ONE of the best parts of sharing my scientific interests with the public is how engaged people are with the ideas. In my time writing for New Scientist , I have received lots of lovely and thoughtful notes from diligent readers. Occasionally, there is a question that I quietly respond to in a future column. Since I only write once a month, I can’t address all of the questions, but I was particularly pleased last month to get an email asking me in what ways calculations in cosmology are similar to calculations involving fluids. The person who sent …