局钱德拉X射线天文台、欧洲航天局的XMM-牛顿卫星和日本先进宇宙学与天体物理学卫星所观测到的近850个星系团的最新数据与档案数据。

  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.

  宇宙的各向异性将动摇物理学的基础,并会对当前关于宇宙演化的观点提出重大修改的要求。未参与此项研究的密歇根州立大学天体物理学家梅根·多纳休(Megan Donahue)说:“如果[宇宙的增长]确实在不同方向上有所不同,那么这将给整个宇宙学界关于宇宙在足够大的空间区域内均匀膨胀这一假说带来一个新的棘手问题。”她补充道,不均匀的膨胀“将是出乎意料而又令人沮丧的”,因为这表明我们对宇宙的宏观结构和演化的理解或许永远都会是极其片面的。

  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.”

  “如果发现暗能量在宇宙的不同区域具有不同的强度,那将是一项引人注目的发现,”该项研究的共同作者,波恩大学的托马斯·里普里希(Thomas Reiprich)在最近的一份声明中说,“但是,需要更多的证据来排除其它可能的解释,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.”


  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!”

  “使用星团作为宇宙学探针的研究导致错误结论已有一段时日。”与米格卡斯团队无利益相关的约翰·霍普金斯大学的天文学家亚当·里斯(Adam Riess)说道。他的判断是根据其他研究者最近针对基于星团所作的研究与其它测量技术之间的不一致性分析而得出的。这种不一致性表明星团的X射线温度与其光度之间并没有研究者所希望的明确相关性。此外,里斯说,在银河系中还有其它潜在的问题需要解决:即我们这个充满气体与尘埃的磁盘状星系,以各种令人费解的方式限制了天文学家对比银河系更广阔的宇宙空间的看法。这可能并非巧合,他表示,米格卡斯等人所确定的宇宙表现出最明显各向异性现象的区域标志着银河系能够吸收X射线的气体和尘埃最稠密的地方。“他们声称宇宙表现诡异的方向刚好就在我们的盲区中,”里斯补充说:“这似乎令人怀疑!”

  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.

  普林斯顿大学和纽约Flatiron研究所的宇宙学家戴维·斯珀格尔(David Spergel)也怀疑基于星团的测量结果存在错误,部分原因是许多其它技术给出了截然不同的结果。“如果结果是正确的,那么这是一篇非常重要的论文,但很可能并非如此。”他说,“基于[宇宙微波背景]和宏观结构的观测,我们有许多更准确的各向异性测试方法。这些观察结果更简单、清晰,并且能通过多种不同方式再现。”他表示,这项新研究提出的各向异性的规模将导致宇宙微波背景产生波动,导致其亮度比天文学家所观察到的高出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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