杏吧原创

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THE origin story we tell about the universe goes roughly like this: there may have been a big bang (we aren鈥檛 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 air, the dots stay the same size, but, as one would expect, the distance between the dots expands. Pretend those dots are galaxies and, roughly speaking, this is what is happening to space-time.

The expansion of space-time means the space-time between galaxies grows, making the distance between galaxies larger as time goes on. There are exceptions to this. Galaxies that are gravitationally bound to each other won鈥檛 be pulled apart; for example, the Milky Way and Andromeda are on course to eventually collide and merge. The Milky Way also has many satellite galaxies in its orbit or otherwise gravitationally bound to it.

Other than ties through gravity, galaxies are generally 鈥渕oving鈥� away from each other because space-time continues to grow between them. The balloon analogy helps us understand this with some intuition, without needing to take several years of courses in order to fully understand general relativity as a technical subject. However, it also introduces problems that one can avoid if the subject is only looked at in mathematical terms.

As a scientist with deep knowledge of general relativity 鈥� the first advanced physics subject I trained in 鈥� I don鈥檛 necessarily need to spend a lot of time interpreting the equations to suit my intuition. Rather, part of my job is to develop new intuition based on the latest results from mathematical derivations, lab experiments and astronomical observations. The way I lead my life as a scientist involves a foundational commitment to looking at the world as it is and revising my understanding as I gather new information.

In the case of expanding space-time, I have to think carefully about what I mean by 鈥渟pace-time鈥�. General relativity teaches me to think about space-time as a phenomenon that is described by the ruler we use to measure distances, an equation we call the metric. Therefore, expanding space-time means that the metric changes with time in such a way that spatial distances get larger as time goes on. This is something of an idealisation: we know this isn鈥檛 happening on the scales of everyday life here on Earth.

I am lucky because I can work through the equations and get a feeling for how this works mathematically, which makes it intuitive in a language that has more words than everyday English. This raises the question of translation from my technical, mathematical vocabulary into phrases that are comprehensible to broader audiences.

So we return to the balloon, which, yes, gives some good intuition, but can also get a reader thinking. Specifically, I know at least one person who is a regular reader of this column (thank you!) started to wonder where the space-time is expanding into. Certainly that is a limit of the balloon analogy. When we are blowing up a balloon, the balloon is expanding into the room we are in. Another way of putting it is that the balloon exists in a background, where the room is that background.

I introduce this way of thinking because it is one we scientists use when we describe general relativity, which we call a 鈥渂ackground independent theory鈥�. Relativity is described in this way because it doesn鈥檛 matter how we map out the coordinates in this theory, the physical reality it describes stays the same. There is a lot more to unpack here and not enough space (or time!), so let me just point out that this implies the space-time of general relativity doesn鈥檛 have a room. In other words, space-time is expanding, but into nothing in particular. The expansion is simply a growth in distances. It isn鈥檛 a material either, so it isn鈥檛 coming from somewhere. The distances just grow and grow.

Weird, right? If you don鈥檛 think so, that itself is odd. As I write in The Disordered Cosmos (), the universe is always more queer and fantastical than we think. The fundamental nature of space-time is an example of this fact. We exist in space-time and nothing in daily life necessarily makes us think it is a particularly strange phenomenon. Then again, we exist, and we are pretty odd, so maybe we should expect the nature of most things to be, on some level, unexpected.