MOST people have probably heard that the universe is expanding. Certainly 91av readers have, because I keep writing about it in this column. It is perhaps easy to accept the statement that the universe is expanding without thinking too deeply about it. It is just some weird physics indicating that, as time goes on, galaxies get further away from each other. Or maybe you have heard the old race car analogy, that galaxies moving apart are like two cars racing away from each other.
I personally detest the race car analogy and prefer the balloon with slightly magical dots. In this scenario, there are dots all over a balloon that is yet to be blown up. When we blow it up in real life, the dots would increase in size. In this analogy, let’s assume they don’t. What we are interested in is how the distance between the dots on the surface of the balloon grows as we put more air into it. Here, the dots aren’t racing away from each other. The balloon is expanding between them, increasing the distance that separates them.
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This is a pretty good analogy that relies somewhat on our geometric sensibilities. By this, I mean it relies on our sense of shapes and how they change over time. At its core, what we are trying to develop an intuition for is how we measure distances. This is also the fundamental goal of general relativity, Albert Einstein’s theory of gravity. In general relativity, the most important piece of information is what we call the metric. This is an equation that describes how distances are measured, and therefore also tells us about the shape space-time is taking.
The whole idea that space-time is expanding was first noticed as a mathematical consequence of general relativity by Georges Lemaître in 1927, when he solved Einstein’s equation and found a solution for the metric showing that distances grow with time. It became clear two years later that the universe we live in is governed by such a solution when astronomer Edwin Hubble noticed that galaxies appeared to be racing away from each other. Lemaître’s work provided a theoretical explanation for this empirical data: the yardstick for measuring cosmic distance was itself changing with time.
What is delightful about this realisation is it means we can quite reasonably say that space-time’s expansion is a gravitational effect! I enjoy this because it is so deeply counterintuitive to our usual understanding of gravity, which teaches us that it is a force that always draws things together. But in the scenario where gravity is a geometric effect, we are offered a broader range of gravitational possibilities.
It is worth noting that the geometric interpretation of general relativity hasn’t been universally popular. The late physicist Steven Weinberg, whom I admired deeply, wrote in his 1972 textbook on the subject that “the geometric interpretation of the theory of gravitation has dwindled to a mere analogy, which lingers in our language in terms like ‘metric’… but is otherwise not very useful”. I giggled while writing this up because, of course, I have just told you that calculating the metric – the distance measure – is the most important thing in general relativity, and here is a Nobel laureate saying it is a tepid analogy. In my defence, Weinberg goes on to say his views are “heterodox” and that most people would disagree with him.
There is another challenge with the balloon analogy and our reliance on geometric intuition. If space-time is expanding, then why does gravity seem to pull things together in many, if not all, situations? We usually fudge things a bit by saying, “Oh, it’s just on large scales”. This sounds very smart, and it’s true. But it isn’t all that helpful to the non-specialist, I suppose. In fact, a reader wrote in to ask me why the expansion doesn’t operate on the local level.
This is a question that takes me back to my days as a PhD student, when I was tasked with doing a calculation that it turns out I could have looked up in a textbook: the exact moment where local gravitational effects due to the presence of massive objects is so high that they overtake the large-scale expansion effects. This leads to mass clumping into objects that form structures like stars and galaxies – and, eventually, us.
The calculation where space-time is only expanding and this is the only gravitational effect at play is a very idealised scenario where matter was initially spread out perfectly evenly across the universe. In reality, tiny quantum fluctuations caused a little bit more matter to accumulate in some places and a little bit less in others. Those fluctuations in the early universe caused clumping of mass through local gravitational effects, which can overcome what we call the background expansion.
And a good thing too, because that is what makes it possible for us to be here to talk about it.
Chanda’s week
What I’m reading
I’m quite enjoying Shark Heart: A love story by Emily Habeck.
What I’m watching
I’m on a Tom Cruise kick (again).
What I’m working on
How to strike a balance between being a good advocate and taking care of my health.
Chanda Prescod-Weinstein is an associate professor of physics and astronomy, and a core faculty member in women’s studies at the University of New Hampshire. Her most recent book is The Disordered Cosmos: A journey into dark matter, spacetime, and dreams deferred
