What does it mean when we say that space-time is expanding?

When it comes to something as complex as space-time, intuition comes in different ways, depending on the level of science you have access to, writes Chanda Prescod-Weinstein

By Chanda Prescod-Weinstein

THE origin story we tell about the universe goes roughly like this: there may have been a big bang (we aren’t sure), then space-time expanded extremely rapidly (we think) and has continued to expand ever since, even picking up the pace in relatively recent times (we are fairly certain). It is a nice story, and one I largely believe is true. But what does it mean when we say that space-time expanded?

In presentations to broad audiences, I explain by asking people to imagine an unfilled balloon with special dots on it. Imagine that as the balloon fills with …