改变重力：你不能总是得到你想要的
Modifying Gravity: You Can't Always Get What You Want
摘要：
GR和标准模型的结合与我们太阳系以上尺度上的许多观测结果不一致。在宇宙学的和谐模型中，这些矛盾通过引入三个完全独立的新的应力能量成分--暴胀子、暗物质和暗能量--而被消除或缓解。每一个都意味着已经（或目前）主宰了宇宙的动态。对于这些暗区中的任何一个，仍然没有非引力的证据;也没有必要对标准模型进行扩展。另一种方法是想象GR本身必须被修改。某些规模上的巧合甚至表明，人们可能不希望使三个独立。因为他们必须处理最不同类型的数据，所以用修正的引力取代暗物质的尝试是最有争议的。一个现象学模型（或模型族），修正的牛顿动力学，在过去的几年里已经看到了几个协变实现。我们讨论了任何试图用修正的引力取代暗物质的模型都必须面对的一些挑战：伯克霍夫定理的丢失，以及它所暗示的计算简化;无法解释集群，无论是静态的还是相互作用的，以及随之而来的需要引入某种形式的暗物质，无论是热暗物质中微子，还是在修正的引力理论的新领域出现的暗场;宇宙膨胀对修正的力定律的侵入，这正是由于星系中牛顿引力失效的向心加速度与宇宙加速度之间尺度上的巧合而产生的。我们的结论是，虽然修正引力可能确实设法取代暗物质，但它很可能通过成为或合并暗物质理论本身来实现。
英文摘要：
The combination of GR and the Standard Model disagrees with numerous observations on scales from our Solar System up. In the concordance model of cosmology, these contradictions are removed or alleviated by the introduction of three completely independent new components of stress-energy -- the inflaton, dark matter, and dark energy. Each of these in its turn is meant to have (or to currently) dominate the dynamics of the universe. There is still no non-gravitational evidence for any of these dark sectors; nor for the required extensions of the standard model. An alternative is to imagine that GR itself must be modified. Certain coincidences of scale even suggest that one might expect not to have to make three independent. Because they must address the most different types of data, attempts to replace dark matter with modified gravity are the most controversial. A phenomenological model (or family of models), Modified Newtonian Dynamics, has, over the last few years seen several covariant realizations. We discuss a number of challenges that any model that seeks to replace dark matter with modified gravity must face: the loss of Birkhoff's Theorem, and the calculational simplifications it implies; the failure to explain clusters, whether static or interacting, and the consequent need to introduce dark matter of some form, whether hot dark matter neutrinos, or dark fields that arise in new sectors of the modified gravity theory; the intrusion of cosmological expansion into the modified force law, that arises precisely because of the coincidence in scale between the centripetal acceleration at which Newtonian gravity fails in galaxies, and the cosmic acceleration. We conclude with the observation that, although modified gravity may indeed manage to replace dark matter, it is likely to do so by becoming or incorporating, a dark matter theory itself.
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