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重力

重力

重力’s dark side

01 Jun 2006

Despite decades of searching, the "暗物质" thought to hold galaxies together is still nowhere to be found. 马修·查默斯(Matthew Chalmers) describes how some physicists think it makes more sense to change our theory of gravity instead

重力 laid bare

如果您是物理学系的学者,您可能已经习惯于接受自称重写了物理定律的人们的来信。科学杂志的编辑也很熟悉这样的人,其中许多人为受害人着迷。虽然我们不应该自动放弃这些想法–毕竟,爱因斯坦(Einstein)于1905年闯入物理学界时是一位未知的专利书记–大多数人掉下来是因为他们的支持者没有将他们与现有知识相结合。

对于越来越多的认为爱因斯坦的专业物理学家来说,情况并非如此。’相对论的一般理论已经成熟,可以修改。广义相对论是现代物理学的基础。它用优雅的数学术语描述了物质是如何引起时空弯曲的,因此也描述了物体在引力场中如何运动。自从相对论在1916年出版以来,广义相对论已经通过了要求很高的每项考验,并且对许多物理学家来说,认为它是错误的观念是sa讽。

但是,发展替代性引力理论的动机令人信服。在过去的几年中,宇宙学家得出了一个简单而又非常成功的宇宙模型。麻烦在于,它要求大多数宇宙充满我们看不见的神秘物质。特别是广义相对论–或更确切地说,它的非相对论性限制,也称为牛顿引力–如果我们调用大量的星系,就只能正确地描述星系的动力学“dark matter”。此外,有必要使用一种称为暗能量的奇异实体来解释最近发现的宇宙膨胀正在加速的现象。实际上,在宇宙学的标准模型中,诸如恒星,行星和物理学教科书之类的可见物质仅占整个宇宙的4%。

Faced with a lack of direct evidence for 暗物质, a small but growing band of physicists is proposing an alternative explanation: that our description of gravity is wrong. And if the discussions at a recent workshop on 暗物质 and alternative gravities held in April at the Royal Observatory in Edinburgh are anything to go by, they could be in for a bumpy ride.

怀疑种子

1933年提出了暗物质的解释,以解释为什么某些星团中的星系移动得比仅包含星系的星系移动得快。“baryonic”可以看到的事情。几十年后,在单个星系中发现了类似的行为,从而发现最外层恒星的旋转速度没有“drop off”作为距离的函数,但保持平坦(见图)。这些观测结果与牛顿引力直接矛盾,牛顿引力在银河外地区应如地球和太阳系一样正确。但是假设有“haloes”星系结构及其周围的隐形物质’熟悉的平方反比定律得以恢复。

Although firmly embedded in modern cosmology, 暗物质 is viewed by many physicists as a fudge factor. “Astronomers have no idea what 暗物质 is,”圣安德鲁斯大学的赵洪生说。“解释数据所需的一切,而不是最初对粒子物理学的基本预测。”这种情况使人想起了1840年代面临的一位天文学家,他们试图解释天王星轨道的异常现象时,假定了一个新的外行星,而不是废弃了牛顿。’s law. The crucial difference, of course, is that Neptune was discovered shortly afterwards, while 暗物质 remains elusive despite years of dedicated searches.

In 1983, however, Mordehai Milgrom, now at the Weizmann Institute in Israel, claimed he could explain the anomalous rotation of galaxies without invoking 暗物质. Instead, he modified 牛顿’s公式,以便在某些情况下,两个物体之间的重力衰减比它们之间距离的平方反比更缓和。米尔格罗姆的关键属性’s theory –称为修正的牛顿动力学,或MOND–是因为修改后的行为在一定的加速度(而不是距离)尺度以下开始。值得注意的是,米尔格罗姆(Milgrom)能够设置此通用参数的值,以使蒙蒙德(MONDMOND)非常好地描述了星系的动力学,同时保留了其他地方的牛顿引力。

但是,任何值得引以为豪的引力理论都不能仅仅考虑星系动力学。特别是,它需要能够解释光被大型物体弯曲的方式– a central prediction of general relativity that was dramatically confirmed during the solar eclipse of 1919. The most striking manifestation of this effect is gravitational lensing, whereby galaxies or clusters of galaxies cause light from background objects to appear as if it has come from several different sources. As with the dynamics of galaxies, however, general relativity is unable to account for the strength of some gravitational lenses without adding appropriate distributions of 暗物质 “by hand”.

扎根于牛顿力学,蒙德(MOND)没有希望解释光的弯曲。而且,米尔格罗姆’简单的公式违反了物理的一些基本定律,例如动量守恒。这促使1980年代和1990年代的理论家,特别是米尔格罗姆(Milgrom),荷兰格罗宁根大学的罗伯特·桑德斯(Robert Sanders)和耶路撒冷希伯来大学的雅各布·贝肯斯坦,开始着手将MOND转变为成熟的理论。这种情况在2004年达到高潮,当时Be​​kenstein发表了相对论版本的MOND,称为张量矢量标量理论或TeVeS。正是这种理论使许多天文学家,天体物理学家和宇宙学家开始更加认真地对待替代重力理论​​。

几何重力

了解电视–或其他任何引力理论–我们需要深入研究爱因斯坦’的理论。广义相对论是引力的几何理论,这意味着引力场是由时空的几何或曲率引起的。在数学上,曲率由对称张量描述,称为“metric”, which, in Einstein’的理论,纯粹是由局部物质决定的。尽管这是制定引力几何理论的最简单方法,但没有什么可以阻止我们在引力线中添加术语“action” of the theory, which governs the dynamics of the 公制 and therefore the way objects move.

This is precisely what Bekenstein did, by introducing a second 公制 to TeVeS that stretches space-time more globally. In order to connect the two 公制s to produce the physical 公制 experienced by real objects, Bekenstein added two extra terms into the TeVeS 行动. The first was a scalar field, which effectively alters the strength of gravity from place to place, and the second was a vector field that ensures light is affected by the 公制 too.

It may sound ad hoc, but the combined effect of replacing the Einstein 行动 with a scalar, vector and tensor field means that TeVeS has all the desirable 特征 of an alternative theory of gravity: it reduces to Einstein’s关于高速和大加速度的理论(从而解释了引力透镜);低速和小加速度(例如地球上的加速度)为牛顿重力;当加速度仍然较小时,则返回MOND(从而预测观察到的星系旋转曲线)。 TeVeS完全相对论,也可以在最大范围内对宇宙做出预测。

“在TeVeS之前,还有其他类似MOND的理论是相对论的,但是它们都有一些问题,例如违反因果关系,”参加爱丁堡会议的加拿大安大略省周边研究所的Constantinos Skordis说。“TeVeS showed us that you could match cosmological observations without 暗物质 by replacing general relativity with an alternative gravitational theory.”

对于替代重力理论​​,宇宙学也许是最艰巨的挑战。特别是,重力理论需要解释诸如星系和星系团之类的结构是如何形成的。这意味着它必须正确描述宇宙微波背景–源自大爆炸后大约38万年的称为重组的古代辐射。

重组之前,宇宙是一个热的,密集的等离子体,其中光子不断被带电粒子散射。但是,一旦宇宙冷却到足以形成中性原子,光子就能够不受阻碍地穿越太空,从而携带有关原始等离子体中密度不规则性的重要信息。如今,这些不规则现象会导致物质在某些区域比其他区域更聚在一起,在遍布深宇宙的微弱的微波辐射辉光中显得像冷热补丁。

General relativity is very good at describing how cosmic structure evolved from these acoustic oscillations, but only if we assume the universe contained at least as much 暗物质 as it did 重音 matter during recombination. Without 暗物质, which couples to matter but not to light, the density fluctuations would have been smoothed out by collisions with photons long before they had a chance to seed galaxies.

“People wrongly say that because modified gravity theories contain no 暗物质, there is no driving force to sustain the fluctuations through recombination,”牛津大学的Pedro Ferreira解释说,他曾与Skordis合作计算TeVeS的宇宙学含义。“But the extra fields in TeVeS, for example, can produce the same effect as 暗物质 despite having an energy density that is many orders of magnitude lower.”

为了了解TeVeS如何与真实的宇宙学数据相抗衡,Skordis和Ferreira将理论扩展到线性顺序,然后研究了当标量,矢量和张量场受到较小扰动时发生的情况。这使他们能够正确描述宇宙微波背景中温度波动的分布与角尺度的关系,该角尺度由一系列衰减的声峰组成。到目前为止,TeVeS是唯一对重力进行过如此详细预测的替代性重力理论,但费雷拉(Ferreira)敦促从事替代性重力理论的人们超越银河系自转曲线,自己做练习。

这样的人就是周边研究所的约翰·莫法特(John Moffat),他正在与乔尔·布朗斯坦(Joel Brownstein)合作研究一种称为标量张量向量重力(STVG)的理论。尽管像TeVeS一样,在标量场和向量场中引入了广义相对论,但在STVG中,向量场对应于Moffat称为“phion field”. By undergoing a process called Bose-Einstein condensation, the phion场 can produce a superfluid that causes gravity to be 强大 in the centre of galaxies and at cosmological scales yet similar to standard 牛顿ian-Einstein gravity at intermediate scales.

莫法特声称,除了很好地拟合星系数据外,他的理论还可以解释先锋探测器的任性路径。这两个航天器于1970年代发射升空,现在它们位于太阳系最远的地方,似乎正朝着内部太阳系异常加速。尽管这可能具有世俗的技术解释,但它为太阳系尺度上的引力理论提供了经典的检验。

暗能量

While describing the universe without 暗物质 is the main goal of STVG and other alternative gravity theories, it would be nice if such theories could get rid of dark energy at the same time. General relativity has a chequered history in this regard. Einstein initially introduced a constant term to account for the then-observational fact that the universe was static, only to have to remove it again a few years later when Hubble discovered that the universe is expanding. Einstein called the cosmological constant his “biggest blunder”。但是,如果他在1997年到场时看到超新星数据显示宇宙膨胀实际上正在加速,他将不得不再次将其放回原处。–只是被告知它的价值超过了120 orders of magnitude!

应对宇宙膨胀的一种理论是“conformal gravity”. Developed by Philip 曼海姆 of the University of Connecticut, it is perhaps the most radical of all the alternative gravity proposals. “我称之为TeVeS或STVG之类的理论‘Einstein-plus’ theories,” says 曼海姆. “They are designed to reduce to the Einstein equations on solar-system scales, while departing from them on galactic and larger distances. But while they avoid the need for any 暗物质 quite efficiently, none is able to address the cosmological-constant problem.”

曼海姆’的理论仍然是重力的几何理论,但他没有调整广义相对论,而是将其替换为基于四阶的理论“Weyl tensor”. While the mathematics of 共形引力 is complex, the theory has the highly desirable property that gravity is attractive at local scales yet repulsive at cosmological scales. As such, 曼海姆’s theory can account for both the accelerating-universe data and galaxy-rotation curves without 暗物质, dark energy or any “fudging”参数。但是,像STVG一样,仍然需要解决共形引力的全部宇宙学含义。

模式转变

Anyone working on alternative theories of gravity has to justify their ideas to mainstream cosmologists, most of whom simply do not see why there should be alternatives to 暗物质. “General relativity and 暗物质 are the standard paradigm, and until we see something inconsistent with that then there is no reason to look elsewhere,”美国达特茅斯学院的罗伯特·考德威尔坚持认为。

一些天文学家,例如皇家天文台的约翰·皮科克(John Peacock),也提醒我们,标准的牛顿-爱因斯坦引力在毫米尺度到冥王星的轨道上都非常有效,因此人们应谨慎对待“基于凌乱的天体物理学,例如星系形成”。确实,爱丁堡会议上的一位天文学家甚至说他只是参加了,以了解替代性引力理论“shot to pieces”.

But many in the alternative-gravity camp think that the idea of 暗物质 and dark energy has become so embedded that people are no longer looking at the problem scientifically. According to Stacy McGaugh of the University of Maryland, who became sceptical about 暗物质 in the mid-1990s, the best way to face up to the problem is to actually work with the galaxy data. “如果目前正在捍卫标准模型的所有著名宇宙学家在1970年代或1980年代被深深冻结,然后您今天把它们唤醒,并说[暗物质和暗能量]是答案,那么没有一个人会购买它,” he jokes.

但是替代重力阵营也有其自身的问题。例如,虽然TeVeS是桌上最先进的理论,但一些研究人员– notably Moffat and 曼海姆 – point out that the theory contains preferred frames of reference that violate basic relativity principles. Then there is the issue of aesthetics. Is plucking terms from the air and putting them into the 行动 of general relativity any more respectable than fitting the galaxy data with a polynomial or some other function? Finally, 暗物质 could turn up tomorrow in one of the many dedicated searches world wide.

替代重力运动不仅是理论物理学的坚实基础,而且无疑将为科学社会学家证明同样富有。“这是物理学的潜在范式转变,” says Moffat. “当您试图修改一个公认的理论(例如广义相对论)时,您将始终面临反对。”

现在说我们是否确实在目睹这种科学革命还为时过早。但是与此同时,我们应该记住,对替代性引力理论的某些最强烈的抵触曾经是针对爱因斯坦和牛顿本人的。

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