Colliding Clusters Shed Light on Dark Matter

For more than 70 years, astronomers, cosmologists and physicists have known that ordinary matter must be surrounded by vast quantities of an invisible substance—not substantial enough to collide with atoms or stars but massive enough to keep galaxies from flying apart. Dubbed dark matter, the mysterious stuff has eluded detection through any means other than its gravitational impact, leading some to propose that Einstein’s general relativity fails to adequately describe how gravity actually works on galactic scales. Now a relatively recent collision of two galaxy clusters has lifted the veil between ordinary and dark matter, proving the latter must exist. 70多年以来，天文学家、宇宙学家和物理学家一直确信普通物质被大量不可见的物质所包围。这种不可见的物质不够充实，不会与原子或恒星相撞，但它大量地存在，使得星系在飞行中彼此分离。这种神秘的东西之所以被称为“暗物质”，是因为除了造成引力影响之外，它可以逃脱任何手段的检测，导致一些人认为，爱因斯坦的广义相对论没有充分地描述在星系的尺度下引力是如何起作用的。现在，通过前不久的两个星系团的碰撞揭开了夹在普通物质与暗物质之间的那层神秘的薄纱，证实了暗物质肯定是存在的。
Douglas Clowe of the University of Arizona and a team of astronomers observed the colliding clusters—known as the bullet cluster thanks to a cloud of hot gas deformed into a ballistic shape—using the Chandra X-ray Observatory. The eponymous shape formed when the superheated plasma of the larger cluster put a drag on the million-degree hydrogen and helium gas in the smaller one as they smashed into one another. But although they are colliding, the ordinary matter in the constituent galaxies, due to the vast distances between component stars, pass through each other without incident. 亚利桑那大学的道格拉斯·克罗韦和一组天文学家，利用钱德拉X射线望远镜观察到了这次碰撞中的星系团。由于炽热的气体云团变形为弹道形状而被称为子弹状星系团。这种因以得名的形状，是当两个星系团撞到一起时，较大星系团的超高温等离子体拖拽较小星系团中上百万度的氢气和氦气而形成的。然而尽管撞在一起，由于星系组成成分—恒星之间的遥远距离，构成星系的普通物质彼此通过，并没有碰撞发生。
The galactic stars in a given cluster only make up roughly 1 to 2 percent of the mass, whereas plasma comprises anywhere from 5 to 15 percent. As the galaxies pass while the plasma is dragged, the resulting separation allowed the scientists to determine the gravitational strength of the various parts of galactic matter from that of the plasma by viewing how light passing near the cluster is bent—so-called gravitational lensing—thus potentially revealing dark matter for the very first time. Because the gas cloud is more massive, the lensing it creates should be more pronounced. On the other hand, if the galaxies have dark matter halos that continue to travel with them—as predicted by theory—then this relatively small amount of ordinary matter should exert an outsized lensing effect due to its invisible companion. 在某个特定星系团中，恒星只约占星系质量的1%至2%，而等离子体占5%至15%。星系相互通过时，等离子体被拖拽，结果产生了跟恒星的分离。这种分离使科学家能够通过所谓的引力透镜现象，即通过观察星系团附近的光线是如何折射的，将星系物质中各部分的引力同等离子体的引力区分开来。这样就有可能第一次揭示暗物质的存在。由于气云较为庞大，所产生的引力透镜现象也就应该更为明显。另一方面，就象理论上所预测的那样，如果星系拥有暗物质晕伴其运动，那么这种相对少量的普通物质应该将特大的引力透镜效应归因于其不可见的伴随物。
Observations with several other telescopes, including NASA’s Hubble Space Telescope, proved that light from stars behind the colliding clusters responded to the greater gravity of the puny galaxies rather than the plasma clouds. “There has always been degeneracy before: you either needed lots of dark matter to explain the extra gravity or gravity doesn’t obey the same physical laws on these million light-year scales as it does in our solar system,” Clowe explains. “Our observations showed that the dark matter has to be there regardless of how gravity behaves on these large scales.” 好几台其他望远镜，包括美国国家航空航天局的哈勃太空望远镜，都参与了此次观察。观察结果证实撞击中的星系团后面的恒星光线受到小星系较大引力的影响，而没有受到等离子体气团的影响。“以前总会退一步考虑：要么需要大量的暗物质来解释额外的引力，要么就是在这些上百万光年的尺度下，引力不符合类似我们太阳系的自然定律，” 克罗韦解释说，“我们的观察显示，不管引力在如此巨大的尺度下是如何起作用的，暗物质不得不存在。”
This proof of existence, appearing in an upcoming issue of The Astrophysical Journal Letters, does nothing to reveal what dark matter really is, however. “Over the years, we’ve pretty much ruled out all sorts of baryonic dark matter possibilities like brown dwarfs or black holes. We’re pretty much left with some new subatomic particles,” notes team member Dennis Zaritsky of the University of Arizona. “It’s a little embarrassing to claim we know anything about the universe when we don’t know what 90 percent of the matter out there is.” 暗物质存在的证明资料发表在即将出版的一期《天体物理学杂志通讯》上。然而，该资料并没有揭示暗物质到底是什么。“多年来，我们常常将存在各种重子暗物质（如：褐矮星或黑洞）的可能性排除在外。现在关于一些新型的亚原子粒子，我们还有许多工作要做。” 研究小组成员亚利桑那大学的丹尼斯·扎瑞茨基指出，“我们在对宇宙90%的物质尚不明确的情况下，却声称了解宇宙的一切，这真有点令人难堪。”

本站仅提供存储服务，所有内容均由用户发布，如发现有害或侵权内容，请点击举报。

For more than 70 years, astronomers, cosmologists and physicists have known that ordinary matter must be surrounded by vast quantities of an invisible substance—not substantial enough to collide with atoms or stars but massive enough to keep galaxies from flying apart. Dubbed dark matter, the mysterious stuff has eluded detection through any means other than its gravitational impact, leading some to propose that Einstein’s general relativity fails to adequately describe how gravity actually works on galactic scales. Now a relatively recent collision of two galaxy clusters has lifted the veil between ordinary and dark matter, proving the latter must exist.	70多年以来，天文学家、宇宙学家和物理学家一直确信普通物质被大量不可见的物质所包围。这种不可见的物质不够充实，不会与原子或恒星相撞，但它大量地存在，使得星系在飞行中彼此分离。这种神秘的东西之所以被称为“暗物质”，是因为除了造成引力影响之外，它可以逃脱任何手段的检测，导致一些人认为，爱因斯坦的广义相对论没有充分地描述在星系的尺度下引力是如何起作用的。现在，通过前不久的两个星系团的碰撞揭开了夹在普通物质与暗物质之间的那层神秘的薄纱，证实了暗物质肯定是存在的。
Douglas Clowe of the University of Arizona and a team of astronomers observed the colliding clusters—known as the bullet cluster thanks to a cloud of hot gas deformed into a ballistic shape—using the Chandra X-ray Observatory. The eponymous shape formed when the superheated plasma of the larger cluster put a drag on the million-degree hydrogen and helium gas in the smaller one as they smashed into one another. But although they are colliding, the ordinary matter in the constituent galaxies, due to the vast distances between component stars, pass through each other without incident.	亚利桑那大学的道格拉斯·克罗韦和一组天文学家，利用钱德拉X射线望远镜观察到了这次碰撞中的星系团。由于炽热的气体云团变形为弹道形状而被称为子弹状星系团。这种因以得名的形状，是当两个星系团撞到一起时，较大星系团的超高温等离子体拖拽较小星系团中上百万度的氢气和氦气而形成的。然而尽管撞在一起，由于星系组成成分—恒星之间的遥远距离，构成星系的普通物质彼此通过，并没有碰撞发生。
The galactic stars in a given cluster only make up roughly 1 to 2 percent of the mass, whereas plasma comprises anywhere from 5 to 15 percent. As the galaxies pass while the plasma is dragged, the resulting separation allowed the scientists to determine the gravitational strength of the various parts of galactic matter from that of the plasma by viewing how light passing near the cluster is bent—so-called gravitational lensing—thus potentially revealing dark matter for the very first time. Because the gas cloud is more massive, the lensing it creates should be more pronounced. On the other hand, if the galaxies have dark matter halos that continue to travel with them—as predicted by theory—then this relatively small amount of ordinary matter should exert an outsized lensing effect due to its invisible companion.	在某个特定星系团中，恒星只约占星系质量的1%至2%，而等离子体占5%至15%。星系相互通过时，等离子体被拖拽，结果产生了跟恒星的分离。这种分离使科学家能够通过所谓的引力透镜现象，即通过观察星系团附近的光线是如何折射的，将星系物质中各部分的引力同等离子体的引力区分开来。这样就有可能第一次揭示暗物质的存在。由于气云较为庞大，所产生的引力透镜现象也就应该更为明显。另一方面，就象理论上所预测的那样，如果星系拥有暗物质晕伴其运动，那么这种相对少量的普通物质应该将特大的引力透镜效应归因于其不可见的伴随物。
Observations with several other telescopes, including NASA’s Hubble Space Telescope, proved that light from stars behind the colliding clusters responded to the greater gravity of the puny galaxies rather than the plasma clouds. “There has always been degeneracy before: you either needed lots of dark matter to explain the extra gravity or gravity doesn’t obey the same physical laws on these million light-year scales as it does in our solar system,” Clowe explains. “Our observations showed that the dark matter has to be there regardless of how gravity behaves on these large scales.”	好几台其他望远镜，包括美国国家航空航天局的哈勃太空望远镜，都参与了此次观察。观察结果证实撞击中的星系团后面的恒星光线受到小星系较大引力的影响，而没有受到等离子体气团的影响。“以前总会退一步考虑：要么需要大量的暗物质来解释额外的引力，要么就是在这些上百万光年的尺度下，引力不符合类似我们太阳系的自然定律，” 克罗韦解释说，“我们的观察显示，不管引力在如此巨大的尺度下是如何起作用的，暗物质不得不存在。”
This proof of existence, appearing in an upcoming issue of The Astrophysical Journal Letters, does nothing to reveal what dark matter really is, however. “Over the years, we’ve pretty much ruled out all sorts of baryonic dark matter possibilities like brown dwarfs or black holes. We’re pretty much left with some new subatomic particles,” notes team member Dennis Zaritsky of the University of Arizona. “It’s a little embarrassing to claim we know anything about the universe when we don’t know what 90 percent of the matter out there is.”	暗物质存在的证明资料发表在即将出版的一期《天体物理学杂志通讯》上。然而，该资料并没有揭示暗物质到底是什么。“多年来，我们常常将存在各种重子暗物质（如：褐矮星或黑洞）的可能性排除在外。现在关于一些新型的亚原子粒子，我们还有许多工作要做。” 研究小组成员亚利桑那大学的丹尼斯·扎瑞茨基指出，“我们在对宇宙90%的物质尚不明确的情况下，却声称了解宇宙的一切，这真有点令人难堪。”