In Einstein’s special theory of relativity, all observers measure the speed of light, c, to be the same. However, this refers to the round-trip speed, where a clock at the origin times the outward and return trip of light reflecting off a distant mirror. Measuring the one-way speed of light is fraught with issues of clock synchronisation, and, as long as the average speed of light remains c, the speeds on the outward and return legs could be different. One objection to this anisotropic speed of light is that views of the distant universe would be different in different directions, especially with regard to the ages of observed objects and the smoothness of the Cosmic Microwave Background. In this paper, we explore this in the Milne universe, the limiting case of a Friedmann–Robertson–Walker universe containing no matter, radiation, or dark energy. Given that this universe is empty, it can be mapped onto flat Minkowski space-time and so can be explored in terms of the one-way speed of light. The conclusion is that the presence of an anisotropic speed of light leads to anisotropic time dilation effects, and hence observers in the Milne universe would be presented with an isotropic view of the distant cosmos.

中文翻译：

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单向光速与米尔恩宇宙

在爱因斯坦的狭义相对论中，所有的观察者都测量光速，C, 是一样的。然而，这是指往返速度，其中原点的时钟计算从远处镜子反射的光的往返行程。测量单向光速充满了时钟同步问题，而且只要保持平均光速不变C，向外和返回腿上的速度可能不同。对这种各向异性光速的一个反对意见是，遥远宇宙的视角在不同方向上会有所不同，尤其是在观察到的物体的年龄和宇宙微波背景的平滑度方面。在本文中，我们将在米尔恩宇宙中探索这一点，这是弗里德曼-罗伯逊-沃克宇宙中不包含物质、辐射或暗能量的极限情况。鉴于这个宇宙是空的，它可以映射到平坦的闵可夫斯基时空，因此可以根据单向光速进行探索。结论是，光速各向异性的存在会导致各向异性的时间膨胀效应，因此米尔恩宇宙中的观察者将看到遥远宇宙的各向同性视图。