KentuckyFC writes: In the late 1990s, astronomers discovered that not only is the universe expanding but the rate of expansion is increasing. In other words, every galaxy in the cosmos is accelerating away from us. Since then, cosmologists have been intensely interested in studying the effect in more detail. Now one astrophysicist has calculated that quantum mechanics places important limits on how the expansion can be observed. The conclusion comes from a simple thought experiment. Imagine two quantum particles in an accelerating universe like the one we live in. As the universe expands, the separation between these particles increases, an effect that is measurable by their redshift (which is how astronomers measure the movement of distant galaxies). However, quantum mechanics naturally introduces some uncertainty into the position of both particles which can swamp any distance changes caused by cosmic acceleration. And the critical distance at which this uncertainty becomes important is 60 metres. That means there is no measurement, even in principle, that could unambiguously reveal cosmic expansion on scales smaller than 60 metres or so. And there is no sense in which a region of space can be said to expand on these smaller scales. That's interesting because it is the first theory to predict a natural boundary between the quantum and cosmic scales and also predict the scale at which this happens.