“Dark energy,” a mysterious force thatcounters gravity, has been proposed to explain why the universe is expanding atan accelerating rate. Mathematicians at UC Davis and the University ofMichigan, Ann Arbor, argue for an alternative. Galaxy cluster image from theHubble Space Telescope.

 “暗能量”作为反引力的神秘力量，被提出用来解释宇宙正在加速膨胀的原因，但是加州大学戴维斯分校和密歇根大学安娜堡分校的数学家们争论出了另一种原因。图片来自哈勃太空望远镜的星系团簇图像。

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Three mathematicians have a differentexplanation for the accelerating expansion of the universe that does withouttheories of "dark energy." Einstein's original equations for GeneralRelativity actually predict cosmic acceleration due to an"instability," they argue in paper published recently in Proceedings of the Royal Society A.

三位数学家们对宇宙加速膨胀提出了不同的解释，认为不需要“暗能量”理论。他们的论文最近发表在“Proceedings of the Royal Society A”称爱因斯坦原始的广义相对论方程实际上预测了由于“不稳定性”而导致的宇宙加速度。

About 20 years ago, astronomers made astartling discovery: Not only is the universe expanding—as had been known for decades—butthe expansion is speeding up. To explain this, cosmologists have invoked amysterious force called "dark energy" that serves to push space apart.

大约20年前，天文学家发现了一个令人吃惊的现象：不仅是宇宙正在膨胀，这在几十年前就已经知道，而是膨胀正在加速。为了解释这一点，宇宙学家们引入了一种叫做“暗能量”的神秘力量，认为它将空间分开。

Shortly after Albert Einstein wrote hisequations for General Relativity, which describe gravity, he included an"antigravity" factor called the "cosmological constant" tobalance gravitational attraction and produce a static universe. But Einsteinlater called the cosmological constant his greatest mistake.

阿尔伯特·爱因斯坦在为描述引力的广义相对论写下了他的等式之后不久，他列提出一个称为“宇宙常数”的“反引力”因素，用来平衡引力以及产出一个静态的宇宙。但后来爱因斯坦把宇宙常数称为他最大的错误。

When modern cosmologists began totackle cosmicacceleration and dark energy, they dusted off Einstein's cosmologicalconstant as interchangeable with dark energy, given the new knowledge about cosmicacceleration.

当现代宇宙学家开始解决宇宙加速和暗能量问题时，他们将爱因斯坦的宇宙常数与暗能量互换，产生了关于宇宙加速的新知识，

That explanation didn't satisfymathematicians Blake Temple and Zeke Vogler at the University of California,Davis, and Joel Smoller at the University of Michigan, Ann Arbor.

这个解释并不能说服加州大学戴维斯分校的数学家Blake Temple和Zeke Vogler以及密歇根大学安阿伯分校的Joel Smoller。

"We set out to find the bestexplanation we could come up with for the anomalous acceleration of thegalaxies within Einstein's original theory without dark energy," Templesaid.

“我们着手从爱因斯坦原始理论中，没有暗能量的星系异常加速理论里寻求最佳解释，”Temple说。

The original theory of General Relativityhas given correct predictions in every other context, Temple said, and there isno direct evidence of dark energy. So why add a "fudge factor" (darkenergy or the cosmological constant) to equations that already appear correct?Instead of faulty equations that need to be tweaked to get the right solution,the mathematicians argue that the equations are correct, but the assumption ofa uniformly expanding universe of galaxies is wrong, with or without darkenergy, because that configuration is unstable.

Temple说，原始的广义相对论在其他情况下都给出了正确的预测，而且没有暗能量的直接证据。那么为什么把一个“模糊因子”（暗能量或宇宙常数）添加到已经显示正确的方程？与其把该方程当成错误的，而寻求调整以获得正确的解决方案，数学家们认为方程本身是正确的，但是对于均匀膨胀的星系宇宙的假设是错误的，不论有没有暗能量，因为该配置并不稳定。

An unstable solution

不稳定的解决方案

Cosmological models start from a"Friedmann universe," which assumes that all matter is expanding butevenly distributed in space at every time, Temple said.

Temple说，宇宙学模型是从一个“弗里德曼宇宙”开始的，它假定宇宙中的所有物质都在膨胀，但是均匀分布在宇宙中。

Temple, Smoller and Vogler worked out solutions to General Relativitywithout invoking dark energy. They argue that the equations show that theFriedmann space-time is actually unstable: Any perturbation—for example if thedensity of matter is a bit lower than average—pushes it over into anaccelerating universe.

Temple，Smoller和Vogler找出了在没有暗能量的情况下广义相对论的解决方案。他们认为这些方程表明弗里德曼时空实际上是不稳定的：例如，如果物质的密度比平均值低一些，任何扰动都会把它推到一个加速的宇宙中。

Temple compares this to an upside-downpendulum. When a pendulum is hanging down, it is stable at its lowest point.Turn a rigid pendulum the other way, and it can balance if it is exactlycentered—but any small gust will blow it off.

Temple用颠倒的钟摆做对比。当一个钟摆垂下时，在最低点它是稳定的。以相反方向摆动时，正好居中时它可以平衡，但是任何微小的一阵风都会吹走它。

This tells us that we should not expect tomeasure a Friedmann universe, because it is unstable, Temple said. What weshould expect to measure instead are local space-times that accelerate faster.Remarkably, the local space-times created by the instability exhibit preciselythe same range of cosmic accelerations as you get in theories of dark energy,he said.

Temple说这告诉我们，我们不应该想着去衡量弗里德曼宇宙，因为它并不稳定。我们应该期望衡量的是加速更快的本地时空。他说值得注意的是，由不稳定性造成的本地时空的宇宙加速范围和暗能量理论呈现出的完全相同。

What this shows is that the accelerationof the galaxies could have been predicted from the original theory of General Relativity withoutinvoking the cosmological constant/dark energy at all, Templesaid.

Temple说，这表明星系的加速可以从原始的广义相对论中预测出来，而根本不需要宇宙常数/暗能量。

"The math isn't controversial, the instabilityisn't controversial," Temple said. "What we don't know is, does ourMilky Way galaxy lie near the center of a large under-density of matter inthe universe."

“数学计算没有争议，不稳定性也没有争议，”Temple说。 “我们不知道的是，我们的银河系是否靠近宇宙中物质密度较低的中心。”

The paper does include testablepredictions that distinguish their model from dark energy models, Temple said.

Temple说，这篇论文确实包含了区别他们的模型与暗能量模型的可测试的预测。

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