Another re-phrase: how can we detect that space has stretched out if all of our rulers also get stretched by exactly the same amount?<p>The answer is that we have a ruler that <i>doesn&#x27;t</i> get stretched in this way: light. The speed of light is a constant dictated by the laws of physics; stretching out our flashlight to twice its normal size wouldn&#x27;t make the light it emits go twice as fast. So if you just measure the time it takes for a light ray to go from one point to another, you can compute the distance that it must have traveled, and if that distance changes, then you know that the space in between must have been stretched.