原作者：
来源Revised theory of gravity doesn\'t predict a Big Bang
译者ziyuang
题图：宇宙的时间线
(PhysOrg.com) -- The Big Bang theory has formed the basis of our understanding of the universe's origins since it was first proposed in 1927 by Georges Lemaitre. And for good reason: the theory is supported by scientists' latest observations and experiments, and is based on Einstein's widely accepted theory of general relativity. But scientists are always on the lookout for any evidence that might suggest an alternative to the Big Bang. The latest in this area of research comes from astrophysicists Maximo Banados and Pedro Ferreira, who have resurrected a theory of gravity from the early 20th century and discovered that a modified version of the theory may hold some surprises.
(PhysOrg.com) -- 1927年 Georges Lemaitre 首次发表了宇宙大爆炸理论，自那个时候起，它就成为我们理解宇宙起源的基础。这样做理由是很充分的：科学家们最新的观测和实验结果，都为该理论提供了支持，况且它建立在 Einstein 那广为人接受的广义相对论之上。但科学家们从来没有放过任何能通往其他理论的线索，它们将是除大爆炸理论外的别种选择。天体物理学家 Maximo Banados 和 Pedro Ferreira 发表了该领域的最新研究成果，他们将20世纪早期建立的一个引力理论重新带回到人们视线中。经过修正后，他们发现这个新理论带来了一些令人惊奇的结论。
In a recent study published in Physical Review Letters, Banados and Ferreira have reconsidered the theory of gravity proposed by Arthur Eddington, a contemporary of Einstein. Eddington is perhaps best known for his trip to the Island of Principe on the west coast of Africa in 1919, where during a solar eclipse he observed that the Sun's gravity does indeed bend starlight, providing one of the earliest confirmations of general relativity.
在一项最近发表在Physical Review Letters上的研究里，Banados 和 Ferreira 重新考虑了 Arthur Eddington 提出的引力理论。Eddington 被称为当代的 Einstein，他最著名的也许是1919年在非洲西海岸 Principe 岛的日食之旅，他在那儿观察到太阳的万有引力确实弯曲了星星的光线，从而为广义相对论提供了最早的证据。
Although Eddington played a significant role in developing general relativity, during the following decades he became more interested in finding a theory to unify gravity and quantum mechanics - a task that is still being studied today. In 1924, Eddington proposed a new “gravitational action” as an alternative to the Einstein-Hilbert action, which could serve as an alternative starting point to general relativity. In astrophysics, a gravitational action is the mechanism that describes how gravity can emerge from space-time being curved by matter and energy. However, Eddington’s theory of gravity only worked for empty space and didn’t include any source of energy such as matter, making it an incomplete theory.
虽然 Eddington 在广义相对论的发展过程中扮演了举足轻重的角色，但在其后的数十年里，他对寻找能统一引力和量子力学的理论更感兴趣——这是一项至今仍处于研究中的任务。1924年，Eddington 提出了一个新的“引力作用量”，它作为除 Einstein-Hilbert 作用量的另一个选择，可以当作是广义相对论的另一个出发点。在天体物理学中，引力作用量描述了引力在被物质和能量弯曲的时空中将如何出现的机理。然而，Eddington 的引力理论仅对不包含任何如物质等能量源的真空适用，这使得它不能成为一套完整的理论。
Since Eddington’s proposal, scientists have attempted various ways of including matter into the theory, although they have run into problems. In this study, Banados and Ferreira have tried a new way to extend the theory to include matter by using a gravitational action called the Born-Infeld action.
在 Eddington 提出这个想法后，科学家们尝试了各种不同的办法试图将物质整合到他的理论当中，虽然在其中遇到了很多问题。在这项研究里，Banados 和 Ferreira 尝试了一种新办法，即被称为 Born-Infeld 作用量的引力作用量，来使该理论涵盖物质，从而拓展了该理论。
In their analysis, the scientists found that a key characteristic of Eddington’s revised theory of gravity is that it reproduces Einstein gravity precisely in the vacuum conditions (with no matter), but it produces new effects when matter is added. Due to this characteristic, the revised theory has implications especially for high-density regions, such as in the very early Universe or within a black hole. For instance, the theory predicts a maximum density of homogeneous and isotropic space-time, which could have implications for black hole formation.
在他们的分析中，两位科学家发现了修正后的 Eddington 引力理论的一个关键特征是，它在真空（没有物质）条件下精确地重现了 Einstein 引力论，但当加入物质后，它会产生新的效应。由于这个特征，修正后的理论在像在非常早期的宇宙或黑洞等高密度区域中的表现尤为明显。例如，该理论预测了均匀且各向同性时空的最大密度，而这可能隐含了黑洞形成过程的某些信息。
More intriguingly, the theory could lead to an entirely new view of the Universe that doesn't include a Big Bang. In Big Bang theory, the state of the Universe is a singularity in early times, meaning that the Universe was once infinitely small. However, Eddington’s revised theory requires a minimum length of space-time at early times, which means that the Universe could not have been a singularity. The theory predicts that, depending on the Universe’s initial density, it may have loitered for a long time at a relatively small size before growing large enough to be controlled by standard cosmological evolution. Another possibility, depending on the initial conditions, is that the Universe could have undergone a bounce, resulting from the collapse of a previous Universe. Any kind of singularity-free Universe would solve the singularity problem that has bothered scientists about general relativity, since a singularity cannot be mathematically defined.
更引人入胜的是，从这个理论能得出一个全新的不包含大爆炸的宇宙模型。在大爆炸理论里，早期宇宙的状态是一个奇点，这意味着宇宙曾经是无穷小的。然而，修正后的 Eddington 理论要求早期时空的尺度应该具有某个最小值，这意味着那时的宇宙不可能是一个奇点。该理论断言，在宇宙膨胀至服从标准宇宙学演化法则的规模前，宇宙可能曾经在很长一段时间里保持着相对小的尺寸，具体时长取决于宇宙的初始密度。另一种可能是，宇宙可能曾经经历过一次由上一个宇宙塌缩而造成的跃变，具体情况取决于宇宙的初始条件。任何一种没有奇点的宇宙模型都将解决广义相对论中困扰着科学家们的奇点问题，因为奇点在数学上是无法定义的。
“Taking as a starting point what is a very old idea, we have ended up with a theory that has this very interesting property of not having singularities,” Ferreira, a professor of astrophysics at the University of Oxford, told PhysOrg.com. “It was unexpected and definitely not what we were looking for.”“从一个非常古老的想法出发，我们最终得到一套理论，它拥有非常有趣的特性：没有奇点。” 牛津大学天体物理学教授 Ferreira 告诉我们 PhysOrg.com，“它出乎所有人的意料，这绝对不是我们最初要寻找的东西。”
In the future, Banados and Ferreira hope to perform a more detailed analysis of the gravitational Born-Infeld action. While the current study only looks at the classical behavior of the theory, there could also be quantum behavior, such as with the bounce concept. In addition, the scientists plan to look at the possible effects of a cosmological constant, which they did not investigate here. However, they note that the theory is still in the early conceptual stages, and has a long way to go before they know how accurate it is.
在不久的将来，Banados 和 Ferreira 希望能对 Born-Infeld 引力作用量作更详细的分析。虽然现在的研究仅仅集中在这套理论的经典行为上，但是量子行为也可能存在，例如在跃变的概念下。另外，两位科学家计划下一步研究它可能对宇宙常数造成的效应，这项工作没有在这篇论文中展开。不过，他们也指出，该套理论仍然处于概念化的早期阶段，在他们懂得如何去将它精确化之前，还有很长的一段路要走。
“The alternatives to Einstein's theory are all hypothetical possibilities,” Ferreira said. “The goal is to try and find some key observational test that may distinguish between Einstein's theory and the one we have stumbled upon.”
“对于Einstein 理论而言，所有可能的替代品都只存在于假设中，” Ferreira 说，“我们的目标是设法找到一些关键的可观测试验，将 Einstein 理论和这套我们偶然遇到的理论区分开来。”
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